Jig and Fixture Design Manual - Erik K. Hendriksen_3709

8/19/2019 Jig and Fixture Design Manual - Erik K. Hendriksen_3709

http://slidepdf.com/reader/full/jig-and-fixture-design-manual-erik-k-hendriksen3709 1/2237



Henriksen, Erik Karl, 1902-Jig andfixture design manual.

Includes bibliographical references1. Jigs and fixtures-Design andconstruction-Handbooks, manuals,

etc,

I. Title.

TJ1187.H46 621.9'92 73-8810

ISBN 0-8311-1098-8

JIG AND FIXTURE DESIGNMANUAL

Copyright ©1973 by Industrial



Press Inc., New York, N.Y. Printedn the United States

of America. All rights reserved. Thisbook or parts thereof may not bereproduced in any

form without permission of thepublishers.

Contents

The Use of Metric Units Preface

1 Introduction

2 Preliminary Analysis and FixturePlanning



3 The Fixture Design Procedure

4 Locating Principles

5 Preparation for Locating

6 Design of Locating Components

7 Loading and Unloading

8 Chip Problems

9 Centralizers

10 Clamping Elements

11 Equalizers

12 Supporting Elements



21 Universal and AutomaticFixtures

22 Economics

ppendix 1 Measuring Angles inRadians

ppendix II Transfer of Tolerancefrom the Conventional

Dimensioning

System to the Coordinate Systemppendix III The Dimensioning of

Fixtures by Stress Analysisppendix IV Metric Conversion

Tables for Linear Measure Index



I

ii I 5

19

25

32

42

68

82

87

105



136

147

151

154

170

185

194

219

244

259



281

295

298

299 302 304 307

The Use of Metric Units

Dimensions and other data are, as ageneral rule, given in English unitsand in metric units. In the text themetric data are put in parentheses

following the English data; in tablesthe metric units are usually placedn separate columns. The accuracy

with which the conversions are



performed varies with the natureand purpose of the data quoted.Where accurate conversion of

dimensions is made, it is based on 1nch = 25,4 mm EXACT. Several

tables for the conversion of inches

and millimeters, feet and meters,and pounds and newtons arepresented in Appendix IV. Precisench dimensions, written with three

or four decimal places, areconverted, as a rule, to the nearest1/100 or 1/J000 mm. The purpose

s to present the result of theconversion in a mannerrepresentative of the equivalentevel of workshop accuracy. In othe



metric country, the designer wouldnot choose the length as 16 X 25.4 =406.4 mm but would make it an

even 400 mm. Likewise, anmerican component manufacturer

may market an eyebolt 6 inches in

ength, while a Europeanmanufacturer may have anequivalent eyebolt that is 150 mm,not 152.4 mm, long. Where an

merican screw thread is convertedt is to the nearest metric screw

thread. No attempt is made to

convert American standard fits andtolerances. Parts with metricdimensions should be designedwith the ISO Limits and Fits; a



collection of data for this system isfound in Machinery's Handbook,19th ed„ pages 1529 through 1538.

In some cases, such as indimensioned drawings and theiraccompanying calculations, no

conversion is attempted. To writetwo different sets of dimensionsnto the drawings and detailed

calculations would be confusing.

The purpose of such calculations isto explain the method, rather thanto illustrate one particular size of an

object. Also, for some of thecommercial componentsconcerning a specific Americanproduct, only English dimensions



are quoted.

Many of the book's equations are of

such a nature that conversion isunnecessary since they are equally

alid in English and in metric units

Other equations, of an empiricalnature, include numericalcoefficients the values of whichdepend on the type of units used. In

all such cases, separate equationsare given for use with English andwith metric units. In most of the

numerical examples, the given dataas well as the calculated end resultsare stated in English as well as inmetric units.



It should be noted that conversionshave been made to units in theInternational System (SI) which is

rapidly becoming the recognizedstandard throughout the world,Thus the reader will find that the

newton (N) and the kilonewton(kN) are the metric units used forforce while the gram (g) and thekilogram (kg) are used for weight

(mass).

Preface

The book is written as a textbook and reference source, and is meantto be used by the experienced

practitioner as well as the beginner



whether he is a technician inndustry or a college student.

The author concentrates on threemajor objectives: (1) to describe thefixture components in full; (2) to

present the fundamental principlesfor efficiently combining thecomponents into successfulfixtures; and (3) to apply basic

engineering principles to themechanical and economic analysisof the complete design. These three

tasks are supported by acomprehensive description of commercjally available fixturecomponents, a four-point, step-by-

step method and comprehensive



check list for the design procedure,applicable equally to all types of fixtures, and also calculation

methods for the stress anddeformation analysis of the fixturebody and its major components.

The use of a variety of calculationmethods is demonstrated by numerical examples.

The author has avoided presentinga confusion of detailed drawings of complicated fixtures. Instead, there

are 15 actual cases included,ranging from the simplest drill platto some complex and quiteadvanced fixtures for milling and

other operations. For each category



of machining operations, there is adefinition of its characteristicfixture requirements and one or

more typical examples. In addition,the book includes the designprinciples for fixtures of the most

mportant non-machiningoperations, such as welding andassembly.

number of the line drawings inthe book are executed in a recently ntroduced drawing style in which

two line thicknesses are used foredges and contours. The heavierines indicate the contours of

surfaces that are surrounded by air



With the dominant position of themetric system outside of the UnitedStates and the approaching

ntroduction of this system withinthis country, metric units are usedtogether with the English units

throughout the book.

Four informative appendices withllustrations should prove to be

helpful to the reader, they are"Measuring Angles in Radians,""Transfer of Tolerances from the

Conventional Dimensioning Systemto the Coordinate System,""Dimensioning of Fixtures," andastly, "Metric Conversion Tables of

Linear Measure."



CHAPTER

1

Introduction

Definition, Purpose, and

dvantages

fixture is a special tool used for

ocating and firmly holding aworkpiece in the proper positionduring a manufacturing operation.

s a general rule it is provided with

devices for supporting and clampinthe workpiece. In addition, it may also contain devices for guiding the

tool prior to or during its actual



operation. Thus, a jig is a type of fixture with means for positively guiding and supporting tools for

drilling, boring, and relatedoperations. Hence, the drill jig,which is usually fitted with

hardened bushings to locate, guide,and support rotating cutting tools.

The origin of jigs and fixtures can

be traced back to the Swiss watchand clock industry from which,after proving their usefulness, they

spread throughout the entire metalworking industry. Contrary towidespread belief, the recentntroduction of the N/C machine

tools has not eliminated the need



2. It also reduces working time inthe various phases of the operationn the setup and clamping of the

work, in the adjustment of thecutting tool to the requireddimensions, and during the cutting

operation itself by allowing heavierfeeds due to more efficient work support.

3. It serves to simplify otherwisecomplicated op-



Courtesy of Monarch Machine Tool

Co.

Fig. 1-1. Close-up of an aircraft fuelpump body housing mounted in its

fixture on an N/C lathe.

erations so that cheaper, relatively

unskilled labor may be employed to



perform operations previously reserved for skilled mechanics. Jigsand fixtures expand the capacity of

standard machine tools to performspecial operations, and in many cases, they make it possible to use

plain or simplified, and thereforeess expensive, machinery instead

of costly standard machines. Inother words, they turn plain and

simple machine tools into highproduction equipment and convertstandard machines into the

equivalent of specializedequipment.

4. By maintaining or even

mproving the inter-changeability o



the parts, a jig or fixture contributeto a considerable reduction in thecost of assembly, maintenance, and

the subsequent supply of spareparts.

In effect, jigs and fixtures reducecosts and improve the potential of standard machines and the quality of the parts produced.

INTRODUCTION

Ch. 1

Jigs and fixtures represent anembodiment of the principle of the

transformation of skill. The skills o



the experienced craftsmen,designers, and engineers arepermanently built into the fixture

and are thereby made continuouslyavailable to the unskilled operator.One important goal is to design a

fixture in such a way as to make itfoolproof, and thereby contribute toadded safety for the operator as welas for the work.

pplication and Classification of Jigs and Fixtures

The obvious place for jigs andfixtures is in mass production,where large quantity output offers

ample opportunity for recovery of



the necessary investment. Howeverthe advantages in the use of jigs andfixtures are so great, and so varied,

that these devices have alsonaturally found their way into theproduction of parts in limited

quantities as well as intomanufacturing processes outside ofthe machine shop, and even outsideof the metalworking industry. The

many problems of geometry anddimensions encountered within theaircraft and missile industry have

greatly accelerated the expandeduse of jigs and fixtures.

Within the machine shop, jigs and

fixtures are used for the following



operations: Boring, Broaching,Drilling, Grinding, Honing, LappingMilling, Planing, Profiling,

Reaming, Sawing, Shaping, SlottingSpot-facing, Tapping, and Turning,

systematic master classification

of machining fixtures according tothe characteristics of the operations shown in Table 1-1.

Outside of the machine shop,fixtures may be applied toadvantage for: Assembling,

Bending, Brazing, Heat treating,Inspecting, Riveting, Soldering,Testing, and Welding. Such fixturescan be characterized as manual

work fixtures and may be classified



Quite often, drill jigs are built toallow a sequence of operations to bperformed at one location, such as

drilling followed by tapping orreaming, or drilling to increasingly arger diameters, or drilling

followed by countersinking orboring, etc. Less frequently, afixture may be designed with

nterchangeable or adjustablenserts, such that it can be used for

several parts of slightly modified

shape or dimension. This concepteads logically to the "universalfixture," although "universal" may be an exaggeration. A universal

fixture is constructed from building



blocks assembled on a commonbase plate to form a fixture for oneparticular operation. After its use is

completed, it is disassembled andthen reassembled to a new anddifferent configuration. Universal

fixtures and jigs of this and other

Table 1-2. Classification of ManualWork Fixtures

Ch. 1

INTRODUCTION

types are commercially available.They may be less rigid than fixtures

of one-piece design but they may,



on the other hand, have economicaadvantages for short-runproduction since their components

can be reused.

This is generally not the case,

however, with ordinary jigs andfixtures of special design.Disassembly and reuse of components is ordinarily not

economically feasible; thus, whenthe production is completed, thetooling costs must have been

written off, hopefully, with a savingand a profit.

Due to the specialized nature of

these tools, their designs are as



aried as the parts which they are toserve. There are undoubtedly nottwo identical fixtures in the whole

world. The design of these tools is,therefore, a task that challenges thedesigner's creative imagination and

draws heavily upon his experienceand ingenuity. Nevertheless, fixturedesign is not a task reserved only for geniuses. It is governed by rules

and these rules can be learned,mastered, and practiced by averagepersons.

s evidenced by the structure of this book, that vast variety of possible configurations of fixtures

can be subdivided into groups of



similar shapes; the groups can bedefined and classified, the classescan be systematized, and each

subdivision within the system canbe evaluated for its good and badproperties, and accordingly assigned

to its optimum application area.

The design process is systematizedto an even higher degree. It is

governed by a logical, step-by-stepprocedure that is time tested andeads to a useful end result. It is a

cookbook recipe. As such, itsupports the beginner, it guides theexperienced practitioner, and it mayeven be of assistance to the expert.



ny mechanical design isncomplete without a

documentation of its structural

ntegrity; that is, a survey of theoads acting on the structure, and

an analysis to the effect that

stresses and deformations fromthese loads remain withinprescribed limits, as defined by recognized factors of safety and

tolerances of accuracy. The penalty for underdimen-sioning is breakageor warpage of the fixture and is

clearly observable. Even one, or afew such cases, would be a lesson tothe design department and result inan upgrading of thickness



standards. The penalty for overdimensioning of a fixture is "only"excessive weight, which is more

ikely to go unnoticed.

Every design activity must never

ose sight of the fact that thepurpose of a fixture is economy.Each design assignment will have a

ariety of solutions with different

degrees of operational economy, a

different useful life; and differentcosts. The deciding factor, whichmust always be taken intoconsideration, is the number of parts to be produced.



Typical Examples

Before entering upon the detailed

discussion of fixture design, asampling of different fixtures willbe shown and described. They have

been selected to represent twocharacteristic types of fixtures,namely, miffing fixtures and drilligs. In addition, they show a

considerable number of typicaldetails and thus serve asntroductory examples for the

subjects following.

The two principal types are shownschematically in Fig. 1-2. Each of

the two sketches shows a part,



typical for the operation, supportedon buttons and clamped by aclamping device likewise

appropriate for the purpose. Theprincipal difference between thetwo types lies in the means for

obtaining dimensional control. Inthe milling fixture (Fig. I-2a), therelative position between cutter andwork is initially found by means of

the tool setting block /, shown tothe left, and from there on theaccuracy of the



Fig. 1-2. The principal types of fixtures. (Top) A milling fixture;(Bottom) A drill jig.



INTRODUCTION

Ch. 1

work depends on the accuracy andrigidity of the machine tool. In thedrill jig (Fig. l-2b), the tool (a twistdrill) is positively positioned by thedrill bushing prior to the start of thcut, and the guidance is maintained

throughout the cutting process.Thus, the relation of cutter to work s self-contained within the jig. The

reason for the need of suchguidance is the well-known fact thaa drill is a relatively highly flexibletool; a milling cutter is not.



The fixture shown in Fig. 1-3 is amilling fixture. The part to bemilled is a flat bracket / of angular

shape, with a rectangular fasteningflange 2. The surface to bemachined is the end surface of the

short leg of the angle. The totalength of the fixture is

approximately 18 inches (460 mm)the weight, approximately 90

pounds (40 kg). It is a very normalsize fixture and can beaccommodated on any milling

machine except the very small onesIt would, however, take two men tosafely lift it up on the table, but aplant that is progressive enough to



utilize well-designed fixtures suchas this one, would probably notdepend on occasional manpower fo

a lifting job, but would providehoisting equipment. Once thefixture is positioned on the table it

does not have to be moved againand is bolted down. The size andweight of the part to be cut presentsno problem.

The fixture body J is a rather solidlydesigned casting; although it could

have been hollowed out at a few places, the weight saving ismmaterial in this case and would

only be offset by increased pattern

and molding costs.



The face of the flange was already machined in a previous operationand permits, therefore, locating on

four buttons 4 without generating aredundancy. The extreme left end othe bracket is supported at one

point only by means of a sliding res5 operated by the plunger 6 andknob 7. This rest is brought intocontact after the actual locating is

completed. Horizontally, the flanges located on two pin loca-

gag£ fin, pf ^-miiiinc curaa *—,'. V ILjH*J

^^tM y~gnk {jag]



{P |g|gg[^gg

Courtesy ofSiewek Tool Co. Fig. 1-3 typical milling fixture designed

with extensive



application of commercially available components.

tors 8 projecting into previously countersunk holes in the bottom ofthe flange. Again, an additional,

adjustable support 9 is providedagainst a small projection on theback of the angle to take the thrustfrom the cutter 10; a standard end

mil). Two straight strap clamps 11and 12, are provided; both are handoperated—the one to the left is

screw activated, the one to the rights cam activated.

The fixture is aligned to the T-slots

n the table by keys 13 and also has



ugs 14 for the holddown bolts. Theugs are C-shaped for easy insertion

of the bolts so that the fixture does

not have to be lifted once it hasbeen placed on the table.

The main characteristic feature of this fixture is the systematical useof quick-action devices for theapplication of the supports and the

clamping. It should be noted thatno spanner or wrench is needed andthat a large number of the parts are

commercially available.

CHAPTER

Preliminary Analysis and Fixture



Planning

The complete planning, design, and

documentation process for a fixtureconsists, in the widest sense, of three phases-design preplanning,

fixture design, and design approval.They are listed here in their naturalsequence, although there may besome overlapping in actual

execution.

The Initial Design Concept

The design concept is, even if notet put on paper, presumably in the

designer's mind at an early stage of

the first phase. As the process goes



forward, the initial concepts arerecorded in the form of sketchesand are gradually developed,

modified, and changed; somedesign concepts will be discardedand replaced by better ones. As a

general rule, there will be developedat one time or another, amanufacturing operations plan,isting among other things, the

sequence of operations, calling forfixtures at the appropriate placeswithin the plan and providing the

machining parameters, cuttingspeed, depth of cut, feed, etc., foreach operation.

It is not the purpose of this book to



deal with this planning process;however, there may be cases wherean operations plan is not available

to the tool designer and in suchnstances his first step must be to

compose the operations sequence.

It is an absolute necessity to havethe sequence finalized prior tofixture design. Whether thesurfaces are rough (cast or forged)

or previously machined makes aradical difference in locating andclamping the part. In the design of a

drill jig it makes a differencewhether the holes are to be drilledbefore or after machining of thesurfaces, and it makes a big



difference whether a cylinder ismachined internally first, andexternally later, or vice versa,

fundamental rule is that thecutting tool must have ready access

to the surface or surfaces to bemachined. The requirement isobvious, but is sometimes forgottenat the start, and a great deal of

redesigning may be required whenthe error is discovered.

There exists a set of general rulesfor selecting the sequence of operations. They are simple andogical, and almost universal;

exceptions to these rules may exist



but they are rare, and usually occuronly under special conditions.

These rules are:

1. Rough machining is done beforefinish machining, followed by grinding, if required.

2. To allow for natural stress relief,

all roughing operations should bedone before any finish machining isstarted; for the same reason, themost severe roughing operation

should be done as early as possible.This last rule has, however, onemportant modification concerning

the clamping or spanning of the



part. Since, for economical reasons,the "most severe roughing"operation should be performed with

maximum possible feed and depthof cut, (and, therefore, large cuttingforces), it requires a strong

clamping in or on the machine toolIf the rough part offers goodclamping surfaces for the "mostsevere" operation, the rule stands.

3. There may, however, be caseswhere the part in the completely

unmachmed condition has nosuitable clamping surfaces for aheavy cut. In such instances, it maybe preferable to machine some

other surface first, which then can



serve as the clamping surface forthe "most severe" operation.

4. Another equally importantconsideration is the avoidance of broken edges in castings and burrs

on ductile parts. This isaccomplished by choosing thedirection of the feed so that thecutter enters the material from an

already machined surface. This rules quite general and can be applied

to parts with combinations of

machined outside surfaces andholes or slots. If holes were drilledand slots were milled first, and theouter surfaces machined

afterwards, then there would be



broken edges or burrs on one sideof each hole and slot. Conversely, ifthe surfaces are finished first, then

the drills and slot milling cutterswould enter the material from the

PRELIMINARY ANALYSIS ANDFIXTURE PLANNING

Ch. 2

machined surface, in accordancewith the rule stated, and brokenedges and burrs would be

prevented. 5. The rule can be statedn its generality as follows: Surface

machining comes before depth

machining.



tn the preplanning phase, thedesigner accumulates and utilizesall available information as far as it

concerns the design assignment.Four areas of information must betaken into account; the part

material

and geometry, the operationrequired, the equipment for this

operation, and the operator. At thisand other design phases, thedesigner may consult elementary

ists of items to be considered.Examples of such lists are given inTables 2-1 through 2-4, while Table2-5 gives a similar list of the

ndividual items concerning the



fixture itself. Such lists may appeartrivial, however experience showsthat they are useful assists to the

designer's memory and help toavoid his overlooking any significant point.

Table 2-1. Part Description Detailsfor Preplanning of Fixture Design

/. Material Class

1.1 Casting

.2 Sintermetal product

1.3 Forging



2.4 Nonmetallic, natural

3. Material Properties

3.1 Machinability

3.2 Hardness

3.3 Strength

3.4 Modulus of elasticity

3.5 Ductility

3.6 Brittleness

3.7 Weight

Specific gravity (density) total



weight weight distribution

ocation of center of gravity for

unsymmetrical or otherwiserregular shapes

3.8 Magnetic properties

3.9 Electric resistivity

3.10 Specific heat

3.11 Thermal conductivity

4. Part Configuration, Shape, andSize

4.1 Solid body of the following

shapes:



4.2 Cylindrical

4.3 Prismatic (bar shaped)

circular cross section

polygonal cross section

structural cross section (angle, tee,etc.)

short and rigid

ong and flexible

4.4

Flat circular square rectangular

triangular trapezoidal polygonal



other shape

4.5 Spherical

4.6 Block of the following shapes:

rectangular sides, square corners

parallelepiped (skew) trapezoidal(in three dimensions) full pyramidtruncated pyramid conical

4.7 Hollow body, box, or container,of the previously listed shapes

thick walled

thin walled

thin walls with heavier parts



(blocks, lumps)

4.8 Baseplate with uprights

4.9 Bracket

4.10 Tube

circular

polygonal

thick walled

thin walled

with eccentric cavity

4.11 Irregular shapes (not listed



above), and combined shapes

5. Special Components of Part

Configuration

5.1 Individual components:

holes splines

bosses localized cavities

blocks undercuts

ribs special surface slots and point

screw threads requirements

5.2 Number of components listed

above



5.3 Dimensions

5.4 Locations

Ch. 2 PRELIMINARY ANALYSISND FIXTURE PLANNING

Table 2-1 (Co/it). Part DescriptionDetails for Preplanning of FixtureDesign

5.5 Accuracy and tolerances linearangular

5.6 Surface finish (roughness)

5.7 Surface coatings, if any

5.8 Other special components, not



isted above

Table 2-2. Classification of

Operations for Preplanning of Fixture Design

/. Machining

1.1 Bore

1.2 Broach

1.3 Drill

1.4 Mill

1.5 Plane

1.6 Ream



1.7 Rout

1.8 Shape

1.9 Slot

1.10 Tap

1.11 Thread

1.12 Turn

1.13 Grind

1.14 Hone

1.15 Lap

1.16 Polish



1.17 Brush

1.18 ECM (elec tro-chem ical)

1.19 EDM(electrical discharge)

1.20 diem mill

1.21 Manual operations

1.22 Other

Blueprints and Specifications

n examination of blueprints andspecifications for the part in theight of Table 2-1 will draw the

designer's attention to the material

size, and weight of the part, and any



unusual conditions. From thematerial properties he can selectthe grade of tool material to be used

and form a first opinion on the sizeand type of fixture required. Table2-2 is self-

2.7

2.8

2.9

2.10

2,11

2.12



2,13

2.14

2.15

2.16

Stitch

Stable

Press-fit

Shrink-fit

Swage-fit

Magneform-fit



Stake

Tab-bend

ssemble with a sea)

O-ring

gasket

flow-sealer Other

3, Inspecting, gaging, measuring

3.1 Linear dimensions

3.2 Angular relations

3.3 Concentricity



3.4 Planarity

3.5 Surface quality (roughness)

3.6 Other inspections

pressure testing for leakage and

rupture

4. Fixtures for other non-cutting

operations

4.1 4.2 4.3

4.4

Heat-treating

Cooling after forming of plastic



parts

Surface coatings

plating

painting (masks) Foundry

operations

Number aspects of operations

5.1 Single operations

5.2 Operations in prescribed

sequence

5.3 Operations to be performedsimultaneously



explanatory, if and when anoperations plan is available, but isalso useful in cases where the

designer must do his ownoperations planning. As the specificoperation, or operations, have been

dentified, the designer will have apicture of the mechanics of theoperation, including thedistribution, direction, and

approximate magnitude of cuttingforces; their character with respectto any tendency for generation of

shock, vibration, and chatter; andsome idea

PRELIMINARY ANALYSIS AND

FIXTURE PLANNING



treatment equipment types

sand blasting equipment

shot peening equipment, etc.Surface coating equipment

plating tanks

painting booths

drying and baking ovens, etc.Foundry equipment

sand preparation equipment

molding machines

core making machines



FIXTURE PLANNING

9

about the cutter life and cutter costto be expected. Table 2-3 brings thedesigner closer to many details. A table of this kind is to be used Inconjunction with lists of the plant'sown machine tools with tables of

their dimensional capacities {tablesize, accessories, horsepower, speedand feed range, etc.) and this shouldessentially conclude theaccumulation of information fromsources outside of the fixture itself.One more aspect should certainly

be considered, namely, the



passed the final test, the economicevaluation.

Table 2-4. Manipulation andOperator Criteria

/. Speed considerations

1.1 Lifting, moving, and loweringfixture

weight

hoisting equipment

1.2 Loading and unloading of part

nto and out of fixture



1.3 Clamping of part 2. Safety of work

2.1 Locating part correctly in fixture(fool-proof concept)

2.2 Adjustment of cutter

J. Operator considerations

3.1 Fatigue

3.2 Operational safety

(accident-proof concept)

4. Miscellaneous

4.1 Supply and return of cutting



fluid

4.2 Chip cleaning and disposal

Table 2-5. General Considerationsn Fixture Design

1. Loading and unloading of part

1.1 Manual lifting or hoisting

1.2 Lowering or sliding part intoposition

1.3 Unloading to floor

1.4 Use of magazines, conveyors,and chutes for receiving and

returning part



1.5 Speed of motions

1.6 Ease of motions

1.7 Safety in manipulations

2. locating parts in fixture for ready

access of cutting tools

2.1 Concentric to an axis

2.2 Vertical and horizontal fromestablished surfaces

2.3 Vertical and horizontal fromdiscrete points

2.4 Other



3. damping of part

3,1 Speed

Size of clamping forces

3.2 3.3 3.4

Direction of clamping forcesLocation of clamping forces

3.5 Manual or power actuation of clamping elements

4. Support of part

4.1 Against clamping pressure

4.2 Against tool forces



4.3 Stability of part and avoidanceof elastic deformation

5. Positioning cutting tool relativeto loaded fixture

5.1 Rotating ("indexing")

5.2 Sliding

5.3 Tilting

6. Coolant supply and return

7. Chips

7.1 Removal of accumulated chips

7.2 Chip disposal



Check List for Fixture Design

No list can cover all conditions in

every company and plant; however,the following list is reasonably comprehensive. It also permits the

reader to make such additions ashis special conditions may require.

The items are listed in their logical

sequence. Those mentioned firstare those which are most likely tobe overlooked during the initialplanning stages. Some items areisted in more than one place.

check list is the book in a nutshel

and has two uses:



1, It is not supposed to bememorized, but if the designer willread it carefully before embarking

Part Details

Select

Pertinent

Items

Discorded Ideas

Pri ma r y Fixture Design

Operation

Classification



Select

Pertinent

Items

Equipment Selection

Operator Criteria

Select

Pertinent

Items

Phase I

Select



Pertinent

Items

Initial Design Concepts

Preliminary Fixture Design

Cost Analysis and Evaluation

Phase II

lternate 1 Fixture Design

lternate 2 Fixture Design

"

r~*~-i



,

Evaluation and Final Decision

Completion o{ Design, Execution oShop Drawings

Pha

Fig. 2-1. Outline of the fixtureplanning process.

Ch. 2


FIXTURE PLANNING

11

on an assignment, it will remain in



his mind as a constant guide.

2. It should be applied

systematically, point by point,whenever a design phase iscompleted.

Check List

1. The Part Drawing

1.1 Check date and revisionreferences on part blueprint andmake sure that the print is the

atest edition and that it is up-to-date with respect to revisions.

1.2 Make at least a cursory check of



the part on the blueprint; makesure that all views and sections arecorrecdy oriented.

1.3 Having ascertained the correctshape of the part, check all part

outlines shown on fixture drawingsparticularly for correct orientation.

Z The Shop

2.1 Make sure that there are noobstructions in the shop layout, oraround the work station, as well as

along its access ways, that willprevent or otherwise interfere withthe transportation of the fixture to

ts station.



2.2 Investigate whether the work station is equipped with thenecessary services, such as

compressed air supply, if needed foactivating the fixture.

2.3 For heavy items (fixture as wellas part), a hoist should be availableIt is advisable to check whether it isocated properly for the present

purpose,

2.4 If lifting equipment is neededand no hoist is available at thestation, it is recommended that theshop department provide othermeans of lifting, such as a forklift,



2.5 Once the fixture is properly ocated, examine the shop layout

around the work station to make

sure that it permits transportationand delivery of parts to the work station.

2.6 The same consideration applieswith respect to the loading of thepart into the fixture. Specifically,

watch out for long parts projectingout of the fixture,

2.7 For a work station that is part ofa production line, the fixture, whennstalled, should be correctly ocated and oriented with respect to

the line.



3.1 Be sure that the fixture will fitthe machine tool for which it isntended. Check overall dimensions

and the space within which thefixture is to be installed (the toolingarea). Check dimensions of

machine tool table, and thedimensions, location, width, andaccuracy of T-slots, and comparewith the locating blocks in the

fixture base. Also check T-slotsrelative to holes or slots in thefixture clamping lugs, specifically

for any clearance required for finaladjustment of the fixture locationrelative to the machine tool spindleInspect the condition of the table



slots; if they are worn or mutilatedthey will jeopardize accurate fixturealignment.

The fixture must be fully supportedand must not overhang the edge or

end of the table. 3,2 Investigate thecondition of the machine tool tomake sure that the accuracy issatisfactory for the tolerances

required in the operation.

Make sure that the machine isstrong and rigid enough to carry theweight of the fixture and absorbshocks and vibrations from theoperation. For rotating fixtures

(such as on lathe spindles), check to



necessary that the spindles in thehead can be adjusted to the holepattern prescribed by the drill jig.

3.6 Check the machine too! forrequired cutting speeds and feeds,

3.7 Provide a tapping attachmentfor a drill press, if needed.

4. Cutters

4.1 With respect to generalmaintenance of cutters, it is

required that they can beconveniently removed forresharpening without disturbing

any setting in the fixture and that



they can be adjusted when in place,relative to the fixture. The operatoror setup man must be able to check

the correct setting by visualobservation.

4.2 Check for operator safety.Preferably it should not benecessary for the operator to havehis hands close to the cutter; if so, a

guard should be provided.

4.3 Checfc for cutter and fixturesafety. Make sure that the cuttercannot be damaged by contactingthe fixture. Also, provide that thecutter will not cut into clamps,

stops, or locators if it is set too



deeply or overruns its travel.

Can the cutter {in or out of motion)

be contacted by a clamp when theclamp is being operated (opened orclosed)?

4.4 Check for interference betweencutter and part, particularly duringoading and unloading. A case in

point is drilling with multiple-spindle drill heads and with thedrills on different levels; this may require one or a few exceptionally ong drills which could causenterference.

Check for other interferences when



cutter dimensions (lengths) havebeen significantly reduced by repeated grinding. Long, slender

drills running at high speed shouldbe checked for whipping. A whipping tendency can be

eliminated by providing a bushingat a high level, with the drillrunning in the bushing. Theocation of this bushing must be

high enough to allow the jig to beunloaded and reloaded while thedrill is running. 4.5 Check for

convenience in cutter operation.The fixture should be designed forminimum length of cutter travel. Inthe case of drill jigs, check that all



the greatest possible extent. They should always be kept in stock.

5. Cutter Setting (other than by drilbushings)

5.1 Check all cutters individually;determine whether they requiresetting blocks or not.

5.2 Provide setting blocks whereneeded. Carefully select settingblock material (hardened steel,carbide, etc.)

5.3 Supply feeler gages for settingblock facings where needed. Mark

fixture accordingly, at proper places



5.4 Check to see if a completely andaccurately machined part can beused as the master for the cutter

setting.

6, The Part

6.1 Check the part for such unusualfeatures as overly heavy weight,excessive imbalance, and long

projecting ends, and provide forrequired supports. Hard andabrasive part material may requirecarbide faced locating andsupporting elements. Very soft partmaterial, or parts with prema-chined surfaces may require

protection against scratching or



being otherwise marred by thepressure from clamps and supportpoints. Very thin-walled, or

otherwise nonrigid shapes may require special attention (auxiliary supports, or the like) against

tending or being otherwisedistorted by the clamping andoperating forces. Clamps may beprovided with leather, rubber, fiber

copper, or brass facings. Clampsmay be hand-operated with knurledhead screws to prevent application

of excessive torque.

6.2 Check for the effect of ariations in the shape and

dimensions of the part. Allow



clearance in the jig for normaldimensional variations {forg-ings,castings), and for any protruding

elements (bosses, ribs, lugs, etc.),that are integral parts of the normageometry.

Look for mismatched castings;preferably, have all locating pointson one side of parting line. Look for

flash on forgings; do not placeocators in line of flash.

In case of serious locatingproblems, consult with theengineering department, for thepossibility of permanent

modification of part design to



facilitate production, or temporary modification, such as the additionof lugs or flanges for locating and

clamping purposes, to be removedby subsequent machining.

In drill jigs of simple type, thegathering of all drill bushings inone jig plate will ensure correctrelative position of all holes. 6.3

Some surfaces on castings and dropforgings have draft, ranging from1/2 to 7 degrees, even if the part

drawing may show them as parallelto each other and perpendicular tothe base plane.

Ch. 2




13

7. Locating the Part (Rough Parts)

7.1 Check the part for best possiblesurfaces oi points for locating.Criteria for evaluation of such item

are static stability and compatibilitywith, and good definition of, thesurfaces to be machined; preferablysuch points or surfaces as are

dimensioned directly from themachined surface(s).

7.2 Wherever possible, provide



iewing holes in fixture for visualcheck on locating, or profile platesto assist in locating and chip

cleaning.

7.3 For rough surfaces, buttons are

better than flat blocks. They shouldbe hardened, except for very shortruns.

7.4 Space locating points as widely as possible, particularly on roughsurfaces use three locating points.If a fourth (or more) point isneeded for stability, make itadjustable or a jack type.

7.5 Centralizing devices, such as V-



surfaces for suitability as locatingsurfaces. The basic condition is thattheir tolerances must be fine

enough for the accuracy required inthe following operations. Allsubsequent locating and machining

operations should be based on thesame locating points and/ orsurfaces,

8.2 If a curved surface blends with aflat surface, locate from the flatsurface for the curved surface.

8.3 Locating surfaces in the fixtureshould be kept as small as possible,should be relieved to allow for any

burr from previous machining,



should locate on one diameter only

Provide clearance at the base of all

ocating pins.

9. Clamping

9.1 Evaluate the operation in termsof part weight and cutting forces. Itmay be light, medium, or heavy. In

the case of a light operation, it maybe possible that the part can be heldn the fixture by virtue of its shape

or weight only, without actual

clamping devices.

9.2 Check the support and clamping

system for stability and strength,



specifically the following:Supporting and clamping pointsshould be selected as-wide apart as

possible. The part should besupported directly below theclamping points with solid metal in

between, and close to the points of action of the cutting forces.

The cutting forces should act to

hold the part in position; avoid adesign where the cutting force actsto lift, tip, or tilt the part. Do not

rely on friction to resist the cuttingforce. Preferably, the cutting forceshould be directed against thesupporting points. There are a few

special cases where it is correct to



have the cutting force acting againsthe clamps, Note again that thesecases are few and special. The

cutting and clamping forces shouldnot act to distort the part or thefixture. This must be prevented by

providing adequate support points.The cutting forces should not act toupset, tilt, twist, or otherwisedisplace the fixture on the machine

table.

For milling fixtures, check the

previous points for the two cases ofup-milling and down-milling (alsoknown as climb-milling). Check force and stress analysis to make

sure that the fixture and the clamps



operated simultaneously by oneclamping lever.

void loose pieces; tie them to thefixture by pins, hinges, or chains.

Use compression springs understraps to hold

them up when the bolt is

unscrewed, and provide

elongated bolt holes so that they can be retracted

by a short, straight pull. Use stopsto prevent ro-



fating w he n 1 o osened an d (ightene d.

On large and heavy clamping strapsuse a handle for convenient lifting.

Look for two-hand operations.Examples: with a suitable handle onthe fixture, the fixture is held by one hand while the clamp is

aotivated by the other hand. Onehand clamps while the other handengages the feed lever.

Use toggle clamps for quick action.Use compressed air or hydraulicsfor quick action and/or large

clamping forces.



10.2 Small details in the design of ocating pins may be significant.

Make them as short and as small aspossible. With two locating pins,make one pin longer than the other

so the part is located on one pin at atime. Use the diamond-shapedcross section, where appropriate.Make ends of pins pointed and

rounded for easy catch.

10.3 For heavy parts to be located inpreviously drilled holes, usedisappearing locating pins so thepart can slide over them intoposition.



10.4 Provide comfortable handles oample size for a good grip on hand-operated locators. Knurled handles

particularly small ones, areconsiderably less comfortable tooperate than star-shaped handles.

Combine movable jacks, plungers,and other locators so that they areoperated by one handle and, if

possible, are locked automatically by the clamping operation.

Place all operational devices onoperator's side of the fixture.

10.5 Provide additional stops and

guides so that the part can enter the



fixture in one position only-thecorrect one. Allow for burrs fromprevious operations.

10.6 Wherever possible, pre-position part for easy loading

during machining cycle on theprevious part. Fixtures may be builtwith duplicate space for parts sothat one space is unloaded and

reloaded, while the part in the othespace is being machined.

Or: the finished part may beremoved by one hand, while thenew part is inserted by the otherhand.



Or: The finished part is ejected by nserting the new part.

10.7 The part may be unloaded by means of an ejector which may beautomatically operated when the

clamps are loosened.

11. Drill Bushings

11.1 Check each drilling operationwith respect to the necessity of abushing. Many operations, inparticular, second operations

(countersinking, reaming, tapping)can be guided by a previously provided hole and do not need a

bushing.



11.2 Straight bushings (i.e., withoutflange) are preferred because of their simplicity and low cost;

however, they are totally dependenton their press fit in the plate. If thepress fit fails, they may be pushed

down by the drill. All straightbushings should be checked for thiseventuality and if suchdisplacement can be harmful, they

should be replaced by flangedbushings.

11.3 Flanges on bushings shouldprotrude above accumulation of chips and cutting fluid. Whereverpossible, use flange top as a depth

stop for the drilling operation by



11.7 It is useful to have certainmarkings on drill jigs and on somebushings. The drill jig should be

marked with drill size adjacent tobushings. Slip bushings should bemarked "drill" or "ream," as

required.

11.8 Slip bu shi ngs r equ ire spec iaat ten tion. First,they should only

be used when absolutely necessary (see 11.6). They should beeffectively locked in place. Flanges

should be large and fluted(preferable to knurled) for easy gripping, with room underneath forthe fingertips, and a stop against

turning in the lift-out position, and



perhaps even with a handle.

12. Drill Jigs

12.1 A drill jig for one hole only mayadvantageously be clamped inposition centered under the drillspindle. In other cases, the jig orthe drill spindle (a radial drill) hasto be moved from hole to hole. The

choice depends on size and weightof the jig.

12.2 Only small jigs may be held by

hand. For hole sizes larger than 1/4-inch (6 rnm) diameter, provisionmust be made to resist the drilling

torque. Small jigs may be provided



with a handle for this purpose;arger jigs require positive stops.

12.3 Provide feet under the jig, largeenough to span the table slots, andhigh enough to prevent drilling into

the table. Space the feet so that allcutting forces act inside the feet.

12.4 All bushings should be clearly

isible to the operator when jig ispositioned.

12.5 For holes in a circular pattern,

the jig may be mounted on a rotarytable.

12.6 For holes in several directions,



operating an indexing fixture.

IS, Fixtures in General

13.1 Look for and avoid, difficult orawkward setup conditions whichcould be caused by heavy

weight, uneven weight distributionholddown bolts in difficult

ocations, etc.

13.2 The wrenches again!Preferably, one size only for setup

and one size only for operating.

13.3 Unusual accuracy requirements may call for the use



of dial indicators. Provide bracketsor other means for mounting themFixture may be designed with

adjustment devices to compensatefor machine tool misalignment.

Precision locating from previously bored holes may require expandingplugs to eliminate effect of diameter tolerances,

13.4 Boring fixtures {not the sameas drill jigs) have a special sequencerule: locate part from the smallestbore; reason: this avoids aneccentric cut with the smallestboting bar.



Boring fixtures may advantageouslyuse hall-bearing mounted pilotbushings for small-diameter boring

bars running at high speed.

13.5 Use stock castings or other

stock material for the fixture body,wherever possible.

13.6 For milling operations, use a

standard vise with special jaws,wherever possible.

13.7 Keep the design low.

13.8 Precision components withinthe fixture should be fastened by

means of screws and located by



means of dowel pins or keys,

14. The Chips

14.1 Chips may be continuous(smooth and shiny) ordiscontinuous (short sections,ntegrated into finite lengths, easily

broken), both stringy, and producedfrom steel and aluminum; or

crumbling (small pieces), evenpowdery, produced from cast ironand bronze. Establish the chip typethat is produced when machiningthe part.

14.2 Crumbling chips require space

to escape between surface of part



and end of drill bushing; stringy chips require bushing carried closeto surface to guide chips up through

bushing.

14.3 Cutters should have ample

space in flutes to allow chips toform; flutes should be carried wellabove surface of fixture to allow chips to escape. Pilot ends on tools

should have grooves or flutes forchips, to prevent binding inbushings.

14.4 Chips tend to collect in thebottom of the fixture. Avoidforming corners and pockets that

can collect chips. Provide openings



and inclined paths for chip escapeand chip cleaning. Cutting fluid mabe directed so as to assist in chip

removal.

Lift surfaces of locators and

supports above possible chipaccumulations. Keep such surfacessmall in area and provide forcleaning.

-blocks and other locators withreentrant surfaces should haveclearance for chips and burrs.

16




FIXTURE PLANNING

Ch. 2

14.5

14,6

14.7

14.8

Prevent chips from entangling inclamp lifting springs, Protect

movable parts such as plungers,acks, index plates, etc., from chips.

If necessary, provide shielding.

If necessary, provide chip cleaning



surfaces.

15. Cutting Fluid

15.1 The cutting fluid must reachthe edge of the cutter. If necessary,provide channels or guides for thispurpose.

15.2 Provide channels, guides, and

guards to prevent cutting fluid fromrunning to waste, from being spilledon the machine or the floor, andfrom hitting the operator.

15.3 Utilize the flow to wash chipsaway.



15.4 Cutting fluid may serve asubricant for movable fixture parts.

Provide necessary holes for this

purpose, as required.

16. Safety

16.1 Make a general review of allmanipulations and operations to beperformed on the part to ensure

that they do not present any hazard

16.2 Make a similar cheek withrespect to the cutter; in particular, i

the part should be incorrectly oaded, will this cause damage to

the cutter? Take a last and critical

ook at the'dimensions of the cutter



and its support {arbor, chuck,toolholder).

16.3 Check fixture and part forisibility at all times (loading,

positioning, clamping, cutter

approach, cutting, possible overruncutting fluid, chips, un-clamping,and unloading).

16.4 Check fixture design for eventhe most trivial hazards: sharpedges and corners, projecting screwheads, levers and handles, fingerclearance around and underhandles, grips, slip bushings, etc.

16.5 Make sure that hinged and



otherwise movable and heavy parts(leaf plates, clamps, cover plates)cannot accidentally fall upon

operator's fingers; tike-wise, check motion or air-operated clamps.

16.6 In case of accidental failure of a clamp, can the part be thrownout? Specifically, in case airpressure fails.

16.7 See that operator is protectedagainst flying chips ■ and any splashing cutting fluid.

16.8 Consider the use of notices of caution on the fixture.



17.3 An unusual, but notmpossible, case occurs when the

fixture containing the part must be

removed from the machine andbrought to the inspection device.This calls for close cooperation both

n the design of the fixture and of the inspection device.

18. Manufacture, Maintenance,

Handling, and Storage of Fixtures

18.1 With the design finalized,check the cost analysis. Beware of the following two cases: (a) the coss too high for the productionolume anticipated (b) the

production volume is high enough



capability with respect to thisfixture, including its prescribedtolerances.

18.4 Parts to be heat treated shouldhave provision for suspension; if

necessary, drill a small hole for thatpurpose,

18.5 Check tor lubrication

possibility of all movable parts, forcorrect material selection andspecification of heat treatments, forhard surfaces on all wearing parts,and for easy removal andreplacement of parts subject towear or accidental damage. Be

certain that replacements can be



made without interference withother fixture elements. Make sureof access to the "inner end" of parts

with a press fit.

18.6 Provide aids for lifting, such as

ugs, eyebolts, hooks, chain slots onheavy fixtures, and handles onheavy loose parts.

Ch. 2


17

18.7 Secure small loose parts and



hand toots against loss in storage(chains, keeper screws, etc.); do notrely on tape.

18.8 For storage, provide protectivecover, case, or a box for weak and

delicate parts, precision andpolished surfaces, or the completefixture.

18.9 When in storage, the fixtureshould rest in a well balanced andstable position. If necessary,provide a suitably profiled baseplate or crate for this purpose.

18.10 Provide all necessary

dentification marks, including



marks on any hose items.

18.11 Check design drawings for

correct sections, projections, andiews, and a clear description of all

functions, for complete

dimensioning, completenformation notes, and clear,correct, and complete title block. Donot forget tool identification

number,

18.12 During this final check, look once again for the

following details: Strength, rigidity,simplicity, safety, under operating

conditions as well as under



accidental overloading.

Sufficient mass to absorb shock and

ibration.

Tolerances, as liberal as possible fothe purpose.

Support surfaces, datum surfaces,and grinding clearances for the

toolmaking. Provision foralignment of precision press-fittedprecision parts. Avoid blind holes.Do not forget countersinks where

desirable. Avoid square andpolygonal holes (for plungers andsimilar moving parts). Provide

breathing holes on bores for



plungers. Dowel pins, precisionpins, locating keys for partsrequiring repeated accurate

positioning, spaced as widely aspossible.

18.13 For precision operation, thefixture may require a scraped fit tothe machine table.

18.14 When future changes in partdesign can be anticipated, provisionshould be made in the fixturedesign.

CHAPTER

The Fixture Design Procedure



The process of fixture design differsdrastically from the design processusually applied to a machine part.

The conventional method of machine design consists, in broadterms, of first making an

approximate sketch of the outlineof the part, with axis lines andsignificant points indicated, andassisted by appropriate sections,

The loads are then applied, bendingmoments and stresses arecalculated, and the dimensions are

altered where necessary until thestress analysis is satisfactory.

Steps in the Fixture Design Process



In fixture design, the outline of thefixture is about the last step in theprocess. The sequence is locating,

clamping, supporting, applyingcutter guides, and, finally, drawingthe fixture outline as the envelope

that combines all the previously drawn elements.

In the practical design process,

however, one operates continuouslywith the part, and it isrecommended that the part outline

or appropriate sections in thefixture sketches, should be drawneither in thin lines or, preferably, incolored lines.



Locating and Degrees of Freedom

Locating the part is, at this stage, a

geometrical concept; the actingforces (weight, clamping pressures,and cutting forces) are not taken

nto consideration with respect totheir magnitude, but only as to theidirection, so as to ensure that thepart is located in a position of static

stability.

"small" particle (a point), whenunsupported, has three degrees of freedom in space. It can move inany direction, but any motion itmay perform is fully defined by its

components in three directions; in



mathematical language, by threecoordinates. If these three motioncomponents are arrested, the

particle cannot move; it has beendeprived of its three degrees of freedom.

"body" consisting of several, ormany, particles, can move as aparticle, but it can also rotate; it has

three axes of rotation, and as any rotary motion of the body can bedescribed by three rotation

components around three axes, thethree linear motions plus threerotations make six degrees of freedom. The six degrees of

freedom can be made up by



components other than those justdescribed, and several other suchcomponent sets, at various places in

the following, will be applied to theproblem of locating a part in afixture.

s a simple and fully realisticexample, consider a rectangularblock with unmachined rough sides

Le., a casting or a forging. Locatethe bottom surface on three pointsnot in a straight line (see Fig. 3-1 a)

and assume holddown forces to soact on the block, that it cannot beifted off. These three points

prevent motion in the vertical

direction and also prevent rotation



around a longitudinal and acrosswise axis. In other words, theyhave deprived the block of three

degrees of freedom. The block canstiU slide in two directions in theplane defined by the!three points

(two degrees of freedom) and canrotate around a vertical axis (thethird degree of freedom). Now add(see Fig. 3-1 b) two locating points

against one of the vertical sides, non the same vertical line, and again

with holddown forces. This

prevents motion in the crosswisedirection and also rotation aroundthe vertical axes, and thus deprivesthe block of two degrees of



freedom. It has one degree of free-dom left, namely, motionengthwise. Finally, apply one

ocating point (with holddownforce) against the end (see Fig. 3-1c); this eliminates the sixth degree

of freedom and the block is now fully located. The addition of afourth point at the bottom surface(see Fig. 3-1 d) would theoretically

make the system redundant, but isused under certain conditions tomprove the stability, as explained

n Chapter 4, Locating Principles.

Ch. 3

THE FIXTURE DESIGN



PROCEDURE

19



Fig. 3-1. The principle of locating apart by single locating points, a, b, c

pplication of three points on the

bottom surface, d. Application of four points on the bottom surface.

In actual fixture design, the very first step is to deprive the part of itssix degrees of freedom. To apply sixndividual locating points, as

described above, is perfectly possible, but the six degrees of freedom can be eliminated in many

other ways, each of which isgeometrically equivalent to the sixocating points.

Using the Clamping Elements



The hold-down forces referred toabove were imaginary forces. Inactual design, the next step would

be to apply real clamping elementssuch as bolts, straps, cams, etc., insuch places that the part is held

firmly against the locatingelements, not only as it is beingocated, but also at the time that the

cutting forces become active. The

number of clamping elements usedare not necessarily equal to thenumber of locating points. For

example, in the illustration shown,one vertical clamp over the centerof the block (arrow C, Fig, 3-1 a)would suffice to take care of the



three locating points; one additionaclamp against the center of one side(arrow C, Fig. 3-1 b) would take care

of two points; in fact, one clamp(arrow C, Fig. 3-lc) acting on acorner and directed along the

diagonal, would take care of all sixocating points.

This is, however, a purely

theoretical concept. One importantrule at this point is that a clampingforce must be applied as directly as

possible, and without causing any elastic deformation or "springing"of the part. In case the block in Fig.3-1 is of solid metal, it may well be

clamped with two or three clamps



only; but if it should be hollow, itmay be necessary to allow foradditional clamps so that each

clamping pressure is transmittedthrough solid material (preferably)to its reaction point.

Providing Support

The next step involves "support."

The term, as used here, is slightly misleading because some supporthas already been provided by theocating points. However, theocators have only provided

sufficient support to secure thegeometrical stability of the part, and

this may not be sufficient to absorb



all acting loads without causingelastic deformation of the part. Thesupports to be supplied in this

design step must be sufficient innumber and strength to absorb allacting loads. On the other hand,

they must not interfere with theocating of the part, as already

established. They are, therefore,made adjustable and brought to

bear against the part withoutsignificant pressure and withoutproducing any geometrical

redundancy.

THE FIXTURE DESIGNPROCEDURE



Ch. 3

Cutter Guidance

The following step is to provideguidance for the cutter. For drilligs, this means to draw the drill

bushings in their proper place inthe design. For other types of fixtures (milling, planing, etc), the

tool guides are actually points forpositioning the tool prior to thestart of the cut.

Completing the Body

Until now, all the elements are

drawn "floating in the air," so to



speak; the last remaining designstep is to draw a jig or fixture body that carries all the individual

elements and has enough strengthand rigidity to hold them in theirproper places under load.

The fixture body must also fulfill anumber of other conditions-it mustfirst accommodate the part, have

clearance for loading andunloading, and for chips; it shouldhave feet or some other supporting

surfaces to carry it on the machinetable; have locating elements foraligning it with the machinespindle, and have an adequate

number of lugs for holddown bolts.



Categories of Fixture Materials

The following categories of

materials are used in fixtures:

Steel, not formally specified

Steel, specified by the SAE and A ISI classifications and standards

Tool steel

Cast iron

luminum and magnesium

Sintered tungsten carbide

Plastics



Other nonmetallic materialsDetailed analyses, mechanicalproperties, and heat treatment

nstructions of these materials are,as a general rule, not listed in thefollowing sections. The data for

standardized materials are readily available in reference books, i.e.,Machinery's Handbook 1 . Data formaterials that are not covered by

standards, are available from themanufacturers.

1

Steel, Not Specified

Much steel material is used withou



standardized and identified by theprefix SAE (Society of AutomotiveEngineers) or AISI (American Iron

and Steel Institute), followed by afour-digit number. The last twodigits indicate the carbon content

(SAE 1020 contains from 0.18 to0.23 percent carbon). The first digitndicates the class to which the

steel belongs. The classes are:

Erik Oberg and F, D, Jones,Machinery's Handbook (New York:

Industrial Press Inc., 1971.) 19th edpp. 1985-1978.

The large class of carbon steels, 1

Oxx, can be subdivided into the



following groups:

1015 and below, are highly ductile

and are used for press work, but nofor fixtures.

The low carbon group is numbered1016,1017, 1018, 1019, 1020, 1021,1022, 1023, 1024, 1025, 1026, 1027,and 1030. They are available hot

rolled and cold finished; and roundbars are also available with aground finish. The steels are readilyweldableand easy to machine. Thegrades up to and including 1024 arethe principal carburizmg or casehardening grades. The grades from

1022 and up, when car-burized, are



oil hardening, the lower grades arewater hardening. The grades from1025 and up, while not usually

regarded as carburiz-ing types, aresometimes used in this manner forarge sections or where a greater

core hardness is required.

The medium carbon group isnumbered 1030, 1033, 1034, 1035,

1036, 1038, 1039, 1040,

Ch. 3


21



1041, 1042, 1043, 1045, 1046, 1049,1050, and 1052. They are availablehot rolled and most of them are

also available cold finished. They are readily machinable and havehigher strength than the previous

group although they still retainsatisfactory ductility and toughnessTheir mechanical properties arefurther improved by heat treatment

Steels with less than 0.40 percentcarbon cannot be hardened above45 Rockwell C. The high carbon

group is numbered 1055, 1060.1062, 1064, 1065, 1066, 1070, 1074,1078, 1080, 1085, 1086, 1090, and1095. Available hot rolled, most of



them are also to be had as drill rod;that is, round bars, ground to closetolerances. They have higher

strength and hardness than theprevious groups, have satisfactory toughness, but low ductility and are

used where higher strength andtoughness or greater wearresistance is required. They are, tosome extent, hardenable, but the

hardness obtained depends largely on the rate of cooling during thequenching operation. The high

cooling rate (critical cooling rate)required for maximum hardness isonly present in the surface layerwhere the metal is in direct contact



with the cooling medium; in thenterior of the material, the cooling

rate is less than critical, and the ful

hardness is therefore confined to ashallow surface layer. This is knownas the "mass effect." In heavier

sections, the mass effect is sodominating that even the surfacecannot attain the theoretical fullhardness of 62-64 Rockwell C. A

arge number of stainless steels arestandardized, but are not includedn the list above. Stainless steels

arecorrosion resistant and mosthave excellent mechanicalproperties. Due to their higher costand lower machinability, they are



not used to any significant extent infixture construction. However,some fixture components are now

commercially available in stainlesssteel. Stainless steel is also used forpurposes where its lack of magnetic

properties is of value, such as forseparating elements in magneticchucks and for backing bars inwelding fixtures. Alloy steels, in

general, have greater toughnessthan carbon steels of comparablestrength. Also, the wear resistance

s greater than for a carbon steel of the same carbon content. They havebetter har-denability than carbonsteels, which works out in two



ways: (1) They harden in depth, notust in the surface, and for the

hardening they require a less severe

heat treatment than carbon steels;(2) They distort less, therefore,during heat treatment, and

have better dimensional stability after heat treatment because they contain less residual stresses. The

cost is higher, however, and alloy steels are only selected where acarbon steel cannot be used.

Tool Steels

ll tool steels normally used in jig

and fixture construction have a high



dimensions.

Tool steels differ widely with

respect to distortion during heattreatment. For fixture parts it isdesirable to keep distortion at a

minimum and this is a majorconsideration in the selection of thetype of tool steel to be used.Practically all fixture requirements

can be met with the selection of tool steels listed in Table 3-1,comprising two water hardening

steels (W-), three oil hardeningsteels (0-) and SAE 52100 which isnot really a tool steel, but very suitable for some fixture purposes.

High-speed steels are not included



n the list because they are seldomused in fixtures.

Table 3-1. Selected Tool Steels forFixture Components

Steel Material Selection

Shop language has its own non-standardized nomenclature for

arious classes of steel. Machinesteel is hot rolled, low carbon steel,as opposed to cold rolled steel. Lowcarbon tool steel is carton steel of

ess than 0.60 percent carbon; theower

THE FIXTURE DESIGN



PROCEDURE

Ch. 3

imit is undefined. High carbon toosteel is carbon steel of 0.60 percentcarbon, and higher. Low alloy toolsteel means the chromium-molybdenum and nickel-chromium-molybdenum steels in

the41xx and 43xx class. It is actuallya misnomer because the carboncontent in these steels never evenreaches 0.55 percent.

Tool steel is usually purchased hotrolled and annealed and is therefore

decarburizcd in the surface. The



material because it is work hardened. It also produces asmoother surface when machined,

however, since it contains higherresidual stresses than hot rolledmaterial it may warp if it is

asymmetrically machined. For thisreason, hot rolled material ispreferred for parts that are to befinished by grinding, or machined

on one side only.

Selection of Proper Steels for Parts

Small and medium-size drillbushings are made of oil hardeningtool steels, grades 02 and 03, or of

52100 steel because of their low



distortion in heat treatment. Largerdrill bushings are made of 8620steel, carburized and case hardened

ery large bushings are made of lowalloy steels in the 41xx group.Cheap bushings are made of 1060

or 1065 carbon steel. Locating partssuch as buttons, pins, and pads, tharequire a high hardness and arefinish ground after heat treatment,

are made of water har-

dening steel W] or W2, quenched in

brine and tempered at 300 to 37 5F(149 to 191C). Cams that cannot becontour ground after heattreatment and, therefore, must be

machined or hand filed to final



dimensions are made of oilhardening tool steel, grades 02 or06.

Chuck and vise jaws that can beground after hardening are made of

grades Wl or W2; if finish grindings not possible, they are made fromgrades 02 or 06.

Miscellaneous parts of small andmedium size that do not carry any significant load but requirecomplete or partial case hardeningfor wear resistance are made of 1018, 1020, 1133, or 1144 carbonsteel. Surfaces that are exposed to

ight wear only, can be case



hardened by cyanizing and do notneed subsequent grinding unless ahigh degree of accuracy is required.

Highly stressed parts, such asarbors or mandrels, boring bars,

highly loaded clamps, and fixturedetails are made from alloy steelswith medium carbon content suchas 2340, 2345, 3140, 3145, 4140,

5140, 6140, and 6145 with orwithout heat treatment, dependingon the stress level during operation

For particularly heavy duty, oilhardening 06 tool steel is used.

Collets and expanding arbors are

particularly critical because they



Dowel pins are made of carbonsteel, either 1035 or a high carbonsteel hardened to 60 Rockwell C,

Keys for alignment of parts withinthe fixture or for aligning thefixture on the machine table, are

made from 1045 carbon steel. If hardened surfaces are required forwear resistance, they are made froma tool steel.

ssembly screws and bolts aremade of various grades of machine

steel, however, it is preferred to usehexagonal socket head cap screwsmade of heat treated alloy steel,usually with 160,000 to 170,000 ps

(1100 to 1170 N/mm 2 ) tensile and



100,000 to 105,000 psi (690 to 724N/mm 2 ) shear strength. They require less space and are


Ch. 3


23

The straps in clamping devices aremade of 1020 carbon steel for

average conditions, and of 1035carbon steel in such cases where aower strength material would

require excessive dimensions. The



tips of the straps are often casehardened. Hardened washers areused under the nuts in these

devices. The bolts are made of 1020carbon steel; for large loads they armade of a heat treated low alloy

steel to avoid excessive dimensionsThe nuts are made of a free-machining low carbon steel andcase hardened for heavy loads.

Fulcrum pins for swinging clampsare made of carbon steel, or toolsteel, heat treated to 54-58

Rockwell C. Large pins are made of case hardened, low carbon steel.Eccentrics and cams are made of ow carbon steel, case hardened to



at least 60 Rockwell C.

Castings

Small secondary parts such as handknobs, crank arms, etc., are made omalleable iron. Large castings (andsometimes these are very large), areused for parallels, raiser blocks,fixture bases, and in some cases, fo

fixture bodies; particularly whereibration damping capacity and

absence of stresses are of mportance. Castings in the first

category are not severely stressedand are made of gray cast iron inclasses 20, 25 and G2000 with

20,000 to 25,000 psi (138 to 172



N/mm 2 ) tensile strength. Largerfixture bases, and fixture bodies ingeneral, are made of gray cast iron

n classes 30, 35, and G3500 with30,000 to 35,000 (207 to 241N/mm 2 ) tensile strength. Very

arge fixture bodies are made of ductile (nodular) cast iron in class65-45-12 with 65,000 psi (450N/mm 2 ) tensile strength, and type

SP80 Meehanite Ductliron with80,000 to 100,000 psi (550 to 690N/mm 2 ') tensile strength. Type

GA50 Meehanite with 50,000 psi(345 N/mm 2 ) tensile strength isalso used. Because of its structuralhomogeneity, it retains high



dimensional accuracy in service. Itresponds to heat treatment and canbe hardened locally or on the

surface by either flame or thenduction process. The design

should avoid thickness variations in

adjacent sections beyond the ratioof 3:1, otherwise the thick sectionwill have a porous center. Steelcastings are

rarely used in fixtures.

luminum and Magnesium

These two light metals are widely used in the form of tooling plate.

They are supplied with finish



machined surfaces, are weldableand easily machinable; for many purposes they are competitive with

steel.

luminum tooling plate is a cast

product of a chemical compositionequivalent to the 7000 series of aluminum alloys. They are furnacestress relieved and machined

(scalped) on both sides to athickness tolerance of ±0.005 inch,(0.13 mm), a flatness tolerance of

0.010 inch on 8 feet (0.25 mm on 21/2 m), and 25 to 40 micro-inch(0.6 to 1.0 jum) surface finish.Standard thicknesses range from

1/4 inch to 6 inches (6 to 150 mm).



Thicknesses up to 16 inches (400mm) are available from stock;above 16 inches by special order.

The material weighs 0.101 poundper cubic inch (2.8 g/cm 3 ), and itstensile strength is 24,000 psi (165

N/mm 2 ).

Magnesium tooling plate is a rollingmill product. It is made of AZ31B

alloy containing A I, Zn, and Mn. Its thermally stress relieved at 700F

(371C) and machined on both sides

to a thickness tolerance of ±0.010nch (0.25 mm) and a typicalflatness tolerance of 0.010 inch inany 6 feet (0.25 mm on 1.8 m).

Standard thicknesses range from



1/4 inch to 6 inches (6 to 150 mm).The material weighs 0.0642 poundper cubic inch (1.78 g per cm 3 ) and

ts tensile strength is 35,000 psi(240 N/mm 2 ). The compressive

ield strength is only 10,000 to

12,000 psi (69 to 83 N/mm 3 ). Themachining of magnesium involves aserious fire hazard, becausemagnesium is combustible in the

atmosphere, and the chips igniteeasily and burn violently.

Sintered Carbides

Drill bushings for extraordinary ong service life are usually made o

sintered carbide materials. Two



rehardened.

Plastics

Plastic tooling is made by castingand by laminating. Plastic toolingmaterials include phenolics,polyesters, polyvinyls and epoxies.They differ

2 Trade name, proprietary toSintercast Division of Chio-malloy

merican Corp., West Nyack, New ork.




applied in the form of cloth, mats,or rovings.

Cast phenolics have a compressivestrength of 12,000 to 1 5,000 psi(83 to 103 N/mm 2 ); the

compressive strength of castpolyesters varies from 12,000 to35,000 psi (83 to 241 N/mm 2 ).These plastics exhibit a significant

inear shrinkage. Cast epoxy has acompressive strength of 15,000 to25,000 psi (103 to 172 N/mm 2 )

with an elastic limit of 5000 psi (34N/mm 2 ), a modulus of elasticity (E) of 0.1 X 10 6 to 0.8 X 10 6 psi(690 to 5520 N/mm 2 ), and a

inear shrinkage of 0.001 Jo 0.004



nch per inch (0.025 to 0.10mm/mm), which is considerably ess than the shrinkage of other

cast-able plastics. The use of epoxies as a tooling material isgrowing rapidly. For this

application, epoxies are superior tomost other plastics and they arenow used almost exclusively foraminates. Epoxy laminates have an

elastic limit of about 15,000 psi(103 N/mm 2 ), and a modulus of elasticity ranging from 1.5 X 10* to

3.5 X 10 6 psi (10,300 to 24,100N/mm 2 ). The shrinkage isnegligible and they aredimensionally stable after curing.



Rigid polyurethane foam is used asa core material and a backupmaterial for fixtures constructed in

the form of plates and shells. It hasts optimum strength/weight ration the density range of 6 to 10

pounds per cubic foot (96 to160kg/m 3 ). Representative valuesof physical properties at 8 pounds

per cubic foot(128 kg/m 3 ) are 200psi (1.4 N/mm 2 ) yield strength,250 psi (1.7 N/mm 2 ) ultimate

compressive strength; B=> 10 X 103 psi (70 N/mm 2 ) at yield, and Ehas the near constant value of 7.5 X10 psi (52 N/mm 2 ) over the

operating range from 0 to 150 psi (0



to 1.0 N/mm 2 ) compressive stressPotting compounds used forfastening drill bushings in bored

holes in plastic drill jigs are high-temperature-resistant epoxy resinsEpoxy resins with metallic fillers

("liquid steel," "liquid aluminum")are used for repair of plastic toolingand for locators in fixtures for useat elevated temperatures (up to 500

F, or 260 C).

Other Non-metallic Materials

Large plane drill jigs for use in theaircraft industry where light weights essential for easy handling, and

high strength is not required, are



Masonite Corp., Oiicago, Illinois.

Locating Principles

CHAPTER

Locating Principles, Flat Surfaces

part without scribed lines andpunched centers can only be located

from its surfaces and this is done byproviding them with the necessary number of restraints. To restrainthe part on a surface against only

one direction of motion, as wasshown in Fig. 3-1, is termeddefining the part and implies the

addition of a subsequent clamping



action to maintain positive contactwith the restraining element. Thepart is single defined as long as it is

restrained on one surface only,double defined when it is restrainedon two surfaces, and fully defined

when it is restrained on threesurfaces. It is a condition here thatthe defining surfaces are notmutually parallel; generally, but no

as an absolute condition, the threedefining surfaces are perpendicularto each other.

The rectangular block shown in Fig4-1 is a general example of definingand locating from flat surfaces. The

block is single defined as long as it



only rests on the horizontal base ofthe fixture (position a), doubledefined when it is moved to full

contact with the verticalongitudinal strip (position b), and

fully defined when it is also moved

endwise to contact with the endstrip (position c).

To define a part from two parallel

and offset flat surfaces results in



overdefining. Because of thetolerances, the part cannotsimultaneously be brought into

effective contact on the twosurfaces. It will either hang or tilt(Fig. 4-2), depending on the nature

of the clamping. A similar situationexists for parts with two or moreconcentric cylindrical surfaces.When the part has to be located on

two offset surfaces, it can be donesatisfactorily by locating them onthree points.

?-



Fig. 4-1. Defining and locating a parfrom flat surfaces.

Fig. 4-2. Over defining a part fromtwo parallel flat surfaces.

Nesting

part may be located on, orrestrained between, two or more

surfaces such that motion isprevented in the two oppositedirections on at least one line. The

part is said to be nested, to be



nesting, or to nest within, therestraining elements. In Fig. 4-3a,the part is fully defined and single

nested, in b and c it is doublenested, and in d it is fully nested.Full nesting requires that the

fixture has a detachable

LOCATING PRINCIPLES

Ch. 4

cover to provide access to thenterior of the fixture, and openings

n the fixture walls to allow theoperations to be performed.



Fig. 4-3. Single, double, and fullnesting.

Nesting requires that it be possibleto move the part to a positionbetween locating surfaces or points

On the other hand, it must also fitas closely

as possible between the mating

surfaces when in position. Any clearance, no matter how small, wilallow motion between part andfixture that will generate a certainsmall amount of displacement andmisalignment. An interference fitwould define the location without

ambiguity, but does not readily



permit the part to be moved intoposition. Therefore, the class of fitactually selected must be a

clearance fit, equivalent to one of the tighter classes of these fits. Forthe sake of clarity, the clearances

shown in the illustrations aregrossly exaggerated. Nesting cantake many forms. Nesting surfacesdo not have to be parallel and

opposite. If the fixture in Fig, 4-1 isrotated around one edge, it can beseen (compare with Fig. 4-4) that

the part is actually double nested ontwo perpendicular surfaces when its in position C. In addition, the

diagonal plane in the part is now



centered with respect to the fixtureThe concept of centering is of greatmportance and will be discussed in

detail in Chapter 6, in the sectionon Circular Locators, and inChapter 9, Centralizes. If the fixture

s set on a corner with a cornerdiagonal vertical, then the part isfully nested on three cornersurfaces, and is also centered.



a b c

Fig. 4-4. Modifications of the

principle of nesting.

The example in Fig. 4-1 is anllustration of the elimination of th

six degrees of freedom by means ofcontact between large surfaces. Itpoints back to the application of the

same principle as is shown in Fig, 31, The base plane is equivalent tothe first three locating points, theside strip is equivalent to the nexttwo points, and the end strip isequivalent to the last point. This seof equivalences can be formulated

as the "3-2-1 locating principle." Th



ocating function of the side andend strips and points is somewhatdifferent from the function of the

base plane and base points. Tounderline this difference, allocating points above the base are

termed "stops."

Other equivalences, however, arepossible. A set of two points is

equivalent to one strip; a plane isequivalent to two parallel strips, orto one strip and a point (see Figs. 4-

5, and 4-4a, b and c). A locatingpoint is not a mathematical point, is often a small flat surface (a pad).

The locating elements should



Ch. 4

LOCATING PRINCIPLES

27

Fig. 4-5. Locating by means of twostrips, or one strip and one point.

be spaced as widely as possible.

This open spacing provides the bestobtainable stability against theacting loads (gravity, clamping andcutting forces), and minimizes any



error that may be caused by a smallmisalignment or displacement of aocating element.

The 3-2-1 Principle

The 3-2-1 principle represents theminimum requirements for locatinelements. The locators, togetherwith the clamps (represented by

arrows C in Fig. 3-1) which hold thepart in place, provide equilibrium oall forces, but do not necessarily also guarantee stability duringmachining. Usually, stability issatisfactory if the three basebuttons are widely spaced and the

resultant cutting force hits the base



from a triangle to a rectangle, asshown in Fig. 3-id, and provides therequired stability. The principle

may be termed the "4-2-1 locatingprinciple." For rough castings, oneof the four base locators may be

adjustable. Such locators aredescribed in Chapter 12, SupportingElements,

If the locating surface is machined,all locators may be fixed, and thisoffers an advantage in another

respect. When the part is properly seated on its four locators, it feelsstable, but if a chip or some otherforeign matter has lodged itself on

ocator, or if the locating surface is



warped, the part will rock. This isnoticeable to the operator andserves as a warning that there is a

defect in the set-up which must becorrected.

Error Possibilities

The use of large locating fixturesurfaces is only feasible when the

matching part surfaces arecompatible with respect totolerances and geometry. This is nonecessarily the case, even forsurfaces already machined, becausefixture surfaces are usually finishedto closer tolerances than are most

production parts. The consequences



of incompatible tolerances will beexplained in Chapter 5 with respectto Fig. 5-2.

The most common errors in partsurface geometry are convex and

concave curvature, twist, andangular errors. The effects of curvature and twist are shown inexaggerated form in Fig, 4-6,

Convex curved and twisted surfaceswill not accurately define theocation since they may cause the

part to rock. With curved surfacesand insufficient rigidity, the partmay also be distorted (bent) whenclamped in the fixture but after it is

released from the clamp it will



spring back and the previously flatnew surface will now be curved.Even with distortion-free clamping

the curved part may still bensufficiently supported and may

deflect under the cutting forces.

7l~) ) / > rT7\> >

rrr) >///;>/,

sTTV / > s J >T7 s

r; ? > > / > / s > r

rTT/ / s / ^ ■7 = T 7 -



Ch. 4

-

rn

Fig, 4-7. The effects of angular



errors on locating.

ng system is incorrectly designed

or operated. Some examples areshown in Fig. 4-7, where the largearrow indicates a clamping force,

and the resulting dimensional errorn the locating is indicated by adouble arrow. Perhaps the mostdangerous case is that of Fig, 4-7d,

because the error occurs at a placethat is not easily observed. Nestings no guarantee against the effect of

angular errors, as is shown in theexample of Fig. 4-8. The offsetshown is per-

mitted by the clearance in the



nesting but it is caused by theangulaT error.

Locating Principles, CylindricalLocators

Cylindrical surfaces will usually beocated by nesting in or on

completely or partly matchingsurfaces. A fixture base with a side

and an end strip can almost but notcompletely locate it, as shown inFig, 4-9. In position a, the partstands on the base and threedegrees of freedom have beenremoved. When moved to positionc, two more (but not three) degrees

of freedom have been removed. The



cylinder is now nested in a V-bloek n the same way as shown

previously in Fig. 4-4a, and it is also

centered with respect to the V. Thesixth degree of freedom, rotationaround a vertical axis, has not yet

been removed.

Fig. 4-8. The effects of angularerrors on nesting.



Fig. 4-9. Locating a cylinder againstflat surfaces.

The same incomplete locating canbe accomplished by placing the partnside a matching cylindrical holder

—an outside cylindrical locator (seeFig. 4-10), but it still is free torotate. Rotation can now beprevented and the part locked in

position by means of a clampingdevice employing friction. If thesignificant part configuration

consists entirely of cylinders andperpendicular planes, it has nopreferred diameter, atid any position is as good as any other

position with respect to the



machining operations to beperformed in the fixture. If,however, the part has a projecting

or a receding surface, no matterhow it is shaped, then it has one orseveral preferred diameters to

which the machined surface mustbe related in the way determined inthe part design, and such apreferred diameter, or diameters,

must be held to a predeterminedocation within the

Ch. 4

LOCATING PRINCIPLES

29



preferred diameter(s). This can bedone in a great variety of ways and few representative examples are

shown in Fig. 4-11.

cylindrical locator can also be

applied to the inside of a cylindricalcavity and takes the shape of amandrel, a plug, or a flange. Someexamples are shown in Fig. 4-12.

Two factors common to allrotational locators are: that they acton a point of a radius in the part,and that they restrain motion of that point in a tangential direction.These will be termed "radial

ocators." Usually, and preferably,



the direction of the actual contactpressure should be perpendicular tothe radius at the point of contact, a

condition which is fully satisfied inFig. 4-12b; approximately satisfiedn Figs. 4-1 la, b, and c, and 4-12a

and c; but not, however, in Figs. 4-11 d and 4-12d.

The radial locator may be a small

pin, fitting in a hole, or it may bearge and formed as another plug.dditionally, the cylindrical locator

may be so small that it also takesthe shape of a pin. Locating by suchsystems, illustrated in Fig. 4-13, istermed "dual cylinder location," and

s widely used.



ny location using cylindricalocators involves nesting, and

therefore requires clearance which,

n turn, affects the locatingaccuracy. As indicated in Fig. 4-14,sketch a, the position of the center

of the part may vary as much as theclearance and may be offset from itnominal position as much as one-half

l

—*

a ici

N$h



Z\



Fig. 4-11. Complete locating of acylinder by means of an outsidecylindrical locator and a radial

ocator.

of the clearance. The application of

a clamping pressure (see sketch b),forces the offset to one side, butdoes not eliminate it, and the poornesting at the contact point

opposite the clamping pressurepermits the part to shift slightly toone side or the other. If the part

does not have a good locating basesurface, but, for example,terminates in a point (as shown insketch c), it is also subject to

misalignment resulting in a



maximum angular variation 6 of thaxis direction determined by:

2{D F -D P ) 360

B = j radians ■" —-—-

D F -D p

degrees

once again confirming thefundamental rule that locatingpoints should be as far apart as

possible. It

LOCATING PRINCIPLES

Ch. 4



m&

n.

Fig, 4-12. Complete locating of a

part fay means of an insidecylindrical locator and a radialocator.

s strongly recommended, that theocating points be placed in

mutually perpendicular planes. If a

ocating plane is inclined against



the perpendicular, as shown in Fig.4-15, a transverse force components generated that tends to lift the

part from the base points. A dirtaccumulation on the locator of thickness T produces a locating

error E=T when the locating planes perpendicular, but a larger error T

~ cos~a occurs wnen tne locatingplane is inclined

an angle a against theperpendicular.

Offset and misalignments, asdiscussed above, are eliminated by the use of conical (tapered)

ocators, because they do not



require clearance but providepositive contact. They belong to theclass of centralizing devices to be

discussed in Chapter 9.

0

J3&



-F=Fr"

Fig. 4-13. Examples of dualcylindrical location.



Fig. 4-14. The effect of clearance incylindrical locating.

Ch. 4



LOCATING PRINCIPLES

31

Fig. 445. The effects of locatingagainst a perpendicular and annclined plane.

Preparation for Locating



CHAPTER

Locating Unmachined Surfaces

One basic purpose of a fixture is toproduce parts that are withinspecified tolerances. It is themachined surfaces on thendividual parts that define and

determine the distances to all

principal axes and other systemines and planes within the finished

product. It is obvious that all suchdimensions must be correct, of course, but it is also necessary thatany remaining unmachinedsurfaces maintain their proper

ocation relative to system lines and



to each other to avoid interferencewith each other and with movingparts of themachine, to secure the

required material thicknesses, andto provide uniform machiningallowances with full cleanup on all

machined surfaces, A drastically exaggerated example of a violationof this rule is shown in Fig. 5-1.

Fig. 5-1, Correctly and incorrectly

ocated center lines. The cylinder to



the left was machined with thecenter lines correctly located withrespect to the outer surfaces. The

cylinder to the right was machinedwith a gross error in the locationand the direction of the center line

with respect to the outer surfaces.

In job shop production, theseconditions are met by the layout of

the parts prior to machining.System lines and centers are scribeand punched into

the surfaces of the part which isthen set up in the machine tool by measurements taken to these lines

and centers. One important purpose



of a fixture is to eliminate thisayout operation; the raw part

usually comes to the fixture

without such lines, centers, or othemarkings, and all locating has to bedone from the surfaces and

contours as they exist. It istherefore important for the designof the fixture, and particularly forts locating elements, to know the

dimensional tolerances that may beexpected (or even better, may beguaranteed) on the raw part.

They will vary from case to case,according to application andpurpose of the product, from plant

to plant, and from supplier to



supplier. In the specific case,however, the applicable toleranceswill normally be made available to

the fixture designer.

Tolerances will usually be fairly

consistent within each group of materials, depending on the typeand class, and also the size of thepart. General rules for tolerances

and other dimensional variationsare presented in the sectionsfollowing. They will be found usefu

for the fixture designer in theabsence of specific prescribedtolerances, and may also serve as abase for the valuation of any given

or proposed tolerances.



Machining allowances are, in a wayrelated to tolerances and must alsobe taken into consideration by the

fixture designer. The maximumtolerance on a surface is thetheoretical lower limit for the

machining allowance. Wherepossible, the actual machiningallowance should be obtained fromthe production planning

department or from suppliers of raw parts. As a substitute, anestimated value may be used.

For an order-of-magnitudeestimate, it may be assumed thatmachining allowances increase with

the overall size of the raw part. For



gray iron castings made in greensand molds in sizes from 20 to 100nches (500 to 2500 mm), average

machining allowances vary from3/16 to 7/16 inch (5 to 10 mm),

Ch. 5

PREPARATION FOR LOCATING

33

they are a little higher Cor surfacesocated in the cope, a little lower for

surfaces located in the drag.Practice varies between differentfoundries; some consider 1/8 inch

(3 mm) as the minimum machining



allowance, also for smaller castingsMalleable iron and nonferrous alloycastings require 33 percent less, and

steel castings 50 percent more, thangray iron castings.

For forgings, the weight W (poundsor kg) is the parameter by which thmachining allowance may beestimated. Small hammer and press

forgings (hand forged) require from1/16 to 1/8 inch(l 1/2 to 3 mm) oneach surface. For this type of

forging (from 15 pounds [7 kg], andup) the allowance per surface canbe taken as

0.05 %/l7inch



1,65 VH-" mm

where: W = weight in pounds, and

W — weight in kilograms.

For closed die forgings (dropforgings and other machine forgedparts) the allowance required isfrom 40 percent (for solid andbulky shapes) to 60 percent (for

elongated shapes) of the valueestimated for a hand forging of thesame weight. Minimum allowancefor all forgings is 1/32 inch (0.08mm) because of scale pits and otheocalized surface defects and

decarburization.



Castings

casting is by no means a

mathematical reproduction of thepattern; not even of the mold cavitySome cast materials will shrink,

others will expand duringsolidification. All of them shrink during the subsequent coolingperiod; the resulting total shrinkage

depends on type and composition othe metal, the pouring temperatureand the cooling rate. Slight

ariations in the composition may occur from charge to charge and canaffect the shrinkage. Unevenshrinkage often results from

differences in wall thickness and



may cause warping.

Common values for shrinkage are

shown in the following:

On large castings the apparentshrinkage will be less than themetallurgical shrinkage, becausethe pattern is rapped in the moldbefore it is drawn and thereby

slightly expands the mold cavity.This is of significance for largecastings only.

With respect to warping, only a fewgeneral rules can be formulated.The complete process of

differentiated shrinkage rates



during solidification is complicatedHeavy sections, and sections thatare shielded against loss of heat,

will lag behind during cooling, andthe end result is that such sectionswill show increased apparent

shrinkage. An I-beam-type gray ironcasting with one thick and one thinflange will, consequently, beconcave lengthwise (hollow) on the

side of the thick flange. An upperimit for the maximum deflection r

raax of such a beam of length L and

height H is:

1

ma * 3200 H



Greater warpage may be expectedfor malleable and steel castings, A channel- or U-shaped section may

open up at the top, because thebottom shrinks and the free edgesare held in position by the mold or

core.

The lower limit for tolerances oncastings can be taken as one-half of

the shrinkage. This assumesfavorable conditions, such asregular shapes without tendency to

warping. However, while closetolerances sometimes may bedesired for some functional reason,the economical viewpoint calls for

the most liberal tolerances that



design considerations can allow.Unnecessary close tolerances add tocost and increase the scrap hazard.

ny conscientious production manwill select the widest tolerancesthat he can get away with, and the

fixture designer should be aware ofthat.

Representative and rather realistic

tolerances are:


Ch. 5

Material



Iron castings, gray, white, andmalleable:

Tolerance

Range,

nch

Tolerance

Range,

mm

more specific set of rules,applicable to green-sand ironcastings up to 16 inches (400 mm)

n



size, is the following:

Description

Overall external dimensions withinthe same part of the mold parallelto the parting plane

On dimensions perpendicular toparting plane

Tolerance

+0.030 inch (0.8 mm) for the first 3

nches (75 mm) ±0.008 inch (0,20mm) for each additional inch (25mm)



from +0.02 (for small castings) to+0.06 (for large castings) inch pernch (mm/mm)

Description

Cored (usually internal)dimensions

Core location

Concentricity of a cored hole

Tolerance

of maximum overall castingdimension

+0.020 inch (0.5 mm) for the first 3



nches (75 mm) ±0.012 inch (0,30mm) for each additional inch (25mm)

±0.050 inch (1.3 mm) for the first 3nches (75 mm) ±0.008 inch (0.20

mm) for each additional inch (25mm)

0.090 inch (2.3 mm) T1R (total

ndicator reading)

The values are for oil-sand bakedcores. For shell cores, the accuracy

s about 25 percent better. A comparison of these values shouldwarn the fixture designer that

critical conditions are more likely to



For sand-cast aluminum,magnesium, and copper alloys, thegeneral tolerance is ±0.005 inch pe

nch (mm/mm), minimum ±0.015nch (0.38 mm).

Casting tolerances, as listed above,do not apply to a dimensionmeasured over the "gate." The gates the passage leading to the mold

cavity and also includes any remnant of solidified metal fromthat passage. The gate is broken off

as the casting is shaken from themold, or sawed or sheared off later,and the remnant gate is usually cleaned up by grinding; either flush

or to a tolerance that may range



Deep castings do not permit a very arge draft as it would too greatly

distort the dimensions.

Minimum values are: CastingMethod For pattern drawn from

mold: external surfaces, curved

external surfaces, flat

ribs and webs, curved

ribs and webs, flat

small holes

For mold lifted from pattern:

alue



1000

1

500

1 100

1 200

«P«o_

u " t0 20

rough measure of uniformity ineastings is provided by someaverage weight tolerances, which,for gray iron castings, are 5 percent

when made from solid patterns, 10



percent when made with sweeppatterns, and for malleable ironcastings, 5 percent when machine

molded, and 10 percent when handmolded.

The uniformity and accuracy of castings (gray iron, malleable iron,and modular or ductile iron) ishigher from permanent molds, and

molds with metallic cores andnserts, than from sand molds;

higher from machine molding than

from hand molding; higher fromdry-sand molds than from green-sand molds; and significantly higher when the castings are made

n shell molds.



Handmade hammer forgings will,as a rule, not be manufactured inquantities that justify machining in

a fixture. However, hammer andpress forged parts from ferrous andnonferrous metals are used in

moderate quantities in variousndustries such as the weapon and

aerospace industries, and theseforgings may need fixtures because

of intricate and accurate machiningrequirements. The most commonforged raw parts are impression die

forgings which, again, may be dropforgings (closed die forgings) andupset forgings.

For estimating forging tolerances,



materials can be classified by stiffness as follows: Low stiffness-aluminum, magnesium, copper, and

brass; Medium stiffness—carbonand low alloy steel, stainless steel(400 series); and High stiffness—

stainless steel (300 series),titanium, super-alloys, andrefractory metals (Columbium, Cb;Molybdenum, Mo; Tantallum, Ta;

Tungsten, W). Tolerance data listedn the following without material

specification may be applied to all

three classes.

Hammer and press forgings areseldom fully freehand forged, but

are made with the use of flat and



simple open-face dies. For suchforgings, tolerances can beestimated from nominal

dimensions and weight.

For elongated shapes of length L

(inches or mm) and any transversedimension (width, height, diameteretc.) D (inches or mm), estimatedtolerances are:

on length—

T L =±[0.05 + 0,003(1+ 10/>)] inch

T L =± [1.3 + 0.003 (L + 10 D)] mm

on a transverse dimension-



accurate than are other dimensions

Die cavity dimensions depend on

the initial accuracy to which the diewas sunk and polished, and theamount of subsequent wear. Initial

dimensions

*Co = Cobalt, Mo = Molybdenum,Ta = Tantalum, and W = Tungsten

Ch. 5


37

*Co = Cobalt, Mo = Molybdenum,



Ta = Tantalum, and W = Tungsten

can be held to relatively very dose

tolerances which are consideredncluded in the shrinkage

tolerances. However, dies are also

subject to severe wear and are, foreconomic reasons, allowed to wearconsiderably during their usefulservice life.

Shrinkage tolerances, also known a"length-width" tolerances, are:±0.003 inch per inch (mm/mm) of nominal dimension. Weartolerances on external and internaldimensions are: wear factor (from

Table 5-1, below) multiplied by



greatest external dimension (lengthor diameter). On external dimen-

sions, the wear tolerance is plus, onnternal dimensions it is minus.

Wear tolerances do not apply to

center-to-center distances.

Thickness tolerances are based onpart area in the parting plane and

can be taken from Table 5-1. Tablealues apply to parts not exceeding

6 inches (150 mm) of depth withinany one die block, as measuredperpendicular to the parting plane.For such parts of forgings thatexceed this limit, an additional

tolerance is applied, equal to:



±0.003 inch per inch (mm/mm) of any such dimension.


Ch. 5

Thickness tolerances are alwayspositive, meaning that incompletefilling of the die cavity is not

acceptable.

Mismatch tolerances and flashextension, which is the maximum

distance that the flash may protrude from the forging body, areboth positive; they are based on the

forging weight after trimming, and



can be taken from Table 5-1. Flashthickness ranges from 1/16 to 1/4nch.

Straightness tolerances mean theimitation imposed on deviations of

surfaces and centerlines from thenominal configuration and areadded to previously estimatedtolerances. Forgings and parts

within a forging can be classified byshape as elongated, flat, or bulky,and one forging may well comprise

parts belonging to more than oneclass.

Straightness tolerances are: for

elongated shapes—0.003 inch per



nch (mm/

mm) of length for flat shapes—

0.008 inch per inch (mm/mm) of ength, width, or diameter. Bulky

parts require no straightness

tolerance.

The values are for medium stiffnessmaterials and assume that the

forgings have been mechanically straightened as required. For low stiffness materials, deduct 33percent; for high stiffnessmaterials, add 33 percent.

ll die forgings must have draft. In

some extreme and special cases (in



aluminum and magnesium for-ingsof the extrusion type) the applieddraft may be 1 degree or even zero.

However, the most common valueson external surfaces are 5 to 7degrees for medium-stiffness

materials, down to 3 degrees forow-stiffness materials, and up to

10 degrees for high-stiffnessmaterials. Internal surfaces

(pockets) require higher drafts,from 10 to 13 degrees. All draftscarry a +2, —1 degree tolerance.

Overall tolerances for closed dieforgings range from 5 to 15 percentof thicknesses, and from 0.5 to 1.5

percent of widths, lengths, and



diameters. In comparison,tolerances on upset forgings are 25percent higher on axial lengths and

flange diameters, but 25 percentess on some individualntermediate axial dimensions, such

as flange thicknesses. Mismatchtolerances are the same. Cavitiesrequire a TIR concentricity tolerance of 1.3 percent of cavity

diameter. Upset forgings do notshow flash and, in many cases,require little or no draft.

Tolerances quoted are"commercial." Finer tolerances,known as "close" can be obtained.

The values are approximately 33 to



50 percent less than "commercial."Calculated tolerances are roundedoff to two decimal places, then

converted to nearest higher 1/32nch (1 mm) and entered on

drawings.

The fixture designer must beprepared to encounter some minordefects which are considered

acceptable and passed by inspection—such as scale pits, shallow depressions caused by scale

accumulation; mistrimmed edges,where the flash protrudes unevenlyaround the forging; small fins andrags, driven into the metal surface;

cold shuts, produced by material



folded against itself; small unfilledareas; and conditioning pits, wheresurface defects have been ground

away.

The dimensionally most reliable

configurations are those formedwithin one die block, and flatsurfaces parallel to the partingplane. All forgings produced within

one life period of the die are usuallyery uniform. The same applies to

sheared flash contours. Slight

differences may be expected when adie is reconditioned, and also if more than one die is in use.

Weldments



Production parts to be machined ina fixture will usually be fabricatedby methods entailing closer control

and better uniformity than job shopwelded parts and can therefore bemade to closer tolerances,

particularly when the welding isperformed in fixtures. Toleranceson finished welded parts dependargely on the distortion during and

after welding. The tolerancesobtained must be ascertained fromcase to case, and only broad and

general statements can be madeabout them.

utomatic welding results in less

distortion than hand welding. Arc



welds distort less than gas welds.Heavy welds distort more than lighwelds, but heavy sections distort

ess than light sections. On theother hand, weldments from lightsections are easier to straighten

mechanically. Resistance weldsdistort less than fusion welds. Leasdistortion is found in flash-buttwelding, where length tolerances

can be held to ±0.02 inch (0,5 mm)When the dies are not self-centering, a maximum offset equal

to the sum of the tolerances on thepart diameters or thicknesses may be expected.

In the absence of specific



nformation, tolerances forresistance weldments can be takenas for die forgings, and tolerances

for arc welded parts can be taken as50 percent of the tolerances forcastings of comparable dimensions

Torch-cut parts will display thethickness tolerances of the stock material with an addition for the

burr which, after proper cleaningfor slag, may be from 0.01 to 0.06nch (0.25 to 1.5 mm) on either

side. Contours can be held to+0.015 inch (0.38 mm) on smallparts, and ±1/16 inch (1.5 mm) on

Ch. 5




39

arge parts with automatic andtracer control, and ±0.1 inch (2.5mm) with manual feed. The cutedges may deviate 1/4 degree fromthe perpendicular position. Withnert-gas tungsten cutting at high

feed rates, edges may be beveled asmuch as 5 degrees.

Mill Products

This class comprises rolled, drawn,and extruded shapes. Detailed

tolerances are available from



suppliers' catalogs, a few illustrativeexamples only, are shown in thefollowing:

Material

Steel red, low carbon and low alloy,round or square: Hot Rolled I-inchdiameter

or side 2-inch diameter


or side

Cold finished

1-ineh diameter



or side 2-tnch diameter


or side

Tolerance, inch

±0.009

±1/64

+1/16,-0

Carbon

Round I Square

luminum rod, round or square:



1-inch diameter



or side

luminum hollow shapes, extruded

wall thickness overall dimensionson a hollow section

—0.002 -0.004 -0.003 -0.005 -

0.005 -0.006

lloy

Round



Square

-0.003 -O.005 -O.004 -0.006 -0.005

-0.007

Noie: Minus tolerances only.

Rolled

Round Square

±0.006 ±0.016 + 0.031

-0.016

•0.020

Cold Finished



Round Square

±0.002 ±0,0025 ±0.004 ±0.005

±0.012

15% of nominal

1.5-2.5% of nominal

With respect to tubes and pipes, thefixture designer should know thatthey not only have diam-

eter tolerances, but also liberaltolerances on out-of-round and walthickness variations.

Press Products



This class comprises sheet metalparts produced by shearing,punching; stamping, drawing, and

pressing with dies in a mechanicalpress. Some basic tolerances areusually very close. Thickness

tolerances for cold rolled carbonsteel sheets range from below ±0.001 inch (0.03 mm) to ±0.005nch (0.1 3 mm) for thicknesses up

to 1/4 inch (6 mm). Contours of punched flat parts may vary 0.001to 0,002 inch (0,03 to 0.05 mm) as

ong as the same tool is usedwithout reconditioning. The samemay be expected for small stampedand drawn parts.



Machined surfaces have closertolerances than raw parts and aretherefore, a priori, more suitable fo

ocating a part within a fixture forfurther machining. They shouldnot, however, be indiscriminately

accepted for this purpose. The basicrequirement is that the tolerance onthe already machined surface mustbe satisfactory for the correct

tolerance to he obtained in thefollowing operation within thefixture. As an illustration (see Fig.

5-2) assume surfaces A and B arealready finished to a tolerance of ±0.005 inch (0.13 mm), and surfaceC must hold ±0.002 inch (0.05



mm) against B. Surface A,presenting a wide bearing area,would appear desirable for locating

but the presence of the ±0.005 inch(0.13 mm) tolerance from B to A prohibits machining of C to ±0.002

nch (0.05 mm) from B, no matterhow close tolerance r is taken, and

must therefore be rejected as theocating surface.

Blueprint tolerances, if uncritically accepted without part inspection,

could cause the fixture designermany disappointments. Nominally plane surfaces could be convex,concave, or twisted, from improper

clamping; gradual tooi wear;



naccurate setting of a millingcutter; or distortion (warp) fromstress relief. Broached

configurations might be offset or


Ch. 5

a ±0.005

Fig. 5-2. Consideration of tolerancen locating.

tilted, due to the elasticity of thebroach. Ground surfaces on thinparts could show heat distortion.Nominally square edges and



corners could be out-of-angle fromncorrect clamping. Sawed surfaces

even when machine sawed, are

neither straight, flat, nordimensionally correct. All machinededges will have a burr on the side o

tool exit.

Heat Treated Parts

Such parts may distort and show relatively large deviations fromnominal shape and dimensions.

lmost any geometrical elementmay be affected, Overalldimensions, including centerdistances, may increase or decrease

hole diameters may become larger



or smaller, circles may go out-of-round, and flat or straight parts maycurve or twist. The distortions

cannot be predicted except in rathergeneral terms and may well vary from piece to piece. Control of

distortion requires careful stressrelief of parts prior to hardening,and depends also on the experienceof the heat treater and his skillful

application of time-honored tricksof the trade. More reliable andconsistent control of heat treat

distortion is effected by having thepart clamped in a fixture during theprocess.

s a rough estimate, tolerances



required to cover for heat treatdistortion can be taken as ±0.05 to±0.15 percent, additive to prior part

tolerances.

Plastic Parts, Molded

Parts made in various sizes fromplastics (thermoplastic as well asthermosetting) are widely used and

are manufactured in largequantities. They have excellentsurface quality as formed, but thats not necessarily identical with

high dimensional accuracy, becauseof shrinkage, uniform ornonuniform, and sometimes

dimensional changes during aging.



While they can be formed (cast orpressed) in the mold to finisheddimensions for many applications,

other purposes may require somemachining operations such asdrilling and other processing of

holes, where hole location iscritical, or grinding of flat surfaces,

and the fixture designer may well

encounter the assignment of designing fixtures for thesematerials. The amount of shrinkage

aries with the type of plasticmaterial and filler, and may inextremes, range between 0.001 and0.012 inch per inch (mm/ mm) of

nominal dimension. Representative



average values for single-cavity hotmolds, taken as the dimensionaldifference of mold and part at

ambient temperature, are:

Material

Phenolic with wood-flour filler, andurea

Cellulose acetate

Phenolic with fabric or asbestosfiller,

and methyl methacryiatePolystyrene



nch per inch (mm /mm)

0.006 to 0.010 0.002 to 0.010

0.002 to 0.006 0.001 to 0.003

Resulting tolerances can be taken

as follows:

Parallel to parting plane

Perpendicular to parting

plane Warpage, perpendicular

to nominal surface

±0.005 inch per inch (mm/mm)add 0.015 inch (0.38 mm) ±0.003

nch per inch (mm/mm)



The nominal dimension L (inchesor mm) can also be taken intoaccount in the tolerance T by the

empirical formula:

T= 0.006 \/T inches or r=0.03

imm

Somewhat higher tolerances shouldbe selected for center-to-center

distances of bosses or moldedholes, and for multiple cavity molds, and should be doubled forcold-molded parts.

Some plastics can be moldedwithout draft, when generous fillet

radii are provided. Others may



require a small draft of 1 to 2degrees on smaller pieces, up to atotal maximum draft of 0.04 inch (1

mm) on the side.

Laminated plastic parts are formed

over or inside a die by building upconsecutive layers of impregnatedfibrous material (frequently glassfiber cloth) with a liquid resin as

the impregnating and adhesivematerial. They are cured atmoderate pressure that is provided

by means of an evacuated bag. Theglass fiber reinforcement is strongand rigid and the quantity of resinused is small, thus the dimensional

tolerances on the molded surface



are very close. The man in the shopwill usually say that they are zero,however, the designer should

assume a finite, but small tolerancesuch as ±0.003 inch per inch (mm/mm) for small parts with ±0.010

nch (0.25 mm) as

Ch. 5


41

the upper limit for larger parts. Itshould be remembered that thematerial is flexible and thin parts

can easily be elastically distorted.



Thickness is often defined by thenumber of layers and the thicknessof the stock. This may vary from

0.003 inch to 0.050 inch (0.08 to1.3 mm) for glass cloth; when agreater thickness is required a more

oosely woven matting is used.When properly applied, the resinfills vacancies only and theoreticallydoes not contribute to thickness.

The process is manual and may notalways be closely controlled.Consequently, some tolerance mus

be allowed on the thickness, 1/32nch (0.8 mm) as an upper limit

with glass cloth and 1/8 inch (3mm) with matting.



Plastics, Prefabricated Shapes

The tolerances vary widely with

material, shape, and manufacturingmethod, and catalogs should beconsulted for specific information.

Representative values for thetolerance ranges that may beexpected are given in the chart tothe right. The first figure refers to

the smallest, the last figure to theargest dimension.

For pressed and rolled laminatedplastic plates, the thicknesstolerances are from 0,100 inch pernch (mm/mm) down to 0.030 inch

per inch (mm/ mm). Warp and



Sighting

The simplest method of locating,

not previously discussed, is locatingby sighting to locating lines or othemarkings in the jig. A normal

prerequisite for the use of thismethod is that the part has anacceptable base surface on which itcan rest in a stable position in the

ig. Once this is accomplished, thepart is moved until its contourcoincides sufficiently close with the

markings and is then clamped inposition. The method can be usedfor raw parts such as castings,welded parts, and forgings, where

no great accuracy is required



between the part contour and thesurfaces to be machined. Such partsnvolve large tolerances on the part

contour, and for this reason, eachmarking is made with multiple lineto make sure that the markings are

not totally obscured. With thissimple device, it is always possibleto locate and center the part fairly well. Two simple examples with two

different styles of marking lines areshown in Fig. 6-la and b. For correcocating, the method shown in

these two diagrams dependsentirely on the attention andmanual skill of the operator.However, less manual skill is



required in the modifications c andd, when the part is located



Fig. 6-1. Locating by sighting toines.

by manipulation of finger screws.This dependence on humanudgment is not necessarily always

a liability, since it also permits theoperator to adjust to the correctocation despite bumps or otherocal irregularities on the part

contour.

n example of a differenttechnique, still based on sighting, isa drill jig, shown in Fig. 6-2. Thedrill plate carries sighting apertureswith beveled edges, and the part

contour is lined up with the edges



of

Ch. 6

DESIGN OF LOCATINGCOMPONENTS

43

Fig. 6-2. Locating by sighting to



edges.

these apertures which may be

round or elongated holes or slots.The part is here adjusted to its finalposition by means of cams and

screws.

Nesting

The next logical step, againapplicable to flat parts or parts withat least one flat or fairly flat surfaces to nest it along its contour or

along the contour on its extremeends. An example is shown in Fig.6-3a. The semicircular notches

provide space for the operator's



fingers for inserting and removingthe part. The groove at the contourallows for the burr.

The minimum clearance betweennest and part is determined by the

part tolerance and, obviously,permits some displacement. Thenesting of irregular shapes is,therefore, limited to parts that are

already manufactured with ratherclose contour tolerances. It isusually very suitable for parts

punched from sheet and plate yetess suitable for forgings, partly because of the draft, partly becausethe contour where the flash has

been trimmed off may be offset



with respect to the forging body.

The contour can also be simulated

by blocks with V-notches (Fig. 6-3b). These are cheaper to make andcan also be made adjustable to

accommodate for variations in thepart contour from wear orreconditioning of the tool withwhich the contour was made.

simpler and cheaper method isnesting with pins (Fig. 6-3c). Allpins are shown as cylindrical. Aswill be explained later, locating pinsare, in most other cases, providedwith flat contact surfaces. This is

frequently omitted in contour



nesting fixtures when

—



Fig. 6-3. Examples of nesting alonga contour.

the pins are not exposed to any

substantial load, and also when thepin has to contact a curve on itsconcave side.

Dust and chip fragments which,when accumulated, prevent properseating and cause misalignment of

the part are difficult to clean out of nesting fixtures, particularly the fulnest type. Dirt space allowance istherefore required. A burr groove



Ch. 6

the nest is formed by sealing the

part against the box and pouring acastable material onto the part, Anexample of this is shown in Fig. 6-4

Fig. 6-4. Three-dimensional nestingof an irregular surface.

Castable materials used in nestingare plastics and soft metals. Theplastics are phenolic tooling resin

and epoxy, reinforced, when



needed, with glass cloth. They areight, inexpensive, and easy to

repair if required. The curing

temperature is 300 to 350F (149 to177C). The metals used areKirksite®, a group of zinc base

alloys with a melting range of 717 to745F (381 to 396C), and poured at850F (454C); various lead-lin-antimony alloys with a melting

range from 460 to 500F (238 to260C); and Cerrobend®, an alloy containing bismuth and melting at

158F (70C), that is, below theboiling point of water. The castsurface is ground, if necessary, andpolished to provide some clearance



respect to flatness and dimensions,the simplest locating solution is toprovide mating locating surfaces

ntegral with the fixture. Theprinciple, as applied to a castfixture, is illustrated in Fig. 6-5. The

ma-

fgp»|

chined locating surfaces arendicated by /. The diagram showsarge continuous surfaces as well asndividual pads. Some aspects of th

use of large locating surfaces have



already been discussed. Largebearing areas provide excellentsupport for the part and permit a

great deal of freedom in theplacement of clamping forceswithout danger of elastic distortion

(deflection, springing) of the part;also, as the bearing pressures areow the rate of wear is reduced. On

the other hand, large locating areas

require a high degree of accuracy inthe part as well as in the fixture, foraccuracy is lost if the fixture

distorts as a result of poor stressrelief. Dirt space, however, is only available along the perimeter, thusarge surfaces are apt to accumulate



dirt and chip fragments. It ispossible to subdivide large locatingsurfaces without loss of their

advantages. As shown in Fig. 6-6,the first step is to provide groovesfor the accumulation of dirt; (left)

two sets of crossing grooves changethe original surface into smallerpads without serious sacrifice of supporting and bearing areas; the

ndividual surface areas arereduced, which also facilitatescleaning. The next step (right), is to

reduce the original surface to stripsand finally, not shown, to reduceeach strip to small pads. All thesechanges facilitate the drainage of



Cast or welded integral locatingsurfaces suffer from a commondrawback-they are not directly

replaceable when worn. It is notpractical to harden a cast-ironfixture (steel castings are very

seldom used for fixtures) and theonly means available for controllingthe wear rate is to be as generous aspossible with the dimensions of the

ocating surfaces to keep thebearing pressure low. The sameapplies, in general, to welded

fixtures. However, although notwidely used, it is possible to makethe locating pads and strips fromow-grade tool steel and weld them



nto the fixture body. Apart fromthis, worn surfaces, both on castand on welded fixtures, can be

reconditioned by weld-depositing aayer of material and remachining i

to the original dimensions. The

welding involves some risk of distortion, and a careful inspectionof the fixture is required after therepair. A safer, but more expensive

method, is to remove the worn padsby machining and install new piecemade from hardened steel, secured

by means of screws and dowel pins



Separate Locators

For the reasons explained above, it

s preferred to use separatecomponents for locating purposes,to install them in such a way that

they can be removed and replacedwhen worn, to provide them with ahard working surface, and to protecthem against chip and dirt

accumulation.

Locators have been made frombronze, presumably because of itsuse as a bearing material. Locatingwear strips of synthetic sapphirehave shown a wear resistance

several thousand times that of steel



regular motion, cleanliness) do notapply to fixture locators. With dust,chip fragments, rust, and scale

always involved in their use, theconditions on locator surfaces arefar from ideal, and the type of wear

to be expected is an intermediatebetween contact wear (metalliccontact between clean or corrodedsurfaces, no lubricant, no

significant amount of foreignparticles) and abrasive wear.Locator surfaces do have one

advantage with respect to wear,namely, that they are not exposedto very much sliding motion by thepart. Motion takes place only durin



oading, and the maximum load onthe locators during this period isonly the weight of the part. With

correctly designed clamps, thereshould be no motion when theclamping pressure is applied nor

when the working load from thecutting operation is applied.

It would be desirable if quantitative

data for permissible locator loadscould be quoted but in general, theycannot. The only somewhat

relevant figure is a value forhardened steel of 25 pounds persquare inch (0.17 N/mm 2 ), foundby French and Hersch-man 1 as a

boundary between a lower pressure



region with slow wear and a higherpressure region of more rapid,ncreasing wear. The curve from

which this value is extracted isshown in Fig. 6-8.

1 H. J. French and H. K.Herschman, "Wear of Steel withParticular Reference to Plug Gages,

STM Proceedings, vol. 10,1926.


Ch. 6

u



DL.Z

*£KK

tuLU

a a.

UjUl

3 2

1.0 *10"

0,8

0.6

0.4 —



0.2

0 JO 10 30 W SO

UNIT PRESSURE POUNDS PER SQUARE INCH Courtesy ofH. J.French and H. K. fferschman Fig, 6-8. The rate-of-weai of hardenedsteeL

Example-The largest size of restbutton taken from manufacturer'sstandards has a 1 1/4-inch (32-mm)total diameter. Deducting for the

chamfer, the effective diameter is:

0.92 X 1.25 = 1.15 inches (29.2 mm)



and with three buttons, conformingto the 3:2:1 principle, the maximumoad carried within the 25 psi

pressure limit is:

■4 X 1.15 2 X 3 X 25 = 78.9, or

approximately 80

pounds (36 kg)

very large number of parts weighess than 80 pounds and with the

use of conventional buttons, a longocator service life can be expected.

The fixture designer should notdespair if the locator pressuresignificantly exceeds the limit

quoted, but he must make ample



provision for replacement of wornocators. No fixture is really

expected to last forever, and larger

parts usually do not occur in suchquantities that locator wearbecomes a great problem. When

necessary, larger locators can bedesigned, but under nocircumstances should the designerfeel obligated to employ locators

with excessive bearing areas, chieflybecause it is difficult to keep themfree of chip fragments.

In difficult cases selecting a morewear-resistant material is justified.The ratio of wear resistance of the

four materials—case-hardened



carbon steel, hardened tool steel,cast tungsten carbide (Stellite type)and sintered tungsten carbide—is

1:2:3:40. Any discussion of highwear-resistance applies only to

fixtures for large quantity production. By rule-of-thumb it isaccepted that unhardened locatorsare sufficient for tooling for 100

parts or less.

Buttons

The three most common types of ocating "points" are buttons, pins,

and pads. Conical points are ideal

from the mathematical viewpoint



treating; sufficient relief forgrinding must be provided betweenthe shank and the head. Flat

buttons are used against machinedsurfaces only; crowned buttons areprimarily for use against

unmachined surfaces, but can alsobe used for locating machinedsurfaces. However, they do notprovide a well-defined bearing area

Buttons of these types when usedas base locators are commercially

termed "rest" buttons; when usedfor side and end stops they are thentermed "stop" buttons.

Installation of the button in the



fixture body is done with a press fitn a cylindrical bore (reamed,

precision bored, or ground). For

this purpose the shank ends with a30-degree chamfer. The bore goesthrough the fixture wall; a blind

hole will trap air during pressingand does not permit easy removalof the button for replacement. Thefixture surface is then machined to

provide positive support andadditional alignment for the head.By providing a boss around the

hole, the machining is reduced to aspot facing; on a flat surface, it canbe done by countersinking.

While the shanks on commercial



buttons are supplied withstandardized tolerances, resultingn an oversize ranging from max,

0.0010 to max. 0.0015 inch(0.03 to0.04 mm) within the availablediameter range, there is no formal

standard for the interferencerequired relative to the hole, nor tothe hole-diameter tolerances.However, it is generally assumed

that the hole is finished with areamer with max. oversize of 0.0002 inch (.005 mm) when new.

n analysis of these figuresndicates that the fit actually

obtained will fall in the range fromnterference-fit class LN 3 to force-



fit class FN 2. The

Ch. 6


47



Fig. 6-9. Locating buttons.

class FN 2 fit represents the upperimit, which is in good agreement

with the fact that it (the FN 2 fit) isabout the tightest fit to be used incast iron. It should be remembered

that the reamer, even if it holds the



0.0002-inch oversize, may wellproduce a larger hole, andconsequently a lighter fit, if it is

allowed to wobble during thereaming operation. 2

When a plane (or a line) is definedby three (or two) buttons, they aresurface ground across their facesafter installation to ensure that the

plane (or line) is parallel to thecorresponding outer surface of thefixture,

With a good press fit and amachined surface on the fixturewall, the installation of the button

s accurate, safe, and economical. It



s also proposed (in the literatureand in catalogs of fixturecomponents) to use a threaded

shank in a tapped hole. In this casethe button also has a hexagonalsection for a wrench, as shown in

Fig. 6-1Q. 3 !n general, this practices not recommended. A screw

thread requires clearance and is lesaccurate with respect to location

and direction. The button is notocked and may be loosened by ibration. A fatigue failure or

accidental overload (a blow) may break off the head and make theshank difficult to remove. These

^Detailed information on



definitions and classification of thestandardized fits and numericaldata for their clearances,

nterferences, and tolerances isfound in Erik Oberg and F. D.Jones, Machinery's Handbook (New

ork: Industrial Press Inc., 1971.)19th ed., pp. 1518-1529, followed onpp. 1529-1538 by the metric (ISO)imits and fits.

threaded buttons are for permanennstallation and must be screwed in

tightly; they are not intended to beadjustable in height. Actualadjustable stops and supports willbe described later.



Courtesy of E. Thaulow Fig. 6-10(Left). Locating button with screw thread and a hexagonal section. Fig6-11 (Right). A hollow locatingbutton.

Hollow buttons (Fig. 6-11) fastenedby separate screws, are usedoccasionally as they are a little

cheaper to install. The screw headmust be countersunk safely below the face of the button, which leaves

a small cavity for the collection of



chip fragments and is difficult toclean.

Rest and stop buttons arecommercially available instandardized dimensions. Few case

are encountered within the range ostandardized dimensions

E. Thaulow, Maskinarbejde Gad's

Forlag, 1930) vol. II.

(Copenhagen: G.E.C.


Ch. 6



diameter £>, as no nominally defined bearing area is required.

For flat buttons, H can be selected:

from 1/3 D (but not less than 3/16nch [5 mm]) to 4/3 D (but not

more than 1 inch [25 mm])

B= 3/4 (D-l/8) L = 1/2 (£>+#)

The formulas, except the one for B,are valid in English and in metricunits. With metric units, use:

B = 3/4(D-3)

For crowned buttons, H can be



selected:

from 1/3 DtoD, and

R = 3/2 Z) fl = 3/4 ZJ £-3/41?

Pins

pin is a cylindrical componentthat is contacted on its side. It

follows from this function that theheight of a pin is not a criticaldimension. Buttons can besubstituted for pins, but pins

cannot be substituted for buttons.Pins used as locators are installedby a press fit in the same manner as

a button with or without a shank of



a reduced diameter. Pins are used tomake a nest and, generally, as sidestops and for locating in holes, an

application which will be furtherdiscussed later.

number of typical applications of pins and buttons for side stops areshown in Fig. 6-12. In most of thesketches, no provision is shown for

dirt and chip relief spaces.

Round pins (and buttons) for sidestops can be used on concave andunmachined surfaces. For use onplane machined surfaces, the pin orbutton has a flat to mate the surfac

on the part. For high precision,



these flats are ground afternstallation of the pins in the

fixture. Generally, the use of a pin

as a

a



7!=,

e



Fig. 6-12. Typical examples of pinand button side locators.

a. A simple locating pin used as aside stop.

b. Same, with relieved bearing arean fixture base, c, The conventional

use of a side stop button, d. A button used as a pin for a side stop,

e and f. Pin and button with a flatocating surface used as a side stop.

side stop is a little primitive. The

more usual method of making aside stop is to use a buttonmounted in the side wall of the

fixture, with its face mating the side



sufficient bearing area, as side andend locators, and as nest locators.

Ch. 6


49

The edges and corners of a pad areusually not rounded, beveled, orchamfered as are the edges on abutton, but are only slightly broken

and lightly polished to removeburrs and make them smooth to thetouch. The reason for this

difference is somewhat obscure.



This is a case where a design details based on habit rather than on

calculation or rational logic. Pads

ocated down in the interior of acast or

welded fixture are not chamferedon their edges as they are not easilyaccessible to the machine tool.Loose pads should look like fixed

pads and, therefore, they are alsoeft with their corners and edgesntact. In all fairness, it should be

noted first, that sharp edges on aocating pad may be useful inscraping dirt off the mating part,and second, that the chamfering of

a pad, particularly one of an



rregular



Fig. 6-13. Fastening methods forpads and other locators.


eft. 6

outline, is quite an expensiveoperation because it requires

considerable handwork, while thechamfering of a button is a rapidand inexpensive screw machineoperation.

Pads are fastened by means of screws with well-countersunk

heads, and their position is secured



by means of dowel pins (also othermeans, as required), since screwsare fasteners only and are not

capable of precision locatinganything. The correct use of dowelpins follows certain rules which

apply not only to locating pads butto any loose part to be permanentlynstalled with significant precision. number of representative cases

are shown in Fig. 6-13. (Lower caseetters refer to the particular

diagram.)

In principle, two dowel pins arerequired for locating a componentand they are placed as far apart as

possible (a, c). The holes are drilled



In many cases, it is possible toreduce the number of dowel pins toone, namely, when other locating

surfaces of sufficient precision areavailable to assist in defining theposition of the part. A key and key-

seat (d) or a recess (e) may servethis purpose. One screw, a keyseator recess, and one dowel pin definea position (f). If the position in the

direction of the keyseat or recess isnot critical, a dowel pin is not evenneeded. Two screws and a keyseat

will define the part (g). Two dowelpins substitute for the key-seat,assisted by four or two screws (h).Two screws and one dowel pin may



occasionally suffice (i), namely, if the orientation is not critical. Onthe other hand, a part nested in a

well-fitting recess, held with threescrews and secured by one dowelpin (j) has an extremely well-

defined and secured position. Partswith cylindrical shanks fittingclosely in holes are completely defined by one dowel pin only (k, 1)

s with the shanks for the buttons,dowel-pin holes are drilled through

so that the pins can be driven outagain, when necessary. Therecommended bearing length of adowel pin in each part is 1 1/2 to 2

times the diameter of the pin.



Fig. 6-14 {Left). Standard dowelpins.

Courtesy of E. Thaulaw Fig. 6-15{Right). A tapered dowel pin withextractor screw thread.

Dowel pins are cylindrical(straight), or tapered (Fig. 6-14).The standard taper is 1/4 inch per



foot (1:48), ("Taper" is diameterdifference divided by length.)Straight and tapered pins are

commercially available. The straightype is available unhardened as welas hardened and ground, For

permanent assembly (apart fromthe possibility of infrequentreplacement of a component) the fiof the dowel pin can be a press fit in

each part. This serves the purposeof most fixture applications; incases where occasional disassembly

s anticipated, it is common practiceto give the dowel pin a press fit inone part and a tight sliding (slip) fitn the other part. Again, this is not



ery common in fixture designpractice. Tapered pins are easily oosened by the application of light

pressure or a blow on the small endThey are, therefore, often preferredfor parts that require frequent

disassembly. However, the taperedpin does not produce and maintainas accurate an alignment betweenparts as does the straight part with

a press and a sliding fit. In extremecases where parts must bedisassembled very frequently, the

sliding fit will wear in time andaccurate alignment is lost. In suchcases, a hardened and groundtapered pin gives much better



service. A more sophisticatedersion of the tapered pin, which

greatly facilitates its removal, is

made with a threaded end and a nutas shown in Fig, 6-15. 4 The nut isbacked off when the pin is driven

nto place. When the nut istightened, it gently loosens the pin.

compromise pin design is thetapered pin with a short hexagonal

head. When flat pads used as baseocators are fastened by the means

described, the countersunk screw

heads offer places for theaccumulation of chip fragmentsthat are difficult to clean away.Unbroken pad surfaces can be



obtained if the pads

E. Thaulow, Maskmarbejde

(Copenhagen: G.E.C. Gad's ForLag,1930) vol. II.

Ch, 6


51

are fastened and located by means

of screws and dowel pins from thereverse side and with blind holes.The method is somewhatcumbersome and is not widely



outside locating. In principle, they are nesting devices, and as such,they share the two problems of

amming and clearance versusocating accuracy.

Jamming is mainly a result of friction. If there were no friction,the part would always slidesmoothly into the locator. The

amming process is also affected bythe amount of clearance, the lengthof engagement, and the steadiness

of the hand of the operator.

When jamming occurs, it alwaysbegins when the part has entered a

short distance into an outside



ocator or around an inside locator. case of jamming is shown in Fig.

6-16. The outer cylinder (the

ocator) has diameter W, and thenner cylinder (the part) has

diameter W - C, where C is the

clearance. The part has entered theocator over a short length L, theength of engagement. If the part is

slightly tilted, as shown, then one

side of the leading edge comes intocontact with the inside of theocator and is caught by the friction

If additional pressure is applied tothe part, it serves only to increasethe friction and the tilt, and thusams the part.



ment L, there exist two critical

alues L x and fc Sl which can becalculated. Below L x and above /. 2there is no jamming; the rangefrom L x to L 2 is a no-man's-land

where jamming is possible andikely to occur. This area can,

however, be completely eliminated,

and the locator made jam-free, by providing a relief groove on theocator over a length of at least

from /,i to L 2 , Dimensions can be

calculated from the general theory.



The most important dimension isljwhich is determined by

L 2 =M*

where £t is the coefficient of friction and W is the width of theopening.

The dimensions of the relief

grooves can be standardized. Nosuch standard exists as yet in theUnited States. A German standard(DIN-Norm 6338 in Vorbereitung)

for locating pins has been proposedwith dimensions closely approximating those which can be

derived from the theory and with a



chamfer for pre-positioning.Converted to easy formulas, therecommended dimensions (see Fig

6-17) are:

Li =0,02D L 2 =G.12D

1 3 ss 1/3 \/5 (with L 3 and D innches) L 3 « 1.7 y/D (with 1 3

andZ) in mm) d = 0.97 D

Fig. 6-16, Jamming.

ccording to general theory, the

risk of jamming is associated withthe length of engagement betweenthe part and the locator. For the

ength of engage-



^

1

^-

/k

Fig. 6-17. The significant dimensionof a jam-free circular locator.

The mode of action of a circularocator is modi-fled when it is

combined with a flat locating

surface, a plane perpendicular to itsaxis. The flat surface aligns the partand defines the direction of its axisand the circular locator needs only



to define the location of the axiswith the result that its length can breduced. It is always safe to make

the total length less than thepreviously defined length L t , but its by no means necessary because

the outer


Ch. 6

dimension of the part is also a

factor in determining the maximumpossible angle of tilt, as seen in Fig.6-18, A point A on the locating

surface of the part can swing in a



circle around a center B on theouter perimeter of the part. Any ength A y D of the locator that

makes it stay within the circlearound B is jam-free, even if it isgreater than L lt and the diagonal

CD is longer than the diameter ACnside the part.

Fig. 6-18. The geometry of a jam-free circular locator in combinationwith a flat locating surface.



ny circular locator of a shapecontained inside the sphere willocate jam-free. Such a locator,

consisting of two opposed conicalsurfaces joined by a narrow cylindrical band, is shown in Fig. 6-

19b. This is a solution with practicaapplications,

n entirely different type of

modification of a cylindrical locators shown in Fig. 6-20. Three flats

are machined on the cylinder,

eaving three circular lands 120degrees apart. To provide sufficientbearing area, the width of each lands taken as 30 degrees. This cut

cylinder is now used as an internal



ocator and mated with an externalpart which is assumed to be longerthan the locator. With the same

etter symbols as in Fig. 6-16, theouter cylinder, shown at the left,has diameter W, and the inner

circle through the three lands(shown at the right) has diameterW - C, where C is the diametralclearance. In the concentric

position there is a radial

C clearance of — on each land. This

s also the distance

A i and, therefore, the verticalclearance at A. The



When the circular locator iscombined with a flat for alignment,t does not even have to be

cylindrical. If it is made spherical(see Fig. 6-19a) it still centers thepart. A sphere has one and only one

diameter and no "diagonals," and isam-free at all angles. It is

expensive to machine with goodaccuracy, and the spherical locator

s therefore not a very practicalsolution, but it points the way toother solutions.



a b

Fig. 6-19. Jam-free noncylindrical

circular locators.

0.8536 CW-C)

Fig. 6-20. A cylindrical locator withtriangular relief to minimize

amming.



Ch. 6

DESIGN OF LOCATING

COMPONENTS

53

radii to E and F are drawn at 45degrees with the horizontal. Withthe parts still in the concentric po-

C sition, the vertical clearance at Eand F is 1.4142 2 ,

making the total effective clearancefor vertical motion:

f + 1.4142^ = 1.2071 C



c , .

To jam, first move the part a

distance — up until A i

falls on A 7 , and there is contactwith the locator along thegeneratrix through A. The outercircle, which is the contour of thebore in the part, is projected as the

circle through A^E^Fj. Then tilt thepart around a horizontal axisthrough A?, perpendicular to theaxis of the locator, and located atthe forward end of the part, withthe rear end of the part movingdown. Continue tilting until points

E 2 and F 2 , located further back in



the bore, come in contact withpoints on the rear end of theocator, projected in points E and F

This is the position



WVV\^\wv\^w\\\\\\\\\^

Fig. 6-21. Overdefining and correct

defining of a part with more thanone significant diameter.

Fig. 6-21 Facilitating the entrance oa part with two significantdiameters.

where jamming may begin. In thisposition, the old dimension W isreplaced by 0.8536 W (see left partof the figure) so that the critical

ength £ 2 is now:

L 2 = 0.8536 flW



The triangular shape has reducedthe critical length for jamming by approximately 15 percent but has, a

the same time, increased theeffective clearance by approximately 20 percent and

reduced the locating accuracy of theocator by the same amount.

Locators for parts with more than

one significant diameter must notoverdefine the part. An exaggeratedbad example is shown in Fig. 6-2la.

It is four times overdefined. Thedesign of the locator can bemproved in many different ways;

two correct designs are shown in

diagrams b and a Locators with



Radial locators are those that act ona radius in the part to preventrotation around a fixed center.

Instances where a "radius" is aphysical feature of the workpiecehave been discussed previously.

There are many cases, however,where the configuration of theworkpiece does not provide any opportunity for radial locating, and

other means must be found for thispurpose. Such means fall into threecategories: keys and keyseats, dual

cylinder locating, and indexingfixtures. Any radial locator has acertain tolerance and thereforenvolves the possibility of an



angular error. With tolerance T andradius R {see Fig. 6-23) the angularerror is:

T

8 = ir radians

Since T may be a constant, or ateast a quantity with a fixed lower

imit, it follows that radial locatorsshould be placed on the largestpossible radii for the best angularaccuracy.



t-'ig. 6-23. The radius sensitivity of a radial locator.

Keys and Keyseats

The key with its keyseat is one of the most common elements used inmachine design for the expresspurpose of permanently locatingone machine part radially with

respect to another. Keys andkeyseats are accurately machinedand are capable of transmittingarge forces. The machining of a

keyseat in a part is a fairly expensive operation and keyseatsare not put into parts just for the

purpose of locating them in



fixtures. If, however, the partalready has a keyseat, then thiskeyseat can be utilized for radially

ocating the part relative to afixture.

Keys and keyseats are used for themost part, in connection withcircular mating surfaces, to preventrotation. They are also used

between parts with flat surfaces toprevent transverse shifting. Thesetwo arrangements of keys and

keyseats are shown in Fig, 6-24.Each of them may also be utilizedfor locating the part in a fixture.When used in machinery, a key and

ts keyseat serve as parts of a



permanent

Fig. 6-24. The key as a radialocator.

assembly and are not exposed towear. Where key-seats and keys areused as fixture elements, they arecontinually exposed to wear

because parts are inserted andremoved all the time, as long as thefixture is in operation. Hence it isrecommended that the key, at least



be made of hardened steel and alsof necessary, a hardened insert be

provided for the keyseat. Keys and

keyseats are usually located onsmall radii and have a tight fit;when used for radial locating in a

fixture, they should be made with asliding fit and with the closestpossible tolerances.

Dual Cylinder Locating

Dual cylinder locating uses a flatbase and two cylindrical locators inmating holes. This eliminates all sixdegrees of freedom and providesexcellent mechanical stability with

an accuracy which depends only on



clearances in the holes andtolerances on the hole centerdistance. The interplay between

these tolerances and clearancescreates a specific problem for whichthere exists a specific solution, the

diamond pin.

ssume first, a rather special casewhere the center distances match

so closely that their tolerances canbe ignored. The locating accuracy then depends entirely on hole

clearances which can be minimizedby the use of expanding locators.The expanding locator is shown inFig. 6-25 where A is the bushing,

fitting the finished hole in the work



This bushing is split in severaldifferent ways, either by having oneslot cut entirely through it, and two

more slots cut to within a shortdistance of the outside periphery, oby having several slots cut from the

top and from the bottom,alternating, but not cut entirely through

Fig. 6-25. An expanding locator forminimizing the clearance.

Ch. 6




55

the full length of the bushing. Themethod of splitting, however, inevery case, accomplishes the sameobject, that of making the bushingcapable of expansion so that when

the stud B, which is turned to fit thetapered hole in the bushing, isscrewed down, the bushing willexpand.

It should be noted that the studactually consists of four different

sections, the head; the tapered



shank; a short cylinder; and thescrew thread, The cylindricalsection matches a precision bore in

the fixture base and defines theocation of the axis of the stud.

In more general cases, these almostdeal conditions do not apply. Thereare tolerances on two holes, twoocators, and two center distances;

the tolerances must be adjusted toeach other in such a manner thatthey leave sufficient clearance

around each locator for any permissible dimensional condition,and radial locating must beaccomplished with prescribed

angular accuracy.



To illustrate the problem, considera part with two holes of diameter Dand center distance L ± T. Assume

zero tolerance on all hole diametersand on the center distance L in thefixture. As seen from Fig. 6-26, it is

necessary to reduce the diameter ofthe pin at the right from D to D —27 to make it pos-

sible for the part to be nested in allcases (with all center distancesfrom L — T to L + 1% Consequently

a clearance of IT is introducedbetween the pin at the right and thehole in the part, resulting in anangular error



2T

8 = -j- radians

The case is oversimplified by omitting most of the tolerances butthe result would only be aggravatedby taking all tolerances into properaccount.

It is obvious that the problem couldbe eliminated by elongating thehole in the part (Fig. 6-27). It is alsoobvious that it is not practical to

make elongated precision holes inparts just to fit them into a



Fig. 6-27. Hypothetical locating toan elongated hole.

Fig. 6-26. The general case of dual



cylindrical locating.

DESIGN OF LOCATING

COMPONENTS

Ch. 6

fixture. Any modification to bemade must be with respect to theconfiguration of the pin and it must

permit relative motion between thehole and the pin in the direction of the hole's centerline while it retainsa close fit between these two

members in the directionperpendicular to that centerline.

The Diamond Pin



solution along these lines isphysically possible and technically practical because the fit between

the hole and the major dimensionon the pin must be a clearance fit topermit easy loading and unloading

of the part.

The cross section of the pin isrhombic (hence the name "diamond

pin," see Fig, 6-28) with lengths. Itfits into a hole of diameter D with aclearance C, so that

D=A + C

ssume first that the section

terminates in sharp



points at its upper and lower end(upper part of the

figure). Then using the formula fora circular seg-

C ment in which -j is the chordalheight and T the width

of the segment:

(1% £(d-C\= ™_c^cd

W 2\ 2J 2 4 " 2

T = y/2 CD

ctually the pin does not terminate

n points, but has wearing surfaces



of width W (lower part of figure) sothat A becomes a diameter (thepilot diameter) and

W+T= y/2CD

Fig. 6-28. The geometry of thediamond pin.



sible width W increases with D anddecreases with center distancetolerance T.

arious suggestions have appearedn the literature for the width W.

The recommended value is 1/8 of Dwith 1/32 to 1/64 inch (0.8 to 0.4mm) as a lower limit. All this is nowhistory as these pins are available in

standardized dimensions with W=1/3/4.

proposed N.U.F.C.M. standard(see Chapter 17) comprises sizes upto 1 inch; individual manufacturer'sstandards go up to a 3-inch (75

mm) nominal diameter. The pilot



accuracy and a sufficiently easy sliding fit. T is the total longitudinatolerance; it includes tolerances on

center distances in the part and thefixture and the diameter tolerancesand clearances on the hole and the

pin at the other end. The width W istheoretically selected from wearconsiderations.

It follows from the above formulathat for a given, or desired,tolerance T, the maximum

permissible width W increases withhole diameter D and clearance C. Italso follows that for a given holeclearance C (and corresponding

angular error) the permis-



Fig. 6-29. The use of crosseddiamond pins.

Ch. 6


57

up and down motion at B. Theustification appears somewhat



ncomplete since the up and downmotion at A is not prevented. To usthe crossed diamond pin principle

would require one additionalocator, for example, an external pin

or a button, as indicated by the

dotted lines,

fully legitimate use of a singlediamond pin in combination with

another locator is shown in Fig. 6-30. Up and down and angularocating (not shown) is done by the

fixture base; the diamond pinocates the part lengthwise, whileallowing for the tolerance on thehole center distance a above the

base.



Fig, 6-30. One diamond pin used incombination with a flat locatingsurface.

J1 dual cylinder locating systemscan be designed for easy loading by

application of the two commonprinciples, pre-positioning andsuccessive entering (one at a time).

n illustration of the use of these

two principles is shown in Fig. 6-31



The two pins are shaped for pre-positioning in two different ways;one pin is shown with a long lead

and the other is chamfered. Theength of the actual locating surfaces also different on the two pins.

The part enters first on the long pinto the left, and is supported andguided when it subsequently entersthe short pin to the right.

^#^S^^^N

Fig. 6-31. Dual cylindrical locating

arranged for preposition-ing and



successive entering (one at a time).

Typical Applications of Dual

Cylinder Locating

Dual cylinder locating is simple,reliable, and inexpensive. If a part,as designed, does not have the twoholes that are needed for locatingpurposes, such holes (sometimes

named "tooling holes") can, inmany cases, be drilled and reamedwithout impairing the function of the part. The same tooling holes canbe used for locating the part inseveral fixtures, one at a time-andeven for reconditioning operations

at some later time.



The principle is extensively used inmass production such as in theautomotive industry. For example,

two holes are drilled and reamed inthe pan-rail of the cylinder block tocloser positional tolerances than

required for functional purposes.These holes serve to locate theblock for all operations except formachining the transmission-case

face on the end, the pan-rail face,and the head faces; these havingbeen machined in earlier

operations. The part is then enteredon the conveyor in a transfermachine. Movable "shot pins" enterthe locating holes in the pan-rail to



ocate the block at each station of the transfer machine. All majorautomobile companies in the

United States use this system tomachine engine blocks. Smallerautomotive components mounted

on movable fixtures (also called



Courtesy of The Cross Co. Fig. 6-33

Loading a transmission case on thepallet fixture. Three locating holeswith adjacent bearing surfaces

(indicated by arrows) are machined



n a previous operation. In theoading station, the transmission

case is manually loaded on three

ocating and bearing points in thepallet fixture and the clampingstraps are brought into place. In the

following station, [he clamping nutare automatically tightened to apredetermined a-mount of torque.

Indexing Fixtures

Indexing means to rotate a part to apredetermined angle and secure itn the new position. Very often, but

not necessarily always, the angle ofrotation is a simple fraction of a ful

circle and repeated indexing will



finally bring the object back to thestarting position. Sometimes theword "indexing" is also used

for a straight-line motion of apredetermined length, followed by a

ocking operation-in other words,"indexing in a straight line." Bothtypes of indexing are used in fixturedesign; angular indexing is by far

the most common.

primitive and inexpensive, butnot very accurate, indexing deviceconsists of: a fixture with a bearingpin that fits into a hole in the part, anumber of markings on the

periphery of the part, a target mark



on the fixture, and a clampingdevice. One marking at a time isaligned against the target mark and

the part is then clamped andmachined.

part with a central bore and anumber of holes of equal sizeocated in a circle concentric with

the central bore can function as its

own indexing device. The fixturehas a locator-for the bore and a holewith a pin targeted on the hole

circle. When the pin is brought toenter a hole in the part, the part isocated. This is, essentially, a case

of dual cylinder locating. When the

pin is withdrawn, the part can be



ndexed to the- next position andagain locked with the pin.

Schemes such as these arenexpensive to make, but are slow n operation and not very accurate

as correct operation depends fully on the skill and attention of theoperator. There is also no provisionfor compensation for wear.

Most indexing operations are farmore demanding with respect toaccuracy, fast operation, and fool-proofing. Accuracy means twothings, accuracy in operation, andsustained accuracy during the

entire life of the fixture. Provisions



for the satisfaction of all thesedemands must be built into thefixture. In addition, the fixture

must possess rigidity and strength,and have proper locating andclamping devices for receiving and

holding the part.

The fundamental component inmost indexing fixtures is the

ndexing table. It performs thefollowing functions: It receives andholds the part by means of locators

and clamps. It rotates around anaxis with a minimum of play anderror. It carries the weight of thepart and the load from the

machining forces and transmits



these forces to the fixture base.Finally, it indexes accurately fromposition to position in a manner

uninfluenced by the natural wearon its moving parts.

n example of an indexing fixturethat satisfies all of theserequirements is shown in Fig. 6-34.In this indexing mechanism one of

the chief points in design is toprevent variations in the spacingdue to wear on the mechanism. The

fixture is so arranged that wear onthe indexing points is automaticallycompensated for by theconstruction of the device;

therefore, the provision made for it



upkeep is ex-

Ch. 6


cellent. In addition to this feature,the design is not very expensive andt may be made up at much less cos

than many other kinds of indexingdevices. The work ,4 is a clutch gearthe clutch portion B of which is tobe machined in this setting. As the

work has been previously machinedall over, it is necessary to work fromthe finished surfaces.



Fig. 6-34. A typical and well-

designed indexing fixture formilling clutch teeth.

The body of the fixture G is of cast

ron and it is provided with two



machine steel keys at P; these keysocate the fixture on the table by

means of the T-slots, and the

holddown bolts Q lock it securely inposition. The revolving portion of the fixture F is also of cast iron and

has a bearing all around on thebase, while the central stud C isused as a locator for the work at itsupper end, and holds the revolving

portion down firmly by means of the nut and collar at H. The fittingat this point is such that the fixtuie

may be revolved readily and yet isnot free enough to permit lostmotion. A liner bushing of hardened steel is ground to a nice



fit on the central stud at E and willwear almost indefinitely, while anndexing ring L is forced on the

revolving portion F of the fixture,and doweled in its correct positionby the pin V and held in place by th

four screws/?. The work is helddown firmly on the revolvingportion by means of the threeclamps J, these being slotted at K to

facilitate rapid removal.

steel index bolt M of rectangular

section is carefully fitted to the slotn the body of the fixture, andbeveled at its inner end S so that itenters the angular slots S and T of

the index ring. Clearance is allowed



between the end of the bolt and thebottom of these slots so that wear iautomatically taken care of. A stud

O is screwed into the underside of the index bolt and a stiff coiledspring at N keeps the bolt firmly in

position. The pin U is obviously used for drawing back the bolt andndexing the fixture. Points worthy

of note in the construction of this

fixture are the liner bushing at E,the steel locating ring /., theautomatic method of taking up

wear by the angular lock-bolt M,and the spring N.

With ample bearing dimensions

and a hardened steel liner or liners,



n the bearing, the fixture canoperate year after year withnegligible wear because the amoun

and velocity of the motion-aresmall, and the bearing is practicallyunloaded during indexing. Should

wear ever exceed the permissibleimits, it is a simple matter to

replace the central stud and theiner. The same applies to the index

ring, as it is also a separate item.The part that receives most wear inthis, as in any other indexing

device, is the index bolt. It retainsts beveled shape even when worn

and continues to align the indexring properly as it closes in by the



Fig. 8-11) can be just as accurate asthe flat bolt, as long as it is new; ast wears, however, it forms recesses

and loses accuracy.

Only very large indexing fixtures fo

heavy parts require ball or rollerbearings. The bearings used in suchcases are the same types as are usedn large precision machine tools.

The milling machine dividing headcould perhaps, in a sense, beconsidered an indexing fixture; thesame applies to the various types ofrotary tables, horizontal; vertical;and tilting. They are all work

holders, and so are standard vises,



magnetic chucks and faceplates,athe chucks, and so on. However,

they are designed as general

purpose tools and they are allcommercially available. For thesereasons, they shall not be further

discussed in this book


Ch. 6

except where they may serve as

bases in actual fixtures for specialapplications. Indexing pins withiners or bushings (see Chapter 17)

are standardized and are




In the previous examples, the

pulling of the index bolt and therotation of the work were straightmanual operations. These

operations can, of course, also beperformed by various mechanicalmeans. There is another extremely simple device, which may even be

called a trick, that can bencorporated into the design of anndexing fixture which permits

rapid indexing without the need fornstalling additional mechanisms. Iconsists of selecting a rather largencluded angle for the bevel or tape

on the indexing bolt. On a



cylindrical indexing bolt, this angles zero. On most ordinary indexing

fixtures the angle is 8 to 12 degrees

so that the bolt is self-locking. If thncluded angle is made larger than

two times the angle of friction, the

bolt is no longer self-locking, butcan be pushed back and out, if asufficiently large turning moment iapplied to the index table. Such

devices in the form of spring stops,ball plungers and detents (seeChapter 17), are also commercially

available.

The drill jig shown in Fig. 6-35 wasdesigned for drilling four angular

holes in a brass time-fuse cap. (See



sectional view of cap at lower partof illustration.) The principle of thisig can easily be applied to other

work. The jig consists of a hardenedsteel locating plate A mounted on ahardened spindle, which runs in a

bushing that is also hardened. A ball bearing B takes the thrust of the spindle. At the other end of thespindle is an index plate C in which

are cut four 90-degree notches.Keyed to the index plate, and also tothe spindle, is a ratchet wheel D,

having four teeth. A hand-lever E,which has a bearing and turnsaround a hub on the index plate,carries a spring pawl F that engages



with the ratchet wheel D. The leveralso carries, at the outer ends, twopins G that project downward, so

that when it is pushed back andforth, the pins strike on the body ofthe jig and prevent carrying the

ndex plate beyond the locking pinH. This locking pin is a hardenedsteel sliding pin, one end of whichs rounded and engages with the

notches in the index plate. Back of the pin, and held in place by aheadless set-screw K, is a coil sprin

J, which holds the locking pinagainst the index plate. The tensionof this spring is just enough to holdthe work from turning while being



drilled, but not enough to preventts being readily indexed by a quick

pull on the indexing lever.

The work is held in position againstthe locating plate A by the plunger

L, which rests on a single,



the indexing is done by lever E. Theocating plate A has slots milled int with a radius cutter of the same

radius as the drill to be used. Thisfeature, in connection with the lipon the work, answers the same

purpose as a drill bushing; no othermeans of guiding the drill beingnecessary. The production record ofthis jig was about 4000 caps per

day,

Stability Problems

Some indexing fixtures presentstability problems. Small or flatparts with a short dimension in the

direction of their axis are easily



handled on indexing fixtures with ahorizontal table. Heavy parts of greater axial length cannot be

fixtured with an overhang, butrequire the equivalent of anoutboard bearing. There are cases

where an actual outboard bearingcan be added to the fixture, butusually, this is an impracticalsolution and it is necessary to use a

fixture provided with two trunnionssupported in a separate cradle withtwo bearings, as shown in the

following example.

Ch. 6

DESIGN OF LOCATING



COMPONENTS

61

It is necessary to drill quite anumber of holes in the castingshown in place in the jig illustratedn Fig. 6-36; these holes are located

on different sides and at variousangles to one another. For this

reason, an indexing jig is employedThis illustration shows the cover A of the jig removed in order tollustrate more clearly the position

of the casting, which is located inthe jig by its trunnions. The mainbody of the jig is also supported by

heavy trunnions at each end, and



the large disks B and C enable it tobe held in different positions. Thesedisks contain holes which are

engaged by suitable indexingplungers D, at each end of thefixture.

Fig, 6-36. A large trunnion-mountedndexing jig.



djustable Locators

The term "adjustable locators" is

occasionally used with severaldifferent meanings, and someclarification is therefore required.

In Chapter 3 the difference between"locators" and "supports" wasexplained. "Locators" are theelements that are necessary and

sufficient for full geometricaldefinition of the locating of thepart; they may or may not be

sufficient, however, for the stablemechanical support against all theforces acting upon the part when its being clamped and machined.

ny additional elements that may



be required for this purpose aretermed "supports."

One basic function of the locatorscan be described as the eliminationof the six degrees of freedom. In

mechanical language, this meansthat the locators bring the part intoa statically determinate positionwith respect to the fixture, and any

additional support makes theposition statically indeterminate.

ny such support is said to be

"redundant."

statically indeterminate positionor system is not necessarily bad.

The redundant supports do no



harm if they are compatible withthe part; this means, if they arefitted so closely that they maintain

contact with the part withoutexerting any force upon it. If theredundant supports are

ncompatible with the statically determinate system, there are thenthree possibilities:

1. They fail to contact the part; inthis case, they are ineffective andcould be dispensed with.

2. They lift the part off one orseveral of the locators; in this case,they assume or usurp the locators'

function,



3. They exert significant forcesupon the part, and in so doing, theympose a deformation (deflection,

distortion) within the part andoads (reactions) on the locators.

They "spring" the part (if they do

not bend or break it!).

The various possibilities(compatibility and the three forms

of incompatibility) are shown inFig. 6-37. The part is supported as abeam on two end supports and is, in

this condition, statically determinate. The addition of aredundant intermediate supportmakes the part statically

ndeterminate. Clearly, each of the



three alternative forms of ncompatibility is unacceptable, and

redundant supports are therefore

made adjustable. The variousdesigns are described in Chapter 12

The adjustable locators to bedescribed in this section are thebasic locators conforming to the 3-2-1 principle or its equivalent.

djustable locators are used for thefollowing purposes: Toaccommodate raw parts that exceed

normal or previously establishedtolerances, to adjust fordimensional changes within thefixture from wear, abuse, or neglect

and to use one fixture for more



Dimensional changes within a raw part may occur from time to time.Common causes are change of

supply source, variations(intentional or unintentional) infoundry practice, overhaul or

replacement of forging dies or othetools, etc. If the change

DESIGN OF LOCATING

COMPONENTS

Ch. 6

° (m\-

m



D

\m

Fig. 6-37. Compatible andncompatible statically

ndeterminate I oca tors.



Fig.6-38. A threaded adjustableocating point.

y tightened up and loosened, tohold and release the work, when thntention is that these screws, when

once adjusted, should remain fixedIt is not even possible to dependupon the locknut stopping the

operator from using the screw as abinding screw, A headless screw,therefore, is preferable, as it is lessapt to be tampered with.



different form for the adjustableocator of the screw type is shownn Fig. 6-39. s The head is hexagona

and the top of the screw is rounded(crowned) so that it offers a regularbearing area even when the screw

axis is slightly out of alignment dueto clearance in the screw thread.The bearing area in all

exceeds the fixture tolerances andappears to be permanent, it isnecessary to readjust the fixture.

This is a toolroom operation and isfollowed by an inspection similar tothe inspection of a new fixture. Theoperator should not reset locators

or other vital adjustable parts in a



fixture. Adjustable locators arepurposely so designed that they donot invite, encourage, or facilitate

adjustments "on the shop floor." Incontrast, adjustable supports aredesigned for convenient and fast

operation, preferably without theuse of tools.

djustable Locating Points

The most common form of adjustable locating points is the setscrew provided with a locknut, asshown in Fig. 6-38. The screw A, isa standard squarehead set-screw,or, in some cases, a headless screw-

with a slot for a screw driver; this



screw passes through a lug on theig, or jig wall B, itself, and is held

stationary by a locknut C tightened

up against the wall of the jig. Eitherend of this screw may be used as aocating point, and the locknut may

be placed on either side. By using asquarehead screw, adjustment is

ery easily accomplished, butunless the operator is familiar with

the intentions of the designer of theig, locating points of this kind are

sometimes mistaken for binding or

clamping devices, and the set-screws are inadvertent-



Courtesy oft'. Thauiow Fig. 6-39. A threaded adjustable locating button

with crowned head.

screw type locators is hardened.Screw locators are much longer

than fixed locators. They can beused as side and end locatorswithout difficulty, but not always as

base locators because of the limitedertical design space in the bottomof a fixture.

djustable base locators can be



designed on the wedge principle,The action of a wedge ismechanically equivalent to the

action of a screw, but the wedge hasts major dimension perpendicular

to its direction of action. The wedge

s, therefore, a suitable device foradjustments in a narrow space.

n example of a wedge-operated

adjustable base stop is shown inFig. 6-40. The base stop C is raisedand lowered by the sliding motion

of wedge A . The

S E. Thauiow, Maskinarbejde Gad'sForlag, 1930) vol. II.



(Copenhagen; G.E.C.

Ch. 6


63

Fig. 6-40. A wedge-operated

adjustable base stop.

wedge is provided with a handle B,

so attached that it can easily be



operated. It is held in place by twoshoulder screws that are insertedthrough two elongated slots milted

n the wedge; these screws aretightened after the stop has beenbrought up to position. One

disadvantage in using this type of stop is that owing to the vibrationof the machine while in operation,the wedge is prone to slip back,

causing the stop C to drop down.arious improvements are possible

however, and will be described in

Chapter 12, in connection withsupporting elements of a similartype.

The "sliding point" is another



adjustable locator which is usedextensively in fixtures. It requiresconsiderable design length and

must also be accessible from aboveor from the side. Its principalapplication, therefore, is for side

and end stops. One design is shownn Fig. 6-41, where A represents the

work to be located; B the slidingpoint itself; and C the set-screw,

binding it in place when adjusted.The sliding point B fits a hole in theig wall and is provided with a

milled flat, slightly tapered asshown, to prevent its sliding back under the pressure of the work orthe tool operating upon the work.



This sliding point design isfrequently used, but it is not asefficient as the one illustrated in

Fig. 6-42. In this design the slidingpoint A consists of a split cylindricapiece, with a hole drilled through it

as illustrated in the diagram, and awedge or shoe B tapered on the endto fit the sides of the groove or splitn the sliding point itself. This

wedge B is



Fig. 6-41. A sliding point with a lockscrew.

Fig. 6-42, A split cylinder sliding

point expanded by a wedge and aock screw.

forced in by a set-screw C, for the

purpose of binding the sliding pointn place. Evidently, when the screw

and wedge are forced in, the sliding

point is expanded, and the friction



hard.

djustment for Wear

The adjustable point locators asdescribed in the previous sectionare essentially designed foradjustment to wide dimensional

ariations on raw parts with widetolerances, and locator wear is not a

significant factor.

djustment for wear as well as forocator displacement from other

causes such as overload,carelessness, neglect, misuse, andaccidental damage, is also required

on precision locators to be used on



parts with close tolerances.djustable locators of the screw and

wedge type can be designed with a

DESIGN OF LOCATINGCOMPONENTS Table 6-1.

Dimensions of Sliding Points andShoes or Binders

Ch. 6

Shoe or Binder



Sliding Point

Dimensions, in Inches

B C

% 2 % to 3

7,6

% 1 % to 3

%

%

2 % to 3

7„



% 2 % to 3

%

Screw

B

C

/is

%

%

%

32



Dimensions, in Millimeters

B C

10 57 to 75

5

13 57 to 75

6

16

57 to 75 8

19

57 to 75



10

Screw

B C

8

6 13 1.6

10

7 16 2.4

fine adjustment ratio (fine pitchscrew threads) for this purpose andused in drill jigs and millingfixtures. Lathe fixtures present

special problems as they do not



always provide the space requiredfor screw and wedge locators. They are exposed to wear and also risk

accidental damage when mountedon or removed from the lathespindle, with a resultant

misalignment of the fixture axis.djustment for this type of error

requires certain devices for recen-tering of the locator section of the

fixture.

One fixture for this purpose, which

may also be adjusted to handleseveral sizes of work^, is shown inFig. 6-43. It is essential to be able totrue this fixture when it is mounted

on the spindle nose since absolute



concentricity is required betweenthe machined surfaces. This isaccomplished by four adjusting

screws D and a wedge pin assemblywhich will be described later.

The basic fixture components arethe nosepiece B, which can bedesigned to fit any standard spindlenose in the conventional manner,

and the fixture bodyC. A hardenedsteel locating ring H is mounted

on the fixture body with a sliding fiand clamped in place by the sockethead screws /. The workpiece A isocated and held inside this locating

ring by three strap clamps K. Rings



of several different sizesare made,which can be mounted on thefixture body to accommodate

different sizes of workpiece s.Whenever this fixture is mountedon the spindle nose of the lathe, the

concentricity of the locating ring Hshould be checked with respect tothe rotation of the spindle, using adial test indicator capable of readin

to .0001 inch (0.025 mm). If theocating ring does not run true, it

can be adjusted by means of the

four adjusting screws!) in a mannersimilar to adjusting the jaws of afour-jaw chuck. When adjusting thefixture, only two opposing screws



should be loosened at any one timewhile the other two remaintightened. In this way the fixture

body will remain seated against thenosepiece while the adjustment ismade. The fixture is ready to be

used when the locating ring // istrue within .0002 to .0003 inch(0.005 to 0.008 mm) with all of theadjusting screws D tightened.

Ch. 6


65



§ s

3" I

& 1

I £

3

t


Ch. 6



& E

1 "

a= .y

x a

s o S

2 x;

C

Ch. 6




67

Tightening the adjusting screws D

serves to clamp the fixture body Csecurely to the nosepiece 5, and toocate the fixture accurately in the

axial direction by forcing it toregister against a locating surfaceon the face of the nosepiece. This isaccomplished by the action of the

wedge pin assembly, consistingoi awedge pin E, a wedge-pin seat F,and a wedge-pin seat container G.

The four wedge pins fit closely inthe holes below the adjustingscrews D. These holes should betapped only to a depth that will

allow sufficient room for the



adjusting screws to operate. The tapdrill hole should be reamed to sizeand a hard reamer should be used

to remove any burrs in these holesresulting from the tappingoperation. The round wedge-pin

seat containers G are made of hardened steel and are press fit intothe nosepiece B. An eccentric holes drilled in the seat of these

containers, which must be locatedn the forward position, as shown in

Fig. 6-43, when the containers are

pressed into the nosepiece. A slightnaccuracy in the position of the

eccentric hole is not harmfulbecause it is made much larger than



the pin that is placed in this hole.This pin is pressed into the face of the wedge-pin seat F and it serves

to locate the wedge-pin seat so thatthe bevel ground on the oppositeface will be oriented approximately

n the right direction. The wedge-pin seat F is a very loose fit in thewedge-pin seat container G toprovide it with a limited freedom of

movement. The bevel angle on thewedge pin and on the wedge-pinseat as well, should be 15 to 22

degrees. Thus, when the clampscrews are tightened, the wedges, obevels will cause a reaction of theclamping force, so that it will have



both a radial component and anaxial component. The radialcomponent will hold the fixture

body in the correct radial locationand the axial component will hold iagainst the nosepiece, thereby

providing axial location.

The workpiece A is machined withan 80-degree diamond shaped

nsert L held in a disposable inserttoolholder. The toolholder is held inan adapter that is mounted on the

face of a turret on an NC lathe. Itcould also be held in a conventionamanner on an engine lathe, or on aturret lathe having a cross-sliding

saddle. The cutting tool is used to



machine the faces and the majorrecess. Of compact design and builtclose to the spindle nose, this is an

example of a fixture designed forstandard work that requiresaccurate machining and where the

production lots are small. Althought is heavy, there is so little

overhang that its weight is of smallmportance.

nother fixture incorporating theadjusting screw and wedge-pin

principle is shown in Fig. 6-44, Thisfixture illustrates a different andmore sophisticated clamping devicewhich is an embodiment of the

floating principle. The workpiece is



a bevel gear A and the fixtureconsists of two principal parts, thespindle nosepiece B and the fixture

body C.

The workpiece is mounted on a

hardened steel locating ring H,which is pressed onto the fixturebody. This ring has a clearancegroove to collect small chips and

dirt, enabling the workpiece toregister against the locating face of the fix ture body.

When the fixture is mounted in theathe, the locating ring H must be

trued with respect to the rotation of

the spindle. This is done, as before,



are placed 120 degrees apart andhave slightly oversize holes throughwhich the clamp retaining screws M

pass. These screws have a bailsurface on the underside of thecollar corresponding to a similar

depression in the clampsthemselves. A bronze or steelbushing / is pressed into the fixturebody C, and is threaded with a

coarse-pitch thread whichcorresponds to that on the clampoperating screw J, After the clamps

L have been swung into place onthe ring gear, a few turns of thedamp operating screw tightens allthree of the clamps against the ring



gear A through the action of thespherical floating collar K, whichbears against the inner sides of the

clamps.

Where high production is required,

a machine equipped with a rotatingpneumatic cylinder is used. In thiscase the threaded bushing / wouldnot be used. The screw J would be

threaded directly into an operatingrod that extends through the insideof the lathe spindle, which is then

attached to the pneumatic cylinder.The pneumatic cylinder actuatesthe operating rod which moves thescrew J forward to clamp the

workpiece. However, on lathes that



are not equipped with a pneumaticcylinder, the arrangement shown inFig. 6-44 is very satisfactory.

Loading and Unloading

CHAPTER

Entering the Part

The complete process of fixturing iscomprised of loading, machining,and unloading; the loadingoperation consists of entering and

ocating the part and clamping it;the unloading, of releasing andremoving the part. Each phase has

ts problems. Entering involves



manual handling and requiresspace. Convenient manual handlingdepends on weight and balance.

Light parts are handled by theoperator's two fingers or one hand;heavier parts require two hands or,

n more extreme cases, a hoist,crane, or conveyor. Well-balancedparts require lifting and loweringonly; an unbalanced part, having its

center of gravity at some distancefrom its midpoint, also requires asteadying effort which makes it

ncreasingly difficult to keep thepart level during lifting andowering.

Space must then be provided inside



the fixture for the part, fingers, ahand, possibly two hands (andknuckles!), or two hands and arms.

For heavy parts there must also beclearance from the machine tool toallow the operator to lean over the

fixture, or to admit the load cablefrom the hoist or crane. Althoughthese factors may appear trivial,they are quite serious and it is a

common experience that spacealways looks larger on a drawingthan in reality.

Locating the Part

Locating means bringing the part

nto positive and, correct contact



with the locating points or surfacesChips and dirt on a locating pointprevent direct contact at that point,

but accumulations in other placesn the fixture may well cause such

misplacements or misalignments

that the part cannot be properly ocated. Other causes of insufficien

contact are burrs, part irregularitiesbeyond prescribed tolerances,

amming, and friction. These

68

adverse factors can be directly andndirectly controlled by the fixture

designer who should provide mean

for chip cleaning and for visibility a



the locating point.

part from these considerations,

there is no further problemencountered in locating when theconditions are equivalent to those

shown in Figs. 3-1 d and 4-1,Locating is done in threeconsecutive steps. First, the part isset on the base; second, it is moved

to contact with the side stops; thirdt is moved to contact with the end

stop. Next, the clamping pressures

are applied. A basic andcharacteristic feature in thesesimple examples is that eachocating step is not interfered by,

and does not interfere with, any



other locating step. One result,thereof, is that the individualphases in locating are not sensitive

to the direction of approach.ssume the part is tilted while it is

owered to the base. It then contact

first one of the three points (or onecorner), levels off, contacts thesecond point (or corner), levels off on the axis through these two

points (or the edge between the twocorners), and comes to rest on allthree points (or on the bottom

surface). If it is still misaligned withrespect to the side stops, it contactsone side stop first, then aligns itselfto contact with the second side stop



These observations lead to the basicand very general rule that locatingshould be done on only one surface

at a time, where possible.

Correct and incorrect Loading

Stressing that the part be broughtnto correct contact with theocating surfaces may seem

unnecessary, but it is not. Any partthat has been machined whenocated in an incorrect position isost, and so is the labor that has

been expended. Design steps takento prevent incorrect loading are

Ch,7



LOADING AND UNLOADING

69

termed fixture.

foolproofing," or "mistake-

proofing" the

Symmetry Considerations

Correct and incorrect loading areassociated with symmetry andasymmetry in the part

configuration. With reference toFig. 7-1, planes of symmetry (orasymmetry) are denoted^, BB, andCO the corresponding



perpendicular axes (sometimes, butnot necessarily always, axes of rotational symmetry) are denoted

X., Y, and Z. A completely symmetrical part; that is, a partcontaining three planes of

symmetry, can be loaded in afixture in four differentorientations. From an initialposition it can be turned 180

degrees around the three axesjf, Y,and Z, respectively. In other words,t can be turned end-for-end and

upside down, and there are noorientations other than these four.

part from any surface markingsthere is no discernible difference



between the four positions, and anymachined configuration applied tothe part will produce the same end

result. Every position is a correctposition and incorrect machining issimply not possible in this case,

regardless of how the part wasoaded.

Fig, 7-1. A part with three planes of symmetry.

completely asymmetrical part,



fully nested, will normally be ableto enter the fixture in one positiononly, the correct one; and is,

therefore, always correctly machined. The possible exceptionsare if the configuration of the

nesting points and surfaces con-

tain some degree of symmetry. AMother cases lie somewhere between

these two extremes. Two importantexamples will now be analyzed.

In Fig. 7-2 a part having two planesof symmetry, AA and CC is shown.

lso shown are the three principalaxes X, Y, and Z, in the initial

position. Two sets of surfaces,



namely, the two pads on the topside and the right-hand face, are tobe machined and a hole will also be

drilled, as shown in view b. Toassist in identifying the positionand orientation of the part, one

surface has been labeled A 0 and its called the TOP SIDE SURFACE;

another surface has been labeled B0 and is called the FRONT SIDE

SURFACE. This part is shown infour different positions in Figs. 7-2b, c, d, and e. If no corrective action

s taken, the part may be assumedto enter the fixture in any of thefour positions. The need forcorrective action is evident from an



examination of the fourllustrations. In Fig. 7-2 b, the parts in the initial position; the

ntended correct position forentering the fixture. The surfacesmachined are the correct surfaces,

and the hole is in the correctposition.

The part, in Fig. 7-2 c, is rotated 180

degrees around the Y axis. Noticethat the final configuration of thepart will not change when it is

machined in this position; this isthe result of symmetry on the A A and CC planes.

In the position shown in Fig. 7-2 d,



the part has been rotated 180degrees about the X axis from itsnitial position. When the hole is

drilled and the two surfaces aremachined with the part in thisposition, the relationship of the

hole and the machined surfaces onthe pad will be incorrect. This isshown in the lower illustration,which shows the front side surface.

When this view is compared to thefront side surface in Fig. 7-2 b, it isreadily seen that the machined pads

are on the wrong side. The part inFig. 7-2 e has been rotated from itsnitial position 180 degrees around

the Z axis. Again, the part



configuration will be machinedncorrectly, which can he seen by

comparing the front side views in

Figs. 7-2 b and 7-2 e. The machinedpads are again on the wrong side.

n examination of the figuresshows, that out of the four possiblepart positions within the fixture,there are only two positions

(namely views b and c) in which thesurfaces can be machined to theircorrect relative positions. This

observation serves to illustrate afundamental rule, not generally recognized. There exists a class of operations that is permissible, even

with the part in a prohibited



position. The criteria for this classof operations are that they producesurfaces which consist entirely


Ch. 7

——~T 1 L K >OP SIDE

SURFACE \|



(

__□_

II!

'

TOP VIEW

SHowim \

TOPJIDE SURFACE

CUTTER NO. 2



n.

r j

+i

TT

bFBOWOTEWEW .SHOWING

TT

X



EU i?

1

-

a

Cl/TTER NO. 2

4

.

TTI—

FRONT SIDE VIEW



TJ

FRONT SIQE WgW

n i

itt

TJ

TJ

■<M-

"Q"

J

MEW OF INCORRECT-



FRONT SIDE SURFACE

¥

-/

COTTER NO.!

u

TOP VIEW SHOWING

TOP SIDE SURFACE

«

JfL

TT r



symmetry.

Ch. 7


71

of straight line generatricesperpendicular to the nonsymmetry

BB plane (satisfied for the endsurface, the two pad surfaces, andthe hole), and that they maintainsymmetry with respect to one of the

planes of symmetry, either the A A plane (satisfied for the two padsurfaces), or the CC plane (satisfied

for the end surface and the hole). A



part with only one plane of symmetry radically changes itsposition configuration relative to

the fixture, with each of the threepossible 180 degree rotations, and,normally, these three positions are

prohibited. There also exists,however, a class of machiningoperations that will produce correctsurfaces with two different

positions of the part in its fixture.The part shown in Fig. 7-3 has only one plane of symmetry, plane CC, I

s to be drilled through andmachined on the two pads. With thpart in its initial position, themachined configuration, which is



the correct one, is shown in Fig. 7-3b. When rotated around axis X, thepart is still correctly machined, as

shown in the view of the front sidesurface in Fig. 7-3 c. However, if thepart is rotated around axis Y (see

Fig. 7-3 d) or around axis Z (see Fig7-3 e), the hole is drilled in thewrong place. The criteria for theclass of operations which can

produce a correctly machined partfrom more than one part positionwithin the fixture are that they

produce surfaces which consist of straight line generatricesperpendicular to one of the non-symmetry planes, either the A A



plane (satisfied for the pads), or theBB plane (satisfied for the pads andthe hole), and that they maintain

symmetry with respect to the oneand only plane of symmetry CC(satisfied for the pads and the hole)

Fool proofing

Despite the literally endless variety

of possible asymmetries found inpart configurations, a systematicclassification and the formulationof some widely applicable rules canbe provided. With such rules andsome practice, the fixture designers able to quickly spot and utilize

existing possibilities for



foolproofing and to create themwhere required, by additionalmodifications in the fixture design.

Foolproofing is required for partswith at least one asymmetry. A

completely symmetrical part needsno foolproofing; No matter how it inserted into the fixture, it presents

to the eye, and to the cutting tool,

dentically the same configurationand, no matter where themachining cuts are taken, relative

to the planes or axes of symmetry,the machined parts come out withdentical shapes. Foolproofing is

needed when the part, in addition



to one, two, or three planes of symmetry, also presents one ormore irregularities. Many

workpieces, particularly components of machinery, fail intothis category. The body of the part

may be essentially symmetrical andof substantial dimensions, whichmakes it suitable for locating andclamping. The irregularities can be

convex (going out) or concave(going in). They disturb thesymmetry, and it is these

rregularities that are utilized forthe purpose of foolproofing. Beforegoing into the details of the subjecta few simplified, but typical,



examples will be shown fororientation.

two-times symmetrical part witha projecting lug on one end, asshown in Fig. 7-4 a, can be confined

within a box with a cut in the endwall for the lug. If the cut in the endwall cannot be tolerated, the vacantspace in the box adjacent to the lug

can be taken up by a blocking, asshown in Fig, 7-4 b. With two endugs, as shown in Fig. 7-4 c, the

blocking can be located in the spacebetween the lugs. A cylindricalhollow boss with a plain arm can becentered on a mandrel with the arm

ocated in a straight slot in the



fixture wall. If the arm is formed asa bracket with a T-section, as shownn Fig. 7-5, the contour of the slot

must match the contour of thebracket. A straight slot of constantwidth would not prevent the part

from entering upside down. A partwith an asymmetrically located,downwardly open cavity, as shownn Fig, 7-6, is very simple to

foolproof. All that is required is ablocking on the fixture base, whilean upwardly open cavity would

require a blocking which extendsdownward from above, andtherefore must be carried by aremovable part of the fixture.



Locating elements for foolproofingcan be a part of the fixture base, thecover, or the clamps. In special

cases, it may be necessary to useseparate movable parts for thispurpose. Such parts may

simultaneously serve as centralizer(see Chapter 9), When possible,they should be built right into thefixture base, so that they take effect

mmediately as the part is enteringthe fixture. If placed in a movablepart, they cannot detect an incorrec

oading until after the part has beenntroduced, and additional time is

then required to unload, correct,and reload.



n most cases the foolproofingrequires simple means and is doneat little additional cost. It very often

happens that the major body of thepart, including its natural locatingsurfaces, contains one or several

degrees of symmetry, and theasymmetries are confined to smallareas at isolated locations. In suchcases the locating is performed

according to general rulessupplemented by the necessary fool-


Ch. 7



-a FRONT SIDE

SURFACE

, TOP SIDE SURFACE

1



I I

~

f-^v-o ~t

TOP VIEW

_

I

TTT

TOP W£W

SHOWING I v

TOP JfOE SURFACE



oT^

T

FRONT SIDE VIEW

SHOWING

FRONT SIDE SURFACE

cmunNo.2

c-r- -1—e

CUTTER HO. I

_EL

FRONT SIDE VIEW



CUTTER NO 2

Bt L --^-^

W£Vf Of

BMW J/Of SURFACE

-u

1(1

n.

CUTTER NO.!

-VA

ML



%-

LJ ^incorrect

FBOHT SIDE VIEW

SHOWING

FRONT 5/Of SUPFACE

CUTTER NO. 2

0

\ Y I

-TTJ^-

CUTTER NO. I



n

U

"Q~

FRONT SIDE VIEW

"<feB—"

-ft

CUTTER NO. 2

L cr J —ir-

f°a

-INCORRECT Q



W£ W Oif FRONT SIX SURFACE

Fig. 7-3. A part with one plane of

symmetry.

Ch. 7


73

XZE

TZL

ft

XZE



P

M

m

HI

m

3=1

Fig. 7-4. Foolproofing a part withprojecting lugs of simple shape.

proofing elements. Localizedasymmetries are convex or concaveWhen convex, they take the form of

projections, such as arms, bosses,



brackets, ribs, etc.; when concave,they may be recesses, depressions,notches, slots, holes, perforations,

or cavities of other shapes.

3 kIU^\(^

scctiova-a Fig. 7-5 Foolproofing apart with a contoured bracket.



Projections on the part can, in mostcases, be located or embracedbetween projections in the fixture,

two at a time, forming a fork. Thefoolproofing elements may beblocks or pins. They can be given

wide clearance against the partbecause they are not locators in thestrict sense of the word; all they dos select one out of two or more

possible orientations, each of whichs closely defined by the actualocators. From the viewpoint of

simplification, economy, andefficiency it is, of course, desirablef the real locators can also do the

foolproofing, or participate in it.



This happens when a locator ismade to serve as one of the two;prongs of the fork that embraces

the projection, the other prongconsisting of an additional block orpin. However, instead of

constructing a fork to embrace theprojection, the same result can beaccomplished by a cavity in somepart of the fixture such as the wall,

a bracket, or a rib.

Reversing the principle of a fork

embracing a projection, constitutesthe "blocking" principle; noconstraining means are provided fothe projection at its correct

position, but every incorrect



position is blocked. The blockingprinciple is often simpler to apply than the principle of the embracing

fork.


Ch. 7

symmetries in the form of cavitiesare simpler to handle thanprojections because it takes only

one block or pin to mate a cavity,



but two to make a fork. A simpleand very common case is where thecavities are a group of holes in a fla

surface. The part is effectively ocated but not necessarily fool-

proofed by two pins mating with

two holes. If the holes are of different diameters, the locating isfoolproof. If not, it may be possibleto provide one extra hole of a

different diameter for the doublepurpose of locating and foolproofing, or the designer may find it

possible to make an existing holeoversize, for the same purpose.

symmetries, whether projectionsor cavities, also may be located



essentially on a horizontal contour,n a vertical orientation, or

tangentially; that means an

asymmetrical or otherwise irregulaspacing of the projections orcavities on a circle.

The example shown in Fig. 7-7 is acylinder block with asymmetries(the flange contour and the hole

pattern) in a horizontal plane. Theocating elements between the drillig A and the surface of the upper

flange, and the side stops andclamping screws on the two longsides, have symmetry. But if nofurther steps were taken the jig

could nest on the block in two



opposite positions, and one of thesemust be prohibited. This isaccomplished simply by carrying

the end stop B, sufficiently close tothe end lug C, so that It cannot neston the triangular extension D, on

the opposite end.

shown in Fig. 7-8, where it isrequired to drill and countersink

the hole in the arm. Even withoutthe countersink, a skilled mechanicwould not slide the

LOCATING PI WORKPIECE

SLIP BUSHING



OPTIONAL DESIGN OF LOCATOR

Fjg. 7-8. Foolproofing by means of the bracket configuration.

part on in the wrong position and

attempt to drill it without support.In the case shown, the part canswing into position under thebracket with the drill bushing only



when it is first correctly located onthe locating pin. An analogousexample is shown in Fig. 7-9, where

a hole in the arm can be located on

Fig. 7-7. Foolproofing an

asymmetrical part by means of anasymmetrically located block.

Parts with vertical asymmetries are

ery common and are also



frequently associated with surfacesof revolution. Perhaps the mostdangerous configuration is one with

an asymmetrical arm projectingfrom a boss with a cylindrical bore,since the bore can slip on a

cylindrical locator in two oppositepositions. A simple and typical caseof this category is



Fig. 7-9. Foolproofing by means of astep on the cylindrical locator.

Ch.7




75

the pin without interference with

the drill bushing only if the part isheld in the correct position.

The part shown in Fig. 7-10 has itsboss centered between a lower andan upper conical locator. It canenter the lower locator only when

the arm coincides with the notchprovided in the cylindrical wall, andn no other position.



Fig. 7-10. Foolproofing by means ofa notch in the wall of the drill jig.

Parts with radial locating and

tangential asymmetries are shownn Figs. 7-11 and 7-12. In each case

the part is centered on its axis,

clamped with a knob K, and radially



ocated by a pin X, in a slot. In Fig.7-11, a hole is to be drilled oppositeflat,4. Foolproofing is done by the

pin B, set tangentially to flat A. Forradial locating, this arrangementwould be the poorest possible, as it

permits a small but finite tangentiamotion. However, the actualocating is done by the pin X, which

can be made to fit in a machined

slot with close tolerances.

In Fig. 7-12 the hole is to be drilled

opposite a projecting bracket A, Thepart is clamped by means of ahinged clamp C, with a knob K, Theclamp has two lugs; a slot in the

ower lug forms it into a fork F,



matching the bracket A when it is ints correct position. The damp,ncluding the lugs, blocks all other

positions of the bracket.

Examples with asymmetrical

cavities are shown in Fig. 7-13. A blind hole is to be drilled on thesame side as the cavity or on theopposite side. In each case the

foolproofing is done by means of asmall block that matches but doesnot have to fit closely into the

cavity. With the hole and the cavityon



Fig. 7-11. Foolproofing by means of

a pin.

the same side (Fig. 7-13a), the blocks mounted on the underside of the



hinged leaf of the drill jig; with thehole and the cavity on oppositesides (Fig. 7-13b), the block is

mounted on the base of the jig. Ineach case, incorrect loading isprohibited.

A



Fig. 7-12. Foolproofing by means ofa slot in the hinged clamp.


Ch. 7

With vertical loading andunloading, the lifting is usually aittle more difficult than theowering. It is a matter of getting

the proper grip or hold on the part,



which applies to very heavy parts aswell as to very small parts. Oncethis is understood, the operator

must always make sure that there ienough clearance inside the fixturefor getting the proper grip on the

part. Clearances must also bechecked against the possibility thatthey have caught chips that canbind between part and fixture, when

the part is to be removed.



Fig. 7-13. Foolproofing by use of acavity in the part.

Most cases of asymmetry aresimple to handle by application of the general rules and illustrative

examples mentioned, but there aresome exceptions requiring differentsolutions and one of these is thecase of punched parts. Even with a

symmetrical contour, such partscontain one asymmetry, namely,the burr, A well-designed fixture

will provide burr clearance butrequires also that the part enterwith the correct up and downorientation. The solution is to

provide an asymmetrically located



hole in the part and a matching pinn the fixture. The hole is punched

simultaneously with the part and is

therefore inexpensive, and thepunched part can now only beocated with the burr in the right

place.

Removal Problems

The part removal operations, aftermachining and release of clamps,are nothing more than the reversalof the entering and locatingoperations, and no seriousproblems are encountered here,provided the part is identically the

same as when it entered the fixture



(less the metal removed). Althoughthis proviso might be consideredtrivial, there are two very real

conditions that can cause trouble ifthey are not given properconsideration—new burrs and

warping due to heat and relief of residual stresses.

Burred Parts

s explained later, in Chapter 8, twotypes of burrs, the major burr andthe minor burr, are formed in amachining operation on ductilematerials. Burrs along externalsurfaces (see Fig. 8-2a and b)

present no problem as they are out



n the open; nor do burrs on drilledholes (see Fig. 8-2cJ if the part isifted out. However, burrs from any

operation, particularly from drillingdo present a problem if they areformed on a surface which has to

slide with a narrow fit in or on amating locating surface. The mostcommon case is a part with acylindrical surface, to be drihed

perpendicular to the cylinder axis.Drill jigs for this operation areshown in Fig. 7-14.

The part shown in diagram a is ashort shaft, located in a cylindricalbore in the drill jig. The hole is a

blind hole and only a minor burr is



formed adjacent to the drillbushing. The body of the drill jighas a keyseat which provides

clearance for the burr to form andallows it to slide out. The body of the drill jig can be completely

machined,including the cutting of the keyseat, before the end plug iswelded on.

The part shown' in diagram b is abushing with a hole through on adiameter. It it located on a plug. Th

plug has a hole to clear the drill,and is machined with upper andower flat surfaces which provide

clearance for the major burr on the

upper hole in the part, and the



minor burr on the lower hole.

The part shown in diagram c is a

stepped shaft with one throughhole. The drill jig is machined fromthe solid. Clearance for the minor

and major burrs is provided by twokey seats inside the bore in the jig.Two holes A are drilled to provideend clearance for the cutting of the

keyseats. Hole B is used to clear thedrill.

Burr problems are also encounteredn milling operations, particularly

those involving slotting operations.n example is shown in Fig. 7-15.

The



Ch. 7


77

operation is to cut a slot lengthwise

from one end of part A , a cylinder.The part is located on a mandrel Band is clamped by means of a large

washer C. The mandrel has anoutboard support D, which isremovable for loading andunloading. The milling operation is

climb milling (down milling) sothat the work is held against themandrel. A slot E is machined in

both mandrel and washer to provid



clearance for the cutter. At the samtime it provides clearance for themajor burr which is being formed

on the inside of the<part.

Ejectors

Despite all the fixture designer'scare and foresight, it is not alwayspossible to ensure free and easy

removal of the machined part and amechanical ejector then becomes anecessity. A warped and bindingpart is not the only Justification forthe use of an ejector; in fact, theejector should enjoy much widerpublicity in the literature and much



n \T7Ttfk

-'ig. 7-14. Drill jigs with bunclearances.

Fig. 7-1S. A milling fixture with a

slotted mandrel for burr clearance.

wider use in industry than ispresently the case. The ejector is

not a convenience but an economic



asset. It reduces the time forremoval of the part. As an example,t took 0.20 minute to grip a part,

ift it out of the fixture and place itn the tote pan. With an ejector

used for lifting, the same operation

was reduced to 0,08 minute. Thistime saving may appear small;however, for large-volumeproduction with short duration

operation cycles, every savingrepresents a significant percentageof the total. By eliminating the need

for finger and hand space forgripping the part, the ^ejectorpermits a reduction of the overalldimensions-thus the cost-of the



fixture. The ejector pins can beocated to suit the part

configuration so that side-heavy

parts are lifted free and clear, inperfect balance. In this way,ejectors eliminate inconvenient and

awkward hand manipulation andreduce operator fatigue. There areseveral reasons why parts may bindn the fixture. It may be from

distortion during machining, aspreviously discussed, or it may bebecause the part has a locational

nterference fit (class LN 2 or 3)against the locator to ensure closetolerances. The part is tapped downn place when loaded into the



fixture;removal requires theapplication of some force, but iseasily done with ejectors.

For maximum time saving, theoperation of the ejector can be

automatic and coupled with therelease of the clamp. The combinedmechanism can be poweredhydraulically or by compressed air.

Ejector Details

Ejectors are inexpensive since there

s no need for close tolerances;most machine work is by turning




Ch. 7

Fig. 7-16 {Left). A single ejector. Fig7-17 (Right), A multiple ejector.



and (for hardened parts) cylindricalgrinding; and holes are finished by reaming. Details that come in

contact with the part are usually made from hardened tool steel orfrom a case hardening steel. If the

part surface must be protected fromscratches or other markings, theejectors may have contact headsmade of copper, brass, or

aluminum.

It is a prerequisite for the use of

ejectors that the locators must bedesigned jam-free, as previously explained in Chapter 6. Sinceejectors are moving parts, their

bearing surfaces must be protected



against dirt and chip fragments. Inmost cases ejectors are shieldedagainst chips by the part itself; if

necessary, shields or seals also canbe installed, as described at the endof Chapter 8.

The basic elements in an ejectorsystem are the pin and the spring, Asingle ejector is shown in Fig. 7-16.

The ejector pin A is manually operated by means of knob B, and ireturned by spring C acting on the

shoulder D, which also stops it inthe return position. The pin issupported at E and F, The distancebetween the two supports must be

several times the diameter, for jam



free guidance; and to provide, at thesame time, the necessary space forthe spring.

If the part has a central hole, amultiple ejector must be used with

two or more ejector pins. Theejector shown in Fig. 7-17 has twopins A, fastened to a flange on knobB. The return spring C is centrally

ocated and acts on the flange. Thearresting shoulders D are parts of the pins, which also provide the

nner bearing surfaces E, while theouter bearing surface F is in a boren the knob. Other arrangements

are also possible. Each pin could

have its own two bearing surfaces



and its own spring. The simplestmeans of operating an ejector is thehand-operated knob, as shown, but

t requires that the ejector bearranged in the side of the fixture.

When ejection is in the upwarddirection, the knobs are inaccessiblfor direct hand operation, and aever must be used. The example

shown in Fig. 7-18 is a singleejector. The design is simplified by the absence of a spring as the pin

will retract by its own weight.

In the multiple ejector shown inFig. 7-19, the pins A are carried by a

ring B, and the lever C is forked.



Fig. 7-18 (Top). A lever-operatedsingle ejector.

Fig. 7-19 (Bottom). A lever-operated



multiple ejector.

Ch.7


79

The ring with the pins is returnedby means of springs D, Pins and

springs must be evenly spaced onthe ring; on large diameters thereshould be at least three pins. Twosprings are theoretically sufficient;

however, three are recommended.

Small parts can be automatically ejected by direct spring pressure.



Two arrangements for this purposeare shown in Fig. 7-20 a and b. Indiagram a the part is located agains

the fixture base by means of clamps; the ejector pin A is forceddown flush with the base when the

part is loaded. In diagram b thedefining surfaces B are above thepart which is lifted into properposition by the force from the

ejector spring. In each case the parts automatically ejected as soon as

the clamps are removed.

heavy part, it must also be designedso that it stays in the elevatedposition under load. In this way the

operator has both hands free for



removing the part. This isautomatically accomplished (thescrew is self-locking) when the

helix angle of the thread is less thanthe angle of repose (the angle of friction, 8'/j to 14 degrees,

corresponding to fJ. = 0.1 5 - 0,25)and may require the use of arelatively large pitch diameter.

Ejectors can also be cam operatedor wedge operated, which isparticularly applicable to fixtures

for multiple parts, A representativeexample is shown in Fig. 7-22. Thefixture holds four parts, and thefour ejectors are operated by one

push-rod A, with four milled



notches, tapering on one end.

Fig. 7-20. Examples of spring-actuated ejectors.

Spring-operated ejectors, however,



are insufficient for heavy parts andfor parts that bind on the locators.In such cases, screw ejectors are

used. An example is shown in Fig.7-21. The screw A has a handle B,Since this handle must project

through a window in the wall of thefixture base, it can only be operatedthrough a small fraction of arevolution (30 to 45 degrees) and it

s, therefore, necessary to use ascrew thread with a large lead,which in turn, may require a

double-or triple-thread screw.When a screw ejector is used forifting a



Fig. 7-21. A screw-operated ejector.

The pins Bare bored and slotted toreceive and guide the ejector pins C

The four ejector pins are operatedsimultaneously by pushing in rod A

The principle also can be modified

n various ways:

1. Two (or more) pushrods can beoperated by one knob



2. The ejector pins can operate onone large part instead of onndividual small parts

3. The push-rod, or rods, can beoperated by a lever to provide a

greater lifting force with normaloperator effort.

n extremely simple, almost

primitive, but very effective yetnexpensive ejector system is the

following: The necessary number oejector pins are fastened into a platn the desired pattern, and the plates then clamped to the machine

table. The system is only applicable

to drill jigs so small and light that



they can be lifted without undueeffort. Holes are drilled in thebottom of the drill jig

corresponding to the pattern of theejector pins.


Ch. 7

m

^^



Fig. 7-22. A wedge-operated ejectorfor a multiple-part fixture.

When the drilling operation iscompleted, the operator opens theclamps so that the part is free, then

ifts the jig and lowers or forces itdown over the pins until the parthas been pushed free of the jig.



Loading of Large and Heavy Parts

To facilitate handling when loading

arge heavy parts, the hard physicalwork should be done in aconveniently accessible space

unencumbered by the machine toolThis is frequently accomplishedthrough the use of dual fixtures seton opposite ends of a machine tool

tabic, or with one or several fixturemounted on an indexing rotary table, but a means can also be

ncorporated in the design of asingle fixture. General rules for thedesign cannot be formulated, but afew representative examples of

such devices, which take many



different shapes, will be shown.

The fixture has a movable receiver,

which is moved to the outer stationfor loading and unloading andreturned to the actual fixture

station for machining. Being part ofthe fixture, the receiver must havedevices for accurately locating itselfwhen it is back in the fixture

station, firmly supporting it and, if necessary, locking it in thatposition.

Receivers, Sliding or Rotating

When the weight of the part still

permits it to be manually moved on



a smooth horizontal surface, areceiver may not be needed, but thefixture base can be extended

sufficiently outside of the machineheadstock area to allow the part tobe conveniently set off and then

pushed into the fixture space asshown in Fig. 7-23.

Rotating or swinging receivers may

take many forms and may performadditional functions within thefixture. The receiver R shown in

Fig. 7-24 is actually a swinging drillig. With the receiver in

Z=P-^ \^J



Fig. 7-23, A fixture with an extendedbase for easy removal of a heavy

part.

Ch. 7


81



Fig. 7-24. A drill jig with a swinging

receiver.

the open position, the part is

clamped on by means of clamps C,the receiver is swung back andocked in position, and drilling is

done through the bushings in the

receiver. The fixture shown in Fig.7-25 is also a drill jig. The receiver Rcarries a central locator /,. With R

n the outer position, the part is



ocated over I and clamped by means of clamps C. R is swung backand the drilling is done

through drill jig bushings mounted

n the stationary bracket B. Thesame principle is frequently used inthe design of broaching fixtures.



somewhat different use of thereceiver principle is shown in Fjg, 726. For small parts, the problem is

not weight, but quantity, and thereceiver functions here as aprepositioner for a surface grinder

with a magnetic chuck or faceplateC. The use is therefore limited tomagnetic materials. The receiver R s a plate, supported in trunnions

and equipped with permanentmagnets. With the magnet side up(diagram a), R is loaded then

rotated 180 degrees and loweredonto the magnetic chuck (diagramb).

a



MINI

to

3»

T I I I I I I . ^-^ .

1"%. 7-25. A drill jig with a swingingreceiver.

Fig. 7-26. A combined receiver andprepositioner.

The permanent magnets carry theweight of the parts, but the strongemagnetic chuck, when switched on,pulls the parts down to its own



surface in position for grinding. Thereceiver, now empty, is returned tots loading position. It is also a

time-saving device in that it permitthe loading of a new batch of partsduring the grinding of the previous

batch.

CHAPTER

8

Chip Problems

Types of Chips

Machining cast iron, bronze, andother brittle materials produces



crumbling chips and a great deal of dust. Steel and other ductilematerials produce several types of

chips. A single-point tool, cutting athigh velocity, as most carbide toolsdo, produces continuous chips

which are long, usually curled into helix of some sort, but sometimessnarling and bundling together,uncontrolled. With the use of a chip

breaker on the tool, the chip flow isbrought under control.

Chip breakers, as a rule, are notused on high-speed steel tools asthe cutting speeds and the volumeof chips produced are much lower;

the chips are of the discontinuous



arge, obtuse angle, so that thechisel edge removes metal by acombination of extruding and

negative rake cutting, thereby generating a large thrust force. Thechips produced by the chisel edge

are small and curling and inthemselves present no problem.The two lips produce the same chiptypes as single-point tools except

that the chip is formed and fed intothe confined space of the flutes of the drill. This eliminates the

possibility of large diameter,helically wound chips and reducesthe chip types to straight chips;tightly wound helices; and short, or



crumbling, chips. With coarse feedsthe first two types have sufficientrigidity to travel through the flute.

With

82

fine feeds, the chips may snarl andpack the flutes. Therefore, anunderstanding of the behavior of

drilling chips is essential to thefixture designer for the correct axiaplacement of drill bushings.

face mill is essentially a plurality of single-point tools set in a circleand produces the normal chip types

with the limitation that no chip can



be longer than the path of the cuttetooth across the work. Most othertypes of milling cutters produce

short chips since the path of thecutter tooth within the material isshort. In ductile materials the chips

are tightly rolled. Long side millswith helical teeth (slab mills)produce stiff, needle-shaped rolledchips as long as the width of the

work.

Burr Formation

Burrs are always associated with -the machining of ductile materials.Truly brittle materials do not

produce burrs, but form crumbled



surface has less support frombehind and no support from oneside. Consequently, it will try to

escape from the pressure in thedirection of no resistance and indoing so it forms a burr. Burrs

develop at the two points B and Cwhere the edge of the cutting toolntersects the metal surface. A arger burr, the major burr, forms a

point B on the leading edge (theside-cutting edge in cutterterminology); and a smaller burr,

the minor burr, forms at

Ch. 8

CHIP PROBLEMS



83

Fig. 8-1. The mechanics of burr

formation,

point C on the trailing edge (theend-cutting edge). No rules can be

given for the dimensions of theburrs, except that larger burrs aredeveloped with coarser feeds, lowercutting speeds, and smaller rake



angles on the tool—and in softerand more ductile materials. Themajor burr develops ahead of the

tool and is removed with the chip atthe next passage of the tool. Theast passage of the tool leaves the

major burr on the exit side of themachined surface. The minor burrs found on the entrance side and

also on each ridge between tool

grooves on the entire machinedsurface where they contribute tothe roughness configuration.

The burrs developed while the parts in the fixture are normally of no

concern to the fixture designer

since they are formed out in the



open. This applies to machiningwith a single-point tool and amilling cutter (see Fig. 8-2 a and b)

but not to drilling (see diagram c)as, in this case, the burrs areformed inside the jig and space

must be provided where they aregoing to form. The necessary spacefor the major

HL



K xi iv v

F(g. 8-2. a. Major and minor burr

formation by a single-point tool; b. milling cutter; c. A twist drill.

burr is automatically provided for ifthe part is located clear of thefixture base; if not, a hole of generous size must be provided in

the fixture base for the drill to clearand the major burr to form. On thetop side, clearance is providedbetween the drill bushing and theupper surface of the part for theminor burr and for other purposesto be discussed later.



Chip Removal

Chips falling on the fixture present

no more of a problem than thosefailing on a part without a fixtureand they are easily scraped or

brushed away. The most efficientassist in chip disposal is a strongflow of coolant, usually in excess ofwhat the cutter requires for cooling

alone.

few minor yet important pointsmust be considered by the fixturedesigner. The means for coolantsupply and coolant return must beadequate. This requires a check on

the pump capacity, pipe and nozzle



dimensions, and the size andextension of the collecting groovesand channels in the machine tool

table and substructure-includingstrainers, sieves, settling tanks, andfilters. The flow pattern of a stream

of coolant discharged to horizontalsurfaces is not adequately controlled by the direction of thenozzle; it will splash and spray by

the impact, spread over larger areasand overflow the edges in anunpredictable manner. This calls fo

troughs and pans to collect theiquid and baffle plates to protect

the machine surroundings,ncluding the operator, Shields and



and bearings in machines-and intothe noses, ears, and clothing of theoperators. The disadvantages of the

use of air blast for cleaning can begreatly reduced by skillful use of shields, baffle plates, chutes, and

ducts. An example is shown in Fig.8-3. A mechanical jig with a fixedocator plate A and a movable jig

plate B is equipped with two airjets

C and an air valve D, actuated by the jig plate in its open position.The air blast cleans the jig plate and

the locator plate before the part E isnserted in the fixture. The chipsm-

CHIP PROBLEMS



Ch.8

Fig. 8-3. Chip removal by an air

blast and with a chute.

pinge upon and are deflected by abaffle plate and slide down chute F.

The inconveniences of air blast are



eliminated when suction is used.This is common in dry grinding andcan be used for cast iron,

aluminum, and magnesium chips.It is also widely used for catchingchips produced when machining

many types of plastics.

Chips that collect in quantity insidethe fixture must be given an exit;

for this purpose the fixture wallsare provided with openings(windows) as large as the stress

analysis can permit, with goodclearance around the part formanual cleaning access with a rakefork, scraper, or brush. Helpful also

s the installation, where possible,



of a gravity chute as chips are easilystarted down the slope by the

ibrations from the machine.

nother, and somewhat unusual,example of chip removal by a chute

nvolves the drill jig shown in Fig,8-4, The part is a cylinder with twoflanges

Fig. 8-4. Chip removal by a chute

built into a drill jig.



that are to have holes drilled inthem. The cylinder is bored and theflanges are faced. The part A enters

the jig through one of the largewindows in the side walls, and iscentered by two plugs B, sliding in

bores in the upper and lower endwall of the jig. With the jig set onend, the part with the plugs inposition slides down until its lower

flange rests on the jig. In thisposition the holes in the upperflange are drilled. To drill the holes

n the lower flange, the jig isreversed. Chips are removed by thechip deflector C consisting of a splitring with a V-shaped cross section.



Only one-half of the chip deflectors removed to unload and load theig.

Relief and Protection

The greatest problem, one thatconfronts the fixture designer at allphases of his work, is caused by chip fragments and dirt; i.e.,

essentially chip dust and particlesof rust, scale, paint, and remnantsof foundry sand from castings.Collecting in corners and cavitiesthey cause misalignment by preventing proper contact betweenpart and locators.



ertical surfaces are, naturally, lessexposed to dirt accumulations.Horizontal locating surfaces are

relatively easily cleaned; this is onereason why locating surfaces shouldbe kept small and why side and end

stops preferably should be installedfrom vertical walls.

When a flat surface with sharp

edges slides over another flatsurface the ieading edge acts as ascraper. In this way the edges of

ocating pads help to clean thesurface of the part. Occasionally,additional grooves with sharp edgesare cut into locating pads to

mprove the cleaning effect. In the



same way, the edge of the partcleans the locator surface, providedthe dirt has a place to go. This leads

to the general rule for dirt-proof design: A contact surface shall besurrounded by a relief space.

Incidentally, relief spaces for dirtwill also serve as relief spaces forburrs from previous operations.They do not accommodate large

chip accumulations, but they allow space for the dirt to escape when its pushed, swept, or scraped ahead

of an edge on an entering part.

ny horizontal locating surfacesatisfies the condition above if it is

elevated sufficiently over the fixtur



base. (Examples were seen in Fig. 65,) Additional constructive actionmust be taken in the corners by

providing relief space either in thefixture base or in the locator. Pinsand buttons can be installed in

bored holes with a chamfer or astraight recess, as in Fig. 8-5a, b,and c. (This solution requires anadditional and rather expensive

machin-

Ch. 8

CHIP PROBLEMS

85



^ I

I

m

1

I IX

i w

^

I



Fig. 8-5. Locators with relief places

for chips and dirt,

ng operation on the fixture; it ischeaper to provide the relief in theocator.) Buttons and pins can be

machined with a flat recess as in d,and e. The latter design has the

advantage that it does not reducethe bearing area between the shankand the hole. The same applies tothe designs in f and g,-where a flatocating surface is provided on the

pin and button. However, thepreferred solution is to turn a

circular recess into the pin or the



button as in h and i. The additionalturning operation to make therecess (which does not have to be

ground) is inexpensive and thebearing area on the shank is notsignificantly reduced. Straight side

and end locators are made with achamfered or straight recess as wasshown in Fig. 6-t2d and e and as inFig. 8-6.

In some cases it is possible to let achamfered recess on a side or end

stop perform two functions asshown in Fig. 8-7. The part is flatand the side locator is made higherthan the part. The locating



Fig. 8-7. A side locator with annclined locating surface.

surface is inclined to a point above

the edge of the part. With theclamping pressure applied, asndicated by the arrow, the part is

not only located and clamped

sideways, but is also forced downon the base. The angle of nclination is from 7 to 10 degrees.

This provides sufficient holding



pressure without risk of jamming owedging. The same arrangementcan be used for other types of

ocators (nesting blocks, V-blocks,etc.). It requires fairly closetolerances on the height of the part

The horizontal line through A doesthe locating. The coordinates x and

are critical dimensions becausethey define the position of A, within

the fixture. In cases where the twoperpendicular base and sideocating surfaces meet in a corner, a

relief groove is cut in the corner.The details may vary from case tocase. Four different configurationsare shown in Fig. 8-8.



DIRT

GOOD



JO

1

Fig. 8-6. A side locator with andwithout relief space.



Fig. 8-8. Typical relief recesses incoiners.

The design of dirt relief spaces forcircular locators follows the sameines as those previously described.

n example is shown in Fig. 8-9. Inthis case, as well as for pins andbuttons, the groove actually

CHIP PROBLEMS

Ch. 8

C

^^



Fig. 8-9. Relief space in a circularocator.

performs three functions; itprovides space for dirt, relief forburrs, and clearance for grindingoperations.

Shields and Seals

fixture with moving parts, such asbearings and sliding pins andwedges, is actually a piece of machinery. As such, it requires

protection against dust getting to it



bearing surfaces. Reasonably goodprotection is offered by the use of dust caps, as shown in Fig, 8-1 Oa.

More effective protection isobtained with felt washers (Fig. 8-1Ob) but they have the disadvantage

of a limited life; they must



be kept oiled and renewed fromtime to time. Modern technology offers a variety of seals with

excellent, although not infinite,service life. The O-ring is a well-known example. Where an effective

dust seal is required, the fixturedesigner should consult catalogs onavailable sealing components.

Indexing fixtures represent anmportant class of fixtures with

moving parts. The required

precision can only be maintained if all bearing surfaces are wel!protected. The typical design of anndexing fixture is shown in Fig. 8-

11, The center pin (king pin) is



shielded by means of a cap, A . SealB are provided to protect the indexpin and the main bearing surface.

Shielding and sealing, as described,s required when a movable

component is in an exposedposition. Many fixture designspresent an automatic shieldingdevice at no cost, namely, the part

tself. A large flat part providesquite an effective shield againstchips falling on base locators and

ntermediate supports. To fully utilize this effect, locators areplaced a short distance inside thecontour of the part.



Fig, 8-10. Chip protection by ashield (a), and a seal (b). Fig. 8-11.Shield and seal applied to anndexing fixture.

Centralizers

CHAPTER

Definitions

Centering is an advanced method o



ocating. While locating, aspreviously described, brings onesurface at a time into the proper

place relative to the fixture,centering is applied to two surfacesat a time and locates a plane within

the part— almost always the middleplane between the two surfaces.

Centering has three degrees. Single

centering is when one middle planes located, double centering is when

two middle planes (usually

perpendicular) are located, fullcentering is when three middleplanes (likewise usually perpendicular) are located. The

components for centering are



termed "centralizers." Arrows 1,1;2,2; and 3,3 in Fig. 9-1 indicatethree pairs of centralizers. Single

centering with one pair of centralizers 1,1 locates the middleplane aa and nothing else. Double

centering with the additional pair ocentralizers 2,2 locates two middleplanes aa and bb, as well as the axis3,3 where aa and bb intersect. Full

centering with the further additionof centralizers 3,3 locates threemiddle planes aa, bb, and cc, three

axes 1,1; 2,2; and 3,3; and thecommon center for middle planesand axes.

dvantages



In the introductory discussion toocating, it has been stated that one

purpose of a fixture is to

approximately substitute for thescribed lines and punched centersprovided in the initial layout of a

rough part—prior to machining. Itwas also explained that the partocates within the fixture from one

or several of its surfaces. With

tolerances, sometimes quite wideon a rough part, there is noguarantee that the physical middle

planes, axes, and centers of therough part will coincide with thecorresponding planes, axes, andcenters of the fixture. The fixture



can only guarantee the correctocation

of machined surfaces relative to theocators and to each other,

regardless of rough part tolerances.

The application of centering deviceshas advanced the function of thefixture to the point where itprovides a true substitute for the

scribed and punched layoutmarkings. The physical middleplanes, axes, and centers within the

part are now exactly located in thefixture (within tolerances).Machining allowances are evenly distributed, depth of cut is constant

on all sides, and excessive cutting



forces are avoided. Center of gravitys also correctly located and any

unbalance of rotating parts (in

turning operations) is eliminated.Surfaces that remain unmachmedare more accurately located relative

to system lines and planes in thepart and in the completed product.Entire machining operations may be eliminated by the use of cold

rolled and cold drawn stock locatedand clamped in accurately centeringdevices.

Centralizers and Locators

Centralizers are single or multiple

components. They act as locators,



as clamps, or both, A fixed singlecomponent centralizer is a locator.Multiple component centralizers

have at least one movable part.They can have one fixed (a locator)and one or more movable

components (clamps). When allcomponents of a centralizer aremovable, they can be considered aseither clamps or locators. Then

there is no real distinction betweenocators and clamps.

Typical combinations of locatorsand centralizers are shownschematically in Fig, 9-2. Doublecentering is accomplished wheneve

an axis is located, regardless of



what system of centralizers is used.By this definition the commonthree-jaw, self-centering chuck and

the collet chuck are double-centering devices (see diagram g),as is the two-jaw chuck frequently

used on smaller turret lathes (seediagram e). Drill chucks are alsodouble-centering devices. The

CENTRALiZERS

Ch. 9


http://slidepdf.com/reader/full/jig-and-fixture-design-manual-erik-k-hendriksen3709 655/2237Fig. 9-1. Single, double, and full



centering.

machine tool vise with two V-

grooves (see diagram f), is a single-centering device. Centralizers arenot nesting components. They

provide positive contact

and pressure without clearance.Locating with centralizers is,

therefore, more closely defined andmore accurate than nesting,particularly on rough parts.

Ch. 9

CENTRALIZERS



/

h

////,,/s

/ / / /

I

-I-I



"!-i


http://slidepdf.com/reader/full/jig-and-fixture-design-manual-erik-k-hendriksen3709 659/2237/77777T?



Fig. 9-2. Typical combinations of centralizers and locators: a. Singledefined, not centered; b. Double

defined, not centered; c. Singlecentered; d. Single centered; e.Double centered; f. Single centered

g. Double centered; h. Singlecentered; i. Single centered.

Classifications Centralizers can be

classified into three categories:

1. The angular block type

2. The linkage controlled multiplecentralizer

(automatic or scissor-type) 3. Self-



centering chucks of commercialtypes.

The term "angular block" is thecommon designation for block-typeocators with converging or

diverging locating surfaces. They are used in three

CENTRALIZERS

Ch. 9

forms, the V-bloek, cone locator,

and spherical locator. The V-block has two flat surfaces. In the mostwidely used type of V-block the V is

concave; the two surfaces form a



slot or groove which receives thepart. An inverted configuration isused occasionally where the V is

convex and forms the two surfacesof a triangular prism whichcentralizes by entering the space

between the prongs of a fork-shaped part. The cone locator has aconvex (male) or concave (cup,female) conical surface. The

ncluded angle within the locatingsurfaces is significant for thefunction of the locator. The locating

surface of the spherical locator isformed as a spherical cap or ringand can be convex or concave.

Linkage controlled multiple



centralizers are those in which themovable components are soconnected that they maintain equal

distance from the middle plane, theaxis, or the center. One example is apair of scissors, and scissor-like

inkages are frequently, but notexclusively, used. The term"linkage" is here used in a widersense; it includes mechanisms that

act with cams, wedges, telescopingrods, symmetrically arrangedsprings, etc. In mechanical languag

these are all called kinematicchains.

Centralizers and Equalizers



In a description of fixture designthere are two terms that must notbe confused: "centralizers" and

"equalizers." Both use linkages, butthey serve entirely different, almostopposite, purposes. In centralizers,

the linkage system controls themotion and position of the locatingand clamping points, and forces thepart into a position defined by these

points. Equalizers are also linkagesand are used for clamps for thepurpose of equalizing the clamping

forces on uneven surfaces of thepart. The equalizers carry theclamping points and enable thendividual points to adjust



themselves back and forth toaccommodate local irregularities.They do not force the part into a

position; they are forced intoposition by the part.

Commercial Centralizers, Chucks

Self-centering lathe chucks haveaws which are made to move in a

concentric relationship to thespindle axis. They can therefore beclassified as centralizers and areavailable with two or three jaws.The four-jaw chuck is also a devicefor centering, but the jaws areadjusted individually and manually

ll these devices are commercially



available, general-purpose work holders; as such, they do not qualifyas fixtures. However, they become

fixtures, or rather

fixture bases or fixture bodies,

when they are provided with speciaaws or jaw inserts and locators tofit specific parts. Machine tool visesare also commercially available,

general-purpose work holders, notfixtures. They are not, inthemselves, centralizers, but they

too can be fitted with special insertand other components for specialwork.

general-purpose work holder,



"borrowed" from its parent machineand mounted on a base, can serveas a major fixture component with

other components built upon it oraround it. Drill chucks are used togood advantage for holding small

parts. Instead of physically acquiring a piece of general-purposequipment, the fixture designermay feel inspired by its design

principle and apply it to theassignment in hand. The principleof the collet chuck with one or two

sets of contracting or expandingelastic fingers can be applied tocentralizers that can be mounted onan arbor or on a base. Fixtures of



this type are excellent for locatingand clamping on internal andexternal cylindrical or conical

machined surfaces where highprecision is required and the loadfrom the machining operation is

relatively light. The particularadvantage is that they exert auniform pressure on the part anddo not force it out-of-round. They

do have one serious limitation,however, as shown in Fig. 9-3; thefingers bend and their slope varies

as they expand or contract.Theoretically, there is only oneposition where they grip and locatewith their full surface. In actual



practice this means

Fig. 9-3. The principle of the colletchuck.

Ch,9



CENTRALIZERS

91

that they operate satisfactorily onlyover a small diameter range; insideand outside of that range they griponly with the edge.

The inclined angle on the conical

surface of the centralizer is 30degrees; the activating (mating)cone is made with 31 degrees if itacts on the inside of the fingers as

shown in Fig. 9-3, and 29 degrees ift acts on the outside. The thickness

of the fingers must be small to

ensure sufficient flexibility. For



work diameters D, ranging from 1/4nch to 6 inches (6 to 150 mm), the

recommended thickness t varies

from 5/64 inch to 13/64 inch (2 to 5mm).

The question may arise whetherhollow parts should be located(centered) on the inside or on theoutside; if so, the following points

may be considered: Any radialocating error (eccentricity) is

reproduced to true size, not more,

not less, regardless of -where thepart is located. A misalignmenterror (a wobble) is reproduced totrue size if the part is centered on

the outside, but is reproduced with



a magnification if the part iscentered on the inside and on asmall diameter-the magnification o

the error increases with thediameter ratio.

For a given clamping pressure, thetransmitted torque and themaximum permissible size of cut isgreater when the part is clamped on

the outside; also, with outsideclamping the part is less likely toslip in case of an accidental

overload in the cut.

Centering by Means of V-Blocks

The common method of centering



TP-r-n

rj-r j Hi

rid-fcd

hole. The underside of the block is

provided with a tongue £>, whichenters into a slot in the jig body tself, the V-block thereby being

prevented from turning sideways.The screw E, passes through thewall of the jig, or through some lugand prevents the V-block from



sliding back when the work isnserted into the jig. It is also used

for adjusting the V-block and, in

some cases, for clamping the work.-blocks are usually made of

machine steel, but when larger size

are needed they may be made of cast iron. Little is gained, however,n using cast iron, as most of the

surfaces have to be machined, and

the difference in the cost of material on such a comparatively small piece is very slight.



Fig. 9-4. Centering a cylindrical

surface by means of a V-block.

I'ig, 9-5. An adjustable V-block usedas a locator and centralizer.

For large size V-blocks it iseconomical to use finish machined,cast iron V-block stock,commercially available in widths upto 4 1/2 inches (120 mm) and inengths from 2 to 3 feet (600 mm to

1 m). When a V-block is used for



ocating round parts, there is somuch empty space left that there isno particular need for a relief

groove for catching chips and dirt.When a relief groove is used, as itoften is, the purpose is to provide

clearance for the grinding wheelused for finishing the flats of the V-block. The groove must be madewith rounded corners or as a

semicircle, to reduce stressconcentration.

Much mathematics has beenapplied in attempts to find and toustify an optimum value for thencluded angle, but no convincing

calculation has yet appeared in the



iterature. Some extreme limits canbe easily established. A V-block with a 30-degree angle will hold a

part very firmly and is near thepoint where the part becomeswedged by friction. A small

diameter variation causes a largeariation in the height at which the

part rests in the V. Thirty degrees isclearly a lower limit, and not even a

practical one. A V-block with a 120-degree angle will receive the partfreely and is not very sensitive to

diameter variations. The position ofthe part is not very stable; it takes arelatively small horizontal force atthe clamping point to roll the part



out of

CENTRALIZERS

Ch.9

ts resting place. 120 degrees is

dearly an upper limit and not adesirable one.

Industry has solved the problem byaccepting, almost universally, thealue of 90 degrees for the included

angle. Most V-block components in

commercial fixtures are made withthis angle, and V-hlock stock (roughand machined castings) is

commercially available with



force can move 45 degrees to eitherside before stability is lost. Otheradvantages of the V-block are that i

s solid, strong, and rigid; itprovides good bearing areas, issuitable for long as well as for large

parts, lends additional stability andstrength to the fixture, is versatilen its applications, and isnexpensive.



Fig. 9-6. The stability range of the90-degiee V-bloek.

machining of any surface andconfiguration that is symmetricalwith respect to the bisector plane,

or which is dimensioned entirely and solely relative to this plane.Examples of such configurationsare (see Fig. 9-7) holes and slots

passing through or across the partn the plane of symmetry, and

planes parallel to that plane. The

words "through or across" aresignificant. Consider a blind hole orongitudinal key seats (see diagram

e). They are machined in perfect

symmetry, but to a depth that



depends on the physical diameter othe part. In a great many cases thisobjection is academic only, as

diameter variations withintolerances are small, and blindholes, keyseats, and similar

configurations are usually designedwith a generous depth tolerancethat can absorb the small errorfrom the diameter tolerance. More

serious, perhaps, is the effect of diameter variations on the locationof configurations that are

dimensioned relative to thediameter; that is, perpendicular tothe bisector (see diagram f), A symmetrically designed hole,



keyseat, or slot will move clearly out of symmetry with a diameter

ariation. This effect is so obvious

that many sources call it a wronguse of the V-block. The criticism iscorrect in principle, but exaggerated

n reality; the real problem is againa problem of tolerances (see Fig. 9-8a and b). In a, the V-block is usedas a centering device for a

cylindrical part. When A is theariation of the part diameter, then

the center of the part is located on

the bisector with a locating error e,determined by

e = yAV2 = 0.707A



In b, the V-block is used as a baseand side locator; a use for which ils eminently suited. In this

application the error in thehorizontal (or vertical) direction isclearly

1 .

Limitations of the V-Block

Since the V-block has so many easily recognizable good points,there is also the danger that it may

be used for the wrong purposes. Itscapability as a cen-tralizer is quiteimited; taken individually it

provides only single centering, but



t does that well. The plane in whicht centers the part is the bisector of

the angle. This centralizing effect is

ndependent of the diameter of thepart, up to the limit of capacity of the V-block, and is used in locating

for the

The Sliding V-Biock

With the few reservations stated,the V-block with a single clamp is

ery suitable for locating andsingle-centering circular andcylindrical parts. Long parts of ample stiffness require a V-block ateach end rather than one long V-

block. The area of application of the



-block principle is significantly expanded by combining one fixedand one movable V-block, the

movable V-block acting also as theclamp. This system is widely usedfor elongated

Ch.9

CENTRALIZERS

93

B3^Kj1



e f

fig. 9-7. Application of the V-block



to cylindrical parts with differentmachining configurati

parts with rounded (partly circular)ends. A typical example is shown inFig. 9-9.

The drill jig shown is designed fordrilling fork links. The form of theinks is indicated by dot-and-dash

ines in both views. The link has around boss at one end and roundedforks at the other. It is accurately held between two V-blocks, oneadjustable and the other stationaryThe adjustable V-block A is clampedagainst the work by a star-wheel

and screw, and it travels between



was accurate, rapid, and easily operated.

The principle of the sliding V-block can be applied to parts of the mostdiversified shapes, as long as they

present bosses ot other contourswith at least a little more than 90degrees of a circle.

-blocks are not often used forocating square and otherwise

prismatic parts. One reason is thatsuch use of the V-block is actually nesting, with its inherent lack of accuracy, mainly in the matching ofthe angles. Parts with flat surfaces

are better located on base points



with side and end stops, on strips,or in a vise-type of fixture. Anotherreason is that the V-block provides

centering with respect to a diagonala feature rarely called for.

Conical Locators

The conical locator is well known inthe machine shop, although not

necessarily by that name. To

CENTRALIZE RS

Ch.9


http://slidepdf.com/reader/full/jig-and-fixture-design-manual-erik-k-hendriksen3709 693/2237Fig. 9-8, The effect of diameter



tolerance on the locating error.

center drill a part prior to a turning

operation and set the part up on theathe centers is actually locating

with conical locators. The tapers in

and on machine tool spindles andthe corresponding tapered shankson drills, arbors, chucks, etc., areconical locators, but these tools are

not fixtures. The lathe mandrel is awork holder with a tapered seat forthe work. The taper is very small,

0.006 inch per foot (0.5 mm per m) common characteristic of thesedevices is that they transmit torqueby friction. The lathe mandrel does

not define the axial position of the



part; it is actually determined by thbore tolerance and the amount of pressure used when the part is

mounted. The lathe mandrel is ageneral-purpose work holder, not afixture.



From this brief resume it followsthat conical locators cannotduplicate these devices. The conical

ocator can center and does thatwell. It can provide some degree of axial locating but not with

precision. Integrating a conicalcentralizer and a flat axial locator(see Fig. 9-1 0) in one piece requireextremely close tolerances and is

mpractical except for special, high-precision work. A good workablesolution is to mount a sliding

conical locator within a flat axialocator and provide independent

clamping devices. The exampleshown in Fig. 9-11 is typical of the



application of this principle andcontains some additional featuresnecessitated by the need for

gripping and clamping the work by a thin rim, without distorting it.

The work A is a special clutchflywheel which has been partially machined, Jn order to obtainconcentricity of the various

surfaces, it is necessary to locatethe work from the taper in the hub.In order



Fig. 9-9. Locating and singlecentering by means of a fixed and asliding V-block.

Ch. 9

CENTRALIZERS

95



Fig. 9-10. Combinations of a conica

centraiizer and a flat axial locator, aProper locating is impossible; b andc. Locating is possible, but notuseful; d. Locating good.

to compensate for slight variationsbetween the taper and otherfinished surfaces, a tapered, shell-



ocating bushing B is centrally ocated on the stud C, which is heldn place in the faceplate fixture E by

the nut and washer at D. A light coispring M insures a perfect contactwith the tapered surfaces, while a

small pin N restrains themovement. As the outside of thework is to be finished during thissetting, it is necessary to grip the

casting in such a way that theclamps will neither interfere withthe cutting tools, nor cause

distortion in the piece itself. Withthis end in view, the three lugsaround the rim of the fixture areprovided with shell bushings K,



each of which is squared up at itsnner end to form a jaw which is

bored to a radius corresponding

with the rim of the casting L. It issplined to receive a dog screw J,which prevents it from turning, and

t also gets a good bearing directly under the point where the work isheld so that there is no danger of itspringing out of shape.

The bolts F pass through the shellbushings and are furnished with

nuts G at their outer ends, the nutshaving a knurled portion 0, whichpermits of rapid finger adjustmentbefore the final tightening with a

wrench. It will be seen that this



construction automatically obtainsa metal-to-metal contact with thethin flange of the casting, without

distorting it in the least, as thefloating action of the bushingsequalizes all variations and yet

holds the work very firmly. Afterthe clamps have been set up tightlythey are locked in position by theset-screw H, at the rear of the

fixture. This application of thefloating principle may be adapted tomany kinds of work, and the results

obtained leave nothing to bedesired. The machine for which thisdevice was designed is a turret latheof the horizontal type.



Conical locators do not necessarily require tapered bores to work with;they also work very well with

circular edges. Any part with acylindrical outer surface or a

cylindrical bore and one or two flatend surfaces can be centered by means of conical locators providedthat the edge is really circular. This

requires that the end surface beperpendicular to the axis. If the parhas been machined, the edges must

be inspected and any machiningburr removed. Castings with coredholes are likely to have fins aroundthe core prints that must be cleaned

away before the edge of the hole



can be used for locating. Whenevert is essential that a cylindrical part

of the work be located centrally

either with the outside of acylindrical surface or with thecenter of a hole passing through the

work, good conical locators can bedesigned as shown in Figs. 9-12 and9-13. In Fig. 9-12 the stud, A, iscountersunk conically (cup locator)

to receive the work. The stud ismade from machine or tool steel,and may, in many cases, serve as a

bushing for guiding the tool. In Fig9-13 the stud is turned conically inorder to enter a hole in the work.These two cone locators are



stationary; they are only used forocating the work and would requir

additional means for clamping.

Clamping with a Moving Cone

Clamping devices for use with coneocators can be separate andndependent, but it is also possible,

and very convenient, to make one o

two locators movable and use it forclamping. The bearing area on theedge is small and the clamping loadmust be kept light to avoiddeformation of the edge, or otherdamage. Clamping by means of amovable cone locator is widely used

n connection with drill bushings.



Drill bushings with an externalscrew thread are known as "screw"bushings and may be used for

ocating and clamping purposes by making them long enough toproject through the walls of the jig

and by turning a conical point onthem, as shown in Fig. 9-14, or by countersinking them, as in Fig. 9-15. In all cases where long guide

bushings are used, the hole in thebushing ought to be counter-boredor recessed for a certain distance of

ts length.

In some instances the screw bushing must be movable sideways

.e., when the piece of work to be



made is located by some finishedsurfaces, and a cylindrical part is tobe provided with a hole drilled

exactly in the center of a lug orprojection, the relation of this holeto the finished surfaces used for

ocating is immaterial. The piece of work, being a casting, wouldnaturally be liable to variationsbetween the finished surfaces and

the center of the lug, particularly if there are other surfaces and lugs towhich the already finished surfaces

must corres-

CENTRALIZERS

Ch.9



B

M

I



2?

-



Fig. 9-12 (Left). The outside conicalocator. Fig, 9-13 (Right). The inside

conical locator.

pond. In such a case, the fixedbushing for drilling a hole that

ought to come in the center of theug, might not always suit the

casting and so-called "floating"bushings, as shown in Fig. 9-16, areused. The screw bushing A isconically recessed and locates fromthe projection on the casting. It is

fitted into another cylindrical piece



conical locator. Fig. 9-15 (Right), A threaded drill bushing used as anoutside conical locator.

adjustment of the jig bushing.When the bushing has been located

concentric with lug E on the work;the nut F having a washer G undert, is tightened. The flange on piece

B and washer G must be large

enough to cover hole C, even if B isbrought over against the side of thehole. It is seldom necessary,

however, to use this floatingbushing, for a drilled hole in a pieceof work rarely can be put in withouhaving any direct relation to other

holes or surfaces.



Fig. 9-16. A floating drill bushing

used as an outside conical locator.

Linkage Controlled (Automatic)Centralizers

For the control of the movingcomponents the following

mechanisms (kinematic chains) areused:

The list is representative, but not

comprehensive; a complete list



would require a kinematicsencyclopedia. There is still room forthe inventiveness and ingenuity of

the fixture designer. As a guide, afew fundamental rules are given:

1. Make it simple

2. Prefer rotation to sliding

3. For rotating members—apply forces perpendicular to radii

4. For sliding members-provide

support below the points of forceapplication

5. Specifically for lever arms in



inkages—

a. Make arms of equal length; if not

feasible, then operate on the longarm and let the short arm performthe clamping

b. Should this not be feasible,operate through a force-magnifyingdevice, such as a screw mechanism

with a handwheel

6. For all mechanisms—

a. Watch for rigidity of all individuamembers, and for

b. backlash in all bearings and othe



points of contact.

To illustrate these rules, a few

typical examples follow.

Kinematic chains do not squeezeand clamp as hard as single clamps,because the available force isdivided between several locatingand clamping points. Primarily used

for drill jigs rather than for millingfixtures, they are recommendedwhen drilling flat plates and coverswhich are not usually machined atthe sides but have to be gripped orocated in a jig by their rough cast

edges. Similarly, such self-

centralizing features will be found



advantageous, time-saving, andeconomical in drilling parts havinga similar shape, but whose overall

dimensions differ.

One of the simplest forms of a self-

centralizing device for a drill jig isshown in Fig. 9-17. This jig is anexample of the type with slidingwedges actuated by opposing

springs. It can also be characterizedas a jig with a split V-block andcontains a rectangular cast-iron

body A, which is flanged at thebottom for hold-down purposes. A swinging arm B is pivoted on a pinC that is pressed into both side

walls of a slotted boss on top of the



body. Arm B carries the drillbushing D. Pressed into theunderside of the arm on each side

of the bushing, are two bearing padE. When the arm is in thehorizontal position, as shown, these

pads will press equally on the workpiece X, holding it firmly to the topof the jig body. The right-hand endof arm B has an open-end slot for

the cylindrical shank of clampingstud F. A

CENTRALIZERS

Ch. 9

c



/^sAvlvu

SO £■ O X C

MIL.

Fig. 9-17. A centralizing drill jig withspring-actuated wedges.



knurled nut G, threaded on theupper end of this stud, enables thearm to be clamped to the work. The

stud can be swung about a pivot pinH pressed into the body. Thecentralizing action on the work-

piece is obtained from identically shaped spring-loaded slides J,which are mounted in a guide holeK, drilled completely through the

body. Springs L are held in pocketsn these slides by stop-plates M ,

which are fastened to the sides of

the jig body by screws. Each slidehas a vertical projection at the inneend, with the hardened end faces ofthese projections inclined at an



angle of approximately 10 degrees.The projections slide in slots whichextend from the top edge of the jig

body into the bearing holes K. Thesprings force the slides inward,toward each other, and the extent o

this movement is limited by eitherthe end faces of the slots or thework-piece, as shown.

large-diameter vertical holeextends down through the body,directly below the hole to be drilled

n the workpiece, to permit chips tofall out of the jig. A short cylindricaplug, cross-drilled as shown, istightly pressed into hole K to lie

midway between slides /. This plug



prevents chips from entering holeK. To mount a workpiece in the jig,arm B is swung upward. The two

opposing sides / will be in theirnnermost positions, in contact with

the end faces of the slots, and the

workpiece is placed between thenclined faces of the slides.

Transverse location of theworkplace is obtained by butting it

against a plate O fastened to the topface of the jig body. Arm B is thenowered into the horizontal position

and clamping stud F is swung intots vertical position, as shown. As

nut G is tightened, pads E beardown on the workpiece and press it



typical design of a fully linkage-operated cen-tralizer is shown inFig. 9-18. It differs from the one

shown in Fig. 9-17 by two very mportant features. The first is that

the centralizing motion is positively

controlled by the linkage, while inthe previous case it was dependenton the symmetry in the springarrangement. The second feature is

that the effective opening has aarge operating range so that this

fixture can be successfully

employed in drilling parts havingconsiderable variation in width orength. As with the jig previously

described, a swinging arm B is



pivoted on a pin C, which is pressednto jig body A. A drill bushing D is

carried in the arm, and located on

each side of this bushing are thedentical bearing pads E. The right-

hand end of arm B is slotted to

admit clamping stud F, which isfitted with a knurled nut G andpivoted on pin//.

The bellcrank locating and clampinevers/ area sliding fit within a

narrow slot in the jig body, and

pivoted on pins K pressed into thebody. Permanently fitted into atransverse slot in the body is aplatform L for supporting the

workpiece X. Vertical clearance



holes are provided in this platform,and in the jig body, to permit thechips to fall through.

The upper inner edges of levers J,which contact the sides of the

workpiece, are rounded andhardened, and can be serrated toprovide a better grip. The lowerends of the levers are reduced to

half their total thickness so thatthey overlap, and the left-hand leves slotted to fit over pin M pressed

nto the tail of the right-hand lever.When the lower halves of the leversare in a horizontal position, thecenter of pin M is aligned with the

ertical center-lines of the jig body



and drill bushing. This arrangemennsures that the levers will be

swiveled equally.

Ch.9

CENTRALIZERS

99


http://slidepdf.com/reader/full/jig-and-fixture-design-manual-erik-k-hendriksen3709 728/2237Fig. 9-18. A fully linkage-operated



centralizei.

ctuation of the levers is obtained

by means of rod N, the slottedshackle end of which is pinned tothe right-hand lever. The cylindrica

shank of rod A r is a running fitwithin an externally threaded sleevO, which is screwed into the right-hand wall of body A* When hand

wheel P is rotated, levers J will beswiv-eled (due to the force of sleeve0} against the shoulder on rod N or

the collar pinned to the rod.

The manner of loading and usingthis jig is similar to the one

previously described. With arm B



raised, workpiece X is placed onplatform L. The contact surfaces of evers J will have been moved apart

by rotating handwheel P.Transverse location of theworkpiece is obtained by butting it

against an adjustably mountedstop-plate Q, The handwheel is thenrotated in the opposite directionuntil the work is firmly gripped and

centralized by the contact surfacesof the levers. Arm B is thenreturned to the horizontal position

shown, and clamped by

tightening nut G. Additionalclamping pressure is thus exerted

on the work by pads E.



n example of the use of rotatingarms actuated by inclined flats isshown in Fig. 9-19. This type of self

centralizing jig has been provedeconomical and accurate in drillinguniformly central holes through

thin cover plates and similar partshaving large variations in width. Inthis jig, a swinging arm B is againpivoted on a pin C, which is pressed

nto the jig body A. The arm carriesa drill bushing D, and its right hands slotted for a ring-head clamping

bolt E that carries nut F and ispivoted on pin G.

spring-loaded cylindrical plug H i

a sliding fit in the vertical bore of



the jig body. Rotation of the plug isprevented by key N. The lowerthreaded end of the plug is screwed

through handwheel K which iscarried in a horizontal slot in the jigbody. The upper head of the plug,

on which the workpiece X rests, hastwo diametrically opposite slotsnto which

CENTRALIZERS

Ch. 9



Fig. 9-19, A cam-operatedcentraliier.

are fitted the triangular-shapedocator pads/. These pads pivot on

pins L, pressed into the side walls othe slots. The lower, outer corner of



each locator pad rests on thetapered bottom surfaces of slots cut

n the side walls of the jig body. Inoperation, arm B is raised, and awork piece is placed on top of plug

H, located against a fixed pin orplate, not shown. Then, by turninghandwheel K, plug H is drawndownward—against the action of

spring M— and pads / are pivotedtoward each other, thus centralizingand gripping the workpiece. Arm B

s then lowered into a horizontalposition and clamped by tighteningnut F.

Wear can be minimized in this jig



by screwing hardened head set-screws into the tapered bottomsurfaces of the slots cut in the side

walls of the jig body. The roundedcontact points of pads/ would thenbear on the heads, and slight

adjustments could be made by tightening or loosening the set-screws.

n effective method of obtainingcentralization of the work by meansof a real linkage (in this case, a

inkage of the pantograph type) isllustrated in the drill jig seen inFig. 9-20. Secured to the top, nearthe rear edge of jig body A, is a

slotted bracket B. A swinging arm C



pivoted about a pin pressed into theuprights of bracket B, carries drillbushing D. The forward end of this

arm is slotted to admit the shank ofa ring-head clamping bolt E thatcarries a knurled clamping nutF.

The ring head of the clamping bolts a close fit in a slot on bracket G

secured



Fig. 9-20. A linkage-operatedcentralizer with a pantographmechanism.



Ch. 9

CENTRALI2ERS

101

to the front of the body and pivots

about a pin pressed into theuprights of this bracket.

Workpiece X is placed on the topsurface of jig body A, bearingagainst stop-plate H secured to thebody for lengthwise location. The

part is centralized transversely andgripped by means of two bars /,which rest on the smooth top

surface of the jig body. The inner



contacting surface of these bars arerelieved slightly to reduce thefrictional pressure on the

workpiece. The ends of both barsare pinned to levers K, forming apantograph mechanism. Levers K

can be pivoted about studs L. Thefront end of the extended right-hand lever K has an elongated slotto fit around a pin M, pressed into

the slotted end of rod N. Thethreaded shank of rod N passesthrough plain holes in both walls of

a slotted bracket O secured to theright-hand edge of the jig body, ands screwed in the internally

threaded handwheel P. Thus, when



the handwheel is rotated, thepantograph lever system is swiveledabout studs L, either moving bars J

together to clamp the work, or aparto permit loading and unloading.

rm C is handled in the same

manner as those on the jigspreviously described; swungupward while unloading andreloading, and down into the

position shown, when theworkpiece is in place.

Centralizes for Gear Wheels

circle. They are the machining of aocalized detail such as the drilling

of a hole pattern or the milling or



broaching of a key seat, and thefinishing of the bore concentricwith the actual pitch circle as

defined collectively by the teeth,

localized detail is usually

dimensioned from a tooth or toothspace and must be locatedaccordingly. This is not just a caseof radial locating from one side

only, but the locator must pick upboth sides of the tooth or toothspace and locate with respect to the

bisector between them. The locatins a centralizing operation and canbe done with a sliding V-block on atooth or a sliding V-prism in a tooth

space (see Fig. 9-21). The design of



these components is based on thegear tooth geometry, as defined by the angles <p, b } , and b 2 .

To ensure contact on the pitchcircle, the included angle a in the

ee is determined by

for a V-block

0 = T + *, fl = 2(0-*i)

for a V-prism

In the manufacture of gear wheelsthere are two types of operationswhich require locating of the gearfrom points on the tooth flanks in



the pitch

a = 2(<j> + b 2 )



Fig, 9-21. Centralizers for gearwheels.

CENTRALIZERS

Ch. 9

<t> is the pressure angle; the

angles b\ and b 2 depend on the



gear data and the tolerances(backlash). All angles are indegrees. Using symbols in

Machinery 's Handbook, 19thedition, the gear data are:

t = D sin b\ s = D s\nb 2

Then, disregarding backlash

h -h 90 °

and with backlash 180°

b, + b 2 =■

N

*a =*!+(§ X57.3°)



Suggested values for backlash:

.. _ 0.030 , 0.040

Minimum B ■ » , Average £ = —=—

Maximum B —

0.050

In connection with the metricsystem, gear dimensions are basedon the module which is denoted m,and is always expressed in mm.

Modules are standardized in simpleand round numbers. The relationsbetween pitch and module are:



R =

2 cos ■

2 cos

+ M

For centering a gear wheel from itspitch circle, a plurality (usually

three, but sometimes four) of centra lizers are used evenly spacedandmountedonthe jaws of a self-centering chuck, If the centralizers

are circular pins, the capacity of thechuck must be at least D Q ,determined by



D Q = D cos b 2 + 2R (\ + sin j\

n t. j, /l + sin(0 + 6 2 )\

= I) CDS b-, + s\ 7,, , —r^M

= Dcosb 2 + s tan 1/2(90° + 0 + b 2

)

Cones and balls are also used as

centralizers for holding helicalgears. Straight solid pins cannot beused as they are not compatiblewith the helix (except when very

short), but pins formed ascylindrical spring rolls and held inposition so as to form a cage, are

used for locating and holding



helical gears between chuck jaws.Figure 9-22 shows four such pins inposition.

The really "natural" centralizer for agear is another gear or a section of a

gear. A fixture for bevel

Courtesy of Heald Machine Div. ,Cincinnati Milacron Inc. Fig. 9-22.

centralizing fixture for a helical



gear, a. Cylindrical spring rolls forgripping the gear tooth spaces; b.The complete grinding fixture.

Ch. 9

CENTRALIZERS

103

gears is shown in Fig. 9-23.' Eachcentraiizer is an insert with onegear tooth operating in a toothspace. The inserts can slide radially

and are forced inward by the outercone when the base plate is pulledengthwise toward the spindle. The

fixture shown in Fig. 9-24 has a



cluster of small pinions

Example-A spur gear with 20 degre

pressure angle has 6 diametral pitchand 72 teeth. A V-block locator shalbe designed with and without

considerations of average backlash.

D =•

1 2 inches



Without backlash

ft -2£-

61 "72 -

3 = 2(20' With backlash 0.040

is

-1°15') = 37°30'



B =

= 0.0067 inch

Courtesy of E, Thaulow Fig. 9-23. Acentralizing fixture for a bevel gear.

bi=^p: rrrvrX 57.3 =1 15 —58

= ri4'02" a = 2 (20° - 1°14'02") =

37°3l'56"



Courtesy of Heaid MachineDiv.,Cincinnati Mitacron Inc. Fig. 924. Centralizing fixtures for

clamping helical gear workpieces bymeans of a cluster of small pinions(The Garrison gear chuck.) a. The

fixture open, helical gear workpiecen foreground; b. The fixture with

helical gear work-piece in place.

acting as centralizers for external orn.ernal spur

and helical gears. Kadi pinion ismounted on an eccentric pivot andthe work is clamped ar.d centeredrelative to its pitch circle when all

pinions are simultaneously rotated



E, Thaulow, Maskinarbejde Gad'sForlag, 1930) vol. II.

(Copenhagen: G.E.C.

Using symbols in Machinery'sHandbook, 19th edition, a helicalgear is characterized by the helix(spiral) angle a, also known as the"tooth" angle, which is the acute

angle between a tooth and the axisof the gear. The number of teeth A',and the pitch diameter D, have norelation to the helix angle. Thechordal thickness t c and the toothspace chordal width s, refer to thepitch circle as shown in Fig,



CENTRALIZERS

Ch.9

9-21. In a section perpendicular tothe tooth, the following dimensionsare defined: The normal pressureangle <j>, the normal diametralpitch P n , the normal chordalthickness t n , the normal tooth

space chordal width s n , and thebacklash B. 0, P n , and B areselected from the conventional

alues as for a spur gear. In asection perpendicular to the gearaxis are defined the tangentialpressure angle <t> t , the diametral

pitch P, " and the tangential



backlash B t . The followingrelations hold:

, tan 0

tan ffl. =

cosct

P — P n cos oe = j:

B f =

cos a

ametral pitch, 60 teeth andminimum backlash. A circularocator (spring roll or ball) must be

designed.



tan 20 0.36397

tan <p f = 5—; =

r cos 33 33 0.83340

= 0.43673

O - _ f _ _ it

0 r = 23 35 33

P = 6cos33°33' = 5

60

D — — — 12 inches

n 0.030



J=—g—= 0.005 inch

0.005 ^ = cos33°33' = 0 006inCh

90 , 0,006., „,„ o , „ ^ = 60 + 2^T2X57 ' 3=1 3052

s n = s cos 0£

The included angle a for a

"conjugate" (imaginary) V-sectionn the plane perpendicular to thegear axis, is calculated from thespur gear formulas and is converted

to the included angle a' for theactual V-section (in the planeperpendicular to the tooth) by



t a a

tan -_- — tan — cos a

Dimensions J? and D a relating to apin or ball centralizer are calculatedas before, by substituting a and s nfor a and s.

Example-A helical gear with 33°33*

helix angle and 20 degree normalpressure angle has 6 normal di-

f = <23°35'33" + 1°30'52") =

25°6"25" tan ~= tan 25°6'25" X cos33°33' = 0.39052 y=21°19'55"

s = 12sinl°30'52" = 0.3172inch s n =



Clamping Elements

Classifications

Here, as in other cases within shopterminology, the word "clamp" hasmore than one meaning; it is thecommon designation for all devicesby which a part is secured in afixture against the acting forces and

t is also the name for a specifictype of holding device described asa "strap." While the most importantphase in fixture design is theocating phase, the clamping phase

takes its place as second inmportance. Its technical

mportance lies in the fact that



clamps must generate and directthe acting forces in such a way thatthe part is securely locked in place

without suffering injury in the formof elastic distortion (springing). Theeconomic aspect of clamping is just

as significant, because clampingand releasing the part absorbs aportion, perhaps the largest portionof the total operating time.

Therefore, clamps must be designedfor safe and fast operation.

To accomplish this, clampingdevices have been developed in anextremely wide assortment of diverse types and details. Almost al

of them are now commercially



available and are, to some degree,standardized." The fixturedesigner's task is no longer to

nvent and design new clampingcomponents, but to select the righttype and size from those on the

market. A comprehensive display ofclamping components is found inChapter 17 and the presentdiscussion is confined to an

explanation of principles, thedescription of representativeapplications, and references .to

details.

The basic types of clamping devicesare: screws, straps, wedges, cams,

toggles, and rack and pinions.



Racks and pinions are discussed inChapter 21. The action of mostclamps is based on friction, and

they are actuated either manually,pneumatically, or hydrauiically.Predominantly, clamps are of the

strap type; structurally, they aremodifications of the simple beam.

The Mechanics of Wedges

The action of wedges, screw threads, and cams are all based onthe same friction relations. A screwthread is a wedge rolled around acylinder. A flat cam is a wedgefolded around a circle. To clamp a

workpiece by means of a wedge



requires, in the simplest case, thatone side of the wedge bears againstthe work surface, as shown in Fig.

10-1 a, while the other side issupported by a surface in thefixture. With the coefficient of

friction £<, the forces on the twosliding surfaces are P and fiP, and itrequires the force shown as F t tonsert the wedge. To calculate F x

the forces P are resolved into theircomponents with respect to the axisof the wedge, as shown in Fig, 10-lb

Equilibrium requires that the sumof all force components parallel tothe wedge axis equals zero, fromwhich we get



Fi = 2P sin — + 2 M P cos~

2 ' ~ r " ""2 + n cos ■

sin — + n cos -z I

To withdraw a wedge that holds the

work with a pressure P requires aforce F 2 found by

F 2 =-2i > sin^+2M^cos-

Since the parenthesis includes aminus term, it may become zero,

which gives

a a

— sin^r + fX cos -r = 0



tan^ = P

CLAMPING ELEMENTS

Ch. 10

Table 10-1. Coefficients of Friction

j. for Wedges, Cams, Pivots andBearings Made of Hardened Steet

Pi sin;



sin £ Fig. 10-1. The mechanics of the wedge.

The result, F? = 0, means that thewedge is no longer self-locking. Thecondition for self-locking dependsstrongly on the coefficient of

friction, but so do the values of F\



and F 2 . The coefficient of frictiondepends not only on the contactingsurfaces, but also on the pressure

(ij increases with increasing unitpressure) and the operatingconditions. Everything in a machine

shop carries an oil film (unless ithas been recently degreased). Thiscondition is assumed for clampingoperations. While clamped under

pressure the surfaces have atendency to break through the filmand "bite," and the coefficient of

friction for release is significantly higher. On the other hand, if exposed to vibration, the surfacesmay work loose resulting in a



reduced coefficient of friction.Recommended values of (i forwedges, cams and pivots and

bearings are given in Table 10-1.

Self-locking wedges and cams are

made with tapers from l:2G(a =2°52')through 1:10(<* = 5°44')andup to a wedge angle of 7 degrees(taper 1:8.18). For wedges exposed

to vibrations it is recommendedthat the taper should not exceed1:15 (a = 3°47'). Wedges designed

within these limits have a shortoperating range and are only usedon parts with fairly close tolerancesWhen the wedge is positively held

n place, larger wedge angles can be



used. Wedges that actuate plungersor other movable components aremade with al least 15 degrees (taper

1:3.8) and up to 45 degrees wedgeangle (plungers with ends cut unde45 degrees).

Example - Assume a 7-degree wedgacting on a cast iron part with fi =0.15 for clamping and p = 0.1 8 for

release.

F } = 2i>(sin3 1/2° + 0.15 cos 31/2°) = 0.422/"

The pressure on the part is, in thiscase,«s2 1/3 times the applied force



F 2 =2P(-sin3 1/2°+ 0.18 cos 3 1/2°)= 0.237F

The force to release the part is, inthis case, approximately one-half ofthe clamping force. The wedge is

self-locking for coefficients of friction down to

u = tan 3 1/2° as 0.06

Ch. 10

CLAMPING ELEMENTS

107

The Mechanics of Screws



When a screw is used as a clampingdevice, the clamping force P on thework, or on a strap, is developed by

application of torque T to the headof the screw or to the nut (see Fig.10-2). In addition, the torque must

overcome friction on the thread andunder the head. All forces can becalculated by



Fig. 10-2. The mechanics of thescrew thread.

use of the wedge formulas with the

helix angle and the thread angletaken into account. Detailedanalysis of the system is availablefrom text books on machine



elements. However, considerablesimplification is possible, becausethe standardized screw thread

systems have constant thread angleand nearly constant ratios betweenthe significant dimensions

(nominal, pitch, and root diameter,etc.). With a coefficient of frictionof 0.15 as representative of averageworkshop conditions and nominal

screw diameter D, the formulaconnecting torque T and clampingpressure P is

T= 0.2 DP

(H = 0.15)



With well cleaned and lubricatedscrew threads, the coefficient of friction can go down to around 0.10

which is about the best that can beexpected under workshopconditions. Values for T for various

alues of 11 are

T= 0.164 DP T =0.139 DPT=0,U$DP

(ju=0.12) 0i = 0.10) (li = 0.08)

The formulas as written are valid

for coarse threads. For fine threadsT is 3 to 5 percent less; a rathersurprising result. One would expect

a greater difference, but the gain in



mechanical advantage is partly absorbed by the increased frictionosses. All standard UNF, UNC, and

all of the machine screw series of screw threads are designed to beself-locking and no further analysis

s required with respect to thispoint.

Manual Forces

The torque on a clamping device isproduced by manual force.Mechanical clamping devices do nouse screws but apply their pressuredirectly, or through a simpleinkage.



The force, that can be provided by an operator, is not a constantquantity and cannot be calculated

as are mechanically generatedforces. Nevertheless, numerousattempts have been made to

measure them: their average andmean values and their upper andower limits. Most of these have

been directed towards the

manipulation of machinery controln general and have little bearing on

the actuation of fixture clamps with

the exception of a few results of ageneral nature. Test results show acceptable correlations and normaldistributions around a mean value;



the maximum value is 3 to 4 timesthe minimum value, and theminimum value is 0.45 times the

mean.

Data for two operations are needed

n manual fixture design: to pull (orpush) a lever with one hand, and toturn a knob. The lever can be awrench, a handle on a nut, or a cam

ever. From evaluation of laboratorydata it appears that a mean valuefor pulling a lever with one hand is

90 to 110 pounds (400 to 490 N).Data of this nature, however, mustbe taken with some reservation.The force a man exerts during a

aboratory test is not necessarily th



same as that which he would usewhile on his job, where he(unknowingly) is influenced by his

physical condition, his willingness,the frequency of the effort, hisdegree of fatigue, and his age. It wa

found in one case that themaximum one-hand pull was 125pounds (556 N) for a 25-year-oldman and 103 pounds (458 N) for a

60-year-old man. It is not known towhat extent the equipment used inthe various tests is representative o

common shop equipment. Theength of the lever is of importance

because it affects the mode of thegrip and the geometry of the



motion. A rule-of-thumb, applicableto levers of up to 8 inches (200mm) in length, gives 2.8 pounds pe

nch (5 N per cm) lever length asthe average applied force in one-hand operations.

CLAMPING ELEMENTS

Ch. 10

Laboratory results agree well on thesignificance of fatigue. The forceapplied repeatedly by one hand

should not exceed 30 to 40 pounds(135 to 175 N). European designpractice uses 145 to 175 newtons

(33 to 40 pounds) as the force



applied to a lever. American practics to take 30 pounds (135 N) for

calculating the operating clamping

force, but to calculate dimensionsfor an occasional maximum force o90 pounds (400 N).

The turning of a knob is clearly aone-hand operation. The torquethat can be exerted depends on the

shape and dimensions of the knob;the maximum torque in repeatedapplication can be taken as 47.5

nch-pounds per inch (211.3 N mmper mm) knob diameter for roundand four-lobe hand knobs.

The above are design values which



means limiting values, and notnecessarily operating values, andmust be applied with due

consideration of all facts includingthe dimensions of the equipment.No workman would use from 30 to

90 pounds on a wrench to tighten a3/8-inch screw. The operatoradjusts his force to the job at hand;he pulls the wrench until he "feels"

that the screw is "tight."

The results of a test program

performed under actual shopconditions 1 are listed in Table 10-2The operator was instructed to "pulnormally until the screw is tight."

Screw thrusts were measured by a



3 shows a cam actuated by a force Fon a handle of length I, measuredfrom the pivot center C, and

exerting a clamping force P and africtional force n l P at the point of contact D. These forces generate a

reaction R with a frictional force /i3 R on the pivot. Any point on thecam contour is defined by radius

ector r and position angle 6

measured from the low point A. Theocation of D, relative to C, is

defined by the initial eccentricity e

and the height H, which vary

Table 10-2. Clamping Force fromHand-Operated Screws—

Experimental Results



JS1

0

a.

cs

according to the cam geometry andas a function of 6. The surface andubrication conditions at D and on

the pivot are different and are

reflected by the two differentcoefficients of friction pn and fi 2(Mi > Mi; see Table 10-1).



Tomco, Inc., Racine, Wisconsin.

'V T

— ! —! p

Fig, 10-3. The mechanics of a cam



of arbitrary contour.

Ch. 10

CLAMPING ELEMENTS

109

ssume, for a moment, that there isno friction; the acting forces are

now F 0 and P; the reaction R doesnot enter into the equilibriumequation which is

as the theoretical condition for self-ocking. For any practical

application a safety factor (FS) isrequired, defined by



F 0 L=Pe

F„ e

The pressure P exists only as longas the force F Q is maintained.When F 0 is removed, P subsides.This situation is unusable andunrealistic. Frictions are alwayspresent. Their effect is to drastically

ncrease F for a given or required Pand to provide the possibility forself-locking.

With friction, the equilibriumequation for the clamping phase is

Pe +HtPH + n 2 R r i =FL



R is composed of contributionsfrom P and F. Cam devices aredesigned so that P is much greater

than F (1 0 to 20 times) anddominates the value of R which, asa good working approximation, can

be taken as 1.03 P. Then

P(e+li 1 H+ 1.03M 3 r 2 ) = FL

HiH +n 2 r 7

-(FS)

With the assumption that the cams clamped in a self-locking position

with the force P, it is now desired to

release the cam by applying a force



F to the handle. This means that Fchanges direction and so do thefriction forces fi^P and l* 2 R. By

changing sign for these; terms inthe equation it becomes

Pe—ti l PH-n 2 Rr 2 =—F'l

F' —e+(i 1 H+ 1.03fJ 2 i-a

P ~ L

s long as the equation gives apositive F the cam is self-locking.

The theoretical limit for self-lockins when F' = 0, which makes R ■= P

and gives



—e + iX t H + ti 2 r 2 =0 Miff + Vl r2 =

where (FS) is taken as 1.5 to 2. Forft, it is recommended to use theower range of values (column

"clamping" in Table i0-1). Thefriction losses are evaluated by thecam efficiency

■ o ~F'

e + HiH + l.03u 2 r 2

The cam contour has five points of nterest; the low point A, the high

point B (dead center), the lower and

upper limits E and G, and the



midpoint M, of the operating rangeThe upper limit G must never getclose to B because of the danger of

the cam snapping through in case ooverload. This danger increases asthe cam and its pivots and bearings

wear. Safety against snap-through iobtained by proper selection of theocation of M and the length of the

operating range EG, as measured on

the contour, or expressed as 2j3 inangular measure. Recommended

alues for (3 are from 30 to 45

degrees.

Example-A cam clamp is designedwith the following values:



In English Units

1 = 5 inches, r 2 = 0.25 inch

H = 0.84 inch, e - 0.081 inch

/i! = 0,18,ju 2 = 0.05, required P =

600 pounds

600 5 __

F "^ 0.081 + (0,18X0.84) +(1.03X0.05X0.25)

F = 29A pounds

(0.18 X 0.84) + (0.05 X 0.25) _

(FS) = efficiency =



0.081

0.081

■= 2.02

0.081 + (0.18 X 0.84) + (1.03 X 0.05

X 0.2S) X 100 = 33.0%

CLAMPING ELEMENTS

Ch. 10

In SI (Metric) Units

L = 125mm r% — 6mm

ff= 21mm e = 2mm



Hi = 0.18, ju 2 = 0.05, required P =2670N 2670N 125

The slope of the tangent and thenormal to the curve is determinedby

F ~2 + (0.18X 21) +(1.03 X 0.05 X6) ItSF = 2670 X (2 + 3.78 + 0.31)I25F= 16260 F= 130N FS =

(0.18X21) + (O.Q5X6) ^ % Q4

efficiency =

2 + {0.18 X 21)+(1.03 X 0.05 X 6) =32.8 percent

X 100



SPIRAL CAMS

Fixture clamping cams are formed

as Archimedes spirals or as circulareccentrics.

The fundamental equation for anrchimedes spiral (see Fig. 10-4) is

where the angle 8 is in radians, and

/ is the lead, that is, the increase inradius vector r for one revolution.



Fig. 10-4. The mechanics of therchimedes spiral cam.

(The angle equivalent of onerevolution is 8 = 1m radians which,when substituted in the aboveequation, gives r = I). The rise is thencrease in r along a given length of

contour. The rise over the entireoperating range is the "throw."



tan a —

dr rdd

I 2nr

The angle a is equivalent to a wedge

angle. While / is constant, r and aary with 6; a increases with I and

decreases with increasing r. For

fixture clamping cams the variationof the angle a over the useful lengthof the cam is small, usually only around 1 degree. It is not difficult to

make the cam self-locking over awide operating range. There is noow or high point on a spiral.



For practical applications it isconvenient to measure 8 from afixed point A with radius vector t 0

This modifies the equation to

W « + S«

t an arbitrary point D we have

e = r sin a H = r cos Of

There are several empiricalrecommendations for the selectionof the lead / for fixture clamping

cams. One such recommendationsays 0.O01 inch (0.025 mm) perdegree per inch radius which, on a

2-inch (50-mm) cam radius, gives /



= 0.72 inch (18.3 mm) and tan a =0.0573. This cam is self-locking forti = 0.06. Another recommendation

says that the throw over 90 degreesshall be 1/6 times the radius. Thisgives f=2/3r and, when applied to a

1 1/2-inch (38-mm) radius, tana =0.106. This cam is intended to beself-locking for /J = 0.1.

Eccentric Cams

The circular eccentric cam (see Fig.10-5) is characterized by theeccentric radius R and the fixedeccentricity E. This cam has low andhigh points A and B. The point of

contact D, is defined by r and 8 as



before, height H and eccentricity ehave, likewise, the same meaning.For calculating // and e it is

convenient first to find angle \j/,which is done by

sin jg

sin (180-0) R

sin $ = — sin 6

Ch. 10

CLAMPING ELEMENTS

111

n Table 10-3. The location of these



ranges is shown in Fig. 10-6.

Fig. 10-5. The mechanics of the

eccentric cam.

n auxiliary angle 0 is neededtemporarily; it is found by

9O + 0 + (l8O-0) + ^= 180 0 = 0-^-90

Then

tf - R = E sin 0 = —£ cos (0 - 4>) H

= R-Ecqs(0~t1j) e = E cos 0 = £ sin(0 — i/0

It should be noted that e is



ndependent of R and depends onlyon the fixed eccentricity E and theangle. 8 — $ is the cam rotation

angle. ^ is always a small angle andf = 0 degrees at 8 - 0 degrees and Q= 180 degrees, e is zero at 0 degree

and 180 degree cam rotation andhas a maximum e = E at 90-degreecam rotation. $ has here itsmaximum value determined by



Fig. 10-6. The location of self-ocking ranges on an eccentric cam.

Table 10-3. Circular Eccentric Cams

Self-Locking Operating Range for fi= 0.1

sin 4/ = ^



Since tan tf> is the slope of thetangent, it follows

that the cam is least likely to beself-locking in this

position, or, if it is self-locking inthis position, it is

always self-locking,

£ For (1 = 0.1 a cam is self-lockingfor — = 0.09961

2R or -7T = 20.8. The preferredoperating range is sym-h

metrical with respect to the position



for maximum e.

2R The self-locking range for other

alues of -=• is listed

The Mechanics of Toggle Clamps

Toggle clamps are linkage operatedclamps and are based on the samekinematic principle as the eccentric

clamp but with widely differentdimensions of the moving parts.They possess enough elasticflexibility to allow the actuated link

to pass through dead center. A positive stop just beyond that pointdefines the locking position and the

resistance at dead center secures



the link in that position.

t dead center the initial

eccentricity e equals zero and, inthe absence of friction, themechanical advantage equals

nfinity. This holds for any eccentricam, including toggle clamps.Mathematically

CLAMPING ELEMENTS

Ch. 10

this means that the clampingpressure P becomes in- begenerated by an infinitely small

actuating force finity for any finite



alue of the actuating force F, F. Inreality, we have neither infinitely arge nor or conversely, that a finite

clamping pressure P can infinitely small forces, and the mathematicalmodel



- -6P

S~

zrp-ffi



Fig. 10-7. The action of the toggleclamp, a. A toggle clamp of thepush-pull type in the open and

closed position; b. The force systemn the toggle clamp at the moment

of clamping.

Ch. 10

CLAMPING ELEMENTS

113

means simply that a small (but

finite) actuating force Fcangenerate a large (and still finite)clamping pressure P, as indicated in

Fig. 10-7a.



The technical interpretation of these seemingly paradoxicalstatements is, that a perfectly rigid

eccentric cam actuated by a finiteforce can operate only to the pointof positive contact with the part to

be clamped. The clamping iseffective if this point is near deadcenter. If it is at dead center, thenthe clamping can only be effective

when all dimensions in the systemare mathematically accurate; acompletely unrealistic assumption.

In mechanical language, the systems statically indeterminate; the

height of the part to be clamped isthe redundant dimension.



ctually, there are only twopossibilities, either the cam clampsbefore dead center or it slips

through. Toggle clamps are notdesigned with the same rigidity assolid cams, and their inherent

flexibility drastically changes theforce conditions during theclamping operation. After contacthas been established (a short

distance before dead center)pressure builds up and the entiretoggle mechanism is elastically

deflected. The maximum deflectionand corresponding maximumpressure occur simultaneously asthe actuating link passes through



dead center; the pressure isregulated by means of an adjustingscrew in the pressure pad or

elsewhere in the linkage system.The actuating force required formoving the link from the point of

nitial contact to dead center issmall, because the mechanicaladvantage is large; on the deadcenter, where the mechanical

advantage is infinity, the actuatingforce would vanish if themechanism were frictionless. The

actuating force necessary on deadcenter is the force required toovercome the frictional forces onthe pivots and bearings, and is



calculated as follows: Maximumpressure P produces friction forcesas shown in Fig. 10-7b. Since P is

many times F (example: F = 15pounds, P = 1000 to 2000 pounds),t is permissible to ignore any

transverse reactions from F. Theactuating force is transmitted to thepressure link by a direct force F t ,so far unknown, and a friction

force. First, consider the forces onthe pressure link. Taking momentsabout the center of the right-hand

pin gives

F l B + l±P(,B~R)=pPR F l B =nP(2R -B)



Fj may well come out negative. Thiss not disturbing; it means simply

that the friction force is significant

n the transmission of the actuatingforce. Now, consider the forces onthe operating lever. Taking

moments about the center of itsbearing pin gives FL =F t A +^PR +fiP(A +R) = 2 t iPR A ~=2nPR{j+l

Note that the ratio of F and P doesnot depend on the individualengths A and B, only on their ratio

In English Units

Example-A toggle clamp has: L — 12



nches, A = 3/4 inch, B = 2 inchesand R = 1/4 inch. Assume fi = 0.15,Then

FX 12 = 2X 0.15 X?X 1/4 X (3/8+ 1)

-= = sa»l 16

F

In SI (Metric) Units

Example-A toggle clamp has: L =300 mm, A = 19 mm, B = 50 mm

and D = 6 mm. Assume fx = 0.15,Then

FX 300 = 2X 0.15XFX 6X



-= 120.8 ss 121 F

19 + 50 SO

The Mechanisms of Beams

Straps are beams and are loaded in

bending. The loads are the appliedforce F, the clamping force P and areaction R at the point of support.

The application of straight straps asclamping elements in fixturesncludes the five different force

arrangements shown in Fig. 10-8, a

through e. The angle strap is shownn f.

In the design and stress analysis of



a strap clamp it can be assumedthatF is known, and it is required tocalculate the applied force F, and

the maximum bending moment M,which always occurs at the load thas located in the middle part of the

strap. The formulas for F and Mare:

Case (a)

F~ L 2

M = RLi =P(L 2 -£,) = F-

I

77\



~i

t:

T.

CLAMPING ELEMENTS

I

WA

J R 1 R r

wx



t^ztz^zi '

Ch. 10

T L L i i H pT— l 2— I M £

f

Fig. 10-8. The mechanics of thebeam type strap and angle clamp.

pplied force; F, F lt F 7 ; clamping



pressure: P; support reaction: R, R 1t R 2 .

Case (b)

*■'

M = F

L V L 2

Case (d)

F L 2

M = FLi =PL :

Case (c)



Case (e)

Fx+F 2 l

F L 2

(Li L 2 )L 2

Ch. 10

CLAMPING ELEMENTS

115

( use ff)

17

M = FL t =PL 3



Cases (b) and (c) are normally usedwith L] = L 2 -All six cases can bescrew, cam, or mechanically

actuated. However, screw and camactuation is more common withcases (a), (b), (c), (d), and (f);

mechanical actuation is morecommon with cases (d), <e), and(f).

In case (a) the force ratio (themechanical advantage) is less than1; only cams (d), (e), and (f) have

the possibility of a mechanicaladvantage greater than 1, It isusually desired to make themechanical advantage as large as

possible. Therefore, in case (a), F



should be as close to P as possible,and in cases (d) and (e), F should bas close to R as possible.

Economics

Table 10-4. Representative AverageOperating Time for ClampingDevices

(Time is for clamping only; does nonclude release time)

The clamping devices used in

connection with jigs and fixturesmay either clamp the work to the jior the jig to the work, but very

frequently the clamps simply hold a



oose or movable part in place inthe jig, the part can then be swungout of the way to facilitate removin

and inserting work in the jig. Thework, in turn, is clamped by a set-screw or other means passing

through the loose part, commonly called the "leaf."

M ost clamping devices have some

basic features in c omm on: 1, Theyare made from high-strength

material; 2. Contact surfaces aremade wear-resistant; 3. They aredesigned for quick operation; and 4When released they can be moved

clear of the working area.



When making a sRlp.rtion_betweentwo or more ili£-ferent types of clamp' the^fixture designer must

weigh savings mjaperarirtgjimeagain^Tiabricating (or purchasing)cost. Time-study data are valuable

for this pur pose, if available; if notcomparative TstinTates can bebased on average empirical valuesfor the" com mon~claTnpfhg

operaTionsT"XT isToTsuch v aluess given in Table-10-4. Actuating aalve or "aswifch for an air or

hydraulically powered clamp takesabout 25 percent of the timerequired for manual clamping of the same part, Cams are faster to



operate than screw clamps, but arealso more expensive. Cams arepreferred for large series and short

operations. With operations lastingmore than 5 minutes, theadvantages of the cam clamp

become insignificant.

Example—An operation of 5minutes duration requires either a

screw clamp costing $12.00 or acam clamp costing S50.00.Clamping time for the screw clamp

s 0.16 minute, for the cam clamp,0.07 minute. Release time is 50percent of clamping time. Laborplus overhead is S10.00 per hour.



Higher cost of cam: $50.00 - SI2.00= S38.00

Savings in time per part with camoperation: (0.16-0.07) X 1.50 =0,135 minute

Savings per day in $ (disregardingosses, "breaks," etc.):

^r^X^jp-X $10.00= $2.16

3 Ott

Time required for break-even:

_ " = 17.6 « 18 working days 2.16

Production required for break-even



18 X 8X^= 1728 parts

Clamping Screws

Clamping fasteners (screws andnuts) fall into two groups; fastenersturned with a wrench, and hand-tightened fasteners. Fasteners forwrench operation

CLAMPING ELEMENTS

Ch. 10

are set-screws with hexagonal headcollar-head (square-head) screws,and socket-head screws. Nuts usedare hexagonal nuts, hexagonal



flanged nuts, and acorn nuts.Collars, flanges, and washers areused to spread and reduce the

pressure. Spherical washers areused to equalize the pressure whenclamping on irregular contacting

surfaces. Acorn nuts are used toprotect the thread against damageand dirt. The height of hexagonalnuts should be 1 1/2 times standard

height to reduce the load on thethread.

The acorn nut can be modified asshown in Fig, 10-9. The purpose of this design is to permit lifting thewrench off the "hex," and moving

t'back for a new grip. The round



part of the nut serves to keep thewrench in place to be slipped back onto the hexagon nut, while the pin

at the top of the nut makes thewrench an integral part of thefixture so that it cannot get lost.

Fig. 10-9. Acorn nut for clampingscrew, modified for retention of wrench.



Machine steel of 60,000 psi (414N/mm 2 ) tensile strength issatisfactory in cases where the load

s light; however, most fasteners foclamping purposes are made fromsteel with 120,000 to 150,000 psi

(830 to 1035 N/mm 2 ) tensilestrength. Socket-head screws, areusually made from a 185,000 psi(1275 N/mm 2 ) tensile strength

steel. They are now widely preferredbecause of their strength andreliability, the fact that they require

ess space than older types of fasteners, and because they aresafer in operation; the wrenchcannot slip when it is properly



nserted in its socket.

Hand-tightened fasteners are

designed to provide a good grip forthe hand and at the same timeeliminate the need for a wrench,

which could result in overloadingthe clamp or the part. Simple andnexpensive hand-grip fasteners

that do not require purchase of

commercial items are shown in Fig10-10,

C £I>

=#>



Fig. 10-10. Hook bolt and hand-

actuated fasteners.

special type of bolt used in drilligs is the "hook" bolt shown at left

n the illustration. It is very cheapto make and easily applied. The bol

, passes through a hole in the jig

(having a good sliding fit in thishole) and is pushed up until thehook or head B, bears against thework; the nut is then tightened.

When great pressure is not



required, a thumb- or wing nutprovides a better grip and moresatisfactory means than the knurled

nut (shown at right), for tighteningdown upon the work, and permitsthe hook-bolt to be applied more

readily. When work is removedfrom the jig, using the hook-boltclamping device, the nut is loosenedand the head, or hook, of the bolt is

turned away from the work, thusallowing the workpiece to be takenout and another placed in position.

Figure 10-11 shows an applicationof a bent hook-bolt. Generally speaking, the type shown in theprevious illustration is better suited



to its purpose, as the bearing pointon the work is closer to the boltbody and it can be drawn more

tightly.

Fig. 10-U. Clamping with hook bolts.

screw with a pin through the headcan be used for light clamping, but

s inconvenient to work with. Moreconvenient is the knurled circularnut shown previously, and alsoscrews with knobs of the same type



Because of the better grip obtained,greater force can be applied by theuse of wing-nuts, wing screws, and

hand knobs of various shapes, usedas screw heads and nuts. Anapplication is shown in Fig. 10-12.

These fasteners are allcommercially available. Hand knobcome with 3, 4, and 5 lobes and in arange of sizes. The amount of force

which the operator can apply to theknob increases with its size. Withrespect to shape, test results show

that

Ch. 10

CLAMPING ELEMENTS



117

mended for general use as thescrews are severely loaded inbending, but as a last resort, it may

be acceptable in cases where only ight cuts are taken.

Fig. 10-12. Clamp tag screw with

hand knob.



the 4-prong knob best fits theanatomy of the human hand.

If screws are to be firmly tightenedwithout the use of a wrench, themethod of using a pin through the

screw-head can be used on largefixtures. The pin is 1/2 inch (13mm) in diameter, or more, andrequires the use of both hands (see

Fig. 10-13). A more sophisticatedersion is the speed nut, a

commercial component consisting

of a nut with one or two armsterminating in ball knobs. This isfor two-hand operation and permitsfast spinning and hard tightening.



Fig, 10-13. Clamping screw with prohandle.

The use of manually operated

screws, however, does notcompletely eliminate the possibilityof overloading the clamp. Fullsafety against overload can beachieved by the use of torque headscrews. The screw has a knurledhead with a built-in spring-loaded

clutch that slips automatically when



the maximum load is applied. Manyscrew fasteners are, or can be,provided with swivel ing pads for

even pressure distribution andprotection of the surface of the part

Examples of the direct applicationof screws to substitute for the useof clamps are shown in Fig. 10-14.The method shown in diagram a is

simple and self-explanatory. Themethod shown in diagram brequires screws with conical tips

and is used in milling fixtures foright milling operations; the part isclamped horizontally and verticallyat the same time. The method

shown in diagram c is not recom-



Fig. 10-14. Direct clamping withscrews: a. The general method; b.

n inexpensive method suitable for

ight-duty operations; c, A clampingmethod suitable for light-duty onlynot recommended for general use.



Jack Screws

The o rdinary jack screw is

frequently employed as a"supporting device in ordinary setups on a machine tool, but rarely

n fixtures as it is a loose partartd~ls~quite apt to get lost. In Fig10-15 two simple deviceTamhbwn,that work Oft the same principle as

the jack screw, but they have theadvantage of



Fig. 10-15. Swing jack screws.

CLAMPING ELEMENTS

Ch. 10

being connected to the jig by pin B.t A, a set-screw screws directly nto the end of the eye-bolt, and at

C, a long square nut is threaded on

the eye-bolt. These nuts must be of



a particular length, and madeespecially for this purpose. The eyebolts are fastened directly to the

wall of the jig, and the set-screw, ornut, is tightened against the work.The eye-bolt can be set at different

angles to suit the work, thereby providing a means of doubleadjustment. This is a very convenient clamping device which

works satisfactorily and can beeasily swung out of the way. Severatypes of jack screws for permanent

mounting in fixtures arecommercially available.

Straps



the pressure of the screw,

3*

r~rr

C



n m

■£4 ft

Fig. 10-16. Strap clamps.

t would be certain to bear at theends and exert the required



pressure on the object beingclamped. The strap is also providedwith a ridge at D, located cen-

^^

Fig. 10-17. A self-adjusting strapclamp.

trally with the hole for the screw.

This insures an even bearing of thescrew-head on the clamp, even if the two bearing points at each endof the clamp should vary in height,



as illustrated in Fig. 10-17, left. Theclamp at a in Fig, 10-16 would notbind very securely under such

circumstances, and the collar of thescrew might break off as the entirestrain, when tightening the screw,

would be put on one side,

further improvement in theconstruction of this clamp may be

had by rounding the underside of the clamping points^ (Fig. 10-17,right). When a clamp with such

rounded clamping points is placedn a tilted position it will bind theobject to be held fully as firmly as ifthe two clamping surfaces were in

the same plane.



The hole in these straps is very often elongated, as indicated inFigs. 10-16 a and b by the dotted

ines, which allows the strap to bepulled back far enough to clear thework; making it easier to insert and

remove the piece to be held in theig. In some cases, it is necessary to

extend the elongated hole, as shownn Fig. 10-16c, so that it becomes a

slot (going all the way through tothe end of the clamp) rather thansimply an oblong hole. Aside from

this difference, the clamp works onexactly the same principle as thosepreviously shown.

To suit different conditions, instead



of having the strap or clamp bear ononly two points, it is sometimesnecessary to have it bear on three

points, in which case it may bedesigned similarly to the strapshown in Fig. 10-16d. In order to ge

equal pressure on all three points, aspecial screw, with a half-sphericalhead may be used to advantage. Thehead fits into a concave recess of

the same shape in the strap. Whenthe bearing for the screw-head ismade in this manner, the hole

through the clamp must have agenerous clearance for the body of the bolt.

When designing ciamps or straps of



the types shown, one of the mostmportant considerations is to

provide enough metal around the

holes, so that the strap will standthe pressure of the screw withoutbreaking at the weakest place,

which, naturally, is in a linethrough the center of the hole. As arule, these straps are made of machine steel, although large

ciamps may occasionally be made ocast iron.

Ch. 10

CLAMPING ELEMENTS

119



t e and f in Fig. 10-16, bent clampsand their application to theworkpiece are shown. These clamps

are commonly used for clampingwork in the planer and millingmachine, but are also frequently

used in jig and fixture design aswell.

Screws used for clamping these

straps are either standardhexagonal screws, standard collar-head screws, or socket-head screws

When it is unnecessary to tightenthe screws very firmly, thumb-screws or screws with hand knobsare frequently used, especially on

small jigs as explained earlier.



The simple strap clarnp is now commercially available in a largenumber of very sophisticated

modifications, comprising single-and double-end straps, center andrear-end applications of the

clamping screw, and fixed andadjustable height end support.Without exception the strap has anelongated hole to permit

withdrawal from the work area, anda spring to keep it in the liftedposition when released. Strap

clamps are also made with camactuation instead of screw actuationfor quick clamping, and also withautomatic withdrawal from the



work area.

Strap Clamp Applications

simple and common tyne of fixture r equires clamping with oneor two screws only at the center of the part, combined with easy accessto the work area from above. Thesolution is a clamping screw (or

screws) located at the center of aremovable strap clamp, straddlingthe part. The strap, or clamp, isarranged as shown in Fig. 10-18, thescrew passing through it at thecenter and bearing upon the work,either directly or through the

medium of a collar or a swiveling



pad, fitted to the end of theclamping screw. T his type of clamping arrangement is eom-

monly used for holding work in adrill jig. The strap

used in this type of arrangementcan be improved upon by making itn one of the forms shown in Fig.

10-19. Here the ends of the straps



are slotted in various ways, to maket easy to remove the strap quickly,

when the work is to be taken out of

the jig.

tft

-lb-

\)

pi

-<E3 —f—^

§=5=u]

Fig. 10-19. Strap clamp with slots

for easy removal.



nother way of making the strapremovable is to support it ingrooves in the fixture wall so that it

can slide when the clamping screw s released. Two ways of doing this

are shown in Fig. 10-20, Shown in

Fig. 10-20a is a strap that can slideengthwise through slots in the

fixture walls; the slots must haveclearances below the strap to allow

for passage of the screw. In Fig, 10-20b is a strap that slides in groovesn the fixture walls.

~i r

D"



Rfe. 10-18. Strap clamp with centerscrew.

Fig. 10-20. Strap clamps, sliding, foreasy removal

The clamping pressure can betransmitted to the center of the parby means of the strap, with or

without a pressure pad. Examplesare shown in Fig. 10-21. Diagram a,shows a strap with slots for easy removal. The strap shown in

diagram b, is clamped



CLAMPING ELEMENTS

Ch. 10

Fig. 10-21. Strap clamps with endscrews.

by means of swinging bolts, aprinciple that has numerousapplications. The strap shown in

diagram c, is clamped by means of bolts with C-washers under thenuts and large bolt holes. When thenuts are released, the C-washers ar

easily removed, and the nuts canpass through the holes. The C-washers as shown are loose pieces.

Loose pieces in a fixture are not



desirable as they may be lost. In thepresent case this condition can bemproved by the use of swinging C-

washers. The C-washer is pivotedaround a shoulder screw; when thenut is released, the C-washer is

rotated out of engagement and thebolt can be withdrawn. Swinging C-washers with their pivot screws arestandardized and commercially

available.

ngular Clamps

ngular clamps are those thatredirect the clamping force. In moscases, the force direction is rotated

90 degrees so that, for example, a



ertical clamping screw generatesahorizontal clamping force on thepart. The clamping force is "'turned

around a corner." Some of thesedevices have the double ef-

fect of simultaneously clampingertically and horizontally. Angularclamps are primarily necessitatedby narrow space and restricted

access conditions in the fixture.

The basic, and simplest, form for anangular clamp is the angle strapshown in Fig. 10-8f. Mechanically,t is a bell-crank lever. There are

several other means by which the

clamping force can be redirected 90



degrees or some other angle. Themost important of these are thehinged strap (which is a modified

ersion of the bell-crank lever), theinked strap, the combination of a

strap with a wedge, and the

combination of a strap with aplunger with a 45 degree inclinedend surface. A widely used clampindevice, the "gripping dog" is, in

effect, also a modification of thebell-crank lever. The number of possible combinations is too large

for a systematic clas-



HEIGHT OF PIVOT ABOVE

CLAMPING POINT

PjIflD o



Fig. 10-22, Clamping with "gripping

dogs.'

Ch. 10

CLAMPING ELEMENTS

121

sification, but the principles andtheir application will bedemonstrated by representativeexamples. Irregularly shaped



castings which must be machinedoften present no apparently goodmeans of holding by ordinary

gripping appliances for drilling,shaping, or milling. In such cases,gripping dogs, as illustrated in Fig.

10-22, may be used. The basic types the one shown in diagram a. The

base block C is inserted in the T-sloof the machine table with a sliding

fit and is prevented from slippingbackwards by the backstop F, whichs firmly bolted in position. The

base block is slotted to receive theaw D which is fulcrumed on a

cross-pin. In the tail of the dog aset-screw E is threaded. By turning



this set-screw the jaw is caused to"bite" inward and downward at thesame time, firmly gripping the

casting and forcing it down on thetable. Since the position of thebackstop is adjustable, the same

gripping dog can be used forcastings of different sizes. A gripping dog can also be solidly mounted on a fixture plate,

eliminating the need for a backstop



Gripping dogs have a tendency to

draw the work down firmly and



forcefully onto the rest-pins, orstops, and are useful in all classesof fixtures. A different type is

shown in diagram b. Care should betaken to see that the stop is pivotedabove point A. Another, and more

rigid, device is shown at c. Plunger4, carried in plunger B, is forced

down against the 45-degree side of stop C, compressing spring D. A

fixture that provides two clampswhich exert a "down-and-in"pressure is illustrated in Fig. 10-

23a. Slides B are equalized by strapC and ball-and-socket washers Dand E. This fixture is useful formilling and profiling, as the clamps



and stops are below the surface of the work. A modification of thisfixture is shown in diagram b. It ha

two down-and-in equalized clampsfor holding a round piece of boredwork for a milling operation. Lever

is tapped to receive screw B, andthe clamping pressure equalizeswith lever C by means of rod D.Levers A and C impart a down-and-

n pressure to plungers E.Equalization is necessary as thework is already centered on a

circular locator. This fixture can beapplied to flat work.

In the double-movement clamp

shown in Fig. 10-24, clamp A is



carried by hinge B, pivoted at C.Screw E gives clamp A a down-and-n movement by means of a 45-

degree taper on the contoured andhardened block D, which is alsomilled off at F to



I 7 ig. 10-23. Clamping with "down-

and-in" pressure.

MM IT PIN

Fig. 10-24. A clamp with double



movement.

CLAMPING ELEMENTS

Ch. 10

give the clamp sufficient movemen

to remove the work.

The combined action of a strap and

wedge simultaneously produceshorizontal and vertical clamping. Its simple, strong, efficient, and fast

and has many applications. Figure

10-25 shows a fixture

bears against and clamps the part,as shown in Fig. 10-26b. 2



n example of the use of plungerss shown in Fig. 10-27. Plungers A

and B are built into the strap and

are actuated by means of screw Cwith hand-knob D. In this way it ispossible to reach, with the

\"\g. 10-25. Double strap clampswith wedge action.

where two such straps are clampedby one screw, also resulting in a

centralizing action. The wedge end



of the strap can bear against amating surface on the fixture, asshown in Fig. 10-26a, 2 or the

configuration can be reversed sothat the wedge end

Fig. 10-27. A plunger-actuatedclamp for reaching into anotherwise inaccessible area.



end of the strap, an otherwisealmost inaccessible place in the

workpiece. Another example, Fig,



10-28, shows a small clampingdevice used when drilling rivetholes through beading A and plate

B.

Fig. 10-28. A small clamp withdouble action for drawing andclamping.

Steel bracket C is fastened by



screws to the side of the fixture; thefront face of the clamp bracket isused as a stop for the plate and the

beading; and

Courtesy of E. Thaulow Fig. 10-26.

Single strap ciamps with wedgeaction.

E. Thaulow, Maskinarbejde

(Copenhagen: G.E.C. Gad's Forlag,1930) vol. II.

Ch. 10

CLAMPING ELEMENTS

123



clamp D, with a small hole drilled inone end, is fitted loosely in themilled slot in the bracket. The set-

screw is located a little higher thanthe hole in the clamp and, by a few turns of the screw, the clamp is

brought down against the work andforces the beading up against thestop, ready for drilling.

Swinging Leaves

"leaf" or a "swinging leaf" is ahinged cover on a box-shapedfixture. It is normally used on drilligs. The elementary principlesnvolved in the swinging-leaf

clamping construction are shown in



their simplest form in Fig, 10-29.Loose leaves

fig. 10-29. The general principle of the swinging leaf clamp.

which swing out, in order to permitthe work to be inserted andremoved, are usually constructed insome manner similar to that shown

n Fig. 10-30 in which A representsthe leaf, being pivoted at B and heldby a pin at C, which goes throughthe two lugs on the jig wall andpasses through the leaf, thusbinding the leaf and allowing thetightening of set-screw D, which

bears against the work. The holes in



the lugs of the castings are linedwith steel bushings

Fig. 10-30. Swingleaf clamp withcenter screw.

n order to prevent the cast-ironholes from being worn out too soonby the constant withdrawal andnsertion of the pin. This kind of eaf, when fitted in well, is rather

expensive, but is used not only forbinding but also for guiding

purposes, making a convenient seat



for the bushings. If leaves are fittedwell into place, the bushings in theeaves will guide the cutting tools in

a satisfactory manner.

nother method of clamping down

the leaf is shown in Fig, 10-31, inwhich A is a thumb-screw, screweddirectly into wall B of the jig, andholding leaf C down, as indicated.

The thumb-screw is a quarter-turnscrew. To swing the leaf out, thescrew is turned back about a

quarter of a turn, so that the headof the screw stands in line with aslot in the leaf, which is both wideand long enough to permit the leaf

to clear the head of the screw. This



s a very rapid method of clamping,and is frequently used.

4

fl

€-1

B

Fig. 10-31. A quarter-turn screw



used for locking a swinging leaf clamp.

The lower side of the screw headwill wear long before the headfinally turns in line with the slot

when binding. It can then easily befixed by turning off a portion of thehead on the underside, so that itwill bind the leaf again when

standing in a position where thehead of the thumb-screw is at rightangles to the slot. The size of these

thumb-screws is made according tothe strain on the leaf and the sizeand design of the jig.

The hinged or latch bolt, shown in



Fig. 10-32 is also commonly used.Here,j4 represents an eye-boltconnected with the jig body by pin

B. The leaf or movable part C of theig is provided with a slot in the end

for the eye-bolt, a trifle wider than

the

Fig. 10-32. An eye-bolt used for

ocking a swinging leaf clamp.



CLAMPING ELEMENTS

Ch. 10

diameter of the bolt. The threadedend of the eye-bolt is provided witha standard hexagon nut, a knurled-head nut, or a wing-nut, accordingto how firmly the nut must betightened. When the leaf is to be

disengaged, the nut is loosened justenough to clear the point at the endof the leaf, and the bolt is swungout around pin B, which is drivendirectly into lugs projecting fromthe jig wall; a slot being providedbetween the two lugs, as shown, so

that the eye-bolt can swing out



freely. At the opposite end, theeaves or loose parts of the jig swing

around a pin, the detailed

construction of this end being, moscommonly, one of the three typesshown in Fig. 10-33. It must be

understood that to provide jigs witheaves of this character involves a

great deal of work and expense, andthey are used almost exclusively

when one or more guide bushingsare held in the leaf.



Fig. 10-33. Details of leaf hinges.

hinged jig cover may also be

conveniently held in place by asemiautomatic spring latch of thetype shown in Fig. 10-34. The body of the jig is shown

n

L_i_ i—



_ i "offll

LB

M==t

IE

■>-■"--

S:

-h

7T

S-

at A; the hinged cover at B. This



When the leaf is used to transmitclamping pressure to the part, aatch-type lock is insufficient, and a

screw-type lock is required. Thequarter-turn screw (see Fig. 10-35)s used where the height of

Fig. 10-35. Leaf jig with quarter-

turn screw.



the part is within close tolerancesso that no significant heightadjustment is required. Should

some height adjustment benecessary, such as in a case wherethe leaf clamps upon a rough

surface, a clamping screw with aonger travel can be used. A typical

arrangement, using a swinging bolts shown in Fig. 10-36, Since, in this

case, the pari requires two clampingpoints; the clamp is pivoted so thatthe total clamping force is equally

divided between the



devices, leaves, and boxes arecommercially available.

"Clamping Mandrel-mounted Work

The "natural" and convenient way of holding small parts that havebeen bored through and faced onthe ends, is to mount them on amandrel and make the mandrel an

ntegral part of the fixture. When inposition, the part is clamped. Thesimplest way of clamping it is witha washer and nut on the free end ofthe mandrel. This is a cheap andreliable, but slow, operation,because the nut must be run off and

on each time a new part is placed in



the fixture. The time required forrunning the hexagon nut on and offs saved as shown in the design in

Fig. 10-37, using a quarter-turnknob. Stud B has a flat milled onboth sides of its threaded end

portion. The slot in knob A slides onover this flat and a quarter turnclamps the work. If the variation inthe length of the work is not too

great, this makes a rapid clampingarrangement.



^=3

c

Fig, 10-37. Clamping on a mandrelwith a quarter-turn knob.

Figure 10-38 shows another means



of clamping the same piece inwhich the variation in length of thework and the time required for

turning the knob to match the flaton the stud has been considered.The slotted washer A and knob B

are dropped over stud C; A is heldagainst B, which can then bescrewed up as freely as a solid knobThis can be used for a variety of

bushings of various lengths; stud Cbeing made to suit the longest pieceof work* Using a square or Acme

thread is recommended, since thesehave less tendency to tilt the nutthan would a 60-degree thread.

'ig, 10-38. Clamping parts of



arying length on a mandrel.

Cams, Eccentrics and Toggle

Clamps

^ Clamping cams, eccentrics andtoggle joint clamps differ fromclamping screws in that they aremore expensive to buy or make;they ar e faster to operate; a nd they

have only a short effec tiveclamping range. For a cam, theeffective clamping range is 1/8 inch{3-mrn>, tor a toggle joint clamp its 1/16 inch (l.S mm). As these

mechanisms close, they tend toexert a slight lateral movement to

the contacting surface. They are,



therefore, used primarily to operateon another clamping member, suchas a strap or leaf, rather than to

clamp directly on the part. Thereare extensive analyses of therelative merits of the spiral cam and

the circular eccentric cam, andndustry has made its choice: the

commercially available camcomponents are, as a rule, made

with circular eccentric cams. Theres little principal difference between

this type of cam and an eccentric

shaft; a toggle joint can beconsidered to be an eccentric devicewith a very large eccentricity. Inaddition, a toggle mechanism is



fairly elastic, and this featureenables the toggle clamp to movebeyond dead center when closing.

Eccentric shafts are often used formoving and closing a clamping

strap. In Fig. 10-39 two applicationsof the principle of the eccentricshaft are shown. In diagram a, theeccentric shaft A has a bearing at

both ends; the eye-bolt B isconnected to it at the center and isforced down when the eccentric

shaft is turned, causing the two endpoints of clamp C to bear on thework. This clamping arrangementhas a very rapid action with good

results. The throw of the eccentric



shaft may vary from 1/16 inch (1.5mm) to about 1/4 inch (6 mm),depending upon the diameter of the

shaft and the accuracy of the work.In cases where it is required thatthe clamp bear in the center, an

arrangement such as that indiagram b may be used. Here theeccentric shaft A has a bearing inthe center and eye-bolts B are

connected to it at each end. As theeccentricity

CLAMPING ELEMENTS

Ch. 10



Fig. 10-39. Clamping with eccentricshafts.

s the same at both ends, the eye-



bolts or connecting-rods will bepulled down evenly when lever C isturned, and strap D will get an even

bearing on the work in the center. Ithe force of the clamping stress isrequired to be distributed equally a

different points on the work, a yokemay be used in combination withthe eccentric clamping device.

When it is essential to use strap Dfor locating purposes, guides, whichare necessary for holding it in the

required position, must be providedfor the strap. These guidingarrangements may consist of rigidrods, ground and fitted into drilled

and reamed holes in the strap, or



square bars held firmly in the jigand fitted into square slots at theends of the strap. The bars may also

be round, and the slots at the endsof the strap half round, theprinciple in all eases remaining the

same; but the more rigid theguiding arrangement, the moreaccurate the locating.

The ordinary eccentric lever workson the same principle as theeccentric rods described above.

There are a great variety of eccentric clamping devicesfrequently used and commercially available in several different

models. For convenient and



efficient operation the cam oreccentric lever should be located sothat it is actuated by a straight

horizontal pull and rotated to itsposition of maximum

mechanical advantage with littlechange in body position. At any position of the handle it must,however, have a finger clearance of

at least 5/8 inch (16 mm). Figure10-40a shows a cam specially ntended for clamping finished

work. It is not advisable to use thistype of lever on rough castings, asthe castings may vary to such adegree that the cam or eccentric

would require too great a throw for



rigid clamping.

Fig. 10-40. A cam and an eccentricfor clamping, a. For clamping on thpart; b. For locking a leaf clamp.

The extreme throw of the eccentric

ever should, in general, not exceed1/6 of the length of the radius of theccentric arc, if the rise takes placeduring one-quarter of a complete



turn of the lever. This would give anextreme throw of, say, 1/4 inch (6mm) for a lever having 1 1/2 inches

(38 mm) radius of the cam oreccentric. It is obvious that as theeccentric cam swivels about center

, the lever being connected to theig with a stud orpin; face B of the

cam, which is struck with theradius^? from the center C, recedes

or approaches the side of the work,thereby releasing it from, orclamping it against, the bottom, or

wall, of the jig. The lever for theeccentric may be placed in any direction, as indicated by thebroken and unbroken lines.



nother eccentric lever, is shown indiagram b. It is frequently used onsmall work, for holding down straps

or leaves, or for pulling togethertwo sliding pieces, or one slidingand one stationary part, which, in

turn, hold the work. These slidingpieces may be V-blocks or somekind of jaws. The cam lever isattached to the jig body, the leaf, or

the jaw by a pin through hole A.Hook B engages the stud or pin C,which is fastened in the opposite

aw, or part, to be clamped to thepart in which this pin is fastened.The variety of design of eccentriccam levers is so great that it is



mpossible to show moie than theprinciples, but those exampleswhich are shown embody the

underlying action of all thedifferent designs.

Ch. 10

CLAMPING ELEMENTS

127

Intermediate adjustable supportsrequire a quick-acting, safe, and

hand-operated locking device. A cam-operated locking device forthat purpose is shown in Fig. 10-41.

The actual support member is the



plunger >1. loaded by spring D.Plunger .4 has a tapered (conical)shank while the binder plunger B

has a matching tapered flat. Whenthe fixture is loaded, spring D keepsplunger A up against the work; by

actuating cam C, the binder ispulled outward, and the tapered flatengages and locks the taperedshank on A. The double taper on

both plunger and binder makes itmpossible to press the plunger

down, away from the work.



Fig. 10-41. A cam-operated locking

device for an intermediate support.

The clamp is, by its design, a quick-acting device, This property can be

combined with other design



elements to produce devices thatperform more than one function inone stroke,

The quick-ac ting jig clamp, Fig. 10-42, has a handle A, threaded to fit

screw B, and a cam lobe E thatengages strap C. As handle A isturned, cam E applies pressure tostrap C. A movement of handle A of

approximately 90 degrees, producesthe clamping action on the work.This allows for a variation in the

thickness of the piece to beclamped, equivalent to one-fourththe lead of the screw threadadvancement. For example, with a

5/8-11 screw, a tolerance of plus or



minus 0.011 inch would be allowedn the

Fig, 10-42. A quick-acting, cam-operated strap clamp.

work. A groove is cut in the upper



surface of strap C, and when thestrap is loose, the cam rests in thisgroove (see sectional view). About

30 degrees movement of handle A s required to cause cam E to ride

on top of the strap, as shown by the

sectional view at the left.

The head of screw B has six grooves(lower right-hand corner), which

are engaged by set-screw D toprevent it from turning. To adjustthe lever or tighten the strap when

parts wear, screw B is turned to anew position and locked in place byset-screw D which also serves tokeep screw B from dropping out of

the jig. It is advisable to make a



positive stop for handle A, so thatthe cam will not fall into the grooveby a 180-degree turn and so loosen

the strap.

Work can be clamped with one

quick stroke, in the milling fixtureshown in Fig. 10-43, by a cam-actuated clamping device. Theworkpiece is shown secured

between the clamp and the formblock, ready for the millingoperation. It will be noted that the

cam is provided with a handlehaving a ball at one end. At thecompletion of the cut, the handle israised to a vertical position. This

causes a tooth on the underside of



the cam to enter a notch in the topof the clamp, thus moving it away from the form block and permitting

the part to be unloaded from thefixture. The ciamp is held in contacwith the cam by a spring-loaded

support finger which slides up anddown on a dowel-pin. Whenanother part has been placed on theform block, the ball is again lowered

to the position shown. The toothprovides a positive engagementbetween cam

CLAMPING ELEMENTS

Ch. 10



Spri/lf

Fig, 10-43. A cam-operated clamp

for quick withdrawal.

and clamp, moving the clamp to theeft, over the part. The cam surfaces

then force the clamp down on thepart, holding it securely during themilling operation. The weight of the

ball prevents the part from working



oose due to chatter or vibration.arious modifications of this type

of quick-acting clamp are


bayonet-lock is a type of cam and

the bayonet-lock type of clampingdevice, Fig. 10-44, is fast inoperation and positive. The bayonetslot is milled in ram C. and the

point of screw D (which is locked inplace by a check-nut), slides in it. Inoperation, the part is slipped over

stud A with one hand, while withthe other hand, handle fc, attachedto the ram, is pushed in and rotatedwith a single continuous motion.

The shoulder stud A, extends into



the work for about two-thirds of theength of the hole. This insures

accurate location of the work and

provides ample support against thethrust of the drill. The stud isflattened, as shown, to give ample

drill clearance. The revolving cap Bturns on a crown at the end of theclamping ram C and provides for aslight



amount of float to compensate forpossible variations in the work. Asclamp B remains stationary during

the actual turning or clampingmotion of ram C, scoring of the faceof the work is avoided. The drill

bushing F, in the jig illustrated, ispermanently fixed to the base.

Toggle clamps are commercially

available in so many types andmodels that they satisfy all, oralmost all, ordinary fixture

clamping requirements. They occupy quite a large space, howeverand the need for the design of aspecial toggle device arises when

the clamping device has to fit



within narrow space limits. Figure10-45 shows a clamping device of this category that has been found

useful on large work. It consists of four arms A with the ends bent to aright angle, and knurled, to bold the

work firmly in place. These armsare pivoted on stud B, and



Fig. 10-44. A bayonet-lock type of cam clamp.

SECTION X-X

Fig. 10-45. A multiple toggle clampactuated from the center of thefixture.

Ch. 10

CLAMPING ELEMENTS

129

their action is guided by blocks C,The spring handle B, is pinned tothe shank of the stud, and the uppe



edge of the handle is beveled to fitrack D, which is fastened to the sideof the base. By turning the handle

n the direction indicated by thearrow the work is securely clampedIf necessary, ordinary straps may be

added for holding the work.

The location of drilled hold-downbolt holes through the steam

cylinder heads for duplex pistonpumps was often inaccurate whenflat bushing plates of the same

shape as the casting were used asigs. These bushing plates wereequipped with vertical pads aroundtheir peripheries to form nests for

the castings. However, due to



ariations in the size of thecastings, many of the workpieceswould have a loose fit in the jigs,

resulting in inaccurate location of the drilled holes. To overcome thisdifficulty, the drill jig seen in Fig.

10-46 was designed to accurately clamp the work at four points by means of a duplex toggle action.

The two clamping arms A aremounted to slide on bushing plate Bby means of studs C; the central

portions of these studs passingthrough large holes in the arms topermit their free movement. Pins Dare a loose fit in the centrally

ocated projections on the clamping



arms, and their lower, enlargeddiameter ends are provided withflats to fit slots milled in the

bushing plate. This permits thearms to pivot about these pins andto slide along the slots when

operating handle E is rotated. CamF, which is rotated by handle Eabout stud G, is connected to

clamping arms A by links //. Theseinks can pivot about the loose-

fitting studs 3 joining, them to the

clamping arms and cam. A spring-oaded latch K holds the cam,evers, and arms in the work-

clamping position shown, or in the

oading position, when the cam is



rotated counterclockwise. As thecam is rotated counterclockwise,atch K will be rotated clockwise

and links H will become alignedwith each other. This forcesclamping arms A outward, away

from each other, so that the jig canbe placed over workpiece X'. Thecam is then turned clockwise to theposition shown, and arms A are

pulled together firmly to clamp thework for drilling. Ten holes 3/4 inchn diameter, are drilled through the

cylinder-head castings in thisoperation.

The Use of Adapters



Relatively inexpensive yet efficientfixtures are made by using acommercial work holder as the

fixture base, then attaching specialnserts to the jaws. Examples of athe chucks converted to fixtures

are shown elsewhere in the book (Chapter 9, Centralizes; Chapter 20Miscellaneous Fixtures). The mostcommon, versatile, and least

expensive work holder suitable forconversion into a fixture is themachine tool vise.



t i x /

Fig. 10-46. A duplex toggle actionclamp with four clamping points.

CLAMPING ELEMENTS



Ch. 10

The cheapest kind of milling fixture

that can be built is a pair of detachable vise jaws, as shown inFig. 10-47. Made of cold-rolled steel

and case-hardened, they arenexpensive. They can be removedfrom the vise quickly and replacedby other jaws. Detachable jaws are

widely used where great accuracy isnot required, such as when cuttingto length or milling clearance cuts.

The jaws shown here are used forcutting off pieces from a bar of stock, which is pushed up againstthe stop and then cut off to the

desired length. However, when the



aws are made with adequateprecision, and the vise is in goodcondition, this type of fixture can be

used for work with tolerances downto ±0.001 inch (±0.03 mm).

- --

T

H

Fig. 10-47. Detachable vise jaws forholding bar stock.

ccuracy is improved if thedetachable jaws are fastened to the

ise jaws by screws and also



secured in position by dowel pins.The fixed jaw is the fixture base. Itcarries the locators for the part, and

the machining pressure mustalways be directed against the fixedaw. With this simple device, the

ise has become a fixture of wideapplicability. The possibility of using a vise with inserts shouldalways be investigated in the early

stage of planning for a small part.

lmost all the rules of locating, and

many of the rules of centralizing,can be applied to the design of viseaw inserts. Round pans are held in-blocks. Detachable jaws can be

made larger than the vise jaws,



thereby expanding the capacity of the vise and, at the same time,reducing its rigidity. Precision

alignment of the two jaws isobtained by providing guide pins ormatching tongues and slots. Insert

faces can be machined to an anglefor parts requiring angular cuts.Parts can be located by stops, pins,and nesls. Inserts can be designed

to hold more than one part,equalizing yokes can be attached toa detachable vise jaw, and even

ejectors can be built in. While mostapplications of the vise with fixturenserts are for milling operations, it

can also be used as the base for a



drill jig by the addition of a drill jigplate with bushings. Various typesof air-operated vises are

commercially available; naturally,they offer

the same conversion possibilities asthe hand-operated vise and, inaddition, faster operation. The sames the case with cam-actuated vises.

Hydraulically operated vises offergreater clamping pressures thanany other type of vise; they are

available with a clamping force of up to 20 tons (178 kN).

Combinations of commercial work

holders can be used to advantage.



For example, a drill chuck, acting asa centralizer, can be fitted to be heldn a vise, thus acting as the fixture

base. A conventional drill chuck may be adapted to hold small work-pieces that might otherwise be

distorted if clamped directly in aise.

MISCELLANEOUS CLAMPING

METHODS

Magnetic Chucks

Magnetic chucks are available intwo main types, as a surface plate(see Fig. 10-48) 3 usually of

rectangular shape and as a face



plate (for mounting on a rotatingspindle) usually of circular shape.The body of the chuck can be

trunnion mounted for precision

)

Courtesy oft'. Thaulow Fig. 10-48,

Magnetic chuck of the surface platetype.

machining. For precision work the

chuck has a reference pin mounted



on one end and a matching flatreference surface on the base. Thedistance between the tw.o reference

surfaces is measured with gageblocks; in this way the chuck functions as a sine plate. The

magnet poles terminate flush withthe face of the chuck, and areseparated from the

E. Thaulow, Maskinarbejde(Copenhagen: Forlag, 1928) vol. I.

G.E.C. Gad's

Ch. 10

CLAMPING ELEMENTS



131

chuck body by strips of

nonmagnetic material (copper,brass, austenitic stainless steel,aluminum, or plastic) of a thicknes

of approximately 1/8 inch (3 mm).The polarity of adjacent polesalternates, or all individual poleshave one polarity, and the

surrounding face of the chuck hasthe opposite polarity. Eitherpermanent magnets or

electromagnets are used. Chuckswith permanent magnets have lessholding power than electromagnetichucks, but have the advantage of

not requiring an electric power



supply. As long as the chuck isempty, the magnetic circuit is openat the chuck face. When a

workpiece of a ferromagneticmaterial is placed on the chuck, thecircuit is closed. In electromagnetic

chucks the-circuit is permanently closed within the chuck; when thecurrent is switched on, the magnetsare energized, and the magnetic flu

passes through the workpiececlamping the work to the chuck.Chucks with permanent magnets

have mechanical devices foropening and closing the magneticcircuit inside the chuck; themagnets may rotate or slide in and



out of their closing position, or theyare connected at their lower

end by a sliding armature withnonmagnetic inserts whichnterrupt the circuit when the

armature is moved to the "off"position.

While most magnetic chucks are

purchased for use as general-purpose work holders, itoccasionally may be necessary todesign a special magnetic chuck forncorporation into a fixture, and

this requires estimating the holdingpower from some given data. The

holding force has two components,



the tensile holding force whichprevents the part from being pulledoff the chuck, and the shearing

holding force, which prevents thepart from sliding along the surface.The shearing holding force is

essentially a friction force and issignificantly weaker than thetensile holding power. The tensileholding force depends on the

strength of the magnets, theposition of the part relative to thepoles, the size of the contacting

surface, and the material,configuration, and surface quality of the workpiece.

The overall strength of the chuck is



expressed by its energy consumption in watts. Specifically,the strength of the individual

magnet depends on the

Table 10-5. Tensile Holding Force

ISI 1018 Steel on ElectromagneticChuck

CLAMPING ELEMENTS

Ch. 10

number of ampere-turns in its coil.

The significant material property isthe magnetic permeability. Low carbon steel has the highest

permeability of the common



materials acquire remanentmagnetism by this clampingmethod and must be demagnetized

There are several means formproving the performance of the

magnetic chuck. They can all bedescribed as field shapers, as they affect the shape of the magneticfield. Their purpose is to draw more

magnetic flux through the work either by increasing the area of access for the flux or by locally

ncreasing the flux density. Figure10-49 shows several such



devices. A is a block of steel called a

"binder block." It is occasionally called a locator although it does notperform the functions of realocators as they are commonly used

n fixtures. It collects flux from thechuck and delivers it to the work,thereby increasing the area of

clamping as well as the clamping



force. Binder blocks arc usedparallel to the work and also as endstops. B is a hold-down plate for the

purpose of clamping thinworkpieces C of nonmagneticmaterial; it is made of steel and

collects some flux, sufficient toproduce a clamping force on thework. The inserts D are flux dams;they are made of nonmagnetic

material and divert the flux in sucha way that the flux density is locallyncreased where greater flux

penetration is required. A fieldshaper of a different design isshown in Fig. 10-50.



-—-■■- J

f * '1 * ~~* lw<

Courtesy ofE. Thaulow Fig. 10-50, Afield shaper of the V-block type forholding cylindrical parts.

It consists of narrow elements with



thickness and spacing matching thepoles in the chuck. The elementsare separated either by air gaps or

by nonmagnetic spacers, andfunction as pole extensions. Fieldshapers of this type come closer to

the original definition of a locator.The design shown is a V-block forclamping cylindrical work.

Electrostatic Chucks

Electrostatic chucks (see Fig. 10-51)work on the principle of theattraction between surfaces withopposite electrical charges. Thechuck body consists of a ceramic

material with an additive to make it



a semiconductor. It is charged toabout 3000 volts from a powersupply while the workpicce "is

electrically connected with thechuck frame which is

Fig. 10-49. Section through amagnetic chuck with field shapers.

E, Thaulow, Maskinarbejde

(Copenhagen: Poilag, 1928} vol. 1.

G.E.C. Gad's

Ch. 10

CLAMPING ELEMENTS



DC OUTPUT

POWER SUPPLT

Courtesy of Electroforce, Inc.,Fairfield, Conn. Fig, 10-51. Anelectrostatic chuck.

permanently grounded. Theamperage of the current delivered

by the power supply is very low, jusenough to make up for any leakagethat may occur in the dielectricfilm. These features eliminate any

electrical hazard. The two surfacesare insulated from each other by ahard resin coating on the chuck

surface supplemented by a film of a



iquid dielectric to prevent air fromentering the interface. The samedielectric can also be applied to the

grinding wheel for cooling andubrication. A small heater

maintains the chuck at a

temperature a few degrees aboveambient temperature to preventcondensation of moisture.

In accordance with Coulomb's Law the attractive force is proportionalto the product of the charges and

nversely proportional to the squareof the distance between the twosurfaces. Since this distance issmall, the force is quite significant,

approximately 30 psi for clean and



smooth surfaces and independentof the thickness of the workpiece.Electrostatic clamping can be

applied to any conductive material;for nonconductivc materials it issufficient to give them a conducting

surface by spraying them with aquick-drying, conductive aerosolacquer, which can be dissolved

after the operation; or by mounting

them on a metal foil, with anadhesive.

dhesive Clamping

Clamping by means of primitiveadhesives such as resin, varnish,

and shellac have been used



presumably since the beginning of the machine shop trade. Moderntechnology has developed a number

of new adhesives for the joining of metallic surfaces. The use of adhesives for clamping in a fixture

s deceivingly simple but has anumber of drawbacks.

The method does not necessarily

provide precision clamping becausethe adhesive film has a finite andsignificant thickness that is not

always easily controlled. The curingof the assembly after adhesive hasbeen applied always takes sometime, it may be a few hours, or it

may be overnight. The adhesive



must have a matching solvent, sothat it can be removed, and theremoval is also a time-consuming

process. On the other hand, theadhesive must not be soluble incutting fluids. Considering all this,

adhesive clamping is selected only when no other acceptable methodcan be found, and is limited to partswith a flat clamping surface and to

ight machining operations.

acuum Clamping

Clamping by vacuum is a versatile,fast, and clean clamping method.There are no requirements with

respect to thickness or size of the



part nor to the electrical ormagnetic properties of the materialThe method is applicable to flat as

well as to curved surfaces and is noery particular with respect to

surface quality, since the clamping

area is sealed off all the way around. The method is distortion-free because the surface of the

acuum chuck is made to match the

work surface. The theoreticalmaximum clamping force is 14.7 ps(0.1 N/mm 2 ) with full vacuum and

s reduced in proportion if theacuum is less. The available

clamping pressure is sufficient formany machining operations on



aluminum, and vacuum clamping iswidely used in the aircraft industry

acuum fixtures are made fromcast iron or aluminum. A flatclamping face is machined; a curved

clamping face is cast to form, thenground and polished smooth, A netof crossing grooves is formed ormachined into the clamping face to

act as a manifold for distributingthe vacuum to the entire surface(see Fig. 10-52). The seal along the

edge consists of a rubber hose in agroove. Vacuum chucks for very thin sheets do not have grooves asthey



Sonar

□□□□[

Fig, 10-52. Design details of aacuum chuck.

CLAMPING ELEMENTS

Ch. 10

would leave permanent markingson the plates. Small parts can be

clamped on individual suction cups



shown at the right in Fig. 10-52; thecenter plug is machined so that itsupports the part without leaving

permanent marks.

Clamping with Low-melting Alloys

Low-melting alloys can be used forclamping parts of awkward shapes,arge overall dimensions, and thin

wall sections. The use of castmaterials for nesting purposes isdescribed in Chapter 6 and severalcast materials differing in meltingpoints and pouring temperaturesare listed. Most of these requireaccess to the facilities of a

nonferrous foundry. For clamping



purposes the preferred metal isCerro Bend®, a bismuth alloy which melts at 158 F and can be

handled in the machine shopwithout the assistance of a foundryOne technique consists in making a

set of nesting clamps. The partserves as the pattern; it is coatedwith a parting agent, and located ina split mold or flask similarly

coated with the parting agent. Themetal is poured, and aftersolidification and removal from the

mold, the metal block is machinedto fit the jaws of a work holder, orfixture. The block is cut in two orthree pieces, the part is removed,



fixture with a cast nest.

ng with three branches. The fixture

s screwed on a lathe spindle formachining. The nest is mounted ona three-position index plate by

means of which the three branchesare aligned in the spindle axis, oneat a time.

n entirely different technique canbe used on large parts that aredifficult to handle. A typicalexample is the fixturing of the fouregged tubular frame A shown in

Fig. 10-54. The part is to bemachined on the four flanges B.

The fixture consists of a base C, two



boxes D, two removable V-blocks Eand two screw jacks G. The boxeshave

E. Thaulow, Maskinarbejde(Copenhagen: G.E.C. Gad's Forlag,1930) vol. II.

Fig. 10-54. Direct clamping by

means of cast metal.



slots for the legs of the frame andserve as molds for the castingprocess. Additional equipment

comprises the two removablecentering arbors F. The arbors areplaced in the tubular legs; the

fixture is placed on a surface plateand the part is set up and leveled bymeans of the screw jacks and arborresting in V-blocks E. The slots

around the legs are sealed with claythe metal is poured and when it hassolidified the part is clamped. V-

blocks«nd arbors are removed, andthe loaded fixture is moved to themilling machine and clamped onthe table with clamps acting directly



on the cast-metal blocks. Aftermachining, the fixture is set in atray, hot water is poured over the

metal blocks, the metal melts awayeasily and is collected in the tray,ready for reuse.

nother, and more sophisticated,application is the clamping of gasturbine compressor blades for the

machining of the root. The shape ofthe blade is an air-foil which, intself, provides virtually no usable

clamping surface. The blade isnserted and located in the castingdie in a regular die-casting machineand a low-melting alloy is cast onto

and



Ch. 10

CLAMPING ELEMENTS

135

around the blade, thereby providing

a set of suitable clamping surfaces.

In the aircraft industry, honeycomb

n the expanded condition isclamped for machining by means ofce. With the honeycomb block

placed on a refrigerated platen, the

cells are filled with water. As the ices formed it supports the cell walls

and clamps the block to the platen.



RESUME AND CONCLUSION

When designing clamping devices,

the fewest number of operatingscrews or handles should be used awill accomplish the desired result.

Making the screw with a double ortriple thread is sometimesemployed to advantage indecreasing the number of turns

necessary to release the piece. Jigids should be hung on taper pins in

order to compensate for wear in the

hinge and to prevent any resultantnac-

curacy due to lost motion in the

hinge. The included angle of taper



on hinge pins should be only one ortwo degrees. The hinge pin shouldbe a tight fit in the central portion

of the hinge, which is usually the jigbody, and a bearing fit in the ears othe lid. All manually operated

clamping screws and similar partsshould be long enough and soocated as to be convenient and eas

to operate, and of sufficient size to

prevent hurting the operator'shands because of the manipulatingpressure necessary. The screws

should be located so that they willresist the tilting action of the blockand dowel pins should be fairly close to the screws and of liberal



dimensions in order to resist theshearing strains to which they willbe subjected. When clamping or

ocating the work in the jig, it isessential to have the clampingpressure exerted in a direct line

against some solid point of supportto prevent the tendency to tilt, andthe thrust should also come at sucha point of the work that it will be

resisted by solid metal.

CHAPTER

11

Equalizers



Definitions

The purpose of an equalizer is, in

the broadest sense, to distribute asingle force between two or moreseparate points of action. Strictly

speaking, the name implies anequal force distribution; but (hereare also equalizers that distributethe force in a constant ratio not

necessarily equal to one.Predominantly applied to clampingmechanisms equalizers are also

used on locators.

When an equalizer is applied to aclamping mechanism, the force to

be distributed is the "actuating"



strap with a screw in the center,clamping simultaneously on twoparts, as shown in Fig. 10-17, is an

equalizer, as is the three-prongedstrap shown in Fig. 10-16d. A typicaequalizer is shown in Fig. 10-36; it

s the balancing clamp in the centerof the fixture.

In Fig. 10-22a, an equalizing effect

s obtained by a gripping dog. Whenscrew E is tightened, the pressureon the work from the dog D, is

ncreased, as is the pressure on theocator below A. Strap C in Fig. 10-23a is an equalizer for the twoslides B. In the fixture shown in Fig

10-23b, the equalizer is rod D in



conjunction with screw B. Actuatingscrew B has the same effect as anncrease of the length of rod D

which pushes with equal pressureon levers A and C.

Clamp Sin Fig. 10-24 contains anequalizer, namely, the V-notch thatgrips over the corner of the work.The horizontal and vertical

pressures on the work increase inthe same ratio when screw E istightened, assuming that the

coefficient of friction between thetip of the screw and block Dremains constant. The clampingdevice shown in Fig. 10-25 is an

equalizing mechanism because of



ts symmetrica] design.

Many centralizers, including those

shown in Chapter 9, are alsoequalizers. A V-block holding acylindrical part is an equalizer. The

toggle-action drill jig shown in Fig.10-46 contains two equalizingsystems; one equalizes the pressureon the two ends of each clamping

arm A (the arm A being theequalizer), the other equalizes thetwo arms A against each other (the

equalizer is actually the workpieceX). A similar effect is found in theclamping device shown in Fig. 10-45, where it is the workpiece itself

that equalizes the pressure from th



four arms A.

The Floating Principle

n analysis of these examplesshows that in all of them the forcesare transmitted through a floatingcomponent; that is, a componentwhich is movable with at least onedegree of freedom. The floating

component is statically determinateand adjusts its position freely untilforce equilibrium is reached. Insome cases the floating member isrepresented by the workpiece.

Ch. 11



EQUALIZERS

137

The principle of action of anequalizer is often referred to as "thefloating principle." In mechanicalanguage the function of an

equalizer is to eliminate aredundancy. The required

movability in the system is obtainedeither by forming the equalizer as abeam that can swivel around anaxis, or by using a stiff member-such as a rod-that floats betweentwo springs. Although the detailsmay vary, most equalizing systems

are based on one of these two



principles.

Classification of Applications

Basically, equalizers are used forthe following purposes:

1. To distribute an otherwiseconcentrated clamping force moreevenly over the surface of a part

2. To align clamping forces withocators

3. To clamp on rough surfaces

4. To clamp on surfaces (rough ormachined) of different heights



5. To clamp simultaneously on ahorizontal and a vertical surface

6. To spread the clamping forces(and their matching locatorreactions) evenly over a wide area

to avoid distortion of a thin-walledand elastic workpiece

7. To center a part

8. To clamp simultaneously onmore than one part (multipleclamping).

The Problem Areas

The satisfactory function of an



equalizer requires that the designassumptions are fulfilled. Thisrequires attention to several factors

which, under adverse conditions,can restrict the movability of theequalizer. As applied to clamping or

holding methods, the greatest caremust he exercised in order to makesure that the floating action is notconstrained in any one direction,

but will operate equally well, andwith uniform pressure, on therequired area. Frictional resistance

may, in cases of this kind, besufficient to cause imperfect work by reason of unequal pressure onthe work itself.



Friction is invisible although itoccurs wherever there is slidingmotion. Most equalizers close with

a slight rotation and friction forcesare generated at the points of contact. The fixture designer must

isualize the direction of thefriction forces, estimate their size,and evaluate their possible effect.Significant friction forces occur on

rough surfaces and also whereforces are transmitted throughsurfaces that

are not perpendicular to the forcedirection, such as on plungers withtheir ends at 45 degrees.



When clamping action is applied toa rough surface, great care must beused and the amount of float must

be so proportioned that it will takecare of a considerable variation inthe castings or forgings. When a

great number of pieces are to behandled, several patterns are oftenused and these will be found to varysomewhat, thus, there are

differences in the resultantcastings. Allowance must be madefor extreme cases of dimensional

ariations.

When applied to methods of ocating the work, or to supporting

points on which it rests, the



contacts are positively assured, andno tilting of the work will result. Insuch cases, the equalizer may

occupy a tilted position and it isnecessary to check for thepossibility of false contacts between

the equalizer and other parts of thefixture and the workpiece.

There are occasional instances

which require the location of apiece of work from a previously machined surface, in connection

with a threaded portion by which itmust be clamped. In a case of thiskind, the "float" must be made sothat it will take care of a possible

ack of concentricity between the



thread and the other finishedsurfaces and, at the same time,provide means of equalizing

ariations in the alignment of thethread. Locking devices for floatingmembers must be so arranged that

the members can be positively ocked or clamped without causing

any change in their position. A turning action, such as might be

caused by the end of a screw againsa locating point, is often sufficientto throw the work out of its correct

position. The interposing of shoesbetween screws and floatingmembers will prevent any troubleof this kind. The swiveling shoe or



pad on the end of a screw is, intself, an equalizer applied to a

small area, as is the spherical

washer.

The Rocker Equalizer

The basic and most commonly usedequalizer type is the "rocker." Inprinciple, it is a beam, supported

EQUALIZERS

Ch. 11

at the center and loaded at the endsIt can be a straight beam, such asthe double-end strap, or it can be



curved, as a yoke, when it issupported from above and has toreach down to the work piece. This

form is typical and a good examples shown in Fig, 11-1. A is the work

to be clamped, and B is the yoke

which fits into a slot in the center othe strap, or clamp, C. The yoke isheld by pin Z), around which it canswivel to adjust itself to the work.

The amount of pressure at the twopoints E and F will be equal, eventhough the screws at the ends of th

strap may not be tightened toexactly the same height. Pin D takethe full clamping strain, and shouldtherefore be designed to be strong



enough. The strap, which isweakened by the slot and the holen the center, should be reinforced,

as indicated, at this place. It ispreferable lo have spiral springs ateach end of the strap to prevent it

from slipping down when the work s removed. The strap may be of

either cast iron or machine steel,while the yoke should always be

made of machine steel.



F

Fig. 11-1. The rocker-type equalizing

elamp.

The plain rocker acts on two pointsonly. By placing a small rocker oneach end of the main rocker itsaction is expanded to four points;this development can be carried

further and is used forsimultaneous clamping of severalparts (multiple clamping). Partswith large, flat areas requiring threeclamping points can be clampedwith a rocker-type equalizer formedas a plate, as shown in Fig, 11-2. The

fixture is a drill jig provided with a



floating clamp to work on a roughsurface of a piston casting A, whichhas been previously machined at B.

The body of jig G is of semi-boxsection and is provided with feet Don which it may rest, both during

oading and when under the drill. Ahardened and ground steel stud E iset into the casting at one end and

serves as a locating point for the

machined interior of piston B. A stud C is also provided to give thecorrect location to the wrist-pin

bosses.

s the end of the piston is of spherical shape and in the rough

state, it is necessary to provide a



means

Fig. 11-2, A drill jig with a piale type



equalizer.

of clamping which will so adjust

tself to the inequalities of thecasting that an equal pressure willbe obtained so that there will be no

tendency for the work to tilt, A heavy latch M is pivoted on a pin L,and is slotted at the other end toallow for the passage of a

thumbscrew N which is used toclamp it in position. A special screw0 is threaded into the latch, and is

ball-ended at />so that it has aspherical bearing against thefloating clamp Q. The screw S keepst in position, but clearance is

provided to allow for the floating



movement around the body of thescrew. Three pins R are set 120degrees apart, in the face of the

floating clamp, so that a firm three-point bearing is assured. In order toassist in supporting the work under

the pressure of the drill, two springpins T are provided, set in the formof a vee near the front end of thepiston. They are encased in a screw

bushing t/and are locked in positionby means of set-screws (not shownafter they have been allowed to

spring up against the piston casting(In order to avoid confusion in thedrawing, only one of these pins isshown and that at an angle of 45



degrees from its a,ctual position.)

Steel liner bushings F are provided

n the body of the casting so thatthe main bushings, which are of

Ch. 11

EQUALIZERS

139

the removable type, as shown at H,may not produce too much wear in

the jig body itself. A slot is providedn the head of the bushing so that

pin K will prevent it from turningunder the twisting action of the



drill. It should be noted that in theconstruction of the spring-pins,which are used to help support the

casting, the springs themselvesshould be very light so that they will not force the piston out of its

true position, determined by theocating stud.

Equalizing on three points can also

be accomplished by a system of 45-degree wedge-end plungers. A very rigid mechanism of this type is

shown in Fig. 11-3. It is shownapplied to a drill jig, but it is rigidenough to permit its use in millingor planing fixtures. In these cases,

the clamping pins become rest pins



and are subject to the thrust of thecut.

Fig. 11-3. The multiple plunger typeequalizer.

Screw A thrusts against equalizing

plunger B. Plunger B is of smaller



diameter than the drilled hole andrests on piece C. This piece is cutfrom a rod of the same diameter as

the hole and is used to afford a flatbase for plunger B to rest on andnsure full

contact of the wedge end againstplungers D and E. Plunger G is aduplicate of B and equalizes

plungers F and H by means of thesame mechanism.

The Floating Screw Type Equalizer

The second basic principle used inthe design of equalizers is that of

the floating screw. A screw and its



nut are mounted withoutengthwise fixation between two

clamps or other force-transmitting

components. When actuated, thescrew and its nut exert equal butopposite forces on the two adjacent

components. A typical example,shown in Fig. 11-4, is the locatingmechanism for a milling fixture inwhich two pieces are located by two

plungers each, all operated by asingle clamping operation. Lever A draws out plunger B and throws in

sleeve C, operating plungers D and£*. Plungers B are smaller indiameter than plungers D andpermit enough lateral movement to



equalize plungers G throughauxiliary plungers H.

fixture that shows a doubleapplication of the 45-degree wedge-cut plungers is shown in Fig. 11-5.

Two pieces are each clampedsimultaneously, at both ends, all bytightening one nut. The forces arefully equalized regardless of

rregularities on the individualparts. Rod A , running through thefixture, carries ball-and-socket

washers at each end and draws theend clamps B and C together. Theseclamps are given a down-and-inmovement against the 45-degree

wedge ends of rods D and E. The



clamping thrust against the rodsmparts a downward movement to

the inner clamps G and H, pulling

the work down on the inner rest-pins. The clamps are returned by means of plungers K and spring J.

This fixture is also an example of amodification of the strap with awedge end,.which was shown inFigs. 10-25 and 10-26.

Double Movement Clamps

Double movement clamps areequalizers that produce two forcecomponents with a constant ratio,



I-ig. 11-4. A milling fixture for two

parts using the floating screw principle.

EQUALIZERS

Ch. 11



Fig. 11-5. A fixture with doubleapplication of plungers.

usually in different directions. They

are used in a large- variety of formsand applications and offer a widefield for the fixture designer. A few

examples will be shown.

Figures 11-6 and 11-7 illustratesmall, double-movement clamping

mechanisms for hand milling or



profiling use. In Fig. 11-6 theclamping pressure against clamp A also pulls out plunger B, raising

plunger C and throwing the work against stop E, by means of plungerD. Spring plunger C is used to

return plunger D. In Fig. 11-7 thepull-through clamp A on theplunger B throws the work againststop C, by means of plungers D and

E. Figure 11-8 illustrates half afixture for milling a cylindricalconcave surface on an unusual

piece. The work is clamped againstpads A and B, on previously milledsurfaces, by means of twodifferentially operated plungers C



and D. To prevent springing undercut, the work is backed up with thefloating plunger E on one side and F

and G on the other. The plungersare operated by push-rods //and J.These push-rods are hand operated

and are clamped by bushing K andstar knob L.



g>

Fig. 11-6. A small double movement

clamping mechanism.

Fig. 11-7. A double movementclamping mechanism with a



floating screw and a plunger.

Multiple Clamping

Multiple clamping consists of clamping several parts in onefixture for simultaneous machiningThe most primitive form of multiple clamping is simply tomount a number of identical fixture

units on a common base. Eachfixture is equipped with its ownclamping devices and is unloadedand loaded separately. When thefixture is fully loaded, all parts aremachined in one operation. Theobvious first step to improve this

simple setup is to combine the



Fig. 11-8. A plunger-type clampingmechanism operating on fourpoints.

also ensures proper clamping of nonuniform parts. The loading andclamping operation is faster since it

utilizes shorter and more repetitive



machining operations; notably planing and surface grinding.

The largest expense factor in thistype of multiple clamping remainsthe extent of time required for

clamping the parts individually. Itwould be desirable to be able toclamp them all simultaneously,thus the most drastic step to take is

to hold all the parts in one fixtureand, if possible, clamp themtogether in one operation.

n approach to this solution is thestring milling fixture shown in Fig.11-9, The entire package of parts is

clamped lengthwise by one screw;



each piece is clamped sideways by ts own individual clamping screw.ll side screws are mounted in a

swinging bar which can be quickly removed for unloading, cleaning,and reloading of the fixture. The

use of individual screws ensuresthat each part is fully located andaligned with the side wall in thefixture. It also prevents any

"bulging" or "buckling" of the wholepackage by the pressure from theend screw.



BIN ft NO !Cflt*»

SWINGING BAA

Fig. 11-9. A string-milling fixture.

Bulging of a Package

Bulging or buckling of a package of parallel pieces may occur as a resulof accidental irregularities and

thickness variations. However,buckling may occur even if thepieces have flat and parallelmachined surfaces. In that case it is

an instability effect like the



buckling of a column. The tendencyto buckling increases with the ratioLjH (see Fig. 11-10). To prevent

buckling, setting the end screwsunder a small angle with thehorizontal, pointing downward, is

often recommended. Another very simple and effective means of preventing upward buckling is toplace the end screws above the half

height of the package. The clampingforce is now an eccentric load andgenerates an uneven pressure

distribution over the contactsurface. Already having aneccentricity E = 1/6 H, the pressureat the lower edge drops to zero, and



with E > 1/6 H an area along theower edge has zero pressure. The

package is compressed along the

EQUALIZERS

Ch. 11

-J~~TT-Y

7



h ■

H

Fig. 11-10. Bulging of parts in apackage.

upper surface and opens the jointsalong the lower surface. It will try to buckle downward, resulting in a

well-controlled pressure against thefixture base. If no means areapplied to prevent buckling, thenumber of pieces within a clamped

package should not exceed 5.

The principle of clamping, in a

package, can be extended to parts



that are not flat by the use of suitably formed spacers. Anexample of this is shown in Fig. 11-

11. The spacers are mounted in thefixture (no loose pieces) and definethe place where each part is to be

clamped. In the present case, thespacers are formed as half V-blockswhich match the upper ends of theparts while the lower ends rest in

full V-blocks,

Fig. 11-11. Multiple clamping withshaped spacers.



somewhat unorthodox, but very ersatile, design for multiple

clamping of smalt parts is shown in

Fig. 11-12. It can be used for surfacegrinding and for straddle milling,slotting, and form milling. The base

may be fastened to a secondary base so that

the fixture can be easily attached to

the machine table. The work-holding members of the fixtureconsist of two pieces B, right- and

eft-hand, and a piece C, which isdovetailed to match acorresponding dovetail in parts B.Four swinging clamp members D

are mounted on body A and are



arranged so that one end of eachbears against a tapered portion of pieces B. As a result, when the

work-holding unit is forced in thedirection indicated by arrow F,through the action of the eccentric

clamp E, clamps D will pivot ontheir bearing pins and exert a forceagainst the sides of pieces B,moving them slightly on the

dovetail on part Cand clampingthem tightly on the work pieces.When the clamp is first being lined

up for holding a given type of work,clamp E is tightened very slightly.

n attempt is then made to pull theworkpieces at the two extreme ends



from the holder with the fingers.This is done to permit adjusting theclamping members D, so that the

two pairs will have an equalclamping force.

Fig. 11-12. Multiple clamping of aarge quantity of small parts.

If it is easier to pull the workpiecefrom one end than from the other,the clamping pressure is not equal.One of the two clamping members

D at the Light end of the holder is



then removed, and a small amounts ground from one of its pressure

areas, after which it is replaced and

another try is made. By this means,repeating the test process until ittakes a very sharp tug with the

fingers to remove either of the endpieces when E is lightly set, thedevice is considered correctly adjusted, assuming the workpieces

to be uniform in size.

In most cases, operations

performed on work held in a fixtureof the type described will includeonly cuts so light that there will beittle danger of the assembly being

ifted from base A during



machining. Should such trouble beexperienced, however, it is only necessary to mill vees in the edges

of pieces B where the clampingmembers D take their bearing, andgrind the engaging ends of

members D for a good line bearingn the vees.

In the design shown, the two pieces

B were originally one, the holesbeing drilled first and the piece cutn two, afterward. Because of the

way parts B

Ch. 11

EQUALIZERS



143

are mounted on piece C, it is

possible to separate them formilling any desired shape of work seats in these parts. Thus, when the

cross section of the work is notsymmetrical, a work seat of acertain shape can be milled in onemember B, and a differently shaped

work seat in the opposite member.If required, it is also possible tohave two or more sets of pieces 5

for different types of workplaces,which can be used with the sameretainer piece C and the same base

.



device such as that described canbe made without any recesses forworkpieces and used for holding a

number of strips of material in aine pattern for surface grinding or

milling their edges, machining first

one edge and then the other. Otherariations in the design are also

possible. For example, both pieces Bcan be drilled with two pairs of

registering holes in their inneredges to receive compressionsprings, so that the clamping action

will take place against theresistance of these springs. Thedevice will then automatically openpermitting the work to be unloaded



easily and new parts inserted whenthe clamping pressure has beenremoved.

Multiple Clamping with Rockers

The rocker principle finds many applications for multiple clamping.One rocker clamps two parts; twosmaller rockers mounted on the

ends of a larger



f /Z r^O^T s i

rocker clamp four parts. Carryingthe principle one step further leadsto an arrangement for clampingeight parts, all by actuating only onclamping component, a screw, acam, or a hydraulic cylinder.

n example of multiple clamping isthe fixture which was shown in Fig7-22. Each of the two jaws actuates



two rockers, and each of theseclamps, two parts. Another fixtureworking on a modified version of

the same principle is shown in Fig.11-13. The clamping pressure oneight small washers is equalized,

and the washers clamped with adown-and-in movement in thefixture. Rod A clamps the equalizerB and C, which equalize the

pressure against D and E on oneside, and F and G on the other.Clamps D, B, F, and G are given a

downward pull by four plungers //,which also impart a downward pullon the inner clamps /, K, L, andM.The clamps are bored to receive the



washers, and are returned tonormal position by the springplungers N.

±±

//mmm>sA '>;/>/»;/;/)//>

±±



Fig. 11-13. A vise type fixture withrocker type equalizers.

Fig. 11-14. The mechanics of rockertype equalizers.

Rocker systems can be designed for

equalized clamping of any desirednumber of parts, not just 2, 4, 8, etcIn such cases, it is necessary to userockers with arms of unequalengths. Figure 11-14 shows the

arrangements for the clamping of 35, and 6 parts with equal pressure.



calculated with the equations forthe equilibrium of parallel forces.Rollers are solid bodies; the forces

acting on a roller are not parallel,but converge in its axis (the centerof the circle in the drawings) and

the equilibrium condition for eachroller is established by its free bodydiagram.

Hardened steel rollers and sphereshave a high load-carrying capacity,and a roller or ball equalizer can,

with some skill (see Fig. 11-15d), bedesigned within less space then theequivalent rocker equalizer. Thecost is moderate, as the rollers

require no machining other than



cylindrical grinding. There are nopins and bearings and no carryingstructure other than an enclosing

housing which can be made up by the fixture base. There are no partsthat can break. To damage a

hardened roller by direct pressuretakes a high degree of overload andwith proper dimensioning, is a veryremote possibility.

roller-type equalizer looks, at firssight, as if it is statically

ndeterminate with a redundancy ateach contact point on a horizontalcenter line. However,

this condition changes as soon as



the device is actuated. As each rolles forced downward, it attempts to

separate the next two rollers. This

eliminates the contact and fromnow on the system is statically determinate and can be analyzed by

elementary methods. The directionof the enclosing walls is significantfor the mechanics of the system.The device shown in diagram a

produces two clamping forces, eachof: P = 1/2 F; but the device shownn diagram b has two clamping

forces, each of P = 2/3 F; indicatingthat this device is not only anequalizer, but is also a forcemultiplier. The explanation lies in



the fact that each of the two lowerrollers is forced into a V-block by aforce parallel to one side of the V.

The two devices shown in diagramsa and b are true equalizers. There is

symmetry and the two clampingforces P are equal. The deviceshown in diagram c is symmetrical,and the clamping forces are equal,

two and two, but the two innerforces P 2 are greater than the twoouter forces P,:

P, ■ 1/3 F and P 2 = 1 (2 F

The results quoted above assume

absence of friction. Friction,



however, is always present; on aroller the frictional component andthe actuation forces act on the same

radius, while on a rocker thefrictional component acts on theradius of the pivot, but the

actuating force acts on the length othe



7^}////////*



p 2 r 2 'p,

Fig. 11-15. The mechanics of roller

type equalizers.

Ch. 11

EQUALIZERS

145

ocker arm which is several timesgreater. Therefore, the frictionalcomponents are of greater

significance in the roller typeequalizer and should be taken intoaccount in the detailed analysiswith coefficient of friction ju = 0.1



(hardened rollers and someubrication).

Hydraulic Equalizers

With adequate sealing of thehousing the system of rollers can breplaced by a fluid; if pressure isapplied to the fluid it is transmittedequally to all clamping points.

Hydraulic pressure is excellent forthe multiple clamping of identicalparts and single parts of irregularcontour. A general discussion of hydraulic fixtures is presented inChapter 21, Automatic Fixtures. A few simple applications not

requiring an outside hydraulic



power source will be given below.

vise can be equipped with a

hydraulic jaw, as shown in Fig. 11-16, so that a uniform pressure isexerted on all of the castings,

regardless of variations indimensions or irregularities in theirsurfaces, such as are produced by raised part numbers or company

names cast on the work. In the set-up shown, six lever castings A areclamped simultaneously for

machining both sides and the topwith straddle milling cutters B.Hydraulic vise jaw C is drilled toprovide oil reservoirs D, with two

rows of six plungers E fitting into



the reservoirs. The oil reservoirs aresealed by means of pipe plugs F,and the connecting oil passage G is

sealed by welding plug // to the jawafter drilling. Snug-fitting rubberwashers / are placed in the groove

of each piston to prevent oil

eakage. Plate K, machined to fit thetapered sides of the castings, is

screwed to the stationary vise jaw /..

In preparing the hydraulic vise jaw for operation, the reservoirs arefilled with oil to within 3/4 inch of the face of the jaw. The plungers ar

then carefully inserted in the jaw,



and the assembly is clampedagainst some parallel surface toalign the tops of all the plungers.

While clamped, one of the pipeplugs is loosened to permit air andexcess oil to "bleed" from the

system. This plug is then securedtightly, the assembly is undamped,and the hydraulic vise jaw is ready for service. In this case, the

pressure is applied by closing theise. The hydraulic system

preferably is installed in the moving

aw so that the rigidity of the fixedaw is not compromised. The

system is completely self-containedand the vise is actually converted to



a universal fixture.

similar hydraulic system

comprising the oil reservoir D andthe desired number of pistons E,can be installed in other types of

fixtures, however, a modification isrequired to supply the oil pressure.One of the pipe plugs F is omitted;and a straight screw thread is

provided in its place toaccommodate an actuating screw with a piston that has a sliding fit in

the end of the reservoir. The screw and piston constitute a primitivepump; as they are actuated, oil isdisplaced from the reservoir lifting

the plungers. When the plungers



have established contact with thepart, pressure builds up and thepart is clamped. When the screw is

released, the pressure disappears,the plungers retract, and the partcan be removed. The system is

ersatile; clamping plungers can bearranged in any pattern anddirection as long as they



F—1 i j

fV J

n Mii'ic -HuLiLii V V \

rt

Fig. 11-16. A vise type fixture with a



hydraulic equalizer.

EQUALIZERS

Ch. 11

communicate with the pump piston

through the reservoir. High oilpressures can be produced by thesesimple means. The fixture base with

the reservoir must be designed as apressure vessel and dimensionedaccordingly, but this usually doesnot present any problem. The

nstallation of a pressure gage onarge fixtures is recommended. A

practical upper limit for the

pressure in oil-hydraulic systems is



10,000 pounds per square inch(69N/mm 2 ).

The only weak spot in the system isthe possibility of oil leakage. Theres no continual replenishment of

the oil, and even a minor leak cansoon result in loss of the oilpressure. Should this occur duringmachining, the part may be spoiled

Plastic Fillings

The leakage problem is eliminated

by substituting a plastic medium fothe oil and modifying some designdetails accordingly. Paraffin, grease

and beeswax have been used in the



past and are still usable, but arebeing replaced by a polyvinylchloride resin (PVC). 1 It is heated

to 350F for filling the fixture.Theoretically it has no upper limitfor the applicable

PLastiflexdl proprietary to HastingsPlastics, Inc., 1704 Colorado Ave.,Santa Monica, Cal, 90404.

pressure, but for reasons of designsafety it is recommended that thefixture not be designed to operate aa pressure above 15,000 psi(103N/mm 3 ). Since it is a plasticand not a liquid, it does not offer

the same high degree of versatility



n the design as does oil. Thereservoirs must be fairly straightfrom the pump to the most remote

clamping plunger. The cross-sectionarea of the reservoir must not beess than I 1/2 times the plunger

area. Plungers and pistons shall beguided in their cylinders over aength of at least I 1 /2 times the

diameter. The fit shall be an

ntermediate between sliding andrunning; a class RC2 fit is suitable;mating surfaces shall be machined

to 32 A A roughness and thenapped to a true cylindrical shape; a

bell mouth in the cylinder or atapered end on a plunger is not



acceptable because it would providea tapered gap that could provideaccess for the plastic medium. For

operating pressures above 7000 psithe pressure end of plungers andpistons shall be bored out to a cup

shape to provide a 10-degree featheedge sliding against the cylindersurface. It is recommended thatreturn springs be installed on the

plungers. The plastic medium doesnot provide rust protection. Forfixtures that are out of operation fo

ong periods of time, it isrecommended that pistons,plungers, and cylinders be made of a corrosion resistant material.



CHAPTER

12

Supporting Elements

Definition and Classification of

Intermediate Supports

Intermediate supports are those

elements in excess of what isbasically required for geometricallycomplete and statically determinatedefinition of the position of the par

within the fixture. They are usedwhen the part does not havesufficient rigidity to withstand the

operating forces without distortion



In mechanical language thentermediate supports are

redundant, and to avoid displacing

or distorting the workpiece, they must be compatible with theocators, as was explained in the

text to Fig. 6-37. An ideal solutionwould be to make them of a softplastic material which would yieldon contact with the part and freeze

solid after the part has been locatedThis solution is generally hypothetical (except in such

unusual cases as clamping with acast-able metal, see Fig. 10-54), butt illustrates the principal

mechanism of intermediate



supports.

Intermediate supports are manually

operated screws and plungers, andspring, wedge, air, and hydrauli-cally operated plungers.

Method of Operation

Manually operated devices rely on

the "feel" and judgment of theoperator for the correct applicationpressure. Screw-type supports arethumbscrews, wing screws,

knurled-head screws, hand-knobscrews, and torque-head screws.Common to all of them is the fact

that they are operated directly by



hand and usually do not permit theuse of a wrench except in very special cases, such as the one

shown in Fig. 9-11. Thus, theoperator is not deprived of thenecessary "feel" for the proper

contact pressure which enables himto avoid overloading the workpiece.The ideal screw-type device is thetorque-head screw with a spring-

oaded clutch built into the headwhich sets an upper limit for thetransmitted torque and resulting

pressure.

Springs for plungers are weak springs. Air and hydraulically

operated plungers are designed to



exert light pressures only. Anntermediate support always has aocking device for rigid locking in

the operating position. Screw-typesupports have check nuts, whileplungers are locked by means of a

set-screw, a wedge, or a cam. Wherepossible, the locking device engagesa tapered surface of the plunger sothat the locking is positive and does

not depend on friction

only.

Screw-type supports are frequently provided with a swivel head. Onrregular surfaces, this serves to

equalize the support over a larger



area; on previously machinedsurfaces it prevents marring thework with circular scratches. This

can also be achieved by using acopper or nylon tip on the screw.

Commercial Components

Many individual components forntermediate support devices are

commercially available. Severaltypes of spring-loaded plungers areavailable as complete units (jacks)ready for mounting on the fixturebase. Since they contain movableparts and are exposed to chips, theyare protected by caps, shields, or

seals in the manner shown in Fig.



8-10.

There is no very sharp distinction in

the design between adjustableocators and intermediate supports

The two adjustable locators shown

n Figs. 6-41 and 6-42 can also beused as intermediate supports.

Screw Type Supports

The screw type of intermediatesupport is the cheapest one tomake, is rather slow to operate, and

can only be used where there isconvenient access for the operator'shand-in a side wall of the fixture.

The plunger type, much more



ersatile, is therefore usedextensively.

SUPPORTING ELEMENTS

Ch. 12

Plunger Type Supports

Figure 12-1 a shows the simplest

form of plunger support. It isprovided with a helical springbeneath the plunger to press itagainst the work. One objection to

this type of support is that theplunger A, will slip back under thepressure of the clamps or cutting

tools bearing upon the work. There



s also the danger

ig and ailowed to project above the

base. Plunger A is a sliding fit in thebushing. A cap Cis driven onto theend of the plunger and extends

down over the outside of thebushing, as indicated, making thesupport dirt-proof. This device,however, as well as those in Fig. 12-

1, is not entirely satisfactory since iwill shift as it is tightened, althoughwhen tightened, it will remain in

position.



Fig. 12-1. Simple plunger typentermediate supports.

of the milled flat on the plungerbecoming clogged with dirt, so thatt will not work properly.

Considerable time is lost, therefore

n using this type of support. The



method of clamping the plunger isalso slow, as it is necessary to use awrench in tightening or loosening

the set-screw B. Shown in Fig. 12-Jbs a support which is anmprovement over that shown in

diagram a. The flat on the side of plunger A is milled at a slight anglenstead of parallel with the centerine, as in diagram a. This prevents

the plunger from slipping after it isclamped. A pressure shoe fi-madeof hardened drill rod, which is kept

from turning by a small pin C,engaging a flat, milled in piece S-isused between the plunger A and theclamp. A wing nut D is fastened to



the end of the screw, as shown, inorder to eliminate the use of awrench.

In Fig, 12-2 another design isshown, which presents a further

mprovement over those in Fig. 12-1. A bronze bushing B is driven intothe base of the

Fig. 12-2. A plunger type

ntermediate support with bushing



and cap.

Two other improved designs are

shown in Fig. 12-3, diagrams a andb, 1 The end of the pressure shoe indiagram a is machined to fit almost

half-way around the cylindricalsurface of the plunger. Thisncreases the pressure-transmitting

surface and permits the use of a

much larger locking pressure,resulting in a rigid locking of theplunger. The device in diagram b is

an example of the use of the 45-degree wedge-end plunger whichproduces a much smaller side loadon the plunger than the pressure

shoe B, in Figs. 12-lb and 12-2.



Courtesy of E, Thauhw Fig. 12-3.

Improved plunger typentermediate supports.

n intermediate support of the

wedge-operated plunger type isshown in Fig. 12-4. It represents amodification of, and anmprovement upon, the adjustableocator shown in Fig. 6-40. The

design of the device is low and is fouse in cases where the plunger is

ocated in the middle part of a



fixture


(Copenhagen: G.E.C. Gad's Foilag,1930) vol. II.

Ch. 12

SUPPORTING ELEMENTS

149



Fig. 12-4. A wedge-operated,plunger type intermediate support.

and the actuating means must beaccommodated in the fixture base.

The plunger is adjusted up anddown by the horizontal movementof the wedge. In Fig. 6^10 thewedge is located in a groove in the

bottom of the fixture base and



receives its support from themachine tool table. In Fig. 12-4 thewedge and its actuating rod are

housed in a bore within the fixturebase, resulting in a fully self-contained device, a stronger and

more rigid fixture, and a moreaccurate operation of the wedge.The wedge is locked by clampingthe actuating rod in the casting B,

by means of a knurled nut whichalso serves as the handle foractuating the wedge. The wedge is

supported on the lower edges of twoholes in bushing^. For easy alignment, bushing A is made witha sliding fit and is secured in its



proper position by screw C anddowel pin D. Full alignment isfacilitated by the hinged connection

between the wedge and theactuating rod.

Equalizers and Floating Supports

Intermediate supports can becombined with equalizers. An

equalizer can carry two or morentermediate supports, or anntermediate support can carry an

equalizer. Equalizers and "thefloating principle" are used mainly where a plurality of intermediatesupports are applied to a thin plate

or rim. In the lathe fixture shown in



Fig. 9-11 three intermediatesupports are formed by shellbushings K with hook bolts F acting

together as the jaws of a small vise.Each pair of jaws can floatseparately and adjust its position to

the rim of the casting A after thecasting has been located (centered)on the cone locator B. Finally, thethree pairs of jaws are

clamped separately in positionwithout distorting the rim.

The fixture shown in Fig. 12-5 is fora combined lathe operationand.includes three intermediate

supports mounted on a common



floating ring. The work A is to bebored, shouldered, and faced,complete in one setting. Because of

ts length, it was considerednecessary to provide additionalsupporting points besides the jaw

surfaces. A set of special jaws B waskeyed to the sub-jaws in the table aD, with each special jaw shoulderedat C to support the work. The

brackets E are tongued at F to fitthe special jaws and are secured by screws G. These brackets act as a

support for the steel floating ring Mn which the three spring-pins J are

placed.



Fig, 12-5. Intermediate supportsmounted in a floating ring.

SUPPORTING ELEMENTS

Ch. 12

Elongated holes at points N allow for the required floating action, asthe ring is clamped by collar-head

screws. Each bracket on which thering rests is provided with a shelf//which is offset slightly from centerto allow the necessary width for the

screws. In using the device, screwsL and N are loosened, and the works placed in the jaws, which are then

tightened, while the ring floats



sufficiently to allow for variations.It will be noted that the pins, beingspring-controlled, adapt themselves

to the casting and are locked thereby screws L, after which the ringtself is clamped by the collar-head

screws N.

lthough the floating action of thisdevice was satisfactory, the driving

or gripping power was foundnsufficient to hold the work

securely, thus it was necessary to

replace the spring-pins with squarehead set-screws, cup-pointed, andthe ring was then tapped out toreceive them. The ring was also

allowed to float while these screws



were lightly set up on the work,after which the clamping screws Nwere tightened. After this change in

construction, the action of themechanism was much improved,and the driving power was found to

be sufficient.

different case of a floatingntermediate support is shown in

Fig. 12-6, also used for gripping athin wall of a workpiece. The work has a narrow flange



L-J

Fig. 12-6. A floating intermediatesupport applied to a thin wall.

(at x) and clamp A has a hook thatgrips over the flange. The rim of thepart is clamped between point x andthe dog C when wing nut B is

actuated. The left end of the clamp



has an elongated hole that permitsfloating so that each clamp adjuststself to irregularities in the shape

of the part.

CHAPTER

13

Cutter Guides

Definitions and Principal Types

Cutter guides (setting gages, setting

blocks, set-up gages) arc used forcorrectly positioning the cuttingtool relative to the work andthereby eliminating the necessity o



taking trial cuts, measuring thepart, resetting the cutter, etc. Thecutter guides are usually small, flat

or profiled blocks, or completetemplates, and are permanently, orsemipermanently, mounted on the

fixture. When the cutter is correctlyset, relative to the fixture, and thework is

correctly located within the fixture,then the cutter is also correctly positioned relative to the work.

Cutter guides are used extensively on fixtures for: milling, planing, andshaping operations. Cutter guidesfor drilling and boring operations

take the form of bushings and are



described in Chapter 14. On lathefixtures, flat cutter guides are usedfor positioning the cutter for facing

cuts, while curved cutter guideswith special curved feeler gages areneeded for positioning the cutter

for cylindrical turning operations.Turning cuts taken with cutters

m&& '0\ «3,

M



r-tii ir£3 Hi

Fig. 13-1. Principal typesof cutter

guides: F, feeler gage; GB, gageblock; SF, special feeler gage. a.Setting depth of cut for a single-

point cutting tool and a millingcutter, b. Setting depth and sideposition of a milling cutter, c.Setting depth and side position of a

milling cutter with a cutter guidemade for two different positions, d.Setting lathe tool for facing and

cylindrical turning, e. Reversibleguide for two side positions, f.Reversible guide for fourdimensions, g. The use of a gage

block in combination with a cutter



guide.

CUTTER GUIDES

Ch. 13

mounted on a turret do not require

a cutter guide as the tools arealready preset; other turningoperations may require a higher

accuracy than that obtained by using a cutter guide so thatdiameter measurements must thenbe taken. Fixtures for grinding

operations do not employ cutterguides, but may be equipped withpre-positioned grinding-wheel

dressers.



Tooling blocks with preset cuttersdo not, as a rule, require cutlerguides. Cutter guides are always

made of wear-resistant material,usually of hardened tool steel, butsometimes of tungsten carbide.

They are mounted by means of screws and secured in position by dowel pins. They can also bemanually held in position against a

reference surface on the fixture.The reference surface of a cutterguide is usually set back a certain

distance from the path of the cutterand the cutter is positioned againsta feeler gage, or a gage block, placedon the reference surface. In this



way the cutter does not have tocome into contact with thereference surface which is a

hardened precision surface, andboth the reference surface and thecutting edge are protected against

accidental damage as well asexcessive wear. It is arecommended practice tostandardize the setback distance,

which should be not iess than 1/32nch (0.8 mm). However, when the

setback is not standardized, the

required feeler gage dimensionmust be clearly marked at a placeclose to the reference surface. Theprincipal types of cutter guides for



single and multiple operations areshown in Fig. 13-1.

FIXTURE BODY -

LOCATOR FEELER -,

I a stop MITH , S

TONGUE STRIP

Courtesy of Cincinnati Milaeron



Inc. Fig. 13-2. Manually operatedmilling fixture with two grippingdogs and one fixed locator, for

ocating and clamping the part; andwith one centrally located cutterguide for setting the depth.

tween the two cutters.Consequently, the cutters can bepositioned by means of one 0.0625-

nch (1.50-mm->thick feeler gage.

nother, more complicated, casenvolving the simultaneous

positioning of three sets of millingcutters is shown in Figs. 13-4 and13-5. Figure 13-4 shows the part and

the fixture. The part is a pressure



plate with three sets of lugs spaced120 degrees apart. Each lug ismilled on two sides, and the gang o

cutters consists of one slittingcutler and two half side mills (Fig,13-5), The operation is per-

Cutter Guides for Milling Fixtures

n example of the use of a cutter

guide in a milling fixture is shownn Fig. 1 3-2. The operation is the

simultaneous milling of a contourconsisting of seven parallelsurfaces, by one gang of millingcutters. The side positioning is notcritical and is done in the setup by

direct measuring between the



fixture and the milling cutters. Thepositioning for depth of cut must berepeated after each cutter grinding,

and one cutter guide is provided,ocated in the center line of the

fixture. In this case, the cutter guid

s in the form of a button and ismounted by a press fit in the fixturebase.

cutter guide (set-up gage) for sidepositioning is shown in Fig. 13-3.The operation is the straddle

milling of the flanged edges of apinion bearing, shown in chain-dotted lines. The part is located on acylindrical locator and is clamped

between the locator and the



modified movable jaw of a vise.This jaw also carries the cutterguide which is made 1/8 inch (3

mm) less in width than the distancebe-

r

e

"J

1

T

—•—1I FEELER

Qj \



3

(T) r °« 5 FEEL " {—j

<—UhrM-

11

k

•tH^-

DIRECT CN OF FEED

Courtesy of Cincinnati Milaeron



Inc. Fig. 13-3. A modified milling-machine vise for a straddle-millingoperation with one centrally located

cutter guide for setting the sideposition of (he cutters.

Ch. 13

CUTTER GUIDES

153



Courtesy ofCincinnati Milacron IncFig. 13-4. Milling fixture for aspecial three-spindle milling

machine for simultaneous millingof three sets of lugs on a pressureplate (outline of the part indicated)

formed on a special manufacturingtype milling machine with threespindles operating simultaneously

on {he three lugs. The part isocated on a circular locator (the

supporting stud) designed for jam-

free entering and is clamped on itsperiphery by three angular clampssimultaneously actuated by afloating cam for equalized

clamping.



cutter guide in the form of astepped gaging lug is provided foreach of the three places and the

three cutter guides are mounted ona common dummy plate of thesame dimensions as the part itself.

The dummy plate is clamped in thefixture and the cutters arepositioned, one set at a time. Afterremoval of the dummy plate, the

set-up is ready for production. Withthe dimensions shown, the meanthickness of a lug is 0.4965 inch

(12.61 mm), leaving a totalclearance of approximately 1/16nch (approx. 1.5 mm) between

cutters and gaging lug. The



thickness of the feeler gage isselected to be 0.0625 inch (1.5mm), and the reference surface on

the lug must, therefore, be offsetfrom 0,2505 inch (6.363 mm) to 10.2520 inch (6.501 mm), relative to

the center line of the plate.

Courtesy of Cincinnati MilacronInc.

Fig. 13-5.A dummy plate simulatingthe part shown in

Fig. 13-4, provided with three cutterguides

(gaging lugs) for positioning three



sets of milling

cutters.

cutter guide for the setting of aplaner tool is shown in Fig. 13-6. 1The part is a lathe bed, and thecutter guide is formed as a templatefor the complete contour of theways on top of the lathe bed.

Courtesy of k\ Tkaulow Fig. 13-6, A



cutter guide for setting a planertool.

E, Thaulow, Maskinarbejde(Copenhagen: G.E.C. Gad's Forlag,1930) vol. II.

CHAPTER

14

Drill Bushings

Definitions, Action, and

Classifications

Bushings are used as cutter guidesfor drills, counterbores, reamers,



and other cutting tools in the samecategory. They serve a triplepurpose of positioning, guiding, and

supporting the cutting tool.

The twist drill is a tool that is not

well-suited for precision work. Itseading point, the chisel edge, has arake angle of approximately minus60 degrees. With a negative rake

angle of this magnitude, metalremoval is effected more by squeezing it away than by cutting.

The chisel edge, therefore, isconstantly exposed to a large axialcutting force. The angle betweenthe two main cutting edges is 118

degrees (nominally, sometimes



slightly less). An angle of thismagnitude is not very effective incentering the tool. The shank of the

drill is relieved with a slight "back taper," so that there is no contact(and therefore no support) between

the body of the drill and the walls inthe hole in back of the two cornersof the lips. The circular crosssection of the drill is reduced by the

two large flutes, leaving a crosssection similar to that of an I-beamand consequently, each cross

section of the drill has one directionof low rigidity. In addition, thegrinding of a twist drill is, foreconomical reasons, a very fast



operation and, therefore, not ahigh-precision process. Whenstarting the cut, the drill's chisel

edge has a tend* ency to "walk" onthe surface before it starts to bite.

lso, during cutting, the drill is

sensitive to local variations in thehardness of the metal and may react by running out to the side of the softer metal. The result is that

without taking proper precautions,holes drilled with a twist drill, areoversize, out of round, displaced,

out of alignment, and not evenstraight. These deficiencies aregreatly reduced and the quality of the work significantly improved by



the application of a drill bushing soocated that it provides positioning,

guidance, and support to the drill at

a point as close as possible to thesurface of the work.

Bushings are constantly subject towear when in use and must bemade of wear-resistant material. Nosingle fixture component offers

such a large variety of types, shapesand sizes, as bushings. USA Standard bushings arc available in

more than 50,000 differentconfigurations, counting alldifferences in types, sizes, andndividual dimensions. In

comparison, the cross-reference



conversion tables of other fixturecomponents include approximately14,000 items. Drill bushings are

precision parts. They arecommercially available at pricesthai are a fraction of what it would

cost to make them individually.This fact has greatly contributed tothe reduction in the cost of fabricating drill jigs.

Bushings can be classified asstationary press fit bushings and

renewable (loose) bushings. Theterm "fixed bushings" for press fitbushings is not recommended,because it is used with a different

meaning in the text of the USA



Standard. By shape they can beclassified as headless and head typebushings; by use, as liner and

renewable wearing bushings. By basic design they are classified asconventional and special bushings.

Conventional bushings areclassified as standard andnonstandard bushings, dependingon individual dimensions.

Standard bushings satisfy themajority, but not all, of the fixture

designer's needs. In some cases astandard bushing can be modifiedto suit special requirements; inother cases, it may be necessary for

the fixture designer to design a



nonstandard bushing. Empiricalrules for such designs will bepresented. Many of the statements

that will be made in the followingconcerning standard bushings areactually of a general nature and

apply also to nonstandard bushings

Standard Bushings

Illustrations, data, and othernformation about USA Standard

bushings have been extracted from

Ch. 14

DRILL BUSHINGS



155

merican Standard Jig Bushings(

NSI B 94.33-1962, redesignation oB5.6-1962), with the permission of the publisher, The American Society

of Mechanical Engineers, UnitedEngineering Center, 345 E, 47th St.New York, K.Y. 10017. A condensedbut comprehensive extract from

this standard is found in Machinery& Handbook, '

Standardized bushings are shown inFig, J4-1 and the six basic types areshown in Fig. 14-2. They comprisepress-fit and renewable bushings.

Press-fit bushings are either liner



bushings or press-fit wearingbushings, however, for the samediameter, liner bushings are

shorter. Liner bushings areprovided with and without headsand are permanently installed in a

ig to receive the renewable wearingbushings. They are sometimescalled "master bushings." Press-fitwearing bushings to guide the tool

are for installation directly in the jigwithout the use of a liner and areemployed principally where the

bushings are used for shortproduction runs and will notrequire replacement. They are alsontended for use where the



closeness of the center distance of holes will not permit thenstallation of liners and renewable

bushings. Press-fit wearingbushings are made in two types,with heads and without.

Renewable wearing bushings toguide the tool are for use in linerswhich, in turn, are installed in the

ig. They are used where thebushing wiy wear out or becomeobsolete before the jig, or where

several bushings are to benterchangeable in one hole.Renewable wearing bushings aredivided into two classes, "fixed" and

"slip."



Fixed renewable bushings arenstalled in the liner with thentention of leaving them in place

until they are worn out, Sliprenewable bushings arenterchangeable in a given size of

iner and, to facilitate insertion orremoval, they are usually madewith a knurled head. They are mostfrequently used where two or more

operations requiring differentnside diameters are performed in a

single jig, such as where drilling is

followed by reaming, tapping, spotfacing, counterboring, or someother secondary operation.

ll standardized outside diameters



Inside Diameter:

The inside diameter of the hole is

specified by decimal, letter,number, or fraction.

Type Bushing:

The type of bushing is specified by etters:

S for Slip Renewable

F for Fixed Renewable

L for Headless Liner

HL for Head Liner



P for Headless Press Fit

H for Head Press Fit

Body Diameter:

The body diameter is specified in

multiples of 1/64 inch. For examplea 1/2-inch body diameter

= 32/64= 32.

Body Length:

The effective or body length isspecified in multiples of 1/16 inch.For example, a 1/2-inch length =8/16= 8.



Unfin ish ed Bushings: All bushingswith grinding stock on the body diameter are designated by the

tetter U following the number.

Hxample-

5000- S-48- 16

Inside Diameter Hole Size:

1. Decimal

2. Letter

3. Number

4. Fractional



Type Bushing:

1. S - Slip Renewable —

2. F - Fixed Renewable L - HeadlessLiner HL - Head Liner

P — Headless Press Fit H - HeadPress Fit

c. Body Diameter: 3/4 inch = 48/64= 48 J

d. Body Length: I inch = 16/16= 16

Eric Oberg and t'.D. Jones,Machinery's Handbook (New York:Industrial Press Inc., 1971) 19th ed.



pp. 1884-1894.

Tolerance on fractional dimensions

where not otherwise specified shallbe plus or minus 0.010

DRILL BUSHINGS

Ch. 14

HEADLESS TYPE HEAD TYPE

PRESS FIT BUSHIMGS



RENEWABLE WEARING

BUSHING IN HEADLESS

LINER BUSHING

RENEWABLE WEARING

BUSHING IN HEAD TYPE

L.NER BUSHING

LINER BUSHINGS - HEADLESSND HEAD TYPE



Courtesy ofASME Fig, 14-1,Standardized types of drillbushings.

The maximum and minimumalues of hole size, A, shall be as

follows:

Diameter A must be concentric todiameter B within 0.0005 T.l.V. on

finish ground bushings. The body diameter B, for unfinishedbushings, is larger than the nominadiameter fa order to providegrinding stock for fitting to jig plateholes. The grinding allowance is:

0.005 to 0.010 inch for sizes S/32,



13/64, and 1/4

nch, 0.010 to 0.015 inch for sizes

5/16 and 13/32 inch,

and 0.015 to 0.020 inch for sizes1/2 inch, and up.

plus 1/32 inch. The included angleat the bottom of the counterbore is

1 18 degrees, plus or minus 2degrees. The, depth of thecounterbore ranges from 1/4 inchfor the smallest bushings to 5/8

nch for the largest bushings and isadjusted to provide adequate drillbearing length.



Bushings are straight both insideand out. Older literature shows jigbushings that are tapered on the

outside but this can be consideredobsolete. The upper corners, on thenside, are given a liberal radius

(radius D fa Fig. 14-2) to allow thedrill to enter the hole easily, whilethe outer corners, at the lower endof the outside, are chamfered so

that it is easier to drive the bushingnto the hole when making the jig,

and also to prevent the sharp corne

on the bushing from cutting themetal in the hole into which thebushing is driven. In addition, it isrecommended (but not standard



practice) to relieve the outer surfacat the lower end by 0.001 to 0.002nch on the diameter, on a length

equal to 1 1/2 to 2 times the lengthof the chamfer.

Bearing length for the drill withinthe bushing ideally should be afunction of the drill diameter. A bearing length that is too short

causes premature wear, while onethat is too long causes excessivefriction (the twist drill is never a

precision tool!). If there are nooverriding conditions, a bearingength of 2 times the drill diameter

can be taken as a good, workable

average. There are, however, other



considerations, such as the need forsufficient length of press seat in theig wall and for adequate Thickness

of the bushing head. An analysis of the standard tables shows a widerange of bearing length for each

drill size. The ratio of bearingength (taken as the average of the

table values for each drill size) todrill diameter varies from 7 for 1 /4

nch-diameter drills to about 1 1/4for 1-inch-diameter drills and downto 0.8 for large drills about 2 inches

n diameter.

Mounting of Bushings—Press FitBushings



The length C is the overall lengthfor the headless type and lengthunderhead for the head type.

When renewable wearing bushingsare used with liner bushings of the

head type, the length under thehead will still be equal to thethickness of the jig plate, since thehead of the liner bushing will be

countersunk into the jig plate. Allbushings ranging from 0.0135through 0.3125 inch will be

counter-bored to provide forubrication and chip clearance.However, bushings withoutcounterbore are optional and are

furnished upon request. The size of



the counterbore is the insidediameter of the bushing

Stationary bushings of all types aremounted, as a general rule, by apress fit. In any press fit

nstallation, metal is displaced anddistortion occurs in the bushing ann the jig plate. It is recommended

to always use the minimum

nterference necessary to safely retain the bushing in the jig plate. Adiametral interference of 0.0005 to

0.0008 inch is adequate for thenstallation of headless press fitbushings and liners with 3/4-inchto 1-inch OD. For sizes from 1/2-

nch to 3/4-inch OD, the use of



0.0003- to 0.0005-inchnterferences is recommended; for

sizes below 1/2-inch OD, an

nterference of 0.0002 to

Ch. 14

DRILL BUSHINGS

157



2.



•A

Fig. 14-2. The six basic types of drill

bushings.

^~~T

Courtesy oj ASME

DRILL BUSHINGS

Ch. 14

0.0003 inch should be used. Thesealues are for jig plates made of cas

ron and low carbon steel.



Example -When a bushing of 1/2-nch ID, 3/4-inch OD, and 3/4-inchength is pressed into a 3/4 inch

thick jig plate with an 0.0006-inchnterference fit, the !D of the

bushing will be reduced by

approximately 0.0002 inch. Thebore of the bushing ismanufactured with a plus toleranceof 0.0001 to 0.000S inch relative to

the nominal drill size, and thecompression of the bushing quotedabove reduces the bushing diamete

to almost exactly the nominal drilldiameter. At the same time, thedistortion of the jig plate is held to negligible amount.



Head type press fit bushings andiners can be installed with 0.0003-

to 0.0005-inch interference becaus

the contact between the head andthe surface of the drill plateprovides additional support and

rigidity in the assembly. Head typebushings are particularly recommended for installation inrelatively thin jig plates where a

headless bushing would require anexcessive interference fit foradequate retention. The hole in the

ig plate must be finished by meansof a reamer, a jig borer, or a jiggrinder, never with a twist drill. A twist drill cannot be relied upon to



produce a hole of the requiredtolerance and roundness.

The OD tolerances on bushingswere established for the purpose ofmatching holes finished with a

chucking reamer. Reamers arecommercially supplied with a plustolerance and produce holes slightlyarger than their own physical

diameter.

Example -A 3/4-inch chuckingreamer can be expected to measure0.7505 inch when in good conditionand to produce a hole very close to0.7510 inch in diameteT. With the

standard tolerances of 0.7515 to



0,7518 inch on a 3/4-inch ODheadless press-fit bushing, thiseaves an interference of

approximately 0.0005 to 0,0008nch.

Particular precaution must be takenf two bushings are close together;the thin bridge of metal betweenthem will yield excessively, and the

bushings will "walk." In such cases,It is recommended either to usebushings that are so large that the

holes will blend together (andflatten the bushings on one side sothat they can contact each other), oto make a special insert with two

bushing holes.



jig borer or jig grinder can holdtolerances to 0.0001 incheconomically. A psychological

peculiarity of some jig boreroperators is that they sometimesdevelop the habit of working to the

ower limit of the tolerances,resulting in interferences that aresystematically on the high side.

The recommended method of nstalling press fit bushings is with

an arbor press, but if the jig is too

arge, alternate methods must beconsidered such as pulling thebushing into place with a bolt. Thebushing is first carefully started

nto its mounting hole; drilled



carefully cleaned and lubricated.White lead is suitable for this andhas the advantage of facilitating

ater removal of the bushing if replacement is needed.Recommended also is the

handstonjng of the leading ends of the bushing, the chamfer edge, andthe edge of the relief.

Excessive interference may reducethe diameter of the bore to below aworking clearance, which can cause

tool seizure or prevent the insertionof a renewable bushing. Anundersize bore must be relapped,and the operation must be

performed with great care to



prevent the bushing from becoming"bell mouthed." Another method isto forcefully run a drill up and down

n the bushing; this will eitherdestroy the bushing or open it up;at any rate, it certainly damages the

drill and is not a recommendedprocedure.

Distortion of a bushing as a result

of excessive interference isessentially limited to the lowerportion of the bushing. This is due

to an "ironing" effect on the metaln the jig plate whereby smallrregularities in the surface are

squeezed down and the effective

nterference is reduced accordingly



Thus, a point in the surface at thetop has been in contact with and"ironed" by the full length of the

bushing, while points further downsuffer less and less "ironing" effectdue to the shorter length of bushing

to which they are exposed.

Modern technology has providedmeans for eliminating problems

associated with the use of annterference fit. An adhesive 2 is

now available that will bond a steel

bushing in a clearance hole in ametal plate as securely as annterference fit. The surfaces are

carefully cleaned, adhesive is

applied,



2 American Loetite® bondingagent, proprietary to American DrilBushing Co.

Ch. 14

DRILL BUSHINGS

159

the bushing is installed, and theassembly is left to cure for aboutfour hours at room temperature.The curing time can be reduced to

15 or 20 minutes by the applicationof heat. Maximum temperaturerecommended is 25OF and the

curing process can be performed in



an oven or by locally applying heatfrom a heat lamp or other mild heasource. For extremely precise

ocation in the jig plate, theclearance should be 0.0001 to0.0003 inch, resulting in a location

accuracy of approximately 0.0001nch. For average conditions, the

clearance can be 0.0003 to 0.001nch, and for noncritical conditions

t is even possible to go to 0.003-nch clearance with good retention

of the bushing.

Installation of Bushings—Renewable Bushings

When removable bushings are



used, they should never be placeddirectly in the jig body,"unless theig is to be used only a few times.

The hole, however, should alwaysbe provided with a lining that ismade in the form shown in Fig. 14-

3a, If the hole bored in the jig body receives a loose or removablebushing directly, its insertion andremoval (if the jig is frequently

used) would soon wear the walls ofthe hole, and in a short time, eitherthe jig



Fig. 14-3. Mounting of linerbushings.

would have to be replaced, or ateast the hole would have to be

rebored and a new removable

bushing made to fit the now larger-sized hole. In order to overcomethis, the hole in the jig body isbored wide enough to receive a

ining bushing, which is driven intoplace. This lining bushing, in turn,receives the loose bushing, the

outside diameter of which closely fits the inside diameter of the liningbushing, as shown in Fig, 14-3b inwhich A is the jig body, B the lining

bushing, and C the loose bushing.



When no removable bushings arerequired, the lining bushing itself becomes the drill bushing or

reamer bushing, and the insidediameter of the lining bushing willthen fit the cutting tool used. The

bushing may project, as shown inFig. 14-3c, to provide the drill withthe proper guidance and supportclose to the work. If the jig plate is

thin,

t can be locally increased in

thickness by means of a boss, asshown.

Head type press-fit bushings are

used to prevent the bushing from



being pushed through the jig plateby the cutting tool. The mostmportant application is to take the

thrust of a stop-collar, which isclamped on the drill, to allow it togo down to a certain depth, as

shown in Fig. 14-4a ill which C isthe stop-collar, D the wall of the jigand B the press-fit bushing; F is thework. If the work to be drilled is

ocated against a finished seat, orboss, on the wall of the jig, and thewall is not thick enough to take a

bushing of standard length, then its common practice to make a

bushing having a long head, asshown in Fig, 14-4b. The length A,



of the head, can be extended as faras is necessary to get the properbearing. As the bushing is driven

nto place and the shoulder of thehead bears against the finishedsurface of a boss on the jig, it will

give the cutting too! a bearingalmost as rigid as if the jig metalsurrounded the bushing all the wayup.

Removable bushings (Fig. 14-4c)are frequently used for work which

must be drilled, reamed, andtapped. Each of the cutting toolshas its own bushing, and all thesebushings have the same outside

diameter so that they fit into the



iner. There is a sliding fit so thatthey can be gently pressed into theiner by hand. These bushings are

termed "slip bushings." They arealso used when different parts of the same hole are to be drilled out

to different diameters, when theupper portion of the hole iscounterbored, or when a lug has tobe faced off. A slip bushing belongs

to one tool only, and must not benterchanged. Slip bushings for

drills and reamers are nearly the

same size and cannot be recognizedby sight alone. They must,therefore, be identified by conspicuous markings, usually the



etter R, on a reamer bushing.

a be

Fig. 14-4. Head type bushings.

The outline of the bushing shownn Fig. 14-4c is in accordance with

the standardized designs shown in

Fig. 14-2. Attention is drawn to thegroove E,

DRILL BUSHINGS



Ch. 14

that is cut immediately under the

head B, and is also shown in Fig. 142. Although its dimensions are notstandardized, the groove is

mportant because it providesclearance for the grinding wheel, A width of 0.080 inch is suitable.Occasionally, a bushing having a

arge outside diameter is requiredas, for example, when a largecounterbore must be used in a

small hole, which makes itnecessary to have a large opening inthe jig body. If several operationswith tools of different diameters ar

required, then all the slip bushings



for these tools must be made withthe same large outside diameter.

Slip bushings as well as otherrenewable bushings must besecured (locked) in place; otherwise

they may rotate with the tool, wearrapidly on the outside, or be forcedout by the chips. A number of design details for such locking

devices have been developed. Theircommon requirements are that themust be safe, effective, simple,

foolproof, chip-proof, and,preferably, inexpensive.

very strong, safe, and effective

device is that shown in Fig. 14-5a. A



collar with a projecting tail, called a"dog," is press fit around the headof the bushing and is bent at the

end of the tail, with one end restingagainst some part of the jig. The tais

r=^ -i

a

a b

Fig. 14-5. Special bushing details.



sometimes left straight, if there is apossibility for the tail to strikeagainst a lug in the same plane.

Making such dogs involves someextra expense, but they are very effective in avoiding troubles with

bushings turning and working theirway out of the holes. The tail alsoprovides a convenient grip forplacing and removing the bushing.

Large bushings may be providedwith two handles for this purpose.

One solution, which is alsonexpensive, is to work asemicircular groove B in the edge othe head to fit over a pin driven into

the jig plate, as shown in Fig. 14-5b



lthough it is effective againstrotation of the bushing it docs notprevent lifting by chips, nor does it

facilitate the removal of thebushing. The arrangement shown inFig. ]4-5b is commonly used for

making bushings more easy toremove. A step A is turned down onthe head, which, in this case, willhave to be a trifle larger in

diameter. This step permits a tool—a screwdriver, for instance—to beplaced underneath, and with a quick

erk the bushing may be liftedenough to offer a good hold.

Three methods of holding bushings

to prevent them from turning are



shown in Fig. 14-6; all of them onthe same principle described, A shows a bushing with a pin inserted

which then slips into a slot cut inthe lining bushing; B shows abushing with a slot milled through

the collar and a pin is located in theig to engage this slot; and Cllustrates a more elaborate device

that is sometimes used, where the

stop button which is fastened to theig prevents the bushing from being

drawn out of the liner while drills

or reamers are withdrawn, as wellas preventing it from turning.

Standardized bushing locking

components have been developed



for fixed renewable type bushingsand for slip bushings. The fixedrenewable bushing

Fig. 14-6, Devices for preventing a

bushing from rotating.

Ch. 14



DRILL BUSHINGS

161

s provided with a partly circularrecess as shown in Fig. 14-7 and isheld in position against rotationand push-out by a lock screw withts head engaging into the recess.lmost any type of screw could be

used

-g-h



4.

Courtesy ofASME Fig. 14-7.

Standardized locking recess.

as long as it does not have acountersunk head, however, using ascrew with a cylindrical head of substantial dimensions, such as asocket-head cap screw is

recommended. Preferably ashoulder screw of the type to beseen in Fig. 14-10, is recommendedas the shoulder absorbs some of thebending moment from the head.Head bushings without the lock screw recess can be locked by

means of a separate ring-shaped



clamp (Fig, 14-8) that covers theflange of the bushing and isprovided with a recess for a

standard socket-head locking screw

These locking devices require the

removal of the lock screw beforethe bushing can be removed and aretherefore not suitable for slipbushings that have to be changed

quickly. The standardized lockingdevice for slip bushings consists of a bayonet-type combination of a

rounded notch and a curved recess,as shown in Fig, 14-9, and workswith the lock screw shown in Fig.14-10. The bushing is inserted and

ocked with a push and a twist. The



notch clears along the head of theock screw and the bottom of the

recess slides under the head of the

screw and locks the bushing. Thebushing is kept in place by thefriction from the tool as it rotates.

In those rare cases where a leftrunning tool is used, the recessmust be located in the oppositedirection.

r o-



>

3K ' U— W

► H+-

Courtesy of ASME Fig. 14-9.

Standardized bayonet type lockingrecess.

Courtesy ofASME Fig. 14-8.Standardized locking clamp.



Lock screws are only suitable foruse with flush mounted (headlessor countersunk head type) liners,

usually in light-duty applications.For heavy-duty applications clampsprovide a better means of locking

the bushing against the effects of ibration and torque from rotation.

Clamps provide a larger bearingsurface against the jig plate and are

secured by standard socket-headcap screws. The bending momenton the screw is also peatly reduced.

Clamps can also be used for lockingremovable bushings in projectedmounted liners, that is, head typeiners where the head is not



countersunk. Typical

Courtesy ofASME

Fig. 14-10. Standardized lock screw.

DRILL BUSHINGS

Ch. 14

FIXED RENEWABLE BUSHINGS

FLUSH MOUNTED {HEAD OR



HEADLESS LINERS)

LOCK SCREW

• RECOMMENDED

FOR LIGHT DRILLINGPPLICATIONS » SMALL HEAD

DIAMETER PER. MITS CLOSEBUSHING PLACEMENT

ROUND END CLAMP



FOR LOCKING STANDARD FIXEDRENEWABLE BUSHINGS INPROJECTED MOUNTED LINERS

ROUND CLAMP

• PROVIDES BETTER SECURITY

THAN LOCK SCREW IN HEAVIERDUTY APPLICATIONS

• DIAMETER SAME AS LOCK SCREW HEAD FOR CLOSEBUSHING PLACEMENT

FLAT CLAMP



■ PROVIDES MAXIMUMSECURITY AGAINST VIBRATION

ND TORQUE

FLAT CLAMP

PROVIDES MAXIMUM SECURITYGAINST VIBRATION AND

TORQUE

SLIP RENEWABLE BUSHINGS

PROJECTED MOUNTED (HEADLINERS ONLY)



ROUND END CLAMP

FOR ANY APPLICATION USING

SLIP RENEWABLE BUSHINGSINSTALLED IN PROJECTEDMOUNTED HEAD LINERS

Courtesy of American Drill BushingCo. Fig. 14-11. Examples of clampocks for bushings.

examples of clamp locks are shownn Fig, 14-11; although they are

commercially available, they are no

standardized.

Lock screws must be accurately

ocated at the correct distance



(dimension R in Fig, 14-9) from theiner axis. Small drill jigs for this

purpose, shown in Fig, 14-12, are

commercially available. Lock screw

are eliminated by the use of liners

with integral

ocking tabs 3 , as shown in Fig. 14-13. The con- 3 Proprietary to the

merican Drill Bushing Company.

figuration of the tab is similar to apart of a standard lock screw, so

that it engages in the locking recessof a standard slip bushing. In every other respect the liner is standard;

t is pressed in place by means of an



adapter arbor that centers in thebushing and has a milled slot forclearing the lock tab.

Ch. 14

DRILL BUSHINGS

163

Courtesy of American Drill BushingCo. Fig. 14-12. Drill jig for lock screws.



Nonstandard Bushings

These are bushings of conventional

configuration but some or alldimensions deviate fromstandardized dimensions. They are

used where the work-piece presentsdimensional problems for thedesign of the jig.

The theoretical minimum centerdistance between holes is equal tothe outside diameter of thebushing. However, the practicaldistance must be greater to allow for a metal wail between adjacentbushings and practical

considerations require a certain



minimum wall thickness to avoiduneven distortion when bushingsare pressed in place. The center

distance can be reduced if thebushing wall is reduced, and thin-wa!l bushings of the basic types are

available. The wall thickness isapproximately half the wallthickness of standard bushings inthe normal series. "Extra thin" wall

bushings are also available.

When the guide bushings are very

ong, and, consequently, wouldcause unnecessary friction in theircontact with the cutting tools, they may be recessed, as shown in Fig.

14-14a. The distance H of the hole



n the bushing is recessedsufficiently wider than the diameteof the tool so as not to bear on it.

The length L is about twice thediameter of the hole, to provideguiding surfaces for the cutting too

which are long enough to preventts running out. If the outside

diameter of the bushing is very arge compared to the diameter of

the cutting tool, as indicated in Fig.14-14b, the expense of making thebushings may be reduced by

making the outside bushing of coldrolled steel or cast iron andnserting


http://slidepdf.com/reader/full/jig-and-fixture-design-manual-erik-k-hendriksen3709 1168/2237Courtesy of American Drill Bushing



Co. I'lg. 14-13. Slip bushing andiner with integral locking tabs.

Kjg. 14-14, Examples of nonstandard bushings: longbushings and a bushing with an

external thread.

a hardened tool-steel bushing,mounted with a press fit. This

bushing can be considered as astandard press fit bushing. Thereason why a bushing may need tohave so large an outside diameterand so small a hole is that it mightbe necessary to remove it forcounter boring part of the already

drilled small hole by a counterbore



of large diameter, in which case thehole in the jig body has to be largeenough to accommodate the large

counterbore. If a slip bushing isonger than the lining bushing, asllustrated in Fig. 14-14c, it will be

advantageous to make theprojecting portion of the bushingabout 1/32 inch (0.8 mm) smallern outside diameter than that part

of the loose bushing which fits theining bushing. This lessens the

amount of surface which must be

ground, and, at the same time,makes it easier to insert thebushing, forming a point, so tospeak, which will first enter the



ining bushing. This does notnterfere in any way with the prope

qualities of the bushing as a guide

for the cutting tool.

In some cases, the holes in the

piece to be drilled are so close toone another that it is impossible tofind space in the jig for liningbushings. It is then necessary to

make a leaf, a loose wall, or theentire jig, of machine steel or toolsteel and harden the entire jig or a

portion of it.

Removable bushings are sometimethreaded on the outside and made

to fit a tapped hole in the jig, as



shown in Fig. 14-14d. The lower parof the bushing is usually turnedstraight, and ground in order to

center it perfectly in the hole in theig. The head of the bushing is

either knurled or milled hexagon

for a wrench. When these bushingsare used, they are not, as a rule,used for the purpose of

DRILL BUSHINGS

Ch. 14

guiding the cutting too! alone, butfrequently combine the functions oocating and clamping of the work

as well. Examples are shown in



Figs, 9-14, 9-1 5, and 9-16. Thesebushings are not commonly used asslip bushings, as it would take

considerable time to unscrew, andto re-insert into the jig body, abushing of this type.

Drills with Attached Bushings

When machining several small

holes requiring two or moreoperations, changing slip bushingsbecomes relatively time-consumingConsiderable time is saved by attaching them to their respectivetools, so that they participate in therotation and feed. Slip bushings

used for this purpose are without



heads and are termed "guide"bushings. Figure 14-15 shows aguide bushing A attached to a drill.

The free overhang of the drill mustat least be equal to the depth of thehole to be drilled and should not

exceed one inch in order tomaintain the rigid support of thedrill point, particularly whendrilling a rough surface. Since the

guide bushing is rotating againstthe liner, a clearance must beprovided. To minimize the amount

of frictional heat developed, thisclearance should be made as largeas permitted by the requiredaccuracy in the hole location. In



drilling steel, the use of a guidebushing offers the advantage of providing plenty of room for curled

chips.

technique that is widely used in

connection with multiple-spindledrill heads is shown in Fig. 14-16. AZ-shaped bracket is bored to thesize of the drill (or other tool) and

s machined on the outside to forma pilot which enters a drill bushingas the spindle is fed towards the

work.

Some drill presses have provisionfor the mounting of a bushing

bracket carrying a drill bushing



concentric with the machinespindle. The bracket with thebushing is adjusted to a height

slightly above the surface of thework. The bushing guides andsupports the drill, and the work is

clamped or held in a positioningfixture on the drill press table.

combination of these techniques

s found in the aircraft industry arids applied to portable power tools

used for drilling, spot facing,

tapping, etc., of small holes in largeparts. The drill jig is made of relatively thin metal, fiber plate, orplastic laminates and neither the

drill jig, the bushings, nor the drills



are capable of guiding andstabilizing the rather heavy portablepower tool. The operation of this

type of equipment is shown in Fig.14-17a. The bushings in the drill jigare liners. They are secured with a

nut on the far side of the jig and areprovided,

f —<^P



r

3l

Jk



^XT

_oi.Ay r -! h auSHii

BORE IN J10,

Fig. 14-15. (Left) A drill with a guidebushing attached.

Fig. 14-16. (Right) Stationary guidefor multiple drilling

and reaming tools.

directly or indirectly, with twoocking prongs on the forward side.

The drill bushing is a long and



heavy bushing (the tip) that isscrewed into the nose of the powertool; it carries a flange with two

projecting locking lugs matchingthe prongs on the jig bushings. Inessence the tip is a slip bushing

mounted on a power tool; when inuse the tip is pushed all the way nto the liner and the power tool is

rotated counterclockwise so that

the lugs engage the prongs. Thepower tool is now positioned and isso well supported that it takes little

effort on the part of the operator tohold it up and operate it. The drill isrotated and fed through the tip ontothe work.



The locking prongs can be integralwith the jig bushing and for light-duty work they can be made of two

ordinary lock screws of the typeshown in Fig. 14-10. Individual lockbuttons can be used instead of lock

iner bushings where the holes arespaced too closely. When holes areclosely spaced

Ch. 14

DRILL BUSHINGS

165

a



TURN 30° COUNTER-CLOCKWISE

Courtesy of American Drill BushingCo. Fig. 14-17. (a) Operation of portable power drill with thin-

walled drill jig. Liner bushings are



equipped with individual lockingprongs for drill spindle. <b) Pair of ocking strips for drilling of holes,

closely spaced in line.

n a line, locking is done by two

undercut locking strips along theine of holes as shown in Fig. 14-17b.

Empirical Formulas for Design of Bushings

ery wide, very long, and very large

bushings are the three mostcommon types of nonstandardbushings that must be individually

designed and specially made. Very



wide bushings are for sequences of operations such as those shown inFig. 14-18, where one hole is drilled

to two diameters, or where adrilling operation is followed by counterboring, countersinking, or

spot facing to a diametersignificantly larger than the holediameter; perhaps followed by reaming. The diameter of the liner

s slightly larger than the diameterof the counterboring tool. The drillbushing fits the liner with the

sliding fit for slip bushings. Thehole diameter in the bushing is thedrill diameter with normalclearance, and other dimensions on



this bushing can be calculated by the formulas below. Thecounterbore

requires no bushing since it isguided by the pilot in the drilled

hole. The reamer bushing differsfrom the drill bushing only in holediameter.



ery long bushings are of one of thetypes shown in Fig. 14-4b and Fig.14-14. The letter symbols used in

the following formulas are thestandard letter symbols from Fig.14-2 plus the following: Bearing

ength = L, Wall thickness = T = 1/2(B-A) and Flange width on head =G= 1/2 (E-B).

The basic dimension is the holediameter A. According to an oldrule-of-thumb, the bearing length

can be taken as

L =s 2A

This bearing length will generally



work well. It is long enough toprovide sufficient bearing surfacebut not long enough to cause

excessive friction. However, it isarger than is needed for large drills

while small drills, from 1/4-inch (6

mm) diameter on down, can welluse a longer support. A moresophisticated approach would be totake:

Inch Dimensions MillimeterDimensions

L = \Ta + 0.4

nch



L = 5y/~A + 10

mm

for tools with sharp edges, such asdrills; and

L = 0.8 n/T+ 0.4 inch L = 4>/~A+10 mm

for tools with smooth shanks, suchas boring bars, rose reamers, etc.

For minimum wall thickness, take:

T= 0.2 \/~A + 0.04 inch r =7+lram

Courtesy of Technological Institute



Copenhagen Fig. 14-18. Examples oery wide bushings.

Body diameter will then be:

B=A + IT, or B = A + 2T + 1/32

B = A + 2T, or B =A + 2T+0.8 mm

DRILL BUSHINGS

Ch. 14

depending on whether or not the

bushing is counter-bored (Fig. 14-14a.)

For the corner radius at the inlet

end, take: B » 0,1125 VT inches D =



T mm

For the normal height of the head,

take:

F ;= 0.6 VT inches F=7vTmm

This value docs not apply tobushings of the type shown in Fig.14-4b.

Inch Dimensions

For the diameter of the head, take:

E = B+F- 1/8 inch forU< 1/2 inch,and

£= B + F- 1/16 inch for£> 1/2 inch



where F is calculated by theprevious formula for inchdimensions.

For the length of seat, take:

B<C<3B for B< 3/8 inch

0.67B<C<3B for 3/8 <B<^ 3/4 inch

0.6B<C<2B for B> 3/4 inch

The range of values for Ccorresponds approximately to the

alues in the ANSI Standard.Calculated diameters are convertedto multiples of 1/64 inch andcalculated lengths are converted to



multiples of 1/16 inch. Othercalculated dimensions areconverted to the same selection of

fractions as are used in the ANSIStandard.

The following fits based on ANSIStandard Tolerance limits (fromNSI B4.1-1967), are recommended

for liner in jig plate (press fit) H7-n6

for slip renewable bushing in liner

F7-m6 for fixed renewable bushingn liner F7-h6

Tolerance limits for H7, n6, and h6



are found in reference books, suchas Machinery's Handbookf tolerance limits for F7 and m6 are

found in ANSI B4.1-1 967, Appendix1, p. 16 and p. 19.

xamp/e-Calculate dimensions of 3/4-inch diameter drill bushings of the types shown in Figs. 14-4b and14-14a,

= 0.750 inch; vT = 0.8660;

T = 0.9086; VT = 0.9306

Bearing Length: For Fig. 14-] 4a

=Vo.750 +0.4 = 1.2660^1 5/16



nches Corner Radius:

Eric Oberg and l-.D. Jones,

Machinery ',? Handbook (New ork: industrial Press Inc., 1971)

19th ed„ pp. 1523 to 1525.

D = 0.1125 V0.750 = 0.1022 ^ 3/32nch

Total Length : For Fig. 14-4b

L + D= 1.2660+0.1022= 1.3682*= I3/8inches

Wail Thickness:

T = 0.2Vo.750 + 0.04 ■ 0.2132 inch



Body Diameter: For Fig. 14-4b

5 = 0.750+ 2X 0.2132= 1.1764 *= 1

12/64 1 12/64= 1 3/16 inches

For Fig. 14-14a

B =0.750 + 2X 0.2132+ 1/32=1.2077 1.2077 * I 7/32 inches

Height of Head: For Fig. 14-14a

F = 0.6 Vo.750 = 0.5584 « 9/16 inch

Diameter of Head;

E m 1,2077 + 0.5584 - 1/16 = 1.70361.7036 *» 1 45/64 inches



Length of Seat:

0.6 X 1.2077 <C<; 2 X 1.2077

0.7246 <C< 2.4154

3/4 inch < C < 2 3/8 inches

Ch. 14

DRILL BUSHINGS

167

Millimeter Dimensions For thediameter of the head, take:

E = B + F — 3 mm for B < 13 mm

and



E = B + F- 1.5 mm forfl>13mm

where F is calculated by the

previous formula for millimeterdimensions.

For the length of seat, take:

B<C<ZB forB< 10 mm

Q.67B<C<3B for 10 mm <S< 19 mm

0.6B < C -g IB for B > 1 9 mm

For millimeter dimensions, thefollowing fits based on ISORecommendation R286, arerecommended:



Bearing Length: I = 5\AT9 + 10 =31.8^32 mm

Corner Radius:

3 I

D = V 19 = 2.67^3 mm TotalLength:

L+D = 32 + 3 = 35 mm WallThickness:

T = s/~[9 + 1 =4.36+ 1 = 5.36 *= 5.5

mm Body Diameter: For Fig, 14-4bB= 19+ 11 =30 mm

For Fig. 14-14a 5= 19+ 11 + 0.8 =



30.8 =« 31 mm

Height of Head:

For Fig, 14-14a

4i

F=7V 19 = 14.63 =s 15 mmDiameter of Head:

£■ = 31 + 15- 1.5 = 44.5 mm Lengthof Seat:

0.6 X 3KC<62

*lbid.,pp. 1532 to 1537.

19mm<C< 62 mm



Materials for Bushings

Bushings are generally made of a

good grade of tool steel to insurehardening at a fairly low temperature and to lessen the

danger of cracking during heattreatment. Typical examples are theoil-hardening cold work tool steels,types 01 and 02 and AISI 52100,

heat-treated to a hardness of 63 ± 2Rockwell C. They are also made of carbon steel of at least 0.6 percent

carbon content. They can also bemade of machine steel with a lowercarbon content, which will answerall practical purposes, provided the

bushings are properly case-



hardened to a depth of about 1/16nch (1.5mm). Typical examples areISI 1144 and AISI 8620, The

hardness required is 790 ± 50ickers, which is approximately

equivalent to the Rockwell

hardness quoted above. Very largebushings are made from steels inthe 4100 series. Bushings are alsoavailable in a higher quality level,

made of a high chromium, highcarbon die steel. These bushingswill outlast ordinary bustlings 5 to

times. Finally, bushings are alsomade of sintered tungsten carbide,Class C-2, the straight cobalt gradewith 6 percent cobalt and 94



percent tungsten carbide, with ahardness of 92 Rockwell A, Thetotal length of the bushing body is

made of carbide, while the head ismade of steel and is copper brazedaround the upper part of the

bushing. The life of these bushingss about 50 times longer than theife of ordinary bushings. Bushings

are made to the same quality level

from a sintered ferrous titaniumcarbide composition that ismachinable and heat-treatable to a

hardness of 71 Rockwell C.Bushings for guiding toolssometimes may be

DRILL BUSHINGS



Ch. 14

made of cast iron, but only when

the cutting tool is so designed thatno cutting edges come within thebushing itself. For example,

bushings used simply to supportthe smooth surface of a boring-baror the shank of a reamer might, insome instances, be made of cast

ron. But hardened steel bushingsshould always be used for guidingdrills, reamers, taps, etc., when the

cutting edges come in direct contacwith the guiding surfaces. If theoutside diameter of the bushing is

ery large, as compared with the

diameter of the cutting tool, the



cost of the bushing can sometimesbe reduced by using an outer cast-ron body and inserting a hardened

tool-steel bushing, as seen in Fig.14-14b.

Special Bushings

Many jigs are now made of materials other than steel and cast

ron. The commonly used materialsn this category are cast oraminated plastics, plastic or fiber

sheets, aluminum and magnesiumsheets, and tooling plate. Thesematerials do not permitconventional press fit mounting of

bushings.



In jigs made of cast or laminatedplastics, bushings are either cast inplace as the jig is made, or potted in

cavities that are formed by cores asthe jig is laminated. In either case,the bushings are made with an

outer surface texture, or pattern,that permits the plastic to grip andock the bushing. In each case the

surface configuration is such that

the bushing is positively lockedagainst rotation as well as againstaxial displacement. These patterns

are a combination of deep circulargrooves with shallow and sharpongitudinal serrations or

polygonalflanges, or are systems of



two sets of V-grooves crossing eachother to form a pattern of diamondshaped projections similar to a

knurled surface with a coarse pitchSamples of these patterns areshown in Fig, 14-19.

These bushings are also availablewith a 0.030-inch (0.8-mm)-thick ceramic coating to act as a heat

barrier for the protection of thepotting or bonding material againstthe frietional heat developed within

the bushing. Other jig materials,such as aluminum, magnesium,Masonite®, and even wood, whichs occasionally used for lightweight

igs, can be equipped with bushings



that are specially designed for pressnstallation. The lower half of the

bushing is cylindrical and precision

ground for locating the bushing inthe mounting hole. The upper half s larger in diameter and has

ongitudinal serrations (see Fig. 14-19). The step in diameter preventsaxial displacement, and theserrations cut into the jig material

and lock against rotation. Templatebushings



: piiiiu



Courtesy of American Drill BushingCo. Fig. 14-19. Bushings for use in

drill jigs made of nonfenous andnonmetallic materials.

are those used for template toolingthat is, jigs made from metal platesn thicknesses from about 1/16 to

3/8 inch (1.5 to 10 mm). This jig

material is too thin for conventiona



press fit and for serrated bushings.Template bushings also require afastener, which can be a nut or a

deformable fastener. An example isshown in Fig. 14-20, The hole in thetemplate (the jig plate) is drilled,

reamed to 0.001 to

REINFORCEMENT STRIP

LOCK RING

CLINCH-L0K BUSHING

INSTALLATION TOOL(THREADED FOR ADAPTING TORIVET GUN)



BUSHING

TEMPLATE

Fig.

Courtesy of American Drill BushingCo. 14-20, Template bushings.

Ch. 14



DRILL BUSHINGS

169

0.003 inch (0.03 to 0.08 mm)oversize, and countersunk 90degrees. The bushing is placed inthe hole, and an aluminum lockingring is crimped around it and intothe groove. The crimping is done

with a special adapter arbor in anarbor press, or with a rivet gun.Bushings of this type can beremoved by cutting the locking ringand then can be reused.

Circuit board bushings are small

bushings for drills of 1 /4-inch (6-



mm)-diameter, down to #80(0.35mm) for the drilling of circuitboards. With standardized hole

diameters, they are available in aariety of outside configurations to

match the circuit board drilling

machines currently on the market(see Fig. 14-21).



15

Design of Fixture Bodies

Drawings

t this stage the choice of locating

and clamping devices andntermediate supports, if these are

required, has been finalized. A

drawing is made showing thesedevices in their correct positionrelative to the part. The part outlines also shown. Using a color code

for the lines, to differentiate thearious items, is helpful.

Tooting holes are those drilled in



the part for the purpose of locatingt in a fixture or in a series of

fixtures, one after another. All othe

dimensions are directly or indirectlreferenced to the tooling holes.Where tooling holes are used, they

must be shown and identified.

The drawing, as it now stands, is aphantom drawing with the details

floating unsupported; the next steps to outline the fixture body so that connects all loose parts and

satisfies several other requirementsas well.

The Use of Existing Components



Regardless of the type of fixture,the first design step is to examinethe possibility of using existing

equipment or components. Themost promising of these is to use amanually or air-operated machine

ise, with jaw inserts. The vise canbe used as a base for milling andplaning fixtures and for drill jigs, inwhich case it is also provided with a

ig plate. In the case of a drill jig,the next possibility is the universaldrill jig, or "pump" jig. It supplies

the jig structure and clampingmechanism and needs only to beprovided with locating devices and ig plate with bushings. The use of



this type of jig is described in detailn Chapter 21. The third possibility,

applicable to box-type drill jigs, is

the use of a commercially availableig box, an example of which,

equipped here as a leaf jig, is shown

n Fig. 15-1. These boxes are madeof cast iron or of aluminum

Courtesy of Vlier b'xgineeritig CorpFig, 15-1. Commercially available

box-type drill jig with leaf.



with cast-iron corner posts, and areavailable in sizes up to 4 by 8 inche(100 by 200 mm), and 6 by 6 inches

(150 by 150 mm). Plain fixturebases are made in the two stylesshown in Fig. 15-2. The difference

ies in the location of the lugs. Theyare applicable to fixtures and jigs ofmany types, and are available insizes up to 12 by 1 8 inches (300 by

450 mm). In any case, the finalmake-or-buy decision is based on acost estimate. If commercial

components do not fit, however, thedesign procedure continues.

Drill Jigs



In the case of a drill jig, thedesigner has three choices: A plateig, an open jig, or a closed jig. If all

holes are parallel and drilled fromone side, and the part is large andstable, the plate jig is the probable

solution. If all holes are drilledfrom one flat surface, the plate jigtakes the simplest possible form, aflat plate. If the holes are located in

surfaces at different levels, thedesigner has the choice of making aflat plate jig with projecting bosses

of varying lengths, or to form theplate with bends and offsets so thatt follows the contour of the part

Ch. 15



DESIGN OF FIXTURE BODIES

171



Courtesy of Standard Parts Co. Fig.15-2. Two styles of commercially available fixture bases,

predominantly used for millingfixtures.

surface. The first possibility is

usually recommended, except inextreme cases.

[f the part is small and not easily



supported, and all holes are paralleland drilled from one side, an openig is the solution. If there are holes

n more than one direction, a closedbox jig is needed. The designprocedure for these two cases is

described in detail in Chapter 18,Drill Jigs. All drill jigs, with theexception of plate jigs, must havefeet which can be either attached to

the jig body, or integral with the jigbody. The preferred forms of ntegral feet are the L and T; their

overall dimensions must be largeenough to bridge the width of theslots in the machine tool table.

Fixture Clamping



ll other types of fixtures require afixture base that is aligned with andclamped to the machine tool table.

The fixture base must always haveslots, not holes, for the clampingbolts so that the nuts do not have to

be completely unscrewed to removethe fixture. The traditional form forthe clamping bolts is the T-bolt orscrews with T-nuts, which must

slide all the way to the end of thetable to be removed from the T-slots. There are commercially

available clamping nuts and bolts,however, that can be lowered intothe T-slot and rotated into thegripping position.



lignment

lignment is obtained in principle

by means of a key in the fixture andan alignment slot in the ma-

chine table. While T-bolts and nutsmust have a loose sliding fit in theT-slot, a key must have a close fit ints slot, and the fixture designer

must have the data for thedimensions of alignment slots inthe machine tables for which he isdesigning the fixtures. Mostmachine tables do not haveseparate alignment slots, but the T-slots serve both purposes. Since the

clearance in a T-slot may vary, it is



firm rule that a fixture must alignagainst only one side of the T-slot.Several types of combination

clamping and aligning devices arecommercially available. A representative example of an

aligning clamp that is notproprietary is shown in Fig. 1 5-3,

The construction of the clamping

device is as follows; Fitting into theconventional T-slot in the



Fig. 15-3. A nonproprietary, self-

aligning fixture clamp.


Ch. 15

machine table is a hardened andtempered, cast-steel locating T-



provide a clearance of at least 1/32nch (0.8 mm) for the clamping

bolt. Two flats are machined on the

ower end of. this sleeve, with theflat on the left machined so that itwill be in vertical alignment with

the left-hand edge of the T-block.The lower portion of the flat on theopposite side of the sleeve isnclined at an angle of 60 degrees,

as shown, to mate with thepositioning ledge on the T-block.Both the left-hand flat and the

right-hand inclined surfaces shouldbe hardened and polished, sincethey are the parts most subject tofriction and wear. A clearance of



1/16 inch (3 mm) should beprovided between the lower face of the sleeve and the top surface of the

T-block, as indicated.

The upper end of the cam-sleeve,

which projects beyond the top faceof the fixture lug, is provided with afine-pitch external thread toaccommodate the circular ring-nut.

The periphery of the ring-nut isknurled to facilitate manualrotation. A standard hexagonal lock

nut is screwed on the upper,threaded end of the clamping bolt.

In the illustration, the fixture and

parts of the clamp are shown in the



correct relative positions they willtake when the fixture has beenproperly located and clamped to the

machine table. The cam sleeve hasbeen moved downward by simply tightening the lock-nut. This

movement causes the inclined flaton the right-hand side of the sleeveto contact the positioning ledge onthe T-biock, thus pressing the

fixture toward the left until it isstopped by the flat on the left-handedge of the sleeve coming into

contact with the side of the T-slot inthe machine table. Beforetightening the lock-nut, the ring-nut should be backed off slightly to



clear the top surface of the fixtureug and allow the sleeve to pass

through the hole in the lug.

When the fixture is aligned, thering-nut is tightened -by hand

pressure only-and the lock-nut isthen given a final, partial turn tonsure rigid clamping and positive

alignment of the fixture with

relation to one side of the T-slot.The cam-sleeve is prevented

by the ring-nut from pressing tooforcibly against the positioningedge on the T-block.

The Fixture Body



The body of the fixture is now builtup from the base. It consists,essentially, of walls, forming a

channel or a complete box (a pot, inthe case of a lathe fixture), or of ndividual uprights or brackets.

gain, the phantom drawing showswhere material is needed.

The principal consideration, apart

from rigidity and strength, in theapplication of material is clearance.

t this stage, clearance is easily

arranged; later, it may beunavailable. Clearance is requiredat the following places: Around thepart to allow for dimensional

tolerances, around the path of the



part as it is being loaded andunloaded, and around the fixture toprevent collision with any part of

the machine. Wherever theoperator's hand is applied, a fingerclearance of at least 5/8 inch

(16mm) must be provided; more, if a full hand-grip on the part isanticipated. No part of the fixtureshould obscure the view of the

cutter's action area. If possible, theocating areas should be visible.

Windows in side walls may be

needed that also serve to reduce theweight and to provide access forchip cleaning and for the free flow of cutting fluid. Inaccessible



pockets that can accumulate chipsmust be avoided. Projecting points,corners, and edges are a hazard to

the operator; these musl he bluntedand rounded.

Fixtures that require little handlingare made of steel or cast iron; thosethat require a great deal of handlingare made of a selection of

ightweight materials now availablesuch as: aluminum, magnesium,cast or laminated plastics, and

plastic or fiber sheet.

Three Construction Principles

fixture body may be of the built-



up type, a casting, or a weldment.Historically, the first two types havbeen used from the inception of the

use of fixtures, with cast fixturesdominating; the arrival of thewelding process, particularly arc

welding, has changed the picture,and today the welded fixture is thedominant type. It has, however, notcompletely eliminated the two olde

types for each has its advantagesand therefore its limited area of application, therefore a discussion

of the principles and merits of allthree types is justified.

Typical Examples



s an introduction, it will benteresting to see how the three

construction principles can be ap-

Ch. 15


173

plied to the same assignment.lready the simple case of achannel fixture, as shown in Fig. 154, demonstrates some principal

features. It is obvious that therigidity of the three designsncreases in the sequence: built-up,

welded, cast; because the cast



channel is fully integral, the weldedchannel is partially integral, whilethe built-up channel depends for its

rigidity on the size and number of fasteners. A screw joint is nevercompletely solid because of the

required hole clearances, and any screw joint in a built-up fixturemust therefore be additionally andpermanently secured by means of

tightly fitting dowel pins, as shownThe three types differ also withrespect to "clean contours." The

built-up fixture, if made from fully machined or cold-finishedcomponents, presents well definednner and outer contours with clean



nner corners. The welded fixturehas projecting weld beads in thenside corners and on the outer

sides. The cast fixture has roundedfillets in the inner corners andclean, but not parallel, outer sides

because of the draft. These featuresrequire consideration in theplanning and layout of areas to bemachined.

ruitLJ



(LM.T-W CAST »EU>ED

; ig. 15-4. Three designs of a

channel fixture,

somewhat more complicated cases the box jig with hinged leaf

shown in Fig. 15-5, a through c. It isassumed that the three jigs musthave a machined base surface of

dimensions A X B, as shown. Thethree jigs are drawn to the samescale and the thicknesses shown arerepresentative. The built-up jig, Fig



15-5a, is made of mild steel plate. Its designed strictly to dimensions A

and B, because the components are

machined before assembly. All fixedoints are assembled and secured

with screws (shown as larger

circles) and dowel pins (shown assmaller circles). The screws providethe forces that hold the piecestogether, but since screw holes

normally are drilled with aclearance around the screws, they do not guarantee the exact position

of the parts relative to each other.The dowei pins secure the parts intheir exact position. Therefore,

dowel pins are made to fit exactly in



their holes. Normally, it takes twopins to secure a part, and for bestcontrol of the position, the pins are

ocated as

zT=l

c

B

C

§

rc

bqz



s

7^-r

£—

■J.:::)

b -

$

ti

3

I 1




Ch. 15

far apart as possible; therefore,

dowel pins are usually placeddiagonally opposite each other. Inthe present case, the base is closely

fitted into grooves in the end walls;therefore, one pin would besufficient, theoretically, at each endof the fixture; however, most

designers would choose two pins inaccordance with traditional practiceWhere a removable part is to be

secured with dowel pins, the pinsare mounted with a press fit in thefixed part, and holes in theremovable part are made with a

sliding fit over the pins.



Thicknesses of the individual partsare selected approximately equal tothose in the cast fixture to provide

sufficient bearing surfaces in theoints. The two straps improve the

rigidity against longitudinal forces

while they retain accessibility to thebase for chip cleaning.

Dowel Pin Applications

Dowel pins, are used extensively inall categories of tooling, and thecorrect design and application of these small components is of fundamental importance. Dowelpins are made of soft steel or drill

rod. A hardened dowel pin can be



made from commercial hardeneddrill rod. Standard dowel pins canbe purchased either soft or

hardened, and are made with a 5degree taper at the leading end foreasy and safe start. For hardened

dowel pins, the surface hardness is60 to 64 Rockwell C, the corehardness 50 to 54 Rockwell C. Theshear strength ranges from 150,000

to 210,000 pounds per square inch(1035 to 1450 N per squaremillimeter). Diameter tolerance is

plus and minus 0.0001 inch (0.003mm) with a surface roughness of 4to 6 microinches (0.10 to 0.l5jum).Normally, they are manufactured



with 0.0002-inch (0.0005mm)oversize to provide a secure pressfit, but are also available with

0.001-inch (0.005 mm) oversize forrepair work in cases where a holehas been worn or accidentally

machined oversize.

s a general rule for jigs andfixtures, the dowel diameter is

selected one size smaller than theassembly screws. For presswork dies, dowels are made the same size

as the screws because of theconditions of shock and vibrationunder which the dies operate. Theength of engagement, or the

bearing length, i.e., the length



which the pin protrudes into thesecond member of the assembly, ism to 2 times the dowel diameter.

Soft dowel pins can be used forplain locating purposes where noheavy load is applied to the pin.

However, hardened pins aresometimes preferred because they are usually ground to closertolerances. A locating

dowel pin that is also subjected to asevere shear load should always be

hardened.

Locating the Dowel Pin

Dowel-pin locations are not usually



specified by dimensions, but areshown by center lines on thedrawing. The toolmaker usually

knows that he is to locate the holessomewhat at random in the area of the center lines; a note to this effec

s sometimes placed on thedrawing. An exception occurs whenthe dowel holes are to be jig groundan operation which is not

commonly practiced on jigs andfixtures, but is occasionally, on diesDowel pins should be so located

that the assembly of thecomponents is foolproof.Symmetrical parts can be located inmore than one relative position,



and dowel pins are used to makecertain that they go together in theone and only correct location.

In an assembly where onecomponent must be removed

frequently, the use of dowel pinstogether with straight drill bushings sometimes recommended. Using

a hardened pin in a hardened

bushing, results in a precise andwear-resistant fit.

The general rule is that dowel pinsshould be so located that the holespass entirely through the twocomponents. This is done for easy

removal of the dowel pin when



reduce the build-up of air pressurebehind the pin, in the bottom of thehole. The pressure build-up follows

Boyle's Law, but does not have to becalculated.

ssembly Screws

ssembly screws are usually hexagonal socket head cap screws

made of a high-strength steel. Theminimum length of engagement of the screw thread should be asfollows (where D is the screw

diameter):

n steel in cast iron in magnesium



oints permits the use of thinnerplates. On the other hand, the innerspace must exceed the A and B

dimensions with sufficientclearance to avoid removing theweld beads in machining.

The cast jig in Fig. 15-5c is designedwith larger material thicknessesthan the welded jig, since cast iron

has less tensile strength and a lowemodulus of elasticity. The part isfully monolithic and, therefore, has

no weak areas. Again, themachining of the base requires fullclearance all the way around. Thecast design, however, requires less

machining than the other two



designs, and needs no cutting andfitting. In accordance with mostcommon drawing practice, the jig is

drawn without showing the draft,but when proper draft is providedon all vertical surfaces, the casting

can be made from a single pattern.In this case, the side wails would besolid, which is excellent from astructural viewpoint, but sacrifices

access to the base for chip cleaningShould this point be essential,either machined or cast windows

could be provided, as indicated by the chain-dotted lines. Formingwindows in the casting is perfectly possible, but requires the use of



cores, thus complicating thefoundry work. No evaluation orchoice between the three pinciples

will be made here on the basis of these two simple examples, becausesuch a choice would depend on

many factors, such as size, availableequipment, time, etc.

Built-Up Fixtures

For each of the three design types,there exists some general rules andrecommended practices which mayprovide useful guidance in thedesign. Built-up fixtures offer thegreatest freedom in the design,

essentially because there are no



thermal-metallurgical limitationsnvolved. The material is usually ow-to-medium carbon steel, from

ISI 1025 to AISI 1040; steels withery low carbon content do not

machine well to produce smooth

surfaces. Hardened or otherwiseheat-treated steel can, withoutdifficulty, be assembled with softersteels when desired. Small fixture

bodies may be made in one piece bymachining (carving) them from ablock of steel. The whole body may

be heat treated, thereby eliminatingthe need for separate hardenedcomponents such as drill bushings,ocating points, etc.



bout the only two rules regardingmaterial thicknesses refer to thestability and strength of joints.

Experience has shown that whenthicknesses

are selected as if they were intendedfor castings (see later), they willusually provide sufficient bearingareas for stiffness, and they

preferably should be two times theOD of the assembly screws used,with 1.6 times as the absolute lower

imit. There are no upper limits.

When additional rigidity is needed,t is necessary to use straps, as

shown. Gusset plates are not



practical in built-up fixtures. A number of different joint patternsare shown in the composite

structure in Fig. 15-6. A differentexample is shown in Fig. 15-7,consisting of a channel bracket

mounted on a plate flange. Thechannel is machined from a solidblock. Standard commercial rolledsections offer but little opportunity

for use in built-up fixtureconstruction because of theirrounded fillets and thin wall

thicknesses. Therefore, wherechannels and angles are needed in abuilt-up jig, they will have to bemachined from the solid block. As a



rule-of-thumb, this method can beassumed to be economical fordimensions up to 2 to 4 inches by 8

to 12 inches (50 to 100 mm by 200to 300 mm). Beyond thesedimensions, it is cheaper to weld

them.

Contoured flat components can becut advantageously from plate stock

on a contour handsaw



Fig. 15-6. A composite fixturestructure showing different jointpatterns.


Ch. 15



Fig. 15-7. A channel bracket

mounted on a plate flange.

Fig. 15-8. A bracket type drill jig

made of flame-cut plate.



or with a cutting torch, andmachined afterwards wherenecessary. An example is seen in

Fig, 1 5-8, showing a drill jig of thebracket type. The built-up principleoffers the advantage of having the

top

Kt

I TORCI

^4

*

Fig. 15-9. A jjg made of one flame-cut plate and two straight plates.



side of the base machined beforeassembly, an operation that wouldbe more difficult if the jig was

welded together. Modifiedapproaches to related problems areshown in Figs. 1 5-9 and 15-10.

IT

-

TtT l ■ »

mi i I m

n (—rr

I LI



Fig. 15-10. A modification of the jigshown in Fig. 15-9, made entirely ofstraight plates.

Cast Fixtures

The foundry trade has a large bag otricks and devices by which it cansolve almost any design problem,and the use of castings, therefore,

presents a great flexibility of formto the designer. However, thesedevices have their price; there are afew rules to which a casting mustconform in the interest of economical production. Theseconcern the easy withdrawal of the

pattern from the mold, the free flow



of metal in the form, uniformshrinkage, and the avoidance of "hot spots."

The first condition can be stated as"no undercuts with respect to the

direction of withdrawal." Every fixture has, in a sense, a work spacen the form of a more or less

enclosed cavity for receiving the

part, also, a form that permits easy and unobstructed loading andunloading will, usually, also permit

easy withdrawal from the mold.Exceptions occur when the fixturehas localized projections ordepressions perpendicular to the

direction of motion. One example



was the window indicated in thecast

r

ID



Fig. 15-11. A bracket fixture in castdesign. The design to the leftrequires a split pattern; the design

to the right is made from a one-piece pattern.

Ch. 15


177

fixture in Fig. 15-5c. Anotherexample is the bracket fixture seen

n Fig. 15-11. The design to the left,with a circular boss and amachining clearance groove in the

base, requires a split pattern and



the mold to be parted along a-a. By the small changes shown in thedesign to the right, the parting line

can be b-b, and the pattern can bemade in one piece. Numerousexamples of this and similar

concepts are found in foundry iterature. Only a few examples with

direct reference to fixture designshall be given here.

box-type fixture with a dividingwall may be designed as in Fig. 15-

12a. This requires two cores, whichare eliminated by either one of thedesigns in Fig. 15-12b and c. If thepurpose of the upper flange is

additional strength, this is



compensated for in design b, by ncreasing the thickness in the

dividing wall. If the purpose is to

provide a flat surface for assembly with other components, then this isaccomplished by design c.

c

Fig. 15-12. Three different designs



of a box type fixture. View arequires two cores; views b and ccan be made without cores.

Telated case is seen in Fig. 15-13.The box to the left requires a core,

while the box to the right can beformed without a core and is, for all

n a

Fig. 15-13. Two different designs of a flanged channel jig. The design tothe left requires a core; the design

to the right is made without a core.

purposes, at least equivalent to the

design at the left; perhaps even



better, as it eliminates two metalaccumulations in the two T's,

prototype for a widely used classof drill jigs-is the casting shown inFig. 15-14. It contains a number of

details, providing for angular feeton the top and bottom surface, andong straight strips that can serve as

feet on all four sides. Nevertheless,

with the necessary draft, the patterncan be withdrawn and the castingmade without cores. If it is now

desired to reduce the length of thestrip feet by cutting back asndicated at A, and to make these

feet angular by adding horizontal

ribs fi.then the feature of pattern



withdrawal in this casting is lost,and four cores of two differentshapes will be required.

3

c a-

£>



P

Fig. 15-14, A typical drill-jig casting.

Rules for Dimensioning CastFixtures

The condition "free metal flow" isessentially equivalent to the settingof a lower limit to the metal

thickness in walls. If below such aimit, the metal will suffer excessive

heat loss and solidify before thewall cavity is properly filled,

forming what is known as a "coldrun." These lower limits depend onthe length of flow for the metal and

therefore, on the size of the part.



Broadly, the following values arequoted:


Ch. 15

For average size

castings 3/8 to 1 /2 inch (10 to 13

mm)

For smaller

castings 1/4 to 3/8 inch (6 to 10mm)

For very small



castings down to 1 /8 inch (3 mm)

The wall thickness can be more

specifically related to the overalldimensions. In many cases, a wallserves as the web in a beam, either

an I-beam, a T-beam, or an angle, andicated in Fig. 15-15. With beamheight H, the thickness / can betaken as

t — 0.2 \/H inches

= v f /7i



and for double web beams, asndicated in Fig. 15-16, the

thickness in each web can be taken

as

t = 0A6y/H inches

or

0.8 y/H

mm

The reason for this thickness

reduction is twofold; with doublewalls, the temperature in the molds higher and meta! can flow

satisfactorily in a thinner wall



cavity without cold run; statically,the double web beam has sufficientstrength with less web thickness.

1

Fig. 15-15. Dimensions of openbeams.

In many cases, a flat plate within a

casting forms a series of panelswithin a frame. Examples arendicated in Fig. I5-17.The length L

between cross members (ribs,

dividing walls, etc.) may be takennto consideration by taking

f=l/4+ 1/15 y/T inches



or

t = 6+-jy/L

mm

The calculated thickness should be

nterpreted in each case as a lowerimit; in case of a discrepancy

between the two formulas (which

usually will be insignificant withwell-proportioned castings) it is

f > »

H -w

* —^



„

C

S.

Fig. 15-16. Dimensions of double

web beams.

r

1 u '

Fig. 15-17. Examples of beams with

panels, a. I type; b. Channel type.



safer to use the higher value. Theformulas are entirely empirical andcannot be proved mathematically;

experience has shown that theirresults are satisfactory to thefoundry and make a casting of well-

balanced dimensions, which usuallyalso satisfies the static conditionsexcept, perhaps, in extreme cases.

The condition of uniform shrinkagemeans, theoretically, uniformthicknesses; in practice, it means an

upper limit to the thickness ratiobetween adjoining sections. A goodimiting value for this ratio is 2 to 1

normally somewhat less. It is

actually not desirable always to



strive for completely uniformthickness; those parts of the castingthat are exposed to more effective

heat loss to the mold and which,therefore, would tend to cool fastershould be heavier than those parts

where the cooling is slower; the endresult is a casting with uniformshrinkage and low residual stresses

Corners should be rounded; this ismperative for internal corners, to

avoid cracking; it is of lesser

mportance for external corners.The corner radius r, can be relatedto wall thickness t, as follows:

For internal corners For external



corners

r = 0.5 t to 1,0 f r = 0A8 t to 0.2 f

These are minimum values.However, radii in internal cornersshould not be uncritically increasedparticularly not at places where ribsand walls cross or join, to avoidunnecessary accumulation of metal

which would cause "hot spots," i.e.,slow-cooling areas which invariablycollect slag and develop porosities.

While the principal dimensions of acast fixture can be determined orconfirmed by calculation, as

explained in Appendix III, many



details are not

Ch. 15


179

Fig. 15-18. {Left} A weak lug design(Right) A strong lug design.

amenable to such analysis, and willhave to be designed by acombination of experience and

"feel" on the part of the designer.

Lugs for hold-down bolts should beprovided with prongs of generous



width because they may beaccidentally stressed far beyond theimits anticipated in any

calculation. The same applies toribs; in particular, ribs on bracketsand other projecting parts with a

arge overhang. Examples are seenn Figs. 15-18 and 15-19.

Fig. 15-19.

(Left) A weak bracket design, strong



bracket design.

(Right) A

Bends in thin walls should notshow sharp corners, neitherexternally nor internally; in fact,they should be given generous radiiof curvature, exceeding thosequoted previously, as shown in Fig.

15-20.

stresses is obtained. These stressesconsist of the remaining stresses

originally in the casting, plus new stresses set up by the action of thecutting tool. Any casting for a

precision fixture must therefore be



given a stabilizing treatment, eithera normalizing, an anneal, or at leasta stress relief. It is good practice,

but not widely used, to sandblastand paint castings after thistreatment to remove scale, oxide,

and any remaining sand from moldand cores.

Welded Fixtures

Welded fixtures are, with few exceptions, made from low-carbon,hot-rolled steel assembled by electric-arc welding. Thisconstruction principle puts few restraints on the designer. There

are virtually no thickness



imitations, neither down nor up;metal of any thickness large enoughto be used in a fixture can also be

welded, and the proficient weldercan deal with any special problemby, e.g., preheating prior to welding

and selecting a proper weldingsequence to prevent heataccumulation, to cope with heavy sections or sections of widely

differing thicknesses.

Every known type of weld joint may

be used; most frequently employedare those shown in Fig. 15-21.Chamfered corners, and V- and U-oints are used less in fixture

welding than in other structural



welding, since fixtures are usually designed with generous dimensionsso that fatigue is not considered a

serious hazard. Full penetration is,therefore, not necessarily arequirement in joints between

heavy sections.

^

Fig. 15-20. Large radii of curvaturerequired in thin-wall castings.

Effect of Machining on Castings

s a general rule, machinedsurfaces should be as small as

possible, partly because of the cost



of machining, but also because theskin of the casting is of a particularstructural value. The cast iron

mmediately below the skin isstrongest, and this strengthdiminishes gradually towards the

center of the section.

When unstabilized castings aremachined they are apt to distort as

a result of the removal of metal inwhich the stresses were previously balanced against the stresses in the

remaining metal. The castingdistorts (warps) until a new balanceof residual

Fig. 15-21. Types of welded joints



used in fixtures.

I Weld Details

collection of some typical welddetails for fixtures is shown in Fig.15-22, which also gives

nuxt


Ch. 15

1



Wr A *

a



Fig. 15-22. Typical weld details andrelative weld dimensions. Filletsizes are determined by the thinner

of the two adjoining sections.Where extra strength is required,use heavier fillets. The allowablestress under shocks is 5000 pounds

per square inch (34.5 N per mm 2 )



Relative weld dimensions. A few typical fixture details are shown inFigs. 15-23 and 15-24; a composite

fixture structure is shown in Fig. 15-25. Small projections for bossesand pads can be made

n=f\

Fig. 15-23. A welded U-shapc.



Fig. 15-24. A welded bracket.

1 '

Fig. 15-25. A composite welded



fixture structure.

by building up welding material and

subsequent machining o.f thesurface; examples are shown in Fig15-26.

Components for welding should beprecut as far as possible by sawing,milling, shearing (small thicknesse

only), and torch cutting. Largeopenings

I (TlTETU



C I ?"'^

Fig. 15-26. Welded pads.

Ch. 15


181

C )

u u

1P=T

Fig. 15-27. A welded strap clamp.

n plates are precut. Many



contoured components are cut orrough machined before they arewelded. Examples are the three-

point clamp in Fig. 15-27, threedifferent types of hinges in Fig. 15-28, and the built-up T-slot in Fig.

15-29.

and gussets; two reinforcingcomponents which are inexpensive

n their application. Assume, forexample, that a U-shaped open boxsuch as that shown in Fig. 15-23,

Sacks rigidity; this deficiency iseasily eliminated by adding twostraps as shown in Fig. 15-30, orfour gusset plates, as shown in Fig.



J

Fig. 15-30. The use of straps for

ncreased rigidity.

\ pt—^~—A

2



Fig. 15-28. Welded hinges.

Fig. 15-29. A welded T-slot.

Design Rules

Welded design differs from castdesign in one important limitation:Curved shapes should be avoided.

Straight plates, strips, and bars arecheap; and except with thin sectionof no interest for fixture design,bending involves a serious cost



Fig. 15-31 The use of gusset platesfor increased rigidity.


Ch. 15



Fig. 15-32. (left) A bracket with agusset; .and (Right) with a diagonalstrap.

and are available in a great variety of dimensions, including large

thicknesses, The other standardstructural shapes, the channel andthe I- and Z-beam, have relatively small wall-thicknesses and are

seldom used, except in specialcases, such as foT large T-sIotbases, as shown in Fig. 15-33,

Typical examples of combinationsof plates and standard sections areshown in the followingllustrations. A drill jig with angle

egs and an extensive use of flat



sections, a design typical of many box-type jigs, is shown in Fig. 15-34The same is the case with the drill

ig shown in Fig. 15-35, which isbuilt from plates and fiats, withshort lengths of T-sections for feet.

2i

mi_j

L5

Fig. 15-33. A welded T-slot base.

There is no upper limit for the sizeof welded fixtures. For very largewelded fixtures used in the



aerospace industries, tube sectionsof medium wall thickness are

irtually indispensible. The materia

may be of steel or aluminum.Circular as well as square tubes areused, and although square tubes are

easier to cut and fit, they areslightly less economical withrespect to material, in relation tostrength and rigidity, and are not

available in such large sizes as arecircular tubes.

To eliminate the danger of laterdistortion, welded fixtures shouldbe annealed or normalized, thensandblasted and painted before

machining.



Within certain limits, weldingpermits the joining of steels of different hardness within the same

Fig. 15-34. A welded drill jig withangle legs.

structure. Jig feet can be made of



ow-grade tool steel, heat treated,and then welded lo the mainstructure. During annealing or

normalizing, the material in the feewill be drawn and the resultinghardness will be approximately 35

Rockwell C, sufficient to providegood wear-resistance and still bemachinable.

Welded fixtures should be designedto have minimum surface areas formachining. In this respect, the

designer has a little more freedomthan with cast fixtures wherecertain compromises may have tobe accepted for the sake of

simplicity in the pattern design. He



must visualize all machiningoperations and be certain that they do not also remove his weld beads.

Unbelievable as it may seem, thissometimes happens!

Fig. 15-35. A welded drill jig madeof platus and flats.

Ch. 15




183

Comparison and Conclusions

Some general rules can be laid

down for the areas of applicationand relative merits of the threetypes of fixtures:

The built-up fixture, includingthose carved out of one piece, ispreferred for small parts in general

and for medium-size parts wherethe shape is simple; when weldingand foundry facilities are not

available; or when delivery time is



critical. It may take advantage of available standard sections, in thesame manner as the welded fixture

It can be disassembled andchanged, or its components may bereused in other fixtures. The cast

fixture can, in principle, be designedfor any desired size of part;however, its natural area is themedium size. It allows great

freedom to the designer, lends to beheavy, requires access to a foundry,and involves the time and cost of

pattern making, which may besubstantial. On the other hand, thecast fixture may be economically superior if more than one casting



from the same pattern is needed.When a larger number of nearly equal fixtures are needed, it may be

economical to design and stock asupply of standard cast shapes,notably thick-walled channels and

angles. However, it is not practicalto attempt to change a cast fixture.

The welded type of fixture has now

become the most widely used. It isextensively covered in the literatureand its superiority, relative to cast

fixtures, has been widely expressedsome of the statements made arequite correct, while others overly generalize; although they may

signify the trend, in specific cases



they do not always hold true.

The welded fixture can be made

ighter in weight than the castfixture without sacrificing strengthand rigidity. Procurement time can

be less for a weldment than for acasting because of the time requiredfor pattern making. It has been saidthat it takes less time and money to

make a complete weldment than tomake the pattern for it. It isprobably nearer the truth to say

that these two items are, generally,of the same order of magnitude.The foundry trade is full of pitfalls,and perhaps il takes less experience

and skill to design a welded



structure than a successful caststructure.

reas requiring machiningoccasionally may be kept smaller inthe welded fixture; it is also claimed

that a weldment requires lessadditional thickness for machiningallowance; this is, after all, aconsequence of the amount of

distortion to be expected. Oneiterature source suggests

machining allowances of: 1/8 inch

(3 mm) on medium-size and 1/4nch (6 mm) on large-size weldedfixtures. To

this could be added 1/16 inch (1.5



mm) for fixtures consisting mainly of solid blocks with little welding.These figures are actually well in

ine with general practice formachining allowances for castings.Machining of steel takes about 5

percent more time than machiningof cast iron for the same quantity ometal removed. On the assumptionthat weldments do require less

machining than castings, it is alsoclaimed that total machining costwill be 10 percent less, and, in

conclusion, that the completewelded and machined fixture willresult in a saving of 25 percent overthe total cost of the cast fixture.



welded fixture can, in principle,be changed by removing and addingcomponents. This should also be

taken with some reservation,because the altered fixture may welrequire an additional anneal and

renewed machining. Rules such asthese should be taken as guidelinesonly; sometimes they apply,sometimes they do not.

The design of a fixture is notnecessarily confined to any single

type of construction alone. A combined construction may wellpresent the most advantageoussolution. The frame or body can be

welded or cast, and the precise



ocating surfaces can be screwedand doweled onto the frame. Thus,much, but not all, of the precision

machine work can be done on theoose parts before they are attached

to the body; often under more

favorable conditions. Also, repair oralteration work on the fixture ismore easily accomplished.

Solid Fixtures

Solid fixtures means those that aremachined out of one piece of material. Simple jig plates wouldoften fall into this category. Thematerial is machine steel or light

metal tooling plate. If hardened



surfaces are required, the entirefixture is made of tool steel, A practical upper limit for fixtures

made of machine steel isapproximately 2 by 3 by 6 inches(50 X 75 X 150 mm). The limit is

not an absolute one, but increaseswith increased capacity andefficiency of the machiningfacilities available.

simple example of a solid fixtures shown in Fig. 15-36; 1 a drill jig

for drilling, countersinking, andtapping the hole in a split shaftcollar. The jig body is milled squareand bored out, and a drill bushing, a

handle, and a locator for the split in



the collar are installed. The bushings a slip bushing to allow for

countersinking and is beveled to

approximately fit the curved surfaceof the collar.

E, Thaulow, Maskinarkejde(Copenhagen: G.E.C. Gad's l'orlag,1930) vol, II.


Ch. 15



Courtesy ofS. Thaulow Fig. 15-36. A"solid" drill jig.

different type of drill jig is shownn Fig. 15-37. 2 The part is a bearing

bushing; the jig body is a pot with a

handle. The bushing plate has aong stem terminating with a screwthread and the large nut that locksthe bushing plate also provides the

base on which the jig is supportedduring drilling. Almost allmachining operations on the jig are

performed in a lathe. An additionalexample of a solid fixture is shownater in Fig. 18-39.



Courtesy ofE. Thaulow Fig. 15-37. Adrill jig with three "soiid"components.

Ibid.

Plastic Fixtures

Plastic tooling is essentially made

by casting or by laminating. Thestrength of these materials is, atbest, comparable to the strength of cast iron, but often it is less. For



this reason, plastic toolingmaterials are used only Tot toolingthat is not exposed to heavy loads.

Their principal areas of applicationare drill jigs, routing fixtures,nspection fixtures, and various

types of templates. Within theirnatural areas of application they dohave several advantages. They areight and are easy to handle. Since

they are fabricated (cast oraminated) directly from the part,

they lend themselves to forms with

complex contours. They closely reproduce the contour of the mastewithout complicated and costly machining. Tool details such as



bushings and liners, are eitherembedded in the fixture body as its fabricated, or potted in place afte

the plastic has cured. They requireess lead time than metal fixtures

and are also less expensive.

If plastic fixtures are damaged theycan be repaired, and if brokenbeyond repair, they can be replaced

at a moderate cost. Design changescan be quickly and inexpensively ncorporated in the fixture.

Contours can be altered and detailparts added, deleted, or relocatedwithout costly time delays orexpensive machining. They are,

therefore, applicable to prototype



development. The low cost makes iteconomically feasible to use plasticfixtures for even shortproduction

runs. For their application toproduction in large quantities, theyhave the advantage that they can be

duplicated at a low unit cost andwith precise accuracy. Technicaldetails about plastic fixtures arepresented in Chapters 3 and 14.

CHAPTER

16

Drawings, Dimensions, andTolerances



Economical Design

Design and drawing costs represent

a relatively large share of the totalfixture cost since usually only onefixture is made from a set of

drawings. A few shortcuts can beapplied to reduce the cost of thedrawings, but their integrity withrespect to accuracy, completeness,

readability, and clarity cannot besacrificed. The fixture drawings aresuccessively used by the checker,

the planner, the production shops,and the inspection department andmust quickly convey to each, theconstruction as well as the intended

operation of the fixture. Overloaded



drawings will require excessive timfor study and deciphering,ambiguities generate requests for

clarification, and inaccurate orncorrect drawings result in

rejections, rework, and much loss o

material and time.

Economical design requiressystematic work. The first step is to

acquire the detailed and completepart drawing; the next step is toaccumulate additional pertinent

nformation relating to the machinetool to be used and to the availableaccessories and general-purposework holders. AH fixtures required

for the complete machining



program for the part are first laidout in sketches and reviewedtogether, a procedure that quite

frequently results in usefulmodifications of the initialprogram, sometimes even in a

revision of the design of the part.With these steps finalized, the shopdrawings of all the fixtures can beprepared, lessening the risk of

unpleasant surprises.

Fixture Drawing Practices

The layout drawing shows thecomplete fixture with the partocated for machining, including an

outline of the raw material. In this



way, the machining allowance oneach machined surface is

clearly recognized as is theclearance around the part.

djoining details of the machine

tool (table with T-slots, lathespindle nose, etc.) are also shown.The assembly drawing is preparedfrom the layout and shows the

fixture as seen from the operatingside. It is also useful to indicate thedirection of motion of the cutting

tools, the direction of rotation of milling cutters, etc. Whereverpossible, the details, as well as theassembly, are drawn to full size.

Standard parts are not drawn in



detail, but are shown and listed inthe assembly.

Drawings are made in pencil, withstrong black lines to ensureblueprints of maximum contrast. A

blueprint is exposed to roughtreatment in the shop and itsreadability deteriorates rapidly fromwear, repeated folding, and dirt. The

part outline is drawn in phantomred lines. They are highly conspicuous, not only on the

original, but also on blueprintswhere they show as ghost lines.

General drafting practice, as used

for product design drawings, is



followed with respect toprojections, sections, symbols,ettering, title block, material

dentification, heat treatment,surface roughness, standardtolerances, etc. The parts

numbering system used for toolingdetail parts is different than thesystem used for product detailparts. Sections are used generously

as they are more informative thanprojected views. Sections arc cross-hatched for easy recognition in the

drawing. The viewing direction onsections follows establishedpractice or is conspicuously dentified by arrows.



fixture drawing provides somespecific information about the useof the fixture. It lists the number

and type of cutters required for theoperation. Where one fixture servesmore than one operation and the

part is differently located within thefixture for each operation, eachocation of the part is shown or

clearly indicated. The same is done

when a fixture is used for morethan one part.

DRAWINGS, DIMENSIONS, ANDTOLERANCES

Ch. 16



Tricks of the Trade

Templates for all commercial

fixture components are availablefrom the manufacturers and areused at the design stage as well as

n the completion of the finaldrawings.

To improve the contrast of

blueprints, dimensions; dimensionines; arrowheads; and extensionines (witness lines) are drawn in

India-ink. The additional timerequired is negligible and the results well worth the effort.

In some aircraft companies, the



ery large fixtures for welding andassembly are built only fromsketches of the main structure

together with the detail drawings ofthe parts to be welded orassembled.

n layout and assembly drawings itmay sometimes happen that onecomponent, or the work-piece will

obscure another componentbecause their lines coincide. In thiscase, if one component is

deliberately drawn with a slightdistortion, i.e,, a trifle longer orshorter, or to a slightly differentscale, the confusion of lines is

eliminated and the obscured part



will then be exposed to view. Thedrawing gains in clarity, and thepossibility of misinterpretation and

error is prevented by adding one ormore dimensions to the distortedpart.

Placing the drawing paperdiagonally on the board has severaladvantages not generally

recognized. The lower left-handcorner of the sheet is held near thefront edge of the board, the right-

hand edge is raised 20 degrees, andthe corners are then fastened. Thedrafting machine is adjusted towork at this angle. Work is done

faster as shadows are eliminated



along straight edges, and it is easierto see the pencil point. Lettering ismore easily done because of the

slant of the drawing. The paperstays cleaner and the front edgedoes not become worn and tom,

since the draftsman does not haveto lean across the drawing.

Dimensions and Tolerances

ssembly drawings show thedimensions required for thenspection of the fixture; they are

the dimensions to the locating andclamping surfaces, to the toolguides (including drill bushing

centers), and those relative to the



attachment of the fixture to themachine tool. Every part containsone or more critical dimensions,

that are recognized and identified athe beginning of the dimensioningand toleranc-ing procedure. Critical

dimensions are those that aresignificant for the function of thepart or for its compatibility withother parts in the fixture. Typical

critical dimensions are hole centerdistances,

the distance from a hole center to amachined surface, or the distancebetween two machined surfaces. A distance from a machined surface

(or a hole center) to an unmachined



surface is seldom critical, nor is thedistance between two unmachinedsurfaces.

Detail drawings of the fixture partsshow critical dimensions, overall

dimensions, and all others thatdefine machining operations, Stockdimensions are listed in the bill of materials and are not needed on the

detail drawings.

In the conventional dimensioningsystem, all co-linear dimensions arewritten as a long chain that reachesfrom one end of the part to theother. The advantage here is that it

provides a good check on the



nominal numerical values as all thesingle dimensions in the chainmust add up to the overall length of

the part. However, it also has adisadvantage in that the individualtolerances within the chain add or

subtract in an unpredictablemanner. If, for example, alltolerances come out with their plusmaximum values, it would require

an excessively large tolerance onthe overall length of the part.

The conventional dimensioningsystem is now supplemented, and isgradually being replaced, by a new system, the "coordinate

dimensioning system," or the



"coordinate system," Modernmanufacturing practices, asexemplified by the use of jig borers

ig grinders, and N/C (AfumericallyControlled) machine tools, requireeach dimension to be defined as the

distance between the particularpoint or surface and a commondatum or reference line, point, orplane. Fundamentally, two

perpendicular reference lines arerequired for dimensions in oneplane. Three reference planes are

required for complete definition of all dimensions on a three-dimensional part. Where more thantwo reference lines or points are



needed in one plane, their relativeocation must be clearly defined. A

set of two reference lines is

equivalent to the x and y axes usedn analytical geometry.

In a jig borer or jig grinder, thetable with the work can be movedrelative to the spindle in twoperpendicular directions (the

coordinate axis directions), asshown in Fig. 16-1, and the tablesettings (the x and y coordinates)

can be read out with the accuracy o0.0001 inch (0.003 mm). In mostmachines, the readings axe direct,when the table moves toward the

eft and toward the column of the



machine (away from the operator),which again corresponds to anapparent or relative movement of

the spindle to the right (the x axisdirection), and toward the operator(the y axis direction). For

convenience in the operations andas a safety measure to avoid errors,the dimensions shown on the

Ch. 16


187



*effective"spindle movement away from column

CTUAL TABLE

MOVEMENT TOWARD LEFT



EFFECTIVE SPINDLE

MOVEMENT TOWARD

RIGHT

CTUAL TABLE MOVEMENT

TOWARD COLUMN



Fig. 16-1. Movements and positionsof the jig borer table. A. Position of table relative to spindle before table

movement; B. Position of table aftemoving in conventional direction bydirect reading of coordinate

measuring system. Note that the"effective" movement of the jigborer spindle is opposite indirection from the actual table

movement.

part drawing should also go from

eft to right and from top to bottomand the two reference tines in theplan view should originate from apoint at, or near, the upper left

corner of the part.



Numerically controlled machinetools are designed somewhatdifferently. The reference axes on

the

drawings should appear in the

ower left-hand corner; Le., thedimensions go left to right and frombottom to top. However, tooldrawings should be dimensioned as

shown in Fig. 16-2, since toolingcomponents are normally machinedon jig borers, not on N/C machines


Ch. 16



J —f

^#S

3.526

3.600

-#-e



Fig. 16-2. Dimensioning from tworeference lines.

Dimensioning of hole centers fromtwo reference lines (coordinateaxes) is shown in Fig. 16-2. In the

upper view, dimensions are writtenon ordinary dimension lines witharrowheads and leader lines to thecorresponding points in the

drawing. The arrangement of thedimension lines shows dearly where the reference lines are

assumed to be. Direct center-to-center dimensions, such as a in thellustration, are not part of the

system, but are calculated and

entered on the drawing, labeled



"REF," and are to be used fornspection. When the number of

holes is large, some drafting and

ettering work is saved by simply numbering the holes on thedrawing and tabulating them by

number, diameter, and x and y

coordinates. In this case it isnecessary to define the coordinate

axes on the drawing.

different method of dimensioningfrom reference lines is shown in theower view of Fig. 16-2. The

reference lines are defined, and thedimension to each significant point

s written on the leader line. This



method is frequently used, not onlybecause it simplifies the letteringwork, but because if also makes it

much easier to read the numbers.

With the coordinate dimensioning

system, each single dimension andtolerance is now independent of allothers. Sometimes, it is necessary to combine two or more dimension

n a short chain. This is permissibleprovided the individual tolerancesare

Ch. 16

DRAWINGS, DIMENSIONS, AND

TOLERANCES



189

selected so that the total tolerance

on the chain does not conflict withother tolerances within the part.The transfer of dimensions with or

without tolerances from theconventional system to thecoordinate system is governed by the following two principles:

1. For wntoleranced dimensions,the difference of any pair of dimensions (coordinates) on thecoordinate system must equal thedimension that they replace on theconventional system.



2, For toleranced dimensions, thesum of the tolerances of any pair ofdimensions on the coordinate

system must not exceed thetolerance of the dimension thatthey replace on the conventional

system. The procedure is explainedn full detail in Appendix II at the

back of the book.

Fixture Tolerances

The accuracy of a machined part isess than the inherent accuracy of

the machine tool and the fixture bymeans of which the part was made.This is known as the "degeneration

of accuracy." To ensure



nterchangeability of the machinedparts, it is therefore necessary toprescribe closer tolerances of the

dimensions of the fixture than of the dimensions of the part. Thecrucial question is what tolerances

shall be applied to the fixturedimensions to ensure correcttolerances on the part dimensions.The literature is generous with

suggestions. They range from onehalf to one tenth of the parttolerances. Each recommendation

s as good as any otherrecommendation, and none of themwill guarantee the correct result.

Fixture tolerances are determined



the proper functions of the part. A part that functions with widetolerances will also function with

close tolerances, but not necessarilyice versa. An initially selected

tolerance that is found to be too

wide can be reduced without risk.In the search for fixture tolerancest is therefore recommended to star

with those dimensions that offer

the best possibility of using widetolerances.

Tolerances are needed ondimensions relating to thefollowing fixture elements:

1. Part locators and cutter



positioners

2. Devices for attaching the fixture

to the machine tool (positioningkeys, slots for clamping bolts, etc.),for attaching gages to the fixture,

and surfaces for the installation of standard components andnterchangeable or replaceable

fixture parts (bushings, inserts,

etc.).

Example— Two 0.191-inch-diameteholes are to be drilled in the rounddisc by means of the drill jig shownn Fig. 16-3. The part is located by ts periphery in the inverted nest in

the upper part of the jig. There is no



clamping device, but when the firsthole is drilled, a pin is inserted toock the part in position relative to

the jig. The two hole locationdimensions are apparently criticaldimensions. The most liberal

tolerance is the .004 inch on theocation of the hole to the left. If

there were no other considerationsthis would permit the center of the

part to move a total of .004 inchrelative to the jig, and a part withthe maximum diameter of 1.4375

nches could, so far, accept a nestdiameter of 1.437 5 + .004 = 1.4415nches. This dimension, however, is

not acceptable because it does not



nclude any tolerance, it does notallow for wear, and it does notrecognize the existence of parts

with less than maximum diameter.

part of this small size requires a

minimum nominal clearance of 0005 inch to enter the nest whichgives a minimum nest diameter of 1.4375 + .0005 = 1.438(0) inches.

To this is applied a manufacturingtolerance (machining tolerance,"toolmaker's tolerance") of .001

nch resulting in the nest diameter

nches. If the nest is at itsmaximum of



1.439 it provides a clearance of 0025 inch against a part of

minimum diameter 1.4365 inches.

These relationships are shown inFig, 16-4, The selection of thefixture dimension and tolerance is

n accordance with sound economicprinciples. The cost is determinedby the tolerance .001 inch, which isreasonable for this class of work,

and not by the resulting minimumclearance of .0005 inch which is ina sense, incidental only, since it

depends on the fixture and theselected part, not on the fixture assuch. The resulting maximumclearance is within the maximum



permissible value of .004 and alsoallows an ample margin.

s a part of minimum diameter isshifted through the maximumclearance of .0025 inch, the center

of the part is shifted betweenpositions, .00125 inch on

DRAWINGS, DIMENSIONS, AND

TOLERANCES

Ch. 16

DRILL ,191 (= 11 DRILL)



Fig. 16-3. A part with dimensionsand tolerances, and its drill jig.

each side of the center of the nest.Let a be the distance from thecenter of the nest to the center of

the drill bushing, first assumed tobe without tolerance. Then, tosatisfy the part tolerances, we have

a -.00125 > .498 and a + .00125 <502 .49925 < a <.50075

Like any other dimension, a

requires a tooimaker's tolerancewhich again, is selected as .001nch. There must also be an

allowance of .00025 inch for



wear in the bushing (actually alsoncluding the initial clearance

between the drill and the bushing).

These requirements are satisfied byselecting the

center distance as ' 4Qt) c btct,which provides the

required wear allowance of .00025

nch and leaves an unclaimedmargin of safety of .00025 inch.The maximum tolerance on thehole center distance on the part is002 inch; with a tooimaker's

tolerance of .001 inch on thebushing center distance in the jig,

this requirement is comfortably



satisfied.

0005

MINIMUM CLEARANCE

MINIMUM NEST

MAXIMUM PART

0025



MAXIMUM

CLEARANCE

MAXIMUM NEST

00125— * Fig. 16-4, Resultingclearances for the part shown inFig. 16-3.

Ch, 16




191

It is usual practice in such cases tosplit the part tolerance. This is doneby selecting the fixture dimensionas 'qqqj- inches, which provides anallowance

of .0005 inch on each side. Thisallows the 0.00025 inch for bushinwear and still leaves an unclaimed

margin of safety.

Toolroom Tolerances



The toolmaker's tolerance of .001nch (0.03 mm) can be maintained

economically in ordinary toolroom

operations on conventional lathes,milling machines, and grinders.Precision grinders with hydraulic

feed and positioning devices work to tolerances of less than .0005nch (0.013 mm). With the addition

of a high precision gage, tolerances

of ,0001 to .0002 inch (0.003 to0.005 mm) can be maintainedconsistently. The jig borer

maintains .0005 inch (0.013 mm)under average conditions, and thiscan be reduced to .0002 to .0001nch (0.005 to 0.003 mm) by



careful work under the best condi-imis.

Example: A crank arm as shown inFig, 16-5, is to be straddle milled onthe two opposite sides of the

7 50

crank arm boss to a width of "743

nch. One side of

the crank arm boss is to be alignedwith the side of the center boss

(which is already machined) within±.002 inch. The milling cutters arepositioned with a feeler gage from a

setting block which is located from



the same locating surface in thefixture as the center boss.

1 -"a 1

t.m



LOCATOR

I r ig. 16-5. Tolerances for settingblock and milling cutters.

The cutters are mounted on anarbor. The space between the

cutters is checked after eachregrinding

and is maintained at 750 inch. With

a mean cutter .748



space of ,749 inch and a meanthickness of the

feeler gage of ,120 inch, the meanwidth of the setting block is

749-2 X .120 = .509 inch

The mean width of the shoulder onthe setting block is .120 inch, the

same as the thickness of the feelergage. The toolmaker's tolerancesare .0004 inch on the feeler gage,0006 inch on the setting block, and

001 inch on the shoulder. Theactual dimen-

I 1 QQ



sions, are, on the feeler gage ' ( j n ^nch, on the set-

_, , -5093 . . . " . ,. .1205

tmg block, 5Q07 inch, and on theshoulder 1 jq5

nch.

When the right cutter is positionedfrom the right

side of the setting block, the

extreme alignments

with the locating surface (whichmust be within



+ 002 inch) are

Inch

Inch

shoulder, minimum -.1195

maximum -.1205 feeler gage,maximum +.1202 minimum + .1198alignment + .0007 - .0007

Plus and minus alignment meansthat the cutter is positioned to theright and left of the locating

surface.

When the left cutter is positionedfrom the left side of the setting



block, the extreme alignments forthe right cutter are

Inch Inch

cutter space, minimum + .748maximum + .750 shoulder,maximum -.1205 minimum -.1195

setting block, maximum - .5093

minimum - .5087 feeler gage,maximum - . 120 2 minimum -1198 alignment - .002 + .002

which is still permissible.

Cal louts



needed. Some companies forbid theuse of callouts because they preferto have decisions regarding


COMMON AND TYPICALCALLOUTS FOR USE ON PARTDRAWINGS

Ch. 16

Ch. 16




193

machining operations made by the

workshop per- application.Common and typical callouts aresonnel. However, industrial callout

are widely used presented in theisting on the previous page, andand the tool designer should befamiliar with their some typical

operations are shown in Fig. 16-6.



Fig. 16-6. Typical operations,frequently identified by callouts: A.Spot drill, center drill; B. Spot face;

C. Back face; D. Counterbore; B.Countersink; F. Bore,



CHAPTER

17

Standard and Commercial FixtureComponents

dvantages of Standardization

USA Standards

Fixture components are, in generalsmall and are used in largequantities. Their design is closely

determined by the function of theparticular component, and noconsideration of taste or style isnvolved. For these reasons, fixture



components offer a wide field forstandardization.

Standardized components offersignificant advantages to the user."Design" is reduced to selection of a

component of suitable size from atable, and actual design time iseliminated. The quantity of dentical parts required is increased

and production cost is reduced.When the standardization processtranscends the boundaries of

ndividual firms, it opens the way for mass production of componentsby specialized manufacturers withfurther possibilities for cost

reduction and quality improvement



High-precision and high-quality parts (small drill bushings,hardened and ground to close

tolerances) are available in today'smarket at prices from less than adollar; this is less than the cost of

pulling a vellum from the file andhaving a blueprint made!

N.U.F.C.M.

The drill bushing and fixturecomponent industry in the USA istoday a multimillion dollar industryand is steadily expanding. In I 958,the manufacturers within thisndustry organized the N.i.J.F.C.M.

(National Institute of Jig and



Fixture Component ManufacturersOakland, California), A majoractivity of this organization is the

standardization of fixturecomponents. As standards areadopted, specifications are made

available to its members forncorporation into their

manufactured products and arepresented to consumers through

the members' marketing literature.

Standardization of fixture

components in the USA (includingdrill bushings which, traditionally are mentioned separately) is now found on three levels. National

standardization started in 193 5



with the issue of AmericanStandard for Jig Bushings. Thisstandard has been frequently

revised; the current edition is ANSIB94.33-I 962 (R1971), Jig BushingsIt covers the types of bushings

llustrated in Figs. 14-1 and 14-2 anddescribed in Chapter 14.

Proposed Standards

The next level of standardization isa set of proposed standardsprepared by the N.I.J.F.C.M. forsubmittal to the American NationalStandards Institute (ANSI); apackage of individual standard

proposals, covering most of the



components needed for clampingand locating. With the unifiedrecommendation of the industry

concerned, it may be expected thatthese proposals will be approvedand designated as USA Standards in

the near future.

It should be noted thai thisproposal will standardize not only

sizes, dimensions, tolerances, and,n some cases, materials, but also

part numbers, which when adopted

will greatly simplify specifying andordering these items.

Manufacturers' Standards



The third level of standardization isat the manufacturers" level. Muchstandardization and unification has

been going on through the years,and many items have been virtuallystandardized on all significant

dimensions. These are listed inndividual manufacturers' catalogs

which should be consulted for thetems required. One area with the

east

Ch. 17

STANDARD AND COMMERCIALFIXTURE COMPONENTS

195



standardization is the numberingsystem; but this condition will besimplified by the proposed

standards. As it now stands, eachmanufacturer has his own serialnumber system. Cross reference

tables have been prepared by whichnterchangeable or, at least,

equivalent parts from differentsources can be identified.

Drafting templates arc alsoavailable for most commercial

components. When usedsystematically, they can save muchtime in all phases of the design,from the initial layout to the final

ellum.



Proprietary and Patented Fixturesand Components

Most of the devices andcomponents described in this book are in the public domain and may

be utilized freely. However, severalof the commercially availablecomponents and fixtures arecovered by some degree of legal

protection, usually in the form of one or several patents. Wherenformation about proprietary

rights, patents or otherwise, hasbeen available, it is so indicated,either in the text or in captions tothe illustrations. Any device and

design so designated is protected



and the fixture designer is advisednot to copy it nor to utilize it in anyother unauthorized manner.

Commercial Fixture Components

review of the presently availablecomponents with evaluations, briefdescriptions, and data for theirrange of sizes and capacities

follows. Dimension symbols on linedrawings will indicate to the fixturedesigner the dimensionalnformation that is available (from

manufacturers' catalogs) about eachcomponent. Drill bushings are notncluded since they are discussed at

considerable length in Chapter 14.



Bolts, Screws, and Associated Parts(See Fig. 17-1 a through ii.)

Studs (a) are made from high-strength steel and have a number ouses too diversified to be specified.

They can be joined by coupling nuts(f) to create any required length.Threads are made to a nut fit onboth ends. A secure fit at

nstallation is obtained by the useof a bonding agent on the thread.Range: Thread from 1/4-20 to 1-8,

ength from 1 1/2 to 12 inches.

Eye bolts (b), jig latch bolts (c), andswing bolts (d), are used where the

bolt must be swung out of place to



allow a strap to be removed or a jigeaf to be opened. Another use is

with cam clamps where

t is desired that the cam rock back and forth for direct force

application. Range; Thread from1/4-20 to 3/4-10, length from 2 to 6nches.

T-bolts (e) are used with relatedparts such as clamp straps, flangenuts (g), and spherical washers (1)to create various work-clampingarrangements. They are also usedfor clamping the fixture down to thmachine tool table. These bolts are

highly stressed in service and are



made from heat treated alloy steelwith 150,000 psi minimum tensilestrength. Range: Thread from 1/2-

13 to 1-8, length from 1 1/2 to 12nches.

Coupling nuts (f) are long nuts andare used to couple two studs tocreate a stud of desired length. Theyare also used where a nut of

exceptional length is needed forother purposes. Range: Thread from3/8-16 to 1-8, length from 1 to 2 1/2

nches. Flange nuts (g) have a largebearing area to ensure increasedsurface contact with a clampingcomponent. For many purposes,

combining a flange nut with a set o



spherical washers (/) to eliminateunequal load on the screw thread,caused by slight misalignment, is

recommended. Range: Thread from1/4-20 lo 1 1/4-7 and from 1/4-28 toI 1/4-12. Spherical flange nuts (h)

combined with bottom sphericalwashers (1) are used to compensatefor minor irregularities betweenclamp strap and part being clamped

and to eliminate unequal threadoads caused by slight

misalignment. Range: Thread from

1/4-20 to 1 1/4-7.

corn nuts (see Fig. 17-3 e) are nutsthat are closed at one end to protect

screw thread against dirt and



damage. Range: Thread from 1/4-20to 7/8-9.

Knurled lock nuts (i) are used forquick thread locking. They can betightened by hand or with a 1/4-

nch rod. Range: Thread from 3/8-16 to 5/8-11.

T-slot nuts (j) are used with studs

for clamping a fixture down to themachine tool table. They areadapted to standard machine tooltable T-slots. Two series areavailable: "standard" and "N. I. 1. F.C. M. standard." The screw threadn the nut is so designed and cut

that the stud cannot be turned



through the nut and down into thetable. Range: Thread from 5/16-18to 3/4-10.

Interchangeable sine fixture keys(k) are keys for use in the machine

tool table slots and permit the useof the fixture in slots of varyingwidths. They are inserted in the T-slot from above, rotated until they

fit in the wide part of the slot, andocked into position in the fixture

by a slight turn of the set-screw.

Range: Width across flats (forentering the T-slot) from 1/2 to 11/8 inches.

STANDARD AND COMMERCIAL



FIXTURE COMPONENTS

Ch. 17

|M H***»!■ MM W l>(•* M»»H-m

©-

STUD FIT

1

h-

-®-

—li|i|i|il l i| i |i|i l l \

T ^FIT ^ \



SPHERICAL END— 1 ®:THD. SIZE

-]

^^ «U«i

7

- G J

r-



e a-

THRFAO St2E

H B K h— C

g



~\

<

^cj

h ^-a: j-

L_D —J



Fig. 17-1. *Bolts, screws, andassociated parts, a. §Stud; b. ttEye

bolt; c. + Jig latch bolt; d. f Swingbolt; e, tT-bolt; f, ttCoupling nut; g.tt Flange nut; h. § Spherical flangenut.

llustrations courtesy of thefollowing companies: ?AmericanDrill Bushing Co.; ft NorthwesternTools, Inc.; § Morton MachineWorks.

Spherical washers (1) act as a ball-



and-socket to compensate for any slight misalignment between clampstrap and part being clamped, and

to eliminate undue stresses onthreads. Hole size is 1/16 to 1/8nch larger than the corresponding

stud to allow for equalizing action.Range: Corresponding studdiameter from 1/4 to 1 1/2 inches.

C-washers (m) are for easy removalto speed-up clamping and release othe part. A wire hole is provided for

attachment to the fixture. Range:Width of slot A from 9/32 to 1 1/32nches.

Connecting cables (n) are used for



C-washers and other loose parts, foattachment to fixture. They aremade of nylon covered stranded-

steel cable in

Ch. 17


197

THREAD SIZE



\-

+=^k

TABLE , | SLOT'I

3



f—j-VI.

TAPEB V INCH PEKFOOT



fr

Ql

SHOUlDfI SCTiEW F



Fig. 17-1 (Cant.). *Bolts, screws, andassociated parts, i. tKnurled lock nut; j. t+T-slot nut; k.

tInterchangeable sine fixture key (U.S. Pat. 2,707,419); l. § Sphericalwasher; m. §C-washer; n.

tConnecting cable; o. tt Swing C-washer.

llustrations courtesy of the

following companies: 'AmericanDrill Bushing Co.; ''NorthwesternTools, Inc.; §Morton Machine

Works.

15-inch length and provided withferrules for crimping.



Swing C-washers (o) are for easy removal to speed up clamping andrelease of the part. Washer is held

n place by shoulder screws (p or q)Range: Radius B from 1 to 1 3/4nches, thread from 1/4-20 to 3/8-

16.

Slotted shoulder screws (p) providea precision ground shoulder

diameter for use as a pivot for a

swinging or rotating component.Range; Thread from 10-32 to 3/8-16.

Socket shoulder screws (q) provide

a precision ground shoulder



diameter for use as pivot pins for Cwashers, swing clamp straps, andother rotating components. Screw

head contains socket for socket hexwrench. Available as N, I. J. F. C. Mstandard. Range: Thread from 10-24

to 5/8-11.


Ch. 17

F -#



of

-B— 0

q c/ A

^ THREAD SIZE

+ .000

-.001



0 i ' r

r~~~ *

,

THREAD SIZE



a

X

THREAD SIZE

^^^^

QSffilB^

Fig. 17-1 (Con!,). *Bolts, screws, andassociated parts, p. § Slotted

shoulder screw; q. f Socket shoulde



▼

THREAD SIZE \

W



3

THREAD SIZE

-

THREAD SIZE



THREAD SIZE

aa



Fig. 17-1 (Cont). * Bolts, screws, andassociated parts, v. t Torque-he adscrew; w. 1'Quarter-turn screw; x.

tHalf-turn screw;

. * Threaded adjustable locating

button; z. tJack screw; a a. * Barknob. 'illustrations court ?sy of thefollowing companies: AmericanDrill Bushing Co.; **Monroe

Engineering Products Inc.

Hand knobs and screws (r) are usedfor hand-tightened holding andclamping functions. Hand knob iscadmiun plated cast iron; screw isheat treated steel witl black oxide

finish. Range; Thread from 1/4-20



to 5/8-11, length from 1 3/4 to 3nches.

Hand knob screw assemblies (s) areused for hand-tightened holdingand clamping functions. Relieved

tip protects end threads fromdamage caused by slight peening.Range: Thread from 1/4-20 to 5/8-

11, length from 1 to 3 1/2 inches.

Knob swivel screws (t) are used forha rid-tightened holding and

clamping functions. Swivel shoesprevent marring of finishedsurfaces of soft materials such as

aluminum and copper. Shoe stops



rotation immediately upon contactwith workpiece, swivels


Ch. 17

cc



dd

ee

THREAD SIZF

KNURL

ff



SPHER. DIAM.

99

II

Fig. 17-1 fCont.}. *Bolts, screws, andassociated parts, bb. T4-and5-pronghand knobs; cc. ttStar hand knob;

dd. t Knurled knob; ee, 'Speed ballhandle; ff. ttPlastic ball knob; gg. §Finger handle.



Illustrations courtesy of thefollowing companies: ^AmericanDrill Bushing Co.; ^Northwestern

Tools, Inc.: ^Morton MachineWorks.

3 degrees in all directions tocompensate for minor surfacerregularities, pulls off easily and

snaps on for installation in fixture

mounting hole. Range: Thread from1/4-20 to 3/4-10, length from 1 1/4to 4 13/16 inches.

Knurled head screws (u) are usedfor light-duty holding and clampingapplications. Knurled head provides

for easy finger tightening. Relieved



tip protects end threads fromdamage caused by slight peening.Threads are rolled, which provides

ncreased

Ch. 17


201



Fig. 17-1 (Com,). *Bolts, screws, andassociated parts, hh. tHand wheels/Right) with handle; ii, § Machinehandle. Illustrations courtesy of thefollowing companies; 'American

Drill Bushing Co.; § Morton



Machine Works.

strength and wear resistance and a

smoother surface. Range: Threadfrom 10-24 to 1/2-13, length from 1to 3 1/2 inches.

Torque head screws (v) have aspring-loaded clutch which is builtnto the head and releases at a

preset torque, In this way they prevent overclamping anddistortion of the part. In somemodels the releasing torque can beadjusted from the outside. Torquescrews are provided with check nutand can be supplied with swivel

pads and nylon tips. Range: Thread



from 10-32 to 5/8-11, end forcefrom 10 pounds to 28 pounds forfixed torque types and from 0 to SO

pounds for adjustable torque types.

Quarter-turn screws (w) are quick-

ocking fasteners for leaf jigs and jigplates. Corners are chamfered toprovide self alignment of screw head with latch slot when closing

the leaf. Range: Thread from 10-32to 1/2-13, length from 1 to 2 1/4nches.

Half-turn screws (x) are quick-ocking fasteners for leaf jigs andig-plates. Screw-shoulder locks and

unlocks the plate by rotating the



screw one-half turn. Range: Threadfrom 10-32 to 1/2-13, length from 1to 1 1/4 inches.

Threaded adjustable locatingbuttons (y) are used together or in

combination with fixed locatingbuttons and pins to accurately ocate workpiece in fixture. Range:

Thread from 10-32 to 5/8-18, length

from 1 to 3 inches.

Jack screws (z) are used in fixturesto support irregularly shapedworkpieces, such as castings, and toprevent elastic distortion(springing) of thin work-pieces

during machining. Range: Thread



from 3/8-16 to5/8-Il, length B from1 1 /4 to2 1 /2 inches.

Bar knobs (aa) are used in heavy-duty clamping applications. They are made from high-strength

ductile iron and can be tightened bynserting a bar

between the vertical prongs for

maximum leverage. Knobs can beobtained as unmachined blanks oras finished knobs, with choice of tapped or reamed mounting hole.Range: Hole, blank, tapped from3/8-16 to 1-8, reamed from 3/8nch.



Four- and five-pronged hand knobs(bb) are cast from gray iron,tumbled smooth, and cadmium

plated. Faces of reamed or tappedknobs are machined square withhole. Four-pronged knobs-Range:

Hole, blank, tapped from 10-32 to5/8-11, reamed from 3/16 to 5/8nch, diameter A from 7/8 to 2 1/2nches. Five-pronged knobs-Range:

Hole, blank, tapped from 5/8-11 to3/4-10, reamed from 5/8 to 3/4nch, diameter 4, 3 inches.

Star hand knobs (cc) are availablen choice of aluminum or cadmium

plated cast iron. Knobs can be

obtained as unmachined blanks or



as finished knobs with tapped hole.Range: Hole, blank, tapped from1/4-20 to 5/8-11, diameter A from 1

1/8 to 3 inches.

Knurled knobs (dd) are suitable for

adjustment, clamping and locatingdevices. Knurled headprovidesnonslip finger grip. Knobs are madewith tapped holes. Range: Thread

from 10-24 to 3/4-10, diameter Bfrom 3/4 to 2 1/2 inchesL

Speed-ball handles (ee) arebalanced to permit rapid spinningfor quick clamping and release of the work. Wide handle permits

greater leverage. Handles can be



obtained as unmachined blanks oras finished handles with choice of tapped or reamed mounting hole.

Range: Hole, blank, tapped from1/2-13 to 3/4-10, reamed from 1/2to 3/4 inch, width A from 4 3/4 to 8

nches.

Plastic and steel ball knobs.Lightweight plastic ball knobs (ff)

provide a comfortable, rustproof handgrip for actuating levers.Plastic knobs, except


Ch. 17



the largest size, have threaded brassnserts. Steel knobs are

recommended for use where added

weight is desirable for easier leveractuation. They are available asunmachined blanks or as finished

knobs with hole drilled and tapped,ready for mounting. Plastic knobs-Range: Thread from 10-32 to 5/8-18, diameter B from 1 to I 7/8

nches. Steel knobs-Range: Hole,blank, tapped from 3/8-16 to 5/8-18, diameter B from 1 1/2 to 2

nches.

Finger handles (gg) are miniaturehandles to be used as finger grips

for actuation, control, or



adjustment of smaii movable parts.Range: Thread from 10-24 to 5/16-18, length C from 11/16 to 1 1/4

nches.

Handwheels fhh) are made from

cast iron and can be obtained asblanks or finished machined withpolished rims and unpolishedsurfaces painted. Smallest size hand

wheel is solid, larger sizes have fouspokes. Range: Diameter^ from 3 to12 inches.

Machine handles (ji) are used withhandwheels and for variousactuating and gripping purposes

They are machined and polished to



a smooth finish. Shape is designedfor convenient grip, and speed andease of operation. Solid handles can

be obtained" with a press fit, or witha threaded shank, revolving handlesare made with a press fit. Solid

handles-Range: Shank diameter^for press fit from 1/4 to 1/2 inch,shank thread from i/4-20 to 1/2-13,ength B from 1 23/32 to 5 inches.

Revolving handles-Range: Shank diameter A for press fit from 5/16 to1/2 inch, length B from 2 5/8 to 5

1/8 inches

Quick-acting Screw Components(See Fig. 17-2a through d.)



Single locking levers (a) provide aquick and efficient means of permanently locking and clamping

workpicces or movable fixture partswith fairly large clamping pressure.Range: Hole, blank, tapped from

3/8-16 to 5/8-11, reamed from 3/8to 5/8 inch, length of arm from 2 to4 inches.

Double locking levers (b) are usedfor quick and permanent lockingwith large clamping pressure.

Range: Hole, blank, tapped from3/8-16 to 5/8-11, reamed 5/8 inch,width over handles from 4 to 8nches.



Quick-locking levers (c) have a partof the screw thread removed toprovide a smooth hole for fast

removal from the stud when thehandle is tilted. The lever locks orreleases by rotating it one quarter

turn. When in the unlockedposition, the weight of the handleautomatically puts the lever in therelease position for quick removal

from the stud.



Courtesy ofJergeiis Inc. Fig, 17-2,Quick-acting screw components, a.Single locking lever; b. Double

ocking lever; c. Quick lock lever; d.Quick lock knob.

Range: Thread from 1/2-13 to 3/4-10, length of arm from 4 3/8 to 53/8 inches.

Quick-locking knobs (d) have a partof the screw thread removed toprovide a smooth hole for fastremoval when the knob is tilted.The knob locks or releases by rotating it one quarter turn. Range:Thread from 1/4-20 to 5/8-11,

diameter from 1 1/8 to 3 inches.



Screw Clamp Assemblies (See Fig.17-3 a through m.|

Strap clamp assemblies (a throughh) are suitable for a wide variety of clamping applications. They

Ch. 17


FIXTURE COMPONENTS

203

CHOICE OF TIGHTENINGDEVICE

SPHERICAL WASHERS



CLAMP STRAP

FINGER GRIPS CLAMP REST



JAM NUTS

SPHERICAL WASHERS

—I ; » { ] 1



(D

SPRING

g JAM NUT

STUD

KNOB SCREW ASSEMBLY



TAPPED END CLAMP STRAP

REST PLATE

Courtesy of American Drill HushingCo.

Fig. 17-3. Screw damp assemblies, aStrap clamp assembly; b. Strapdamp assembly-single end,

hexagonal nut; c Strap

clamp assembly-single end, acornnut; d. Strap damp assembly-single

end, hand knob; e. Strap clamp

assembly-double end, acorn nut; f.Strap clamp assembly—double end,



hand knob; g. End hand knobassembly.

also offer considerable flexibility since individual parts arestandardized and interchangeable;

e.g., the hex nut can be replaced by an acorn nut or a hand knob, or thesingle end strap can be replaced by a double end strap. The studs are

made of high-tensile steel, andstraps; nuts; spherical washers; andclamp rests are made of heat-

treated steel with black

oxide finish. Hand knobs arecadmium plated. Single end straps

(b, c, d) have machined finger grips



for easy lateral adjustment. Thestraps are spring loaded (lifted) forquick release and for holding them

n position when released. Sphericawashers compensate forrregularities between strap and

part and ensure rigid holding.Double end straps


FIXTURE COMPONENTS

Ch. 17

e

s



? f *fc

L^

^

^F

a



SPECIAL T-BOLT

OPTIONAL FLANGE NUT FOR

USE IN LIEU OF SPEED HANDLEWHERE ACCESS IS LIMITED

WORKPIECE

FIXTURE PLATE

X£j©

J-7T

W



H

C

3

JAM NUT

HAND KNOB



I

D

+ .000 -.001

LTERNATE STOP PIN



LOCATIONS

+ .000

(SOCKET HEAD CAP SCREWSREQUIRED FOR

MOUNTING-SCREWS NOTFURNISHED)

Fig

Courtesy of American Drill Bushing



Co.

17-3 (Cont.j. Screw clamp

assemblies, h. Removable dampassembly; i. Swing clamp assembly for reamed hole

mounting; j. Swing clamp assemblywith flange base; k. Hook clampassembly with socket head cap

screw.

(e, f) are used for clamping flatparts in the position shown in the

llustrations or can be inverted forholding round pieces in V-blocks orn nesting arrangements. With open

hex nuts (a, b) the as-



sembly provides maximumclamping range and has visualndication of thread engagement.

The acorn nut (c, e) protects thestud thread from dirt and damage,but limits the clamping range. The

hand

Ch. 17


205



—c -

THREAD



1/4H

FOR # 10 SOCKET HEAD CAPSCREW


Co. Fig. 17-3 (Cont.J. Screw clampassemblies, i. Hook clamp assemblywith stud and hexagonal nut; m.

Hinge clamp.



knob (d, f) provides quick clampingand release without need of awrench. With a double end strap,

the assembly can clamp two parts inone operation, and can also be usedas a single end strap against a clamp

rest permanently mounted in thefixture. Single end strap-Range:Thread from 1/4-20 to 3/4-10,travel G from 1/2 to 1 3/4 inches,

capacity / from 7/8 to 2 inches.Double end strap—Range: Threadfrom 3/8-16 to 3/4 10, travel from 1

to, 1 3/4 inches capacity from 1 5/8to 2 inches.

End hand-knob clamp assemblies

(g) combine the maximum



clamping range of the hex nut withthe quick operation offered by thehand knob. Rest plate made of heat

treated steel protects fixture body and assists in locating the strapprior to. ■clamping, Range: Thread

from 1/4-20 to 5/8-11, travel from1/2 to 1 1/2 inches, capacity from5/8 to 1 3/4 inches.

Removable clamp assemblies (h)permit complete and fast removalfrom the fixture of the entire

assembly. The hardened bottomnsert is pressed into a recessedhole in the fixture base, and thebayonet-type T-bolt is inserted into

the slot and turned one quarter turn



The hand knob, locked into placewith a jam nut, is used for insertingand removing the assembly. The

speed handle is for clamping andreleasing, and can be replaced by aflange nut for use where space is

imited. Range: Thread from 5/8-11to 3/4-10, capacity from 3 1/8 to 51/8 inches.

Swing clamp assemblies (i, j) aremade both for reamed holemounting and for mounting with a

flanged base. While all strap clampscan be swung free of the part, they require considerable space but theswing clamp assembly swings free

with much



ess space requirements. Theclamping screw has a swivel head toprotect the surface of the work and

adjust to irregularities. The arm canbe adapted to a left or right swingby installing a stop pin in a dowel

hole to one or the other side of thetail lug. Range: Thread on clampingscrew from 5/16-18 to 5/8-11, travefrom 5/16 to 11/16 inch, capacity is

not limited by assembly but isdetermined in the design of thefixture.

Hookclamp assemblies (k,l),available with socket-head capscrew and stud and hex nut, are

deal for use where space is



extremely limited. The hook isspring loaded for quick release andeasily swings away from the

workpiece. The body is an alloy steel investment casting withprecision ground diameter designed

for reamed hole mounting Range:Thread on cap screw from 5/16-18to 1/2-13, on stud from 5/16-18 to5/8-11, capacity from7/8 to 1 11/16

nches.

Hinge clamp assemblies (m) allow

fast, and completely free, access towork area. They can clamp directly on the part or be used for locking aswinging leaf. A hinged mounted

pad bolts to the fixture. A hand



knob swivel screw provides quick clamping and release, protects worksurface and compensates for minor

surface irregularities. Range:Thread from 5/16-1 8 to 3/8-16,capacity //max from 1 1/4 to 2 5/8

nches, throat A from 2 to 3 1/2nches.

Cam Clamp Assemblies (See Fig. 17

4 a through e.)

Long travel cam clamps (a) have ahand knob and a bayonet-type guidgroove in the stem. The hand knobs pushed straight against the work

to close,




Ch. 17

—c—J

— rB



JAM NUTS

SPRING

d JAM NUT

STUD



SINGLE CAM

REST PLATE

USE RISER BLOCK UNDER RESTPLATE TO INCREASE CAPACITY

Fig. 17-4. *Cam clamp assemblies, atLong travel cam lock clamp; b.tCenter cam clamp assembly-single

end; c. Center cam clamp assembly-double end; d. 'End cam clampassembly; e. Automatic cam clampassembly for quick release and

retraction.

* Illustrations courtesy of the

following companies: T American



Drill Bushing Co.; **MonroeEngineering Products Inc.

and rotated to clamp and lock. Thistype of clamp combines long traveland quick clamping better than any

other manual clamping device. Therotating pad on the end of the stemprotects surface of work-piece.Range; Diameter of stem from 3/8

to 1 inch, rapid travel from 1/2 to 21/2 inches, locking travel,equivalent to a cam rise, from 1/16

to 3/16 inch.

Cam-actuated strap clampassemblies (b, c, d) are available

with single end and double end



straps and center cam, and as singleend straps with plain and automaticend cam. The clamp strap is

actuated by a quick-acting clampingand release cam. The strap is liftedand carried by a spring. Spherical

washers permit adjustment of strapto surface irregularities,

Ch. 17


207

Nuts on stud in end cam assemblies

allow for adjustment in height and



compensation for wear. Center camsingle end-Range: Thread from 1/4-20 to 5/8-1 i, travel G from i/2 to 1

1/2 inches, capacity J from 5/8 to 15/8 inches. Center cam, double endRange: Thread from 1/2-13 to 5/8-

11, travel from 1 to 11/4 inches,capacity from 13/8 to 1 5/8 inches.End cam-Range: Thread from 1/4-20 to 5/8-11, travel from 1/2 to 1

3/8 inches, capacity from 7/16 to 11/16 inches.

utomatic end cam clampassemblies (e) have single endstraps. Cam action automatically retracts strap after release and

brings strap forward before



clamping in one movement of handle. Handle may be mounted oneither side of base block.

Range: Thread from 1/4-20 to 5/8-11, travel from 1/2 to 1 inch,

capacity relative to base from 1 3/8to 2 5/8 inches; capacity can bechanged by changing fixturedimensions.

Individual Clamping Components(See Fig. 17-5 a through d, and Fig.17-6.)

ll parts incorporated in theassemblies previously described are

ndividually available for use on jigs



and fixtures. Various other parts forsimilar purposes include plain andserrated end steel clamp straps (a),

matching steel and aluminum stepblocks (b), and chuck jaws (c).



SOFT CHUCK JAW (STEEL OR LUMINUM)

BORING POSITIONER

Fig. 17-S. Individual clampingcomponents, a. ffPlain and stepclamp straps; b. **Aluminum andsteel step blocks; c. "Chuck jaws; d,tChuck jaw boring positioner.

illustrations courtesy of thefollowing companies: 'AmericanDrill Bushing Co.; ''Northwestern

Tools, Inc.: **Monroe EngineeringProducts Inc.




FIXTURE COMPONENTS

Ch. 17

serts are then machined to thedesired diameter and profile.

Range: Clamping diameter from 11/2 to 6 inches.

Use in pairs to support plain clamp

strap.



igs. Plain and serrated end steelstraps-Range: Bolt size from 5/16 to1 inch, length from 2 1/2 to 10

nches, width from 1 to 2 inches,thickness from 1/2 to 1 1/2 inches.Step blocks-Range: Width from 1 to

2 inches, capacity (height of clamped part) from 3/4 to 9 inches.

Chuck jaw inserts (c), to be

mounted on the master jaws of standard 3-jaw lathe chucks, arcavailable in low carbon steel and

2024-T4 aluminum; in each casethey are made of bar stock, not castmaterial Since they are soft, they can be machined to fit a specific

part, thereby converting the chuck



nto a turning fixture. The materialsused permit easy machining. Steelnserts are preferred for most

normal applications; they stand upwell for medium-size productionots; for large production lots, they

can be carburized and casehardened. Aluminum inserts offersome special advantages: They protect highly finished machined

surfaces and parts made from softmaterials, and their light weightreduces the moment of inertia of

the chuck assembly and the loadand wear on the spindle bearings.Range: Length from 2 5/8 to 6nches, width from 1 to 2 1/2 inches



To assist in the machining of softnserts, boring positioners (d) are

available. The boring positioner is a

flat disc in the form of a three-lobecam. It is placed inside the masteraws of the chuck, and is rotated

with a screwdriver until the desireddiameter is obtained. With thisposition of the boring positioner,the jaws are drawn tightly against

the edge of the positioner toeliminate backlash, and the in-

Fixed Locating Components

through k.)

(See Fig. 17-7 a



Rest buttons (a through f) arenstalled in a fixture to provide leve

support of the workpiece and also

to prevent it from resting onaccumulated chips and dirt. Restbuttons arc made from heat-treated

steel, ground to size on principaldimensions.

Round rest buttons (a, b, c) can be

obtained with flat or spherical topsand for various methods of nstallation. The most accurate and

east expensive way of mounting isby a press fit (a, b). Buttons withflat head are made with headthickness (dimension B) precision

ground to final height or



manufactured to 0.010 to 0.014nch oversize to allow for finish

grinding after installation in the

fixture. A less accurate mountingmethod is by means of a screw thread on the shank of the button

(c) and a tapped hole in the fixturebase. These buttons are hexagonaland are made with oversize heightfor finish grinding.

Hollow rest buttons (d) can bemounted hy means of a separate

flat-head or socket-head screw. Thebutton is counterbored for thescrew head and precision ground tofinal height. This type of button is

available in heights up to 2 1/4



nches and is, together with thehexagonal screw-mounted button,also used as feet for drill jigs. It is

best not to use this button with thecounterbore in the top post tion assmall chips and dirt can lodge in the

counter-bore possibly resulting innaccuracy when the work-piece

rests against the button.

Rest buttons are also made withslip-fit shanks (e, f) for quick change. They are secured either by

separate lock screw (e) or by ascrew thread (f) on the outer end ofthe shank. Slip-fit rest buttonsrequire retainer bushings (g, h, i),

press-fit mounted in the fixture



base. Retainers are straightbushings (g) or shoulder bushings(h) for use with buttons with

separate lock screws. The threadedslip-fit buttons require retainers (0with a matching screw thread below

the bore. Buttons of various types-Range: Diameter from 5/16 to 1 5/8nches, height from 1/8 to 1 inch.

Rest pads (j) are used in lieu of buttons for level support of largeworkpieces in heavy-duty

applications. They are precisionground, ready for installation by means of countersunk, socket-headcap screws. Range: LengUi from 2

3/8 to 3 3/8 inches, width from 1 to



2 inches.

Ch. 17


209

+.0000 -.0005'

[



GROUND LEAD

RADIUS

+.010 +.014

FOR SOCKET HEADpR FLATHEAD SCREW SIZE-ISC REW NOTFURNISHED!

^17



THREAD SIZE

CENTER OF LOCK SCREW



1/8—1 *

1

— B — +.010 + .014

+.0000 -.0002

1

+.0000 -.0002

-A



+ .010 + .014


Co.

Fig. 17-7. Fixed locatingcomponents, a. Rest button(locator) for press-fit installation; bSpherical button for press-fit

nstallation; c. Threaded hexagonalrest button; d. Hollow rest button,also used as buttons for jig feet; e.Rest

button for slip-fit installation withock screw recess; f. Threaded

hexagonal rest button with straight



a through d.)

Jack locks (a, b) are used for

actuating (lifting and lowering)plunger-type intermediate supportsand locking them in place when

ifted to contact with

workpiece. Typical applications areto support castings and other rough

workpieces at points between thefixed locating points, to eliminatedeflection during machining. Whenreleased, the jack lock can be movedfreely in either direction. A quick twist of the hand knob locks theack by the expanding action of two

hardened steel shoes. A lock stop,



mounted on the side of the jig by one screw, prevents the unit frombeing rotated or pulled out of the

fixture. Range: Diameter from0.624 to 1.249 inches, travel from3/4 to 1 1/2 inches.

Spring jack locks (c) are completelyself-contained units comprisingplunger with cap, actuating spring,

screw with hand knob for releaseand locking, housing and (optional)base for mounting on fixture base.


Ch. 17


http://slidepdf.com/reader/full/jig-and-fixture-design-manual-erik-k-hendriksen3709 1490/2237r C-DOWEL HOLE DIA fTYPl



FOR (10 SOCKET HEAD CAPSCREW C BORED 1/8 DEEP

1/16— —

-.343 RADIUS




cap. Range: Travel 5/16 inch, heightextended from 2 3/8 to 2 9/16nches.

Eccentric leveling lugs (d) arecircular discs with a pivot hole

ocated off-center. They providepositive support with preciseadjustment. Position after finaladjustment is permanently secured

with a dowel pin and two dowelholes are provided for this purpose.Discs are supplied in soft steel or

carburized and heat treated. Range:Diameter 1 inch, thickness (height)from 1/4 to 3/8 inch.

Locating Pins (See Fig. 17-9 a



through o.)

Locating pins are for the precise

ocating of parts that are already provided with mating holes.Locating pins are either pilot pins

(a) or full-length pins; a pilot pinand a full-length pin are usedtogether; the full-length pin"catches" the part first, then

provides guidance and support asthe part is lowered to "catch" on thepilot pia Most pins are chamfered

on the end (b) or bullet nosed <e)to facilitate catching and entering.Locating pins arc either fully roundor relieved by four flats forming a

"diamond pattern" (b, c). The



diamond pattern provides thecombination of a close tolerance inone direction (the major axis in the

diamond) with a wide tolerance inthe other direction (the minor axis)Pin diameters (full round and

diamond) are

Ch. 17


211

3H

CAVITY FOR JACK PAD OR



CTUATOR

™gG

mi

LOCK STOP AND SCREW

FURNISHED

• WORK PIECE

EXPANSION SHOES



CTUATOR

LOCK STOP



+ .000

r B+.001

3**

-D-f-EHF

1



Courtesy of American Drill BushingCo, Fig. 17-8. Intermediate(adjustable) supports, a. Jack lock;

b. Adjustable support with a jack ock installed in a fixture; c.djustable spring jack locks (press

fit and flange base); d. Eccentriceveling lug.

offered in an "A " and a "B" range;

the "B" range is 0.001 inch smallerthan the "A" range.

Locating pins are press fit (a, b, c, gor slip-fit (e, f) mounted in thefixture. A slip fit is used where thepin must be interchangeable and

requires a matching retainer



bushing press-fit mounted in thefixture. Slip-fit pins are secured by means of a lock screw (e) or by

means of a threaded shank and

a nut (f). Round and diamond pins

are available with a knurled portion(larger than the pin) forembedment in plastic or castabletooling (h, i). Locating pins are also

used for locating of two fixtureparts relative to each other, whichrequires mating liners <d) in the

second fixture part. Standard drillbushings are suitable for matingbushings in most cases. Range:Most types of locating pins are




Ch. 17

c

17>64_J

±.005

h L + .Q0OO -.0005

ED



F~

+.0000

1 -.0002

n R PIL0T A0K H DIAMETER

ROUND

RELIEVED

—j f— 1/3 OF A OR I



SPHERICAL RADIUS

60° CENTER-BOTH ENDS

1/4 OF A— ■> f— SPHERICAL G-HRADIUS



-H r—1/3 OF A OR B PILOT

DIAMETER

r+ 0000 .0002

*.OOO0

-.0002-1 — 1/B 1

RELIEVED

ROUND



-.0003° J e j 1 U F

+.0000 -.0002

LOT

D AMETER A ORB

ROUND



M/30FAORB

r +.0000 0005

ROUND

RELIEVED

0000 0002



Courtesy of American Drill BushingCo. Fig. 17-9. Locating pins. a. Pilotocating pin for press-fit

nstallation; b. Round and relieved(diamond type) locating pins; c.(Left) Round and (Right) relieved

(diamond type) bullet-nose pins; d.Liner for bullet-nose pin; e. Roundand relieved (diamond type)ocating pins for slip-fit installation

with lock screw recess; f. Threadedround and relieved (diamond type)ocating pins with straight shank fo

slip-fit installation; g. Stepped



round and relieved (diamond type)ocating pins for press-fitnstallation; h. Knurled round

ocating pin.

available in diameters from 1/8 or

1/4 inch as the lower limit, to 7/8 o1 inch as the upper limit, and inengths from 1/8 to 1 inch.

Floating pin locators (j), mountedn a press-fit bushing, serve the

same purpose as the diamond pin,providing a close tolerance in onedirection and a

Ch. 17




213

♦ 0000

- 000?

SIOI WIDTH-



J

-1

-if

- A

3 CE

1/8 DIA 1/8 DIA



+.0000 -.0005

+.0000 -.0005

±

C-fDIA OVER KNURL)

m

Fig. 17-9 {Cont). Locating pins

Q



m";;.'/,r-;.>-7~7r//M

Courtesy of American Or HIBushing Co. Knurled relieved(diamond type) locating pin; j.

Floating pin locator, comprising pinwith bushing; k. Slotted hole locatobushing, for use with "L" or "T"pins; 1, Knurled slotted hole locatorbushing for plastic tooling; m. L-pin; n. T-pin; o. Double-actioncaptive L-pin,



wide tolerance (1/8 inch totalfloating travel) in the perpendiculardirection. Float direction is defined

and secured by means of a roll pin.Range: Diameter of floating pinfrom 1 /4 to 5/8 inch.

Slotted hole-locator bushings (k),press-fit mounted in the fixturebase with position secured by

means of a roll pin, serve the samepurpose as the diamond pin. With amating round pin they provide a

close tolerance in one direction anda wide tolerance (1 /8-inch travelallowed lengthwise in the slot) inthe perpendicular direction. Slotted

hole bushings are



also available with knurled outersurface (1) for embedment inplastic orcastable tooling. Range:

Slot width from 1 /4 to 1 /2 inch forpress-fit mounting, from 3/16 to 1/nch for mounting in plastic. L-pins

(m) and T-pins (n) are easily removable locators for temporary precision alignment of pre-drilledworkpieces in jigs and fixtures, or

for alignment of a drill jig plate on apart after the first hole or the firsttwo holes have been drilled.

Conventional L- and T-pins areoose parts and are removed from

the fixture when not in use. They can




Ch. 17

be obtained with a cable forpermanent connection of pin tofixture to prevent loss. Detent pinshave one or (usually) two spring-oaded balls embedded in the pin

body to provide a nonpermanentock and protect against accidental

withdrawal of the pia' Captiveocating pins (o) are provided with a

bushing

n which they slide with a precision

sliding fit. The bushing is



permanently installed in the jigwith a press fit or a slip fit (securedwith locking screw). Bushings are

available with knurled exterior forembedment in plastic tooling. Thepins are captive in their bushings. A

single-action pin can be pushed



a

IP

LINK

BELL CRANK

BELL CRANK



t

THREADED

TAPPED

f

LINK I



BUSHING

HEAD OF PLUNGER PIN LEFTSOFT FOR MACHINING

7

BUSHING NOT INCLUDED;

ORDER SEPARATELY



Courtesy of American Drill BushingCo. Fig. 17-10. Indexingcomponents.* a. Rotary cam

operated tapered (Left), andstraight (Right), indexing plungersfor standard mounting; b.

Suggested methods of adaptinghead of plunger pin to actuatingdevices; c. Spring-loaded straightndexing plunger.

Ch. 17


215



down the full length of the pin,retracted until the pilot end of thepin is inside the bushing, and held

n this position by a groove in thepin. Double-action pins have anadditional but reversed upper

groove that limits the downwardtravel. Range: Pin diameter from3/16 to 1/2 inch, length of travel upto 6 inches. L^ and T-pins can be

obtained with a

screw thread instead of the pilot

end and used as clamping screws,

Indexing Components (See Fig. 17-10 a through i.)



Precision made indexing plungersand matching bushings are themost critical detail required in the

\\\\\WM\\



-B *■

9

&



Fig. 17-10 (Cont.). *Indexingcomponents, d. ^Spring plunger; e.tSpring plunger mounted in a blindhole; f. tSpring plunger mounted in



a through hole; g. § Stainless steelball plunger; h. tliall plunger detent. tSpring stop.


Ch. 17



±0002 '3ALLDIA

6-32x1/4 DEEP

2S00DIA



g

+.0000

-49 DIA

31 DIA

BALL DIA-500 ±,0002



Courtesy of American Drill BushingCo. F%. 17-11. Miscellaneouscomponents, a. Toggle clamp of standard design; b. Toggle clamp fo

heavy duty, built of forged parts; c.ertical angle toggle clamp; d.

Push-pull toggle clamp; e. Standard

tooling ball; f. Shoulder tooling ball



g. Toolmakei's construction ball; h.Tooling ball pad.

construction of an indexing fixture.Plungers are straight or taperedwith a 15-degree included angle (a)

Plain indexing plunger units aremade without actuating devices.The plunger head is soft so that

t can be machined in accordancewith the design of the plungeractuator supplied by the customer(b). The plunger housing ismachined for press-fit mounting inthe fixture. Cam-actuated plungerunits are



Ch. 17


FIXTURE COMPONENTS

217

manually operated. Handle rotationof 180 degrees completely retractsplunger. Plunger housings are

standard mounted in a reamed holen the fixture, and secured in the

"whistle notch" by means of a nylontipped set-screw. Housings are also

available with mounting flange.Range: Housing diameter from 3/4to 2 inches, length up to 3 inches.



the ball plunger (g) where theplunger is a steel ball {low alloy steel or stainless steel); for accurate

positioning the mating part shouldbe provided with a hardened steeldetent (h). Range: Thread from 6-

32 to 1-8 (ball plungers down to 4-48), length from 7/16 to 2 13/32nches.

Spring stops (i) with circularcrowned or rectangular taperedpressure heads are used for holding

parts against locators and for light-duty locking functions. Range:Head size from 3/8 to 3/4 inch, endforce from 1 0 pounds to 32 pounds



Miscellaneous Components (SeeFig. 17-11 a through

Toggle clamps (a through d) aremanually operated linkage clampsbased on the same kinematic

principle as the eccentric clamp.When clamping, the toggle link ispushed slightly past dead centerand stays locked in this position.

Locking and release is done by aquick swing of the handle. Whenreleased, the clamping arm is

swung at least 90 degrees away fromi the clamping position andprovides excellent access to theclamping area. Normal working

position of the clamping arm is



horizontal (a, b), however, modelsare available with working positionof the arm 45 degrees up or down

(c) or 90 degrees down. A push-pullariation of the toggle clamp (d)

performs the clamping by means of

a sliding plunger. Toggle clampsrequire relatively large constructionand operation space; they aremanufactured in various series,

differing in strength and power.Range: Clamping force from 50 to3000 pounds, capacity below

horizontal clamp from 11/32 to 3nches.

Tooling balls (e, f, g) provide a

precision reference point relative to



a part or a fixture, for the adjust-

ment of the machine tool spindle

prior to critical machiningoperations. The ball and shank areconcentric within 0.0002 inch T.I.R

(total indicator reading). Thestandard tooling ball (e) providesmmediately a "visible" and

measurable extension of the axis of

the bore where the shank is locatedBy an additional measurementtaken from the ball to the part, the

ocation of the ball center is definedand available for axial adjustments.The shoulder tooling ball (f)provides a built-in reference

dimension from the shoulder to the



ball center. The tool-maker'sconstruction ball (g) provides thesame dimension and can be locked

n position by a screw in thenternal thread in the shank. Range

Ball diameter from I /4 to 1 inch.

The tooling bail pad (h) is asupplement to the standard toolingball and is used where the part does

not provide a suitable referencehole for the tooling ball shank. Thetooling ball can be adjusted to the

exact position desired and lockedsecurely in place by a cap screw lockwithout marring the tooling ballshank. The pad is drilled for

mounting on the part by a cap



screw, and the pad position can besecured by dowel pins.

Cast Iron Fixture Stock Sections(See Fig. 17-12)

Fixture stock consists of cast ironplates, blocks, and profiled shapesand is used in the design andbuilding of fixture bodies, thereby

saving time and expense forpatterns. The following sections areavailable: V-, U-, H-, L-, and T-sections, flats, and hollow squaresand rectangles. T-sections areavailable in equal and unequalshapes. The material is high-

tensile-strength cast iron; the stock



s machined square and parallel towithin 0.005 inch per foot on allexternal surfaces. The interior of

hoEow shapes is left unmachined.Range: V-sections have a 90 degreencluded angle, an opening width

from 1 1/2 to 4 1/2 inches, andheight from 1 1/8 to 3 inches. Othersections have overall dimensionsfrom 3 to 8 inches, rectangular

hollow blocks have up to a 10-inch-width, wall thickness of openshapes from 5/8 to 1 1/4 inches,

wall thickness of hollow blocksfrom 1/2 to 1 inch. 1

Other commercial products in

arger sizes and for more general



applications are also available. Theyare described in the chapters onfixture bodies, drill jigs, universal

fixtures, and automatic fixtures.

Tables with complete dimensions

for a number of the basic fixturecomponents are found in EricOberg and F.D. Jones, Machinery'sHandbook (New York: Industrial

Press, Inc., 1971) 19thed., pp. 1883-1911.

w

\.-:::■'■■■'



^P I



Courtesy ofEX-CELL-O Corp. l*ig.

17-12. Cast lion fixture stock sections, a. U-Sections; b. L-Sections; c. T-Sections (equal); d. T

Sections (unequal); e. V-Sections; f



H-Sections; g. Flats; h. Squarehollow blocks; i. Rectangularhollow blocks.

>

CHAPTER

18

Design Studies I —Drill Jigs

The Five Basic Design Steps

From the discussion in Chapter 3,The Fixture Design Procedure, itbecomes evident that there is agreat deal of similarity in the design



and design procedures for jigs andfixtures with respect to theprinciples of locating, clamping, and

supporting; the principal differencebeing in the guidance of the cuttingtool and, to some extent, in the

support against the cutting forces.s outlined in Chapter 3, the

systematic design of a fixture iscomprised of the following five

steps:

1. Designing a method of locating in

the jig or fixture which willcorrectly orient the surfaces on theworkpiece, for machining or othermanufacturing operations.



2. Designing a clamping methodthat will hold the workpiece firmly against the locators and against the

cutting forces.

3. [f required, designing additional

ntermediate supports that may beneeded to prevent the work-piecefrom springing or bending when its subjected to the clamping forces

and the cutting forces.

4. Designing or selecting the cutterguides; for drill jigs this means thedrill bushings.

5. Designing the jig or fixture body

to consolidate ail of the



components previously designed,nto one unified structure.

These five steps are fundamentaland of universal validity. In theirapplication, the designer must

consider the dimensions, material,and weight of the part to be handledn the fixture; the already existing

surface finish and accuracy of its

surfaces; the accuracy required inthe operations to be performed; thequantity to be manufactured; the

probability for multiple machining;and safety for the operator, theequipment, and the part.Furthermore, when working on the

design, the designer must keep in



mind the

possible procedures that a

toolmaker will employ to constructthe jig or fixture.

For the purpose of evaluating thedegree of sophistication andperfection to which the fixtureshould be designed, production

quantities can be classified asfollows:

This systematic fixture-design

procedure will be demonstrated by the application of the five basicsteps to a number of cases. This

chapter covers drill jigs; the



following chapters will deal withtypical fixtures for milling andother machining operations.

Case Series

Case J, A Jig Plate for a Small Part

Design a drill jig for drilling two3/8-inch (10 mm) diameter holes in

the part shown in Fig. 18-1. The parmeasures 5 by 2 1/2 by 7/8 inches(127 by 64 by 22 mm), is made of cold-drawn AISI 1030 steel, cut to

ength, finish-machined on theends, and weighs 3 pounds (1 1/2kg). Holes are to be drilled with a

twist drill; reaming is not required.



The quantity to be made isclassified as small-lot productionand called for the cheapest possible

type of tooling. Obviously, the sizeand weight of the part presents nohandling problem. Surfaces and

edges are sufficiently flat andstraight for locating, supporting,and clamping purposes. Thesimplest and cheapest type of

tooling is a plate jig made to thesame length and width as the part,as shown in Fig. 18-2. This solution

correlates with the five steps asfollows:

DESIGN STUDIES I - DRILL JIGS



Ch. 18

O

_1 J

DRILL-PRESS TASLE

Fig. 18-1

Fig. 18-2

Fig. 18-3



Fig. 18-1 Fig. 18-2 L'ig. 18-3 Fig. 18-

4

Fig. 18-4 fig- 18-5

sample part used for thedevelopment of open drill jigdesigns shown in this group.

Fig. 18-6

jig plate with bushings.



Clamping a jig plate with stackedparts.

Sample part, conventionally dimensioned. Fig. 18-5. A jig platewith locating pins. Fig. 18-6. A

modification of the shape of the jigplate.

1. The jig is located by laying it on

the part and lining it up along itsedges. The part is further located bysetting it, directly or indirectly, onthe drilling machine table.

2. Clamping is done by auxiliary components, not integral with the

ig. As shown in Fig. 18-3, the jig



4. For minimum cost, the jig plate imade with holes to match the drillto be used, and it is made without

bushings. This is acceptable forsmall-lot

production. For larger quantities,drill bushings should be used asshown in Fig. 18-2. The thickness othe jig plate can be taken as 3/4

nch (19 mm), equaling two timesdrill diameter, for good bearingength (see Chapter 14, the section

on Standard Bushings).

5. This step is automatically accomplished by making the jig

plate in one piece.



In a conventionally dimensionedpart drawing, the dimensions a, b, cand d, shown in Fig. 18-4, will be

given. For the drawing of the jigplate, the dimensions aretransferred to coordinates, as

explained in detail in Chapter 16.The upper long side and the left-hand short side are selected as theaxes, so that the coordinates

represent the distances from twoedges of the part to the holecenters. Starting from this drill jig,

which represents the simplestpossible design, a number of stepscan be taken to meet the moresevere requirements of larger



production and/

Ch, 18


221

or larger dimensions of theworkpiece. Since the parts have flat

parallel surfaces they can bestacked so that more than one pieces drilled at a time, as indicated in

Fig. 18-3. For other, more advanced

requirements, bushings are used, asndicated before, and the plate is

made of a carbon steel in the AISI

1025 to AISI 1035 range. These



steels are inexpensive and readily machinable and have sufficientnatural hardness to resist wear and

surface damage, caused by accidental nicking and scratching,reasonably well. They arc also

available cold rolled which, for thepresent purpose, would save themachining of the two large surfaces

The operation of the jig is improvedby the addition of locating pointsacting against two adjacent sides of

the work, two on the long side andone on the short side, as shown inFig, 18-5, The locating points arecylindrical pins, installed with a

press fit. Such points are cither



dowel pins or commercial locatingbuttons (see Chapter 17). Since theyact against flat, machined surfaces,

they are provided with flat contactsurfaces, as shown. The addition of pins makes the locating operation

faster, and the positive contact withthree pins ensures that all parts aredrilled identically alike. Figure 18-6shows a modified shape of the jig

plate, suitable for casting, andresulting in a saving in weight forarge jigs. When it is required to

positively clamp the part to the jigsugs A , shown in Fig. 18-7, are

added to the jig plate to carry fingerscrews for locking the part in



position against the locating pins.This eliminates

Fig. 18-7. A jig plate with clampingmeans,



the danger of accidentally losing thecorrect position. For long andnarrow workpieces two lugs on the

ong side are required, to avoidspringing (elasti-cally deforming)the part. A short and rigid part, such

as the one considered here, could bsufficiently clamped with one lugonly, as shown by the dotted linesat B. However, sloppy manipulation

could

result in inaccurate clamping.

Finally, the jig can be provided withfeet or legs, as shown in Fig. 18-8,which are screwed or pressed inplace to the depth defined by

shoulder A. If screwed, they are



ocked by means of small pins orheadless screws. The legs are madeof a carburi/.ing steel or a tool steel

the ends are hardened and, afternstallation, ground and lapped at

the ends to exactly the same length

Commercially available jig feet andegs are described in Chapter 17.

Fig. 18-8. A jig plate with legs.



Case 2. An Open Drill Jig

Design a drill jig for drilling two

5/8-inch {16 mm) diameter holes inthe part shown in Fig. 18-1. The parmeasures 7 by 3 1/2 by 1 5/8 inches

(175 by 90 by 40 mm), is fully machined from a gray iron casting,and weighs 10 pounds {5 kg). It ismanufactured in repeated lots of

100 pieces each with holes to bedrilled and reamed.

The quantities required can beclassified as upper limit of mediumot production, and justifies a

complete jig with all components

ntegral with or attached to the jig,



but without devices for automaticaction. With proper design, suchtooling will ensure

nterchangeability of the partsndependent of the operator's skill

and care. Application of the basic

steps leads to the type of jig knownas the "open drill jig" which is,essentially, a plate jig with the partsuspended underneath, and

standing on legs, as shown in Fig.18-9. Since it is open to one side, its suitable for casting, because it

requires little or no core work. Thedesign shown is cast, and proceedsas follows:

1. Holes can be drilled from one



side. The part has a machined flatsurface that is perfect for locatingagainst a machined surface on the

ig plate, and regular edges for sideocation with two plus one


Ch. 18



—i

IU- i j 1—»

! i I i

Fig. 18-9, An open drill jig.

ocating pins. The jig plate with pincan now be drawn. Minimum

thickness is 1 inch (25 mm), andthe machined face is raised 1/4 inch(6 mm) to provide a clearance allthe way around for machining.

2. The part must be clamped frombelow against the jig plate. It wouldappear natural to provide a strap



clamp at each end; however, thepart is so rigid that it can stand upto the pressure from the drill with

one strap clamp only,located at thecenter. This solution requiressubstantial dimensions. The strap i

2 3/4 inches (70 mm) wide by Inch (25 mm) thick and is slotted

for the 7/8-inch (22-mm) clampingscrew so that it can be pulled back

and clear the locating area fornserting and removing the part.

The heel block for the strap is

provided as a downward extensionof the jig plate. Three finger screwsare provided for locking the partagainst the locating pins.



3. No additional supports arerequired because the part is alreadywell supported over its entire area.

4. Drill bushings are inserted in theig plates. The bushings are press-fi

wearing bushings for use with slipbushings for drilling andsubsequent reaming. The length of each bushing is 1 1/4 inches (two

times hole diameter).

5. 1/4-inch (6-mm) bosses areadded on the upper side of the jigplate to avoid protruding bushings.

ertical side walls are formed alongthe perimeter of the jig plate, to

carry the finger screws. On the four



corners, the side walls are formednto legs with angular (L-shaped)

cross section, and made long

enough to lift all components clearof the machine table. Legs aretapered 15 degrees for maximum

rigidity with minimum weight, asshown in the sketch in the upperright-hand corner. Width b at theower end is 1 1/2 times thickness a

which again depends on the jig sizetaken as overall face area (lengthtimes width of the jig plate), as

follows:

This jig measures 10 1/2 by 7 1/2nches (270 X 190 mm), and the

egs are 3/4 inch (19 mm) thick.



Pads A are provided on one longand one short side, to support thecasting when laid out and

machined. A 4-inch (lQO-mm)-longhandle is cast on one end to give thoperator a secure grip during

drilling and reaming.

Case 3. An Open Jig with anIntermediate Support

Design a jig for a bracket with aarge boss. The part is shown in

chain-dotted lines in Fig. 18-10. It isa gray iron casting, approximately 13 inches

Ch. 18




223



Fig. 18-10. Plan and elevation of anopen jig with an intermediate



support.

(330 mm) long and 8 inches (200

mm) wide, weighing 35 pounds (16kg), with a large boss for a 2 1/4-nch (57~mm)-diameter bearing

hole A, to be faced on both ends. Ithas a flange, already machined flaton its free side, to be provided withthree screw holes B and two dowel-

pin holes C, bosses for screw holesB to be spot faced. The initial lotwill be 300 pieces and may be

repeated at a later date. Theawkward shape, weight, andquantity, call for a good jig withoutexcessive frills. All operations,

except a few facings, can be done



from one side which also offers anexcellent, flat, machined locatingsurface. The solution is an open jig

that can be turned upside down forthe few facing operations from theother side.

1. The jig plate can now be drawn. Is offset (Z-shaped) to conform with

the height difference between the

flange and the end of the boss ands provided with a raised, machinedocating surface to receive the

machined surface on the flange.Endwise locating is against twoocating pins at the small end

of the flange, with a screw at the



3. The boss is overhanging and notsufficiently supported against theheavy cutting-tool pressure to

which it is exposed. No possibility exists for supporting it on the freeside, but a substantial intermediate

support is provided by the 3/4-inch(19-mm) pressure screw E, carriedby a 1 1/2 by 3/4 inch (38 by 19-mm) strap F which is screwed to

the jig by two 5/8-inch (16-mm)screws G. Screw E supports the ribat a point as near as possible to the

boss. Strap F has a hole at one endand a slot at the




Ch. 18

other end, so that it can be swung

clear of the part by loosening, butnot removing, the screws G.

4, Holes B and C are plain drilledholes and require press-fit wearingbushings. Hole A is first drilled 1/8nch (3 mm) undersize, resulting in

1/16-inch (1.5-mm) stock allowancefor the final operation, which isdone with a chucking or machinereamer. These operations requireone press-fit liner bushing in the jigand two slip bushings for,respectively, the drill and the

chucking or machine reamer. The



tools for the facing of A, and thespot facing of B, are guided by pilotn the already finished holes, and do

not require bushings.

5. A long and a short leg is provided

at each corner. The overalldimensions of the jig are 17 1/2 by 10 1/2 inches (450 by 270 mm)which calls for 3/4-inch (19-mm)-

thick metal in the legs, The jig plates 7/8-inch (22-mm)-thick.

Compared to the 1-inch (25-mm)

thickness used in Case 2, this may appear to be on the low side, but thplate is substantially strengthened,first by its Z-shape which virtually

makes it a structural beam, and also



by the boss for the big bushing.Lugs, bosses, and pads are providedfor bushings, screws, and clamps.

Windows are cored out in the largeflat panels for weight reduction.The result is a cast fixture that is

strong, rigid, and as light as it canbe under the circumstances.

Case 4. A Modified Procedure for

Case 3

drastic saving in tool cost, boughtby an increase in the totalmachining time, is accomplished

by a combination of turning and

drilling operations. After a



preliminary layout for the center,the large boss is drilled, perhapsbored, and then reamed and faced,

either on a faceplate in a large latheor preferably, on the horizontaltable of a vertical boring mill (VBM

or a vertical turret lathe (VTL).Holes B and C are drilled by usingthe jig plate shown in Fig. 18-11.The jig is located by a plug in hole

, and manually aligned with theperiphery of the flange; theassembly is then clamped on the

drill table by a strong strap with aack screw for supporting the

overhanging end.

Case 5, Design of a Closed Jig (Box



Jig), of a Relatively Simple Type,with Positive Locating Means

Design a drill jig for drilling thefour 5/8-inch (16-mm) holes in thepart shown in Fig. 18-12.

The part measures 6 by 3 1/2 by 13/4 inches (150 by 90 by 45 mm). Is fully machined from a gray iron

casting and weighs 10 pounds (4.5kg). The quantity is small-lotproduction and calls for annexpensive jig. All holes are drilled

their relative positions must beaccurately maintained, but theirocation relative to the outline of

the part is not critical. Holes A are



drilled through, but holes B and Care blind holes and must be drilledfrom opposite sides.

When holes must be drilled inseveral directions, and bushings are

required, the drill jig must be of theclosed, or box, type, because it hasto be turned around for the drillingoperations. A closed, or box,



DfllLL-FRESS TAGLE

Fig. 18-11. A jig plate with means foocating from a bored hole.

Ch. 18




225

-7-

Hh

Fig. 18-12. A sample part used forthe development of the closed drill-ig design shown in Figs. 18-13

thiough 18-19.



ig can be described as an open jigwith a floor. Since the jig, more oress, completely embraces the part,

a door, gate, or port must beprovided to get the part in and out.With this difference in mind, most

design features in closed jigs are thsame as in open jigs.

The simplest solution is to make

two jig plates as in Case 1, one foreach side of the part, and buildthem together to form a closed jig,

as shown in Fig. 18-13. The part isplaced between the two jig platesand is located by two plus oneocating pins, which have flats to

engage with the machined edges of



the part. The jig plates are held intheir proper position by large-diameter dowel pins which have a

press fit in the lower plate and asliding fit in the upper plate. Toprevent separation, a screw with a

arge flat head or a fully countersunk, hexagonal socket-head screw is installed in one of thedowel pins. Holes are drilled in the

ower plate opposite the bushingsfor holes A to provide clearance forthe drill and for the escape of chips

Case 6. A Closed Jig with allComponents Integral with or

ttached to the Jig



Design a drill jig for the partspecified in Case 5 to bemanufactured in repeated lots of

100 pieces each. The designproceeds according to five basicsteps and results in the jig shown in

Figs. 18-14 and 18-15.

1. The part is located with its flatside on the machined pad R of the

ower plate /,, and sideways andendwise against two plus oneocating pins with flats to match the

machined side and end surfaces of the part, and is locked against thepins by two 1/2-inch (13-mm)screws U and Q. Obviously, one of

these screws may block the part



from entering, once the jig is fully designed and its closed characterhas become apparent. However, thi

problem will be solved in a laterstep.

—I ■=-:"

Fig. 18-13. A simple closed jig withocating pins.

2, The part is clamped against the



pad R, by two 5/8-inch (16-mm)screws J, from above. Thellustration shows an additional

screw / (in dotted lines) to indicatethat for parts of small dimensionsone clamping screw in a central

position may be sufficient.

r U

4 ..J- i. 11

roj

ft 17 ! •?. n r, \ ft (■ r^*

1 j * 1 «?i 5 I 5 1 1 1 f



■ j_;i ■ "

Fig. 18-14. A well-dcvclopcd closedig.

3. The part is so effectively supported that no inU-tnu'tliiLlcsupports are needed.

4. 5/8-inch (16-mm) press-fitwearing bushings are provided,

three from above and one frombelow. Bushings are 1 1/4 inches(32 mm) long for a bearing lengthof 2 times the drill diameter.



5. Upper plate K can now be drawn;thickness is 1 1/4 inches (32 mm),equal to bushing length. No end

clearance is required for burrs andchips, because there is a spacebetween the pan and the plate.

F=&-

JU=

^-fi-

*

-*T-TT

1 \



TT

^U=

s

Fig. 18-IS. A closed jig supported on

parallels.


Ch. 18

Side plates, also marked L (left side

of the illustration), are madentegral with lower plate /,; the

complete lower part can bemachined from a solid steel block



machining of some quite large areaof the jig.

Case 7. A Closed Jig with Legs

Design a drill jig for the partspecified in Case 5, with integralegs for the two operating positions

The jig is shown in Fig. 18-16. Steps

1, 2, 3, and 4 are the same as in Cas6. In Step 5, a boss for the hushingand four foot pads are added to theower part; four legs and a

machining pad around the bushingsand the clamping screws are addedto the upper plate. Machined

surfaces are marked f.



flr-fr

<£=«

Fig. 18-16. A closed jig provided

with feet and legs.

The screw Q is now carried by a 1

1/4- by 5/8-inch (32- by 16-mm)swinging strap E, which issupported at both ends and,therefore, provides a more rigid and

secure position for the screw. A



handle 5 is added on the end of theig to give the operator a safe grip.

Case 8. A Closed Jig Designed as anImproved Type of Leaf Jig

Design a drill jig for the partspecified in Case 5, with a swingingeaf, but without clamping means in

the leaf. While it may be aconvenience to use a commercialeaf jig with the clamping screw, or

screws, placed in the leaf, it also ha

ts valid objections. The clampingpressure is carried by the leaf andcauses an elastic deflection; if the

tight fit in the hinge and in the



seats against the jig body isoosened by use and wear,

thejposition of the leaf may shift

when the clamping screw istightened, causing inaccuracy in thedirection and location of the drill

bushings and thus faulty work. Forbest results it is therefore generallyrecommended to separate bushingsand clamping means and to use the

eaf for only one of these two typesof components, as shown in Fig. 1817 where the clamping is done by

two 1 1/2-by 7/8-inch (38- by 22-mm) strap clamps G with 1/2-inch(12-mm) screws, one at each end.

s usual, the straps are slotted for



easy withdrawal. This arrangements always recommended when the

clamps have to take a heavy drilling

oad as, for example, in operationsusing a multiple-spindle drillingmachine.

<S_M

r

~j

O--

H



ii it ft s n! it

Fig. 18-17. A closed jig with a leaf and clamps.

The leaf is here a 1 1/2- by 1-inch(38- by 25-mm) strap, carrying the

bushing for hole B and locked by means of a thumb screw H,sometimes formed as a quarter-turn screw.



Case 9. A Closed Jig with a One-piece Body and No Swinging Parts

Occasionally, it is possible to designa jig that is sufficiently closed tocarry bushings for all required

drilling directions and which stillprovides an opening, a port, largeenough to bring the part in and outEssentially this depends on the hole

ocation in the part. Chances arebest if the holes are placed near theedges.

n example is shown in Fig. 18-18.The part is the one specified in Case5. An examination of



Ch. 18


227



f±^\

Ftg. 18-18. A closed jig with a port

for loading and unloading the part.

Fig. 18-12 shows that hole B isocated fairly close to the two edges

The jig is now designed with only hole B drilled with the jig upsidedown, and the other holes with the

ig in the upright position. Thebushing for hole B is carried by aboss held on a bracket Dstrengthened by a rib B. With theclamp to the right withdrawn, theend of the part can be lifted and thepart pulled out in a tilted position.

There must be enough clearance,



not only for the part, but also forthe operator's fingers. The designers warned against overoptimism in

this respect. In reality, parts arealways larger and clearancessmaller, than they appear to be in a

drawing.

Case 10. A Closed Jig for Drillingfrom Four Sides

Design a drill jig for the partspecified in Case 5 with anadditional hole in the long side andone in the end.

The jig with the part is shown in

Fig. 18-19. Steps 1 and 3 are the



bushings E and F are added for theside and end holes. In Step 5,additional pads are added to act as

feet for the drilling operationsthrough bushings E and F.

General Definitions andClassifications

Drill jigs are used exclusively for

drilling, reaming. tapping, andfacing operations. Whenever acombination of these operations isrequired on a part, it is usually possible to design one single drillig for all of them. Drill jigs may be

classified as open jigs and closed

igs. Some subclassifications within



these two main classifications canalso be made. The open jig has allbushings mounted in the same

plane and with parallel axes. It hasno removable walls or leaves, thust is easy to insert and remove the

part. The simplest type of open jig ithe template jig, widely used in theaircraft industry, consisting of aarge sheet of aluminum,

magnesium, fiber, or laminatedplastic with the necessary bushingsand the means for locating it on the

part and holding it in position.

The most typical form of open jig inthe average machine shop is the

plate jig; it is applied to, and



supported by, the work. It may require clamping, but in many casethis type of jig is used on a finished

portion of the part which providesmeans for holding the jig inposition. The next step in

development is the plate jig withfeet, with the part clamped below the jig plate. If the four feet arereplaced by two parallel walls, the

ig becomes a channel jig. If twomore side walls are added, the jigmay still be a plate jig, but if these

side plates now are used fornstallation of drill bushings, then

the jig has developed into a box jig.

The open channel jig, as well as any



one of the more closed type jigs, canbe provided with a cover, often inthe form of a leaf, for fast

operation. The box jig is frequently formed as a tumbling jig, that is, ithas feet in several directions and is

simply turned over 90 degrees or180 degrees when the next side isgoing to be drilled. If there are holeaxes at angles other than 90

degrees, the jig can be supported ina cradle, and if there are a largenumber of axis directions, the jig

will be made as an indexing jig.

Operating with Drill Jigs

Before a jig is designed, it must be



decided whether the drillingoperation will go to a fixed-spindledrill press or to a radial drill. Small

parts that are easy to


Ch. 18

handle should always be routed to a

fixed-spindle drill press where theig with the part is moved so as to

bring one bushing at a time in linewith the drill spindle.

The moving of the part is usually done manually, but when

necessary, is assisted by a hoist or a



crane. The use of an air cushiontable greatly facilitates the shiftingof heavy tooling and is gradually

gaining acceptance in workshops. Ifthere are holes in more than onedirection, the jig is designed either

as a tumble jig or as a bracket-typendexing jig, depending on the

clamping possibilities found in thepart.

Large and heavy parts go to theradial drill because this machine

has a spindle that is easily movedfrom one hole position to the next,which permits the jig to be clampedto one of the machine tables. If the

part has holes in more than one



direction, there is again the choicebetween a tumbling jig and anndexing jig. A less than clearcut

situation arises with medium-sizeparts, parts that can be manually handled on the table of an ordinary

drill press, but not without somephysical effort by the operator. Thetendency is to prefer the radial drillf it is available. Modern radial drill

have ample rigidity

and a larger range of speeds and

feeds and usually more poweT thandrill presses of similar spindledimensions, also all handles andevers for the control of the

machine are located within easy



reach of the operator. They aredesigned for convenient and fastmanipulation with a minimum of

physical effort,

most important safety rule

applies to the manual handling of drill jigs. The torque exerted by even a relatively small drill exceedswhat can be safely held by hand.

ny drill jig, If it is not physically clamped to the machine table, mustbe positively restrained against

rotation. A stop block may besufficient for this, but a straight baror rail, contacting one full side of the jig, is better. A set of two such

parallel rails, with the jig sliding



between them, is frequently anexcellent solution. A rule-of-ihumbsays that the work can be held by

hand when drilling holes of 1/4-nch (6-mm)-diameter or less. Evenn this case the jig must be of such

shape and size that the operator hasa good grip on it, or it must beprovided with a handle.

The planner must recognize thatthe drill bushing steals some of theavailable length of drill spindle

travel. The following values areaverage and common:



Fig. 18-20. Using a medium-sizedrill jig.

Courtesy of LeBlond Inc.



Ch. 18


229

Typical examples of medium- and

arge-size drill-jig work are shownn Figs. 18-20 and 18-21. The firstllustration shows a medium-size

part that is to be drilled in twodirections. In the position shown,the jig is supported on a block of such thickness that the height to

the top of the jig equals the lengthof the jig. When the jig is turned 90degrees for the drilling of the end

hole, the height is the same as



before, and no adjustment of theclamps is needed. The secondllustration shows a box jig with a

separate jig plate. The jig ismounted on the indexing table of aarge work positioner that is not

part of the jig.

Placement of Jig Bushings

The fixture designer has no optionwith respect to the placement of thebushings, because they are

determined entirely by the drawingof the part. A recurring problem isthat holes are so close together

there is no room for the drill



bushings. The various possiblesolutions are shown in Fig. 18-22.The use of thin-wall bushings may

solve the problem, or two standardheadless or headed bushings areeach ground with a flat side and are

nstalled with the two flat sides incontact. In extreme cases it isnecessary to include two or moreholes in one single insert.

The distance from the end of thebushing to the surface of the work

s important. Various possiblearrangements are shown in Fig. 18-23. For maximum accuracy, thebushing should contact, or almost

contact, the work to provide



maximum support and guidance tothe drill. When full contact with atight close-up fit on the surface of

the part can be established, thistype of fit is preferable because itpositively prevents chip jamming

below the bushing. It can beachieved with the use of thethreaded type of bushing shown inFig, 14-15d, and later, in Fig. 18-28,

iew A, and also in many cases withplate jigs. In all other cases.it ispreferred, even necessary, to

maintain a short clearance betweenthe bushing and the surface. Thedistance is taken as 0,5 times drilldiameter for materials that produce



short chips,

Courtesy of LeBlond Inc.



Fig. 18-21. Drilling a large part witha drill jig plate.


Ch. 18



EXTRA-TH1NWALL

BUSHINGS WITH OD'S LESS

THAN THOSE OF USA BUSHINGSOF SAME ID

STANDARD HEADLESS OR HEADED BUSHINGS WITHGROUND FLATS ON OD'S

Courtesy of American Drill BushingCo. and Technological Institute,Copenhagen Fig. 18-22. Placementof drill bushings in a limited space.



and 1 to 1.5 times drill diameter formaterials that produce long chips.Materials with long chips can also

be drilled with the bushing incontact with the work. In this case,the chip is continuously guided up

to the surface of the jig. Excessivechip clearance is not used because ireduces the guiding effect of thebushing. Burr clearance for highly

ductile materials, such as copper, is0.5 times drill diameter.

Drilling on non-perpendicularsurfaces is facilitated (see Fig, 18-24) by carrying the bushing down tothe work surface and cutting the

end to the contour of the workpiece



In applications of this nature, thedrill point has a strong tendency toskid or wander. For this reason, the

distance between the bushing andthe workpiece must be held to aminimum so that the full guiding

effect of the bushing can be

NO CLEARANCE

(MAXIMUM PRECISION

DRILLING ONLY)

JIG

NORMAL CHIP CLEARANCE



EXCESSIVE CHIP CLEARANCE

WORKPIECE

EQUAL TO 172 ID (SMALL CHIPS)

1 TO 1-1/2 ID (LONG, STRINGY C

CUTTING TOOL WITH NORMALBACK TAPER



GUIDING

EFFECT

OF BUSHING

REDUCED

WORKPIECE

CLEARANCE



1 WORKPIECE SECONDARY BURR

£ : ig. 18-23. Drill bushing end

clearance.

Courtesy of American Drill BushingCo,

Ch. 18

DESIGN STUDIES I -DRILL JIGS

231

FIXED

RENEWABLE BUSHtNG

0 IRREGULAR WORK SURFACES



HEADLESS PRESS FIT BUSHING.USED FOR SHORT PRODUCTION

RUN .

^1

END OF BUSHING /

FORMED TO '

WORKPIECE J I6 CONTOUR

LOCK SCREW



END OF BUSHING CHAMFEREDTO CONFORM TO SLOPE OFWORK PIECE

FIXED

RENEWABLE

BUSHING

SLOPED WORKPIECE

WORKPIECE



LINER

FIXED RENEWABLE BUSHING

USED FOR HIGH PRODUCTIONPPLICATION

Courtesy of American Drill BushingCo. Fig. 18-24. Bushings for drillingthrough non-perpendicularsurfaces.

obtained. The side load exerted by the drill in applications of this types usually concentrated at a point

near the drill-exit end of thebushing and causes acceleratedbushing wear. Except in short

production runs, the use of fixed



drilling of matching parts. The ringshaped jig shown at A in Fig, 18-25s used for drilling the stud bolt

holes in a cylinder flange and alsofor drilling the cylinder head, whichs bolted to the cylinder. The

position

of the jig when the cylinder flanges being drilled is shown at B. An

annular projection on the jig fitsclosely in the cylinder counterboreto locate the jig concentric with the

bore. As the holes in the cylinderare to be tapped or threaded forstuds, a tap drill, which is smaller indiameter than the bolt body, is used

and the drill is guided by a



removable bushing of the propersize. Jigs of this type are often heldn position by inserting an

accurately fitting plug through theig and into the first hole drilled,

which prevents the jig from turning

with relation to the cylinder, whendrilling the otheT holes. When theig is used for drilling the head the

opposite side is placed next to the

work, as shown at C. This side has acircular recess or counterbore,which fits the projection on the

head to properly locate the jig. Asthe holes in the head must beslightly larger in diameter than thestuds, another sized drill and a



guide bushing of corresponding sizeare used. The cylinder is, of course,bored and the head turned, before

the drilling is done.

c

Fig. 18-25. A reversible, open-plateig.



DESIGN STUDIES 1 - DRILL JIGS

Ch. 18

Fig. 18-26. An open-plate jjg with a

centralizes

The jig shown in Fig, 10-46 is anopen, but not reversible, plate jig.



The jig shown in Fig. 18-26 at A andB is a centralizing open-plate jig ands used for drilling the ring shown

at C. For centralizing the jig on thework, it has three plungers ZJ,which are held against the conical

point of wing screw E by springs F,In operation, the wing screw E isturned back until the plungers Dare well within the body G, at point

H. The ring C is then slipped on andthe wing screw is turned down untithe plungers D are forced out and

nto contact with the inside surfaceof the ring. The ring is then drilledon a sensitive drilling machine, e.g.an upright drilling machine with



hand feed only.

The Box Jig

The leaf jig is a commercially available box jig and one example,designed for drilling a hole havingtwo diameters through the center oa steel ball, is shown

n Fig. 18-27. The work which isshown enlarged at A, is insertedwhile the cover is thrown back, asndicated by the dotted lines. The

cover is then closed and tightenedby the cam-latch D, and the largepart of the hole is drilled with the

ig in the position shown. The jig is



then turned over and a smaller drillof the correct size is fed throughguide bushing B on the opposite

side. The depth of the large holecould be gaged for each ball drilled,by feeding the drill spindle down to

a certain position, as shown by graduation or other marks, but if the spindle has an adjustable stop,t is preferably used. The work is

ocated in line with the two guidebushings by spherical seats formedn the jig body and in the upper

bushing, as shown. The work can benserted and removed quickly, and aarge number of balls can be drilledn a comparatively short time. A



typical, custom designed box jig isllustrated in A in Fig. 18-28, where, B, and C show the three position

n which this jig is being used. A isthe

^"*

>'\

\

U



Kig. 18-27. A box jig with leaf.

Ch. 18


233



Fig. 18-28. A typical box jig fordrilling in three different positions:

, B, and C.

oading and unloading position andB and C are the two differentdrilling positions. The work, in this

case, is a small casting with its form



ndicated by the heavy dot-and-dashines. This casting is drilled at a, b,

and c, with the two larger holes a

and b finished by reaming. Fornserting the work, the hinged cove

of this jig is opened by unscrewing

the T-shaped clamping screw s one-quarter of a turn, which brings thescrew head in line with a slot in thecover. The casting is clamped by

tightening this screw, which forcesan adjustable screw bushing g,down against the work. As this

bushing is adjustable, it can be setto give the right pressure, and, if thheight of the castings should vary,the position of the clamping



bushing is easily changed.

The .work is properly located by the

nner ends of the three guidebushings a L , b lt and c x and alsoby locating screws / against which

the casting is held by knurledthumbscrews m and n. When theholes a and b are drilled, the jig isplaced with the cover side down, as

shown in B, and the drill is guidedby removable bushings, one of which is shown at r. When the

drilling is completed, the drillbushings are replaced by reamerbushings and each hole is finishedby reaming. The small hole c, is

drilled in the end of the casting by



simply placing the jig on end asshown in C. Box jigs which have tobe placed in more than one position

for drilling different holes areusually provided with feet orextensions, as shown, which are

accurately finished to align theguide bushings properly with thedrill.

These feet extend beyond any clamping screws, bolts, or bushingswhich may protrude from the sides

of the jigs, and provide a solidsupport. When inserting work in aig, care should be taken to remove

all chips which mighl have fallen

upon those surfaces against which



the work is clamped and whichdetermine its location.

nother jig of the box type, which isquite similar to the one shown inFig. 18-28, but is arranged

differently, owing to the shape of the work and location of the holes,s shown in Fig. 18-29. The work

has three holes h, in the base, and a

hole at i which is at an angle of 5degrees with the base. The threeholes are drilled with the jig

standing on the opposite end y , andthe angular hole is drilled while theig rests on the four feet k, the ends

of which are at such an angle with

the jig body that the guide bushing



for hole i is properly aligned withthe drill. The casting is located inthis jig by the inner ends of the two

guide bushings w and the bushingo, and also by two locating screws pand a side locating screw q.

djustable screws f and f, in thecover hold the casting down, and its held laterally by the two knurled

thumbscrews u and v.

Jigs for Angular Drilling

When the work is to have angularholes, that is, holes that are to bedrilled at an angle with its basicsurfaces or planes, the jig must be

supported in an inclined position.



For drilling only one angular hole,


Ch. 18

-0=P-

m

tHfA——* -r '-VU



Fig. 18-29. A box jig for a smallcasting.

or a group of such holes withparallel axes, the jig is designed as

shown in Fig. 18-30. The work,shown here as a rectangular block,has one angular hole



Fig. 18-30. A jig for drilling holes atan angle.

Fig. 18-31. A jig for drilling holes at



an angle, and provided with aseparate base (a stand).

which is to be drilled through thebushing A, and the feet on theopposite side are machined to a

plane perpendicular to the axis of the bushing, as indicated by theangle a. The feet B are machined toa plane parallel to the faces of the

work and are used for supportingthe jig for the drilling of perpendicular holes. When it is

required to drill one hole, or agroup of parallel holes, at an angleand other holes perpendicular tothe face of the work and from the

same side, an arrangement such as



that shown in Fig. 18-31 is needed.The bushing for the angular hole is

. On the opposite side, the jig has

feet of equal length for supportwhen the perpendicular holes aredrilled. A separate base (also known

as a stand, an angle block, or acradle) B is provided to support theig in the required inclined position

for drilling through bushing A.

Separate bases are used not only foangular drilling, but sometimes toaccommodate the jig in cases where

t would be inconvenient to providethe jig with either feet, finishedbosses, or lugs, for resting directly on the drilling machine table.



The use of a separate base isapplicable to jigs of almost any sizeIt does require handling and is

therefore constructed as light aspossible, with a

B

Fig. 18-32. A jig with a swinging legfor drilling at various angles.

L_

Ch. 18




235

arge cored hole in the panel andwith ribs and webs for rigidity.

Manual handling is greatly reducedby the use of a jig with a swingingeg as shown in Fig. 18-32. This jig

s designed for the drilling of twoholes, one of which is at an angle.When drilling the straight hole, theig is in the position shown at A; for

drilling the angular hole, theoperator simply lifts the front of theig, and the swinging leg C falls,

bringing the jig into the position



shown at B, and places the hole tobe drilled in line with the drill. By using this jig, extra parts, such as a

separate base or angle block, areeliminated, and the jig is very quickly moved between operating

positions.

Jigs for Large Work

When a jig of large dimensions andweight is to be turned over, eitherfor the insertion or removal of thework, or for drilling holes fromopposite sides, it is advantageous tohave a special device attached to theig for turning it over. Figure 18-33

shows one such arrangement for



use where a crane or hoist is

^<l

ig. 18-33. A jig for heavy work,

provided with trunnions forturning.

available. A represents the jig which

s to be turned over. The two studsB are pressed or screwed into the jign convenient places, as nearly as

possible in line with a gravity axis.



These studs then rest in yoke C,which is lifted by the crane hook placed at D. The jig, when lifted off

the table, can then

easily be swung around. The yoke is

made of round machine steel.

For work of medium size andweight (in the range from 8 to 25

pounds) where crane assistance isnot feasible, much hard work can bsaved and production increased by outfitting the jig with rockers wherethat can be done without interferinwith the drilling operation. Anexample is shown in Fig. 18-34. The

work requires drilling from two



opposite sides, as indicated by thebushings and legs shown, and thethird side is available for the

rockers. They are made from steelplate, machined toa radius,andattached with screws. The

machining of the curved contourdoes not require high precision,since the jig does not rest on therockers during drilling operations.

The Vise as a Drill Jig

The machine vises such as are usedfor milling or planing operationsmay be used for drilling when they are provided with attachments for

holding drill bushings or locating



stops. By using suitable plates inthese jigs, many odd-shaped piecescan be drilled, and Fig. 18-35 is a

typical example. The method of using this plate is shown by thellustration. Bushings A are placed

n plate B at the proper location toguide the drills into the work. Theplate is screwed on top of the vise,stop C is adjusted to the proper

ocation, and the work D is placedn the vise against the stop, after

which the holes are drilled.

nother example of drilling in aise is shown in Fig. 18-36, where a

number of holes are drilled around

a circle. The work is gripped



between the jaws in the vise properand a bushing plate is located by pins A and B in the vise. By sliding

the vise to various positions theholes are drilled in the usualmanner. This bushing plate is

removable for taking out the work

jig construction adapted todrilling holes on an angle is

llustrated in Fig. 18-37. In thiscase, a

not it i h



Fig. 18-34- A drill jig provided with

rockers to facilitate reversing itsposition.

■"

236


Ch. 18



I s

Fig. 18-35, Vise with a jig plate.

swivel vise is fitted with a plate A,

which can be set at the proper anglen relation to the base B by

swinging the vise around axis C.



Jigs for Multiple Spindle Drilling

Multiple-spindle drilling machines

and multiple-spindle drill headsmounted on single-spindle drillpresses have their drills already set

n the required pattern. Some of these machines and drill headscarry their own drill jig, when they are provided with drill bushings

mounted in the manner shown inFig. 14-16; they require only a workpositioning fixture.

When no such devices are present,the multiple spindle machines anddrill heads need a drill jig for

ocating and clamping the part and



for guiding and supporting thedrills. This is particularly importantas the drills, in many cases, are

small and long.

These machines are used in mass

production, and fast manipulationof the drill jig is of the greatestmportance. Depending on the

number of drills in operation, the

oad on the drill jig is usually quitearge.

Indexing Drill Jigs

Indexing devices are described inChapter 6, Design of Locating

Components, and two indexing dril



igs

Fig. 18-36. A viae with V-blocks anda removable jig plate.

Ch. 18




237

Fig. 18-37. A tilting vise with a jig

attachment.

are shown in Figs. 6-35 and 6-36, A

special case of an indexing drill jigwhere the part is its own indexingplate, is shown in Fig. 18-38,

This jig was used for drilling dial



plates of the form employed onautomatic feed mechanisms forpower presses. These dial plates had

the center hole bored and thenotches milled to suit the locatingplungers on the power presses, but

the holes had to be drilled laterbecause they were located withreference to the particular presseson which the dials were used.

Before using the drill jig it wasnecessary to make center punchesto fit the punch-blocks on the

differ-

ent power presses and also to fitbushing A in the jig. Each dial plate

fl was then put on its bed and the



press was set in the usual way, carebeing taken to have the lockingdevice fit properly in one of the

notches. The center punch was thenmounted in the punch-block andone prick-punch mark was made on

the dial in proper relation to one of the notches. The dial plate was nextplaced on the table of a drill pressand the center punch was set in the

chuck in the drill spindle so that theprick-punch mark on the dial couldbe lined up with the spindle. The

plate was then strapped to the tableand stud C



Fig. 18-38. An indexing drill jig.


Ch. 18



driven into the center hole. The topof stud C was machined to fit thepivot hole in the arm D of the jig.

The next step was lining up thebushing A of the fixture with the

center punch in the drill spindle.The bushing was made adjustablerelative to the center C, about whichthe arm swung, so that it could be

set in the required position beforeclamping the binding bolt. Thebushing was located in proper

relation to the notches in the dialplate by means of the locking pawlE, and the eccentric screw Fadjusted the position of the pawl

relative to the arm D of the jig. The



pawl was held in the proper notchn the dial by spring G which was

mounted on pins H and /; and stud

J was used to hold the arm of thefixture true with the face of the dialplate. It will be evident that after

this setting had been made,bushing^ would be located directly over the center punch mark whichwas made on the dial plate while

the prick-punch was mounted inthe punch-block of the power pressThe hole could then be drilled in

the dial plate, after whichsuccessive holes were drilled by simply swinging the dial aroundpivot C, and locking it for drilling



each hole by dropping pawl E intosuccessive notches in the dial plate

Miscellaneous Drill Jigs

Interesting and characteristic typesof drill jigs have been used asexamples in previous chapters. Theig shown in Fig. 11-2 is a clear-cut

example of

a box jig with some good designdetails. Drill jigs for small parts tharequire the hole exactly in the

center of the part are shown in Figs9-17, 9-18, 9-19, and 9-20. A simple,nexpensive, and very versatile jig is

the bracket type of drill jig shown in



Figs. 10-37 and 10-38.

Where a large number of parallel

holes (six or more) all require theuse of one or several slip bushings,the time required for inserting and

removing the bushings becomessubstantial. Considerable time canbe saved by using a bushing holder,a plate carrying all slip bushings of

the same type, where all bushingsare removed and inserted in onemanual operation. The cost of a

bushing plate is negligible.

The smaller the part, the moremportant it is that the operation is

fast, and much can be gained by



using sophisticated, yet simple,devices for closing the jig andclamping the part. The drill jig

shown in Fig. 18-39 is a goodexample of the simplicity of designthat can be attained by using the

bayonet-lock type of clamp. A clampof this type also keeps the loadingtime down to the minimumrequired for economical production

The jig is composed of only sixpieces-the body, clamp, pin, andthree bushings. The side of the bod

opposite the drill bushing for theangular hole in the work piece ismachined at an angle of 90 degreesto the axis of the bushing hole to



serve as a base while drilling thehole. This arrangement eliminatesthe necessity of providing a

separate angle-block.

Wortrpfoc*



Fig. 18-39. A jig with quick-actingbayonet clamp.

Ch. 18


239

The jig is designed for drilling the

angular hole at A and the two holesB and C at opposite sides of thework. The workpiece comprises asubassembly of a high-pressure

alve and stud for a sensitive air-control valve which is part of an airbrake.



The body of the jig is bored to a slipfit for the workpiece. The oppositeend of the jig is flattened on both

sides of the center to meet thebottom of this bore. This providesopenings at D and E for the escape

of chips. A clearance hole F is alsodrilled through this end to clear thestud in the workpiece. The threedrill bushings are pressed into hole

that are accurately positioned in theig body. The clamp is a slip fit for

the hole in the body, which is made

arger than the locating bore for thework, so that the jig will be easier tooad. The top of the hole in the body

and the end of the clamp are



chamfered to facilitate insertion of the clamp. Bayonet slots G on thesides of the body are made a slip fit

foT the pin pressed into the clamp.These slots have a radius bend anda lateral section which permit the

pin to be given a clockwise turn.These lateral ends of the slots aremachined at an angle of about 95degrees to form cam surfaces which

give the bayonet lock its clampingaction against the workpiece. Theateral slot on one side extends in

the opposite direction from that onthe other side. The work should beclamped when the pin is at aboutthe middle of the lateral part of the



slots.

The length and diameter of the hub

on the end of the clamp are suchthat the hub clears the plane of theflat, on the side of the body on

which the jig rests when drilling theangular hole. This flat providessufficient surface beyond the centerine of the bushings to permit

drilling one side hole, and theangular hole, without causing theig to tip. Because it is necessary to

have a small hub at the end of theclamp, a hexagon socket ismachined in it to fit an Allenwrench, so that the clamp can be

easily tightened or loosened.



The jig shown in Fig. 18-40 has anequalizing member for thecombined clamping and closing

operation, and is used for drillingand tapping stud A, which is madefrom 1/4 X 1/4-inch (6 X 6-mm)

cold-drawn steel. The end of thestud enters hole B in the locatingblock, and this hole is milled toprovide clearance for the head of

the stud. The work rests on the drilbushing which is slightly counter-bored to provide clearance for the

tap. The interesting feature of theig is that the cover and clamping

mechanism are both secured by thesame knob; clamp C is swung



around its pivot to hold the studsecurely in place when the knob isscrewed down, and the same

operation tightens the cover. This

principle permits fast opening and

closing of the jig, and can beemployed on jigs and fixtures usedfor holding a great variety of parts.



Fig. 18-40. A jig with an equalizingmember for combined clamping

and closing.

Occasionally a hole must be drilledn the interface between two parts

that are fitted together, and a pin is



driven into the hole to act as a lock or key. In job shop work this isdone at assembly; under

manufacturing conditions it ispreferable to perform theseoperations on the two parts

separately, prior to assembly. Todrill such a "half" hole, it is usually necessary to plug up the hole in thework in some way that will back up

the side of the drill that is notcutting. This is accomplished, asshown in Fig. 1 8-41, by means of a

hardened stud A with a semi-cylindrical groove that matches halfof the surface of the drill. The studhas a push fit in the work and backs



up the drill during the drillingoperation. An angle iron or plate. Bs attached to stud A and held in

position by bolt C; plate B is alsodoweled in place. A hole is drilled inthis angle iron to receive bushing D

which guides the drill in the usualmanner. The remainder of the jigconsists of the key E which locksthe jig in place on the work.

In using this tool, key E is pulledback, clear of the work, and stud A,

which carries the angle iron, ispushed into the hole until the studmoves up against the shoulder of the work. By pushing up tapered

key E until it binds on the flat of th



work then tapping it lightly, the jigs held securely in place.


Ch. 18

Fig. 18-41. A jig for drilling half holes.



When drilling the hole, the work isset up on end on the drill presstable and the drill is then fed

through the bushing in the usualmanner, the bushing holding thedrill in position until it starts to cut

s the drill is fed down, there is atendency to force it away from thework, but this tendency is resistedby the hardened stud A so that the

half hole is drilled parallel with theaxis of the work. Even with a drillig, this is a difficult operation.

When drilling with an ordinary twist drill there is a tendency forthe drill to "hog in," which is apt toresult in the tool breaking, For this



reason, a drill with zero rake angles recommended; either a straight-

fluted or farmer's drill or an

ordinary twist drill ground in such away that it has no rake,

jig that clamps quickly, and withspring pressure, is shown in Fig. 1842. It is a representative example ofa homemade pump jig. The jig is

shown empty. A drill bushing A ismounted in a movable



traverse which, by spring pressure,s forced down and clamps on the

work. One locator is installed underthe bushing and another locator Bs carried on a bracket C. To

unclamp and open the jig, the

operating handle D, is depressed. Itswings around pivots E and liftsrods F which in turn lift the

traverse with the bushing against



springs G.

Tooling for N/C Drilling Machines

In N/C drilling machines, theocating of the spindle is

automatically controlled from atape- or card-operated electroniccontrol unit. Consequently, thesemachines do not require the

conventional type of drill jig withbushings for locating the drillrelative to the work. Most, but notall, N/C drilling



Fig. 18-42. A homemade pump jig.

Courtesy of Heald Machine Div.,Cincinnati Milacron Inc.

Fig. 18-43. A fixture with hand-operated clamps for N/C

drilling and boring of holes in a

circular part.



Ch.18


241

machines are used for job shop or

short run operations. This is ahighly competitive type of businesstooling expenses must be kept low

and the tooling is reduced to thesimplest possible type of fixture forthe sole purpose of supporting andclamping the work. Quite often, the

ob is done entirely without specialtooling.

The clamps used are, as a general



rule, hand-operated strap clamps.nother general requirement is low

construction height to avoid

collision when the table sweepsback and forth under the spindle.Two characteristic examples are

shown in Figs. 18-43 and 18-44. Ineach case, the fixture consistsessentially of an aluminum toolingplate with a few accessories. In Fig.

18-43, the part is centered in acircular recess in the plate whichalso carries four studs for the strap

clamps. In Fig. 18-44, the part isocated against locating pins (visibl

on the right-hand side) andclamped with strap clamps having



knurled-head hand knobs.

Where N/C drilling is applied to

arge-volume production, itbecomes economically feasible touse quick-acting and more

sophisticated clamping devices. A typical example is the drilling of circuit boards. These boards aremanufactured in fairly

arge quantities, in widely differentsizes and are stack drilled. For thesereasons, the clamping devices mustbe horizontally and vertically adjustable and quick acting, asshown in Fig. 18-45. The carriage

provides the horizontal adjustment



for varying board sizes, and the airclamp cylinder has sufficient lengthof travel to accommodate stacks of

ONE Of FOUR ADJUSTABLECIRCUIT BOARD CLAMPSFIXTURE RANGE BOARD SIZE Z"« 3" TO 12" x 24"

FRICTION CLAMP (NOT SHOWN LOCKS CARRIAGE IN POSITION



^CLAMP CYLINDER

Courtesy ofileald Machine Div.,

Cincinnati Milacron Inc.

Fig. 18-45. Air-operated andadjustable clamping device

for a fixture for N/C drilling of circuit boards.



Courtesy of Heaid Machine Div.,Cincinnati Milacron Inc. Fig. 18-44. fixture with hand-operated

ciamps for N/C drilling of a

rectangular casting.




Ch. 18

boards of varying height. Any automatic clamping device shouldcomprise a safety feature againstaccident or damage in case of failure of the operating pressure. Inthe present case, this is

accomplished by a pressure switch.Should the air pressure drop, themachine will stop. In other cases itmay be done by means of a toggleclamp or a self-locking eccentric,cam, or wedge.

N/C drill fixtures, as well as other



ariation of 0.020 inch (0.5 mm) oress, it is necessary first to spot dril

with a short and rigid center drill, o

to use a twist drill with a spiralground point.

drastic and illustrative example othe saving in fixture cost that may be realized by replacing con-

entional drilling machineequipment with an N/C drillingmachine is shown in Fig. 18-46. Thepart is a casting for a fuel pumphousing, and the fixture requiredfor N/C drilling of this part consistsof a base plate, an angle plate, a

bolster plate, and a clamping stud



with nut. The total cost of thisfixture was less than S600.00, whilthe cost of the various drill jigs

required for conventional drillingwas over S 5000.00.

n additional case is the instrumenframe shown in Fig. 18-47a, whichrequires considerable machiningwith end mills, and the drilling of a

arge number



--h \4 J5

I : I I I : j ' ti.

I =•

_SL

1

Fuel-pump housing Numerical

control holding fixture

Courtesy ofMetaiworking MagazineFig. 18-46. An example of



simplification in fixture design thatcan be realized with the use of N/Cdrilling equipment.



Courtesy ofCincinnati Milacron Inc

Fig. 18-47. a. An instrument frameto be N/C machined, b. Thenstrument frame in position on the

fixture for the N/C machining



operations.

of holes some of which also require

tapping. These toggle clamps, andtwo circular locators matching the

operations are performed on anN/C machine tool contour withinthe three small lugs inside the two

n the setup shown in Fig. 18-47b.The fixture is circular openings. By these simple means, the part

built up on a tooling plate as thebase and consists is supported,ocated, and clamped. The clamps



essentially of rectangular blocks,bolted to the base. which are hereshown in their retracted positions,

They carry the part, one cam-operated clamp, two present a low

profile relative to the part.

CHAPTER

19

Design Studies II — MillingFixtures

Milling operations are characterizedby large, periodically varying cuttinforces, producing a large volume of



chips, usually of small size. The toomay be a single milling cutter or aset of gutters. The operation is

normally completed in one pass of the cutter. In most operations thepath of the milling cutter relative to

the work is a straight line. Howeverthe fixture may be clamped on arevolving table for cutting an arc of a circle, or some other curve may be

cut as in tracer controlled andcontour milling.

To meet these conditions, millingfixtures must be sturdy, withrelatively large locating andsupporting areas and very strong

clamps. Wherever possible, cutting



tool pressure is taken up by positivestops, rather than by friction, whichmay fail under vibration. To reduce

oading and unloading time,fixtures for volume production areequipped with pneumatically or

hydraulically operated clamps.Hydraulic operation is preferred,since oil has less inherent elasticitythan air, and because hydraulic

actuators can be made with smallerdimensions for the same clampingforce. Pneumatic and hydraulic

clamping devices must have a safetyocking feature, as explained in

Chapter 1S, page 242, to preventaccidents in case of a power failure.



In principle, a milling fixture is abox, preferably of open design, i.e.,open at the top or at one side for

giving easy access to the cutter forreaching the surface to bemachined, and also to the locating

areas for cleaning away chips. Thestwo cases are illustrated in Figs. 1-2and 1-3, which show most of theocating, supporting, and clamping

components that are typical formilling fixtures. Attention is drawnto the tool setting block (7 in Fig, 1-

2) with which the milling cutter ispositioned for the correct locationof the cut. The gage pin in Fig. 1-3has the same function.



These features are clearly seen inthe fixture shown in Fig. 19-1. Thefixture base is mounted

Courtesy of Monarch Machine ToolCo. Fig, 19-1. A typical millingfixture mounted vertically on anangle plate.



ertically by means of an angleplate, as the face-milling operations done on a horizontal milling

machine. For the same operation ona vertical milling machine, thefixture would be mounted directly

on the machine table.

The milling machine vise withdetachable jaws or inserts,

contoured to fit the part, providesmany opportunities for the designof inexpensive milling fixtures.

Design details are given in Chapter10, Clamping Elements. Inproduction milling it is ofteneconomically advantageous to use

more than one fixture. The



combined length of the run-in andrun-out distance for the millingcutter is usually quite significant

relative to the net length of themachined surface, as illustrated inFig. I 9-2. A considerable saving in

operating time is accomplished by string milling, where a number of dentical milling fixtures are

mounted as closely together as

Ch. 19

DESIGN STUDIES II - MILLINGFIXTURES

245



^£3

<K

+-

K r

-H

.

+

Fig. 19-2. Milling of a single part,



and string milling, showing thesignificance of cutter run-in andcutter run-out.

possible, in a line, on a commonbase. Duplex, milling, i.e., milling o

two parts in one operation, is acommon as well as a profitableoperation. Multiple-spindle millingmachines naturally require multipl

milling fixtures and an example isshown in Fig. 1 9-3.



Courtesy ofCintimatic Div. of Cincinnati Mttacron Inc. Fig. 19-3.Milling with a three-spindle millingmachine, utilizing three milling

fixtures mounted on an angle plate



base.

The systematic fixture design

procedure outlined in Chapter 3,and exemplified in Chapter 18, willnow be applied to three widely

different cases of milling fixtureswith commercial components usedwherever possible.

Case 11. Design a fixture for themilling of the surfaces on the back side of the part shown in Fig. 19-4,the housing for the lead screw driveon a medium-size engine lathe.

The part is a gray iron casting,

weighing 45 pounds (20 kg). In



comparison, the complete fixtureweighs empty, 95 pounds (43 kg).The system of surfaces to be

machined on the back side consistsof an upper and lower longitudinalrecess extending over the entire

ength of the part, and recesses ontwo bearing parts at and near theright-hand-end of the part. They canbe gang milled in one pass with the

set of milling cutters shown in Fig.19-4. This operation is selected asthe first step because it constitutes

the major single operation on thepart, and it provides excellentocating surfaces for all subsequent

procedures. The design develops as



follows:

1. For the first operation, the part

must be located and clampedentirely on raw cast surfaces. Thispresents no problem because the

part has a regular geometry, witharge flat surfaces at right angles toeach other, and, in addition, thereare three bosses in a triangular

pattern on the front. Furthermore,all surfaces to he considered forocating, supporting, and clamping

are free of casting contaminations,such as mismatch and flash. In factthe part offers the possibility of aclassical application of the 3-2-1

principle, using hardened spherical



buttons as the locating points. Theocators are shown in Fig, 19-5 and

are identified by®. Three buttons

carry the part on the three bosses;two buttons on one side align thepart, and one button on the end

ocates it endwise.

2. The part is clamped against theocators by three 5/8-11 UNC (16 X

2mm) bolts, ©, arranged oppositethe side and end locators. Theclamping bolts are inclined 5

degrees, so that they aim below theside and end locators and force thepart down on the three baseocators.



3. A critical examination at thisstage shows that the part is notfully stable. If large forces are

applied outside the locator triangleand near the two corners, the partmay tilt over one side of the base

triangle by slipping slightly underthe clamping bolts. To prevent this,one or several intermediatesupports are needed. Applying two

more base supports near thecorners will provide the classical,rectangular, and very efficient,

support pattern; however, suchsupports must be individually adjustable, as they must act on rawsurfaces. Screw jacks may be ruled



out since they would not be easily accessible. Spring loaded jacks arequite long and would therefore

raise the part a considerabledistance above the machine table,thus substantially


Ch. 19

6,3751005

9.SO0:.009



38 REFTYP .12 ffff TYPMACHINING AlLOMNCt

E&SPTASSHOWH | :l.&MAXREF~i

cn'.Ol

TYP

MtOiiNINS ALLOWANCE TYP



■»3&

;00S -.000 ffff

r-53SP£F-

MIUtHG CUTTER

comouR

10&.0I



£

zzzzzz

SECTION A-A Fig. 19-4. A leadscrew drive housing for a lathe.

sacrificing rigidity in the setup.Endwise, there is no suchdimensional limitation, and a

spring loaded jack is mountedsymmetrically with, and parallel to,



Fig. 19-5. Components for locating,clamping, supporting, and cutter

guidance. The components arenumbered to indicate thecorresponding step in thesystematic design procedure.

the end locator. The jack appliestself to the surface of the part by

spring pressure, and is then secured



by the hand knob locking screw.These parts are identified by Q) ,

4. The relative location of thendividual milled surfaces is defined

by the milling cutter assembly, and

the cutter guide has to locate only one corner of one cutter relative tothe part. The cutter guide, (3), is ahardened steel plug, provided with

90-degree step with horizontal andertical guiding surfaces. To avoid

wear on the precision surfaces, a

0.120-inch (3.05-mm>thick feelergage is laid against the cutter guidewhen the cutter is adjusted.

5. The design of the complete



SPUING JACK LOCK

/.'l INCH EXTENDED L0CK1H0

SCPEVi ADB-V440

PLAINFIXTURE KEY HARB A&TS-

'l A08-*6765,024O4 —

PUPtHASEO COMPONENTS APE



IDENTIFIED 8f MANUFACTUPEPS PART NUMBER, FOL I OWEDBf N.U.FCM, STANDARD NUMBER

WHIPS AVAIL ABU

£~)8UK SM CAP SCREW

HAND KNOB SCPiW

^-—i-l)UNC'l£ ADB -30090

9.000^.002

■ CLAMP SCREW f -II UHC*!f

m„

-c -



LSOtjSou

AWACXK BM

u

wocuooe

r

^C7,

ZOiKQOl

■

SOOtOOtTYP -IM_^_ ..„ PRESSFIT SPHERICAL L -H "™-.<W

LOCATOR BUTTONf'j-JTCtSDIA

STEM A0D~4iSSS



— Je 7,S6l-.0Oi — —-*—ASSMOCB

»r,0j

-4.75 PCF-

SICTION S-S

2.25 PEF-



U9tX8—

IOO:Ot, TYP~

-IJSlfflOIA

fJ-SOZ^TfP

>IA SECTIONA-A p=a ;:=:-■=! r rh"03 T|, f 4--»»f Tf»

2fc»

r

WWW



-f t„-.Op I ^-Mil LI BfAMJTSIWfW

1

n ^1X> fOPPRESS FIT ONIQCATQP STEM

USOWF-

SECTION C-C

SPACE FOR __ . ,j,bjl nfl»

U&ftff FEELER SASE-.\ * 3 ' X -W'



«c

W25*ff W JTJ-'OW =f-

— .tj-'.w t.oozD3

001

-,$li.01 MS

SECTION D-D -

GRINDING ALLOWANCE

010 REE ^-l_-.2»-*«tf,

^@" M '



| L ggy'jiKWjaa

—| ' \-OS*OS

Fig. 19-6. The complete millingfixture.

ifted from the bottom on bosses tofacilitate machining and to ensurethat they stay cleat of chips. The

side walls have windows for chipremoval. The fixture is bolted to themachine table with four T-bolts,and is aligned with two keys in one

T-slot. The closest tolerances on thepart are those controlled by themounting of the milling cutters on

the



arbor; the remaining tolerances arequite liberal. Therefore, there is noplace where the conventional

toolmaker's tolerance of 0.001 inch(0.025 mm) is really needed, and altolerances on the fixture except

those for press fits) are 0.002 inch(0.050 mm) or multiples thereof.

n_

73~

MA J E RIAL: GRAY CAST IRON



L- 325W -

UND/MEHSIONEB RADII25'02 R

Fig. 19-7. A slide base for a specialmachine toot.


Ch. 19



Case 12. Design a fixture for themilling of the upper and sidesurfaces of the part shown in Fig,

19-7.

Detailed drawings of the fixture

body and the clamp strap are shownn Figs, 19-g and 19-9, while thecomplete fixture is shown in Fig 19-10.

clamping surface for thesubsequent milling operations of the upper surfaces. The outline of the milling cutter assembly for thisoperation is shown in Fig. 19-7. Thedesign proceeds as follows:



1. With some modifications, the 3-2-1 principle can be applied. Thefixture body has a large ma-

JSi'.Ol

I *J 10010/

«—4— iS&M

ZiQt.ai t~-TW3 K .



mamuL-eiurctsi iron

Tig. 19-8. Detailed drawing of the

fixture body.

The part is a gray iron casting,weighing 13 pounds <6 kg). Incomparison, the complete fixture,empty, weighs 52 pounds (24 kg).The part is to be used as the base

for a small slide in a specialmachine tool and the surfaces to bemilled form the guideways for theslide.

The first operation to be performedon the part is, naturally, the

machining of the bottom surface.



This operation requires no fixturebecause the size and shape of thepart permit it to be securely

clamped in the milling machineise. And once the bottom is

machined, it offers an excellent

ocating and

^ -10 bNC CLAMPtNQ S TUP

■usacr ecxxim misolggatk



^iooo*-SSP

1JS--.W

1 -"-'.&'

UHDIHlmiOHL i> MWI .15101 S

*F-flv™w r OHMfflj mmmii nit

MAT£WAl ■■ AW 4T40$T{EL HTTOIsajjOOPii ALL su&tt£S T^

Fjg. 19-9. Detailed drawing of the

clamp strap.

chined flat surface to receive themachined bottom surface of the

part. This is the equivalent of the



first three locators. Dimensions andtolerances for the part indicate thata considerable degree of symmetry

s required, which includes the twoun-machined edges of the baseflanges. It is therefore necessary to

provide a system of centralizcrs,acting on the side edges of theflanges. In the present case, this isaccomplished by an unconventiona

design of the clamp straps. Eachstrap is fork shaped and the fork prongs have downward projecting

strips arranged in a V-shape, asseen in Fig. 19-9.

These two V's act on each two

corners of the flanges. To confine



fit in the fixture body. The locatingcomponents, thus described, aredentified by (T) in Fig. 19-10.

2. The part is clamped against thebottom locating surface by the two

fork-shaped clam p straps and two3/4-10 UNC (20 X 2.5 mm) clampstuds with

Ch. 19


249

LLOWANCE FOSFFElFUCAGf



ciampmvbA tointe-tessoi-

CU7 T04-IKtri£MGm GROUND TO7S0,

MXIGOHNW,i~IO

W»aii7s) —— sphih waitiiASBirfori* -m srus



LAMllllS)

SO

TOOL-STEEL

HT. 70S*-56 ROCKWELL C

ftteoijtMtiur, i~to-

1106-25425) *

ROUHDPCSTBUTTOK,

FffsSFlT

t'-£'i-!MB-t623tl

SECTION A-A



Fig. 19-10. The complete millingfixture for the part shown in Fig, 197.

PLAIN FIXTURE KEY-6B71r4-l

IK* ■?* SJH CAPXREn' IABB--H76

-M745;

CCHMEIKiAL COMPONENTS UK IBtHIIttCB Bf HANUfJkCTUKtlfSNt/HBCR

mtwtoaHBB

nuts and spherical washers. Eachstrap clamps on the part at two



points, and is reacted by the restblock under its tail end. The designs such that the stud is located

approximately in the center of gravity for the three pressurepoints, so that the total clamping

force is distributed quite evenly onthese three points. When inoperation, the strap opposite theend stop is moved forward so that

the part is brought into contact withthe end stop. The force from themilling cutter acts in the sa.me

direction. In view of these facts, andthe substantial vertical clampingforces exerted by the two straps, noadditional longitudinal clamping



means are needed. The clampingcomponents are identified by (2) inFig. 19-10. Note the details of the

design of the tail end which permitsthe strap to tilt and adjust itself toany unevenness in thickness of the

flanges without binding of the tailend. Had there been lifting springsunder the clamp straps, theoperator would have found them

convenient. However, the availablespace is too narrow to allow thenstallation of such springs.

3, Because of the rigidity of the partand the uniform support which itreceives from the base, there is no

need for any intermediate supports



4. As in Case 11, the relativeocation of the individual milled

surfaces is defined by the milling

cutter assembly, and the cutterguide has to locate the cutterassembly in the vertical and

horizontal directions. The cutterguide, (4), in Fig. 19-10, is ahardened plug of tool steel,mounted with a press fit in an

extension of the rest block, at thatend of the fixture which is oppositethe end stop. The cutter guide has

horizontal and vertical locatingsurfaces, with dimensions thatallow the use of a

120-inch (3.05-mm) feeler gage



when setting the cutter. In thiscase, no detailed drawing isprovided of the cutter guide, but it

s recommended that the reader, asan exercise, make a completedrawing of the cutter guide

(including grinding clearances, if needed), and calculate the requiredtolerances.

5. The design of the fixture body,shown in detail in Fig. 19-8 anddentified by (?) in Fig. 19-10,

follows almost automatically fromthe previous discussion. Essentiallyt consists of a heavy base with theocating surface for the part, two

rest blocks at the ends, and slots for



the keys and T-bolts which alignand secure it to the machine table.

ll tolerances, except those for the

cutter guide (to be calculated in therecommended exercise), are quiteiberal. There are no closed spaces

and no chip cleaning problems. Thedesign lends itself well to castingand requires no core work.However, it is equally well suited

for welded construction.

rJ^

==j=l '■'■»«-

1 *«3I



^ i ^r

■-1+

T^

T

r Mt.03

Hi TEPtfAl.- GflW CASF WOrV WrOt£MHC£D OtUfftHQHSWOVOt TWO WttMCHfNFP

SUBfJKXS H*VF^



Hg. 19-11. A bracket with twobearing bosses.


Ch. 19

DRILL AND RCAM AT ASSEMBLY -fr-tltmC-t FOR JS DIA DOWEL

PIN, 2PL —, ° /



ZSQ&jOQQ fyp

2,001



{-20UHC ~IOD:.0l KErSEAT

30 a*

mpi in typ

J-- SOU jVC 75fl — UOOOI

-f-HUNCTYP-i



7i CIA f

MATERIAL-.GRAY CAST IRON

UNTOLERANQLQ DIMENSIONSTO ONE OR TWO UNMACHINEDSURFACES HAVE?J$

UNDIMEMSIONEO RADII 2S-.02R

Fig. 19-12. Detailed drawing of thefixture body.

Case 13. Design a fixture for themilling of the base surface of thebracket with two bearing bosses as

shown in Fig. 19-11.



Detailed drawings of the fixturebody, a V-block locator, and theclamp straps, are shown in Figs. 19-

12 and 19-13, while the completefixture is shown in Fig. 19-14. Thepart, a gray iron casting, is a bracke

with two bearing bosses, andweighs 28 pounds (13 kg). Incomparison, the complete fixture,empty, weighs 136 pounds (62 kg).

The part comes unmachined, and its natural to select the machining o

the base surface as the firstoperation since this providesexcellent conditions for fixturingthe following operations. This

choice is not without its problems,



since the part has no other flatsurfaces on which it could beocated and clamped for the first

operation. However, the bearingholes are cored out to 1 3/8-inch(35 mm)- diameter, so that the part

can be well clamped in the coredholes while it is located and carriedon the cylindrical outer surfaces of the bosses. This method of locating

assumes that there is no partingplane with its inherent danger of mismatch across the bosses.

Depending on the type of millingmachine to be used (horizontal or

ertical), the milling cutter is eithera plain milling cutter with helical



teeth, slightly longer than the widthof the

part, or a face milling cutter with adiameter substantially greater thanthe width of the part. It is left to the

reader, as an exercise, to make arecommendation for the diameterof the face milling cutter. When itcomes to the detailed planning of

the operation, the direction of thecutter tooth helix, or the teeth inthe face milling cutter, together

with the rotation of the millingmachine spindle, must be such thatthe side component of



twEftm rtisi toio met ' fiAO R £r

UHTOl 10AI& BtNEHSfONlHAVE

10/

fff -ENYAAHT COAMAS HXVf.oSftfitf ; r

ai t sv*FAcejJ~mAs /.orft? jo JH-*-*z L



-BiOCfr CLAHI

f v rGtASl

•JStZE MiQ m*MH-

*Of* —7

Fig. 19-13. Detailed drawing of the-block locator and the clamp strap

Ch. 19

DESIGN STUDIES li - MILLING

FIXTURES

251

MM NUT 4--IWNC (*DB-25*20)



-MXHEAD SCREW ■$■-ItUNC 2-£LONG

g M^Tti-i Qx, — mm

—/—i i*b

SPHER. LOCATOR BUTTON PRES



FIT

wp aw iuau

COMMERCIAL COMPOHEHTSREIOE/JT/FIEDBY

MAHUFACTURERS NUMBER INPAQIHTHESIS

HAND KNOB SCREW

flOMW ffliST BOT70W PRESS FIT4

grwng ro height wm-ffasG



clamp jroa J -/o iAve-4osm „

CUTTOifLEhlSTH HEX NtlT-i

I rtMb-2537.

SPHER.WASHER I —

rATStYfOP-flQ

/ $7Vt>it£>6-2i?W ~ —

FLAT WASHER I—



2f LOW ,'iO5-:-0C9J,

EJZO'i&LALLQWANCE FOR

■XXX> n£l£ReAe£

-"XT"

400 REF

SJLOIP SCREW a-tlUHCILONG

LSXTOfIT + JWO ZfLOHGtADB-SiOlQi

JAMNlTT^-IOtlMC

■rtky



t*0S-Z34g

■PL* IN FIXTURE XEYM75 *■£»!W-A--JtS.H.CAP

scRE:#0to#~*tnsj

Fig. 19-14. The complete millingfixture for the part shown in Fig. 1911.

the cutting force is acting against

the side stop In the fixture. The



design proceeds as follows:

I, The part can be located and

carried with the two bossessupported in a double V-block,shown in detail in Fig. 19-13. The,3-

2-1 principle is not directly applicable, hut the support in the Vblock eliminates four degrees of freedom; namely, two in the vertica

direction and two in the sidedirection. At the same time, thebearing axis is centered. The part

can still rotate in the V's aroundthis axis, and it can slideongitudinally, thus having two

degrees of freedom. These two

freedoms are now eliminated by the



addition of a side stop and an endstop. The side stop is formed by a5/8-11 UNC (16X2 mm) hexagonal

head screw, acting against the sideof the base. The screw is locked inposition by a jam nut. In this way,

the position of the side stop can beadjusted when necessary, as, forexample, if a batch of castingsshould fall outside of dimensional

tolerances. Only one side locator isrequired, as the direction of thebearing axis is already defined by

the V's. The end stop is a sphericalbutton. The locating componentsare identified by (T) in Fig. 19-14.

2. The part is clamped down into



the V-block by two finger-typeclamp straps, shown in detail in Fig19-13, and two 3/4-10 UNC (20 X

2.5 mm) clamp studs with nuts andspherical washers. The straps haverelief grooves cut across at a

distance of 1/2 inch from eitherend. These details allow the strapsto seek the lowest point in the coredholes, as the nuts are tightened, and

to adjust themselves to slightdimensional variations in the partsLifting springs around the studs are

provided to hold the clamps upwhen in the retracted position, forthe convenience of the operator. A 5/8-11 UNC (16 X 2 mm) hexagonal



head-clamping screw is provided toclamp the part against the side stop

5/8-11 UNC (16 X 2 mm) hand-

knob screw is provided to clamp thepart against the end stop. Here ahand-operated screw is preferred

because it provides more "feel" inclamping, than a hexagonal screw operated with a wrench. Inclamping, the part will be laid down

nto the V's, and manually heldagainst the side and end stops,while the hand-knob screw is

applied. Subsequently, thehexagonal head screws aretightened. The clampingcomponents are identified by (2) in



"feel" so as not to strain the part ouof the position in which it is held bythe previously applied clamps.

These additional supports comprisetwo more 5/8-i 1 UNC (16X2 mm)hand-knob screws, (3) in Fig. 19-14

arranged opposite each other.

4. The cutter can be sidewiseocated by sight, since it is visibly

wider than the part. The verticalLocating of the cutter is done with acutter guide consisting of a round

rest button, installed with a press fin one of the four uprights thatform part of the fixture body. Thisparticular upright is 1/8 inch (3

mm) higher than the three other



uprights and is machined at the topn allowance of .120 inch (3.05

mm) is provided for the feeler gage

used in the setting of the cutter. A recommended exercise, is tocalculate the tolerance on the

height dimension of the cutterguide, @ in Fig. 19-14.

5. The design of the fixture body,

shown in detail in Fig. 19-12 anddentified by (3) in Fig. 19-14,

follows almost automatically from

the positions of the locating andclamping components. It consistsessentially of a heavy base, fouruprights, and two upper cross bars,

carried by the uprights. The base



has the locating surface for the V-block, two rest blocks at the ends,and slots for the keys and T-bolts

which align and secure it to themachine table. The four uprightscarry the side stop and its clamping

screw, and the two additional hand-knob screws. One upright carriesthe cutter guide. The two uprightsn either side are connected by a

5/8-inch (16-mm>thick wall foradded rigidity.

daSf.



WORKPlECE

- fix tunc

HI CH TABLE

Fig. 19-15. a. A milling fixture with

manual clamping, b. cylinder head)milling cutters, locating points, arid

_(—u-rmr—i

TONGUE STRIP

Courtesy of Cincinnati MilacronInc. A line drawing of the samefixture showing workplace (a thegage block.



Ch. 19

DESIGN STUDIES II - MILLING

FIXTURES

253

Courtesy of Cincinnati MilacronInc. Hg, 19-16. A milling fixture



clamped on the machine tool tableby means of end clamps.

Courtesy of Cincinnati MilacronInc. Fig. 19-17. A milting fixturemade by modifying the jaws of a



swiveling milling machine vise.

ends itself equally well to welded

construction or to a combination ofwelded and built-up construction.While the V-block is a precision

part, all tolerances within thefixture body itself are quite liberal.The only dimension that requires asomewhat close tolerance is the

height of the cutter guide.

Typical Milling Fixtures

typical milling fixture is shown inFig. 19-15, a and b. The work is acylinder head, outlined in Fig.

1915b. It has a previously machined



surface and is located by thissurface on five blocks, each withtwo narrow bearing surfaces. The

clamps are hand-operated clamps onormal design. Further locating isdone by two locating pins fitting

nto two holes In the block {toolingholes). The cutter setting gage isocated on a bracket on one side of

the fixture. This fixture is aligned

with the muling machine spindle bymeans of two keys, called "tonguestrips," one at each end, which fit

nto a T-sIot in the milling machinetable. The fixture also has twoordinary slots at each end for theclamping bolts. While this is a



widely used practice, it may be lesspractical when a fixture is expectedto be used on several milling

machines as the spacing of the T-slots may be different on differentmachines. It is, in this sense, more

practical to use end clamps, asshown in Fig. 19-16.

The milling machine vise with

modified jaws provides many opportunities for the design of nexpensive milling fixtures. An

example is shown in Fig. 19-17.String milling is used extensively,and some examples are shown inFig, 19-18.



Duplex milling, that is, the millingof two parts in one operation, isalso a common and profitable

operation.

Contour and Profile Milling

Fixtures

Contour or profile milling fixturesare used on profiling or contour

milling machines. These fixturesare basically similar to otherfixtures; however, a distinctivecharacteristic of this type of



Courtesy of Technological InstituteCopenhagen Fig. 19-18. Examples ostring milling.


Ch. 19



Courtesy of Cincin na ti Milacron Inc. Fig, 19-19. Milling fixtures andtemplate bracket for profile milling

cylinder heads. Fixtures andtemplate are trunnion

mounted.

equipment is that it must providefor a bracket for holding the cam or

template which controls theoperation of the machine. Thefixture must also have setting gagesfor aligning the fixture and thetemplate bracket with the machineand the tracer spindles, as explainedn Chapter 13, Cutter Guides. Two

or three spindles are frequently



found in tracer controlled andcontour milling machines. They require the corresponding number

of identical fixtures forsimultaneous milling of severalparts. An example is shown in Fig.

19-19. Here, two fixtures are used tohold two cylinder heads. Thetemplate bracket is a trunnion-mounted box holding templates for

several different operations. Thisbox indexes between operations sothat only one template at a time is

brought into the active position. Inthis case, various operations requirdifferent



na

_j_

; ig. 19-20. Part to be milled onsides A, B, and C.

angular positions of the part,therefore the fixtures are built astrunnion bases with cradles.

djustable and Movable MillingFixtures



Milling fixtures are made adjustablor movable for several reasons. Anadjustable fixture designed for the

milling of the nonparalle! sides of the block shown in Fig. 19-20, isllustrated in Fig. 19-21. Three

operations are involved; the parallesides A are milled by means of thestraddle cutters, and the two sides Band C are then milled in two

subsequent operations. The threeoperations are all performedwithout requiring more than one

setting of the work. The block is cutoff from bar stock, and drilled andcounterbored to receive twofillister-head screws which hold it



n place on the machine of which itforms a part. These holes are alsoutilized for holding the block in

position on the fixture.

The milling fixture consists of an

upper plate A, which is pivoted onstud B. This stud is mounted in thecross slide C, which operates onbase D. Plate A is provided with two

tapped steel bushings which are aforced fit in holes drilled andcounterbored for the purpose.

These bushings receive the twoscrews which secure the work inposition on the fixture, with thepurpose of preventing the rapid

wear of the threads which would



take place had they been tappeddirectly into the cast iron. Thefixture is shown set in position for

milling the parallel sides A, of thework. There are two tapered pins Eand F, which

Ch. 19


FIXTURES

255


http://slidepdf.com/reader/full/jig-and-fixture-design-manual-erik-k-hendriksen3709 1791/2237ri^-ia



Fig. 19-21. The milling fixture forthe part shown in Fig. 19-20. Plates adjustable to three positions.

are used for locating the work in threquired position. For milling the

parallel sides of the work, pin F isnserted in hole N to locate thecross slide C in the requiredposition. Similarly, pin E is located

n the central hole to locate swivelplated. These pins are merely usedto locate the fixture; bolts G and //

are provided to secure it in therequired position. When the fixtures set for milling the angular side C

of the work, pin E is inserted in

hole /, and pin F in hole O. This set



swivel plated at the required angleand also locates the cross slide C atthe required off-center distance to

enable the work to be milled by theouter edge of the cutter. After thisoperation has been completed,

swivel plate A is then swung over toenable pin E to enter hole K.Similarly, cross slide C is moved sothat pin F will enter hole M. This

brings the work into position toenable the angular side B to bemilled by the outer edge of the

other cutter on the arbor.

The fixture described above is, in asense, a primitive indexing fixture.

Fully indexing fixtures are used



extensively, and dividing heads of arious types are available for the

control of the indexing function.

Movable milling fixtures are thosethat move while the cutter passes

over the work. Radial fixturesperform a slow rotation around afixed center or axis, with the resultthat the cutter generates an arc of a

circle. Other movable fixtures,similarly

rotating around an axis, arccontrolled in their motion by atemplate and are, therefore, capableof generating curved surfaces of any

desired shape.



Locating Pins in Milling Fixtures

Locating pins in milling fixtures can

be fixed or retractable. An exampleof a fixture using fixed locating pinss shown in Fig. 19-22. The part is

an aluminum cylinder head, and thfixture is rotating. The cylinderhead is supported and located onthe large diameter circular locator

which centers it on the inside bore.The final accurate location for mil-ng the fins is obtained by means of

a diamond-shaped pin which can beseen in the background. This pinengages the locating hole in theoining surface of the cylinder head

In this operation, the cooling fins



are milled to the required depth by guiding the cutter with a tracerwhich follows the contour of the

template as the piece rotates withthe fixture.

Retractable pins are used where aworkpiece must slide into positionbefore the pins can engage. Anarrangement of retractable pins is

shown in Fig, 19-23, where theocating pins are mounted on the

ends of a cross bar. This cross bar,

balanced by two springs placed atequal distances from a centrally ocated eccentric, is moved up and

down as the eccentric is operated by

a hand lever. The lever is




Ch, 19

Courtesy of Cincinnati MilacranInc. Fig. 19-22. A rotating fixture fo



milling cooling fins on a cylinderhead with one fixed diamond pinfor locating the part,

ocated at the front of the machine(Fig, 19-24), der block held in a

fixture (not shown in the which isused for milling various surfaces ona cylin- picture).

RETRACTABLE LOCATING PINS



WORKPIECE

REST BLOCK

HANDLE TO OPERATE BAR

Side stop

ECC£NTRIC{ENGA6EDPOSITION}

SECTION A-A



Courtesy of Cincinnati MilscronInc. Fig, 19-23. A milling fixturewith retractable locating pins.

Ch. 19


257



Courtesy of Cincinnati Mikcron Inc

Fig. 19-24. The same milling fixtureshown in position on (tie millingmachine.

The locating pins are normally in



the retracted position. When thework is moved into position, theoperator rotates the lever to raise

the pins, and also moves thecylinder block slightly to ease theengagement of the pins with the

ocating holes. After the part hasbeen located and clamped, the pinsare again retracted. To furtherfacilitate the insertion of the pins

and thus reduce the time requiredfor locating the part, both locatingpins are made with the diamond-

shaped head and both are locatedwith their major axis perpendicularto the surfaces to be milled. Thiswill permit a slight variation in the



ocation of the part in the directionparallel to the surface to be milled.The movement provides ease in

positioning of the parts and has noappreciable effect on the accuracy of location of the cylinder block for

the milling operation.

Gear (-Jobbing Fixtures

Successful gear nobbing dependsequally on the accuracy of the gearblanks and the accuracy and rigidityof the hobbing fixture. A typicalhobbing fixture (Fig, 19-25) for a

ertical spindle hobbing machineconsists of a base bolted to the

machine table, a bottom support



plate, a mandrel, an upper clampingplate, and a clamping nut. Themandrel extends above the nut with

a pilot, which is supported by thesupporting arm of the machine. Thefollowing points are highly

significant: The gear

blanks are accurately centered onthe mandrel; they are supported

and clamped on the largest possiblediameter, and before the base isfinally clamped to the table, the

entire fixture is centered, withrespect to the axis of rotation, by adial indicator. To allow for thiscentering adjustment, the base

must not be solidly centered in the



machine table; there must be about1/8-inch (3-mm)-clearance in thehole in the machine table for the

pilot of the base. The height of thebase must be sufficient to allow fora clearance of about 1 inch from the

work to the cutter at the lower endof the travel. Adequate approachand overtravel must be provided atboth ends of the cutter travel, and

the fixture designer is cautionedthat overtravel, in the case of helicagears, is considerable yet not easily

detected from a drawing.

Fixtures for N/C Milling

The rapidly expanding use of



numerically controlled (N/C)milling machines has focusedattention on the need for reduction

of all phases of non-cutting time.The loading and unloading time isnot a programmed operation, and

even with skillfully designedfixtures, is still a burden on theeconomy of the operation. It can bedrastically reduced, however, by

dual fixturing; that is, by the use of two identical and interchangeablefixtures, a method which is used

quite extensively. While one part,clamped in its fixture, is machined,the otheT fixture




FIXTURES

Ch. 19

Direction of hob rotation forconvention*! 8nd

fU-tl h.Gbrlr,; T.ethbOd* of

euttlnf]'

*■ ^ /



Hob Slide

HI

LTIB



^

I'ig. 19-25. Gear hobbing fixtures, a,

Courtesy of Gould & EberlmrdtGear Machinery Corp. Plain, b.With reversible bottom plate.

s unloaded then reloaded. Whenspace permits, both fixtures aremounted on the machine table.

With very large fixtures, it is



necessary to unload and reload onefixture on the floor, while the part

n the other fixture is machined,and then the fixtures areexchanged. An example of this type

of operation is seen in Fig. 19-26,



Courtesy of Cintimatic Division of Cincinnati Milacron Inc. Fig. 19-26.The use of interchangeable fixture?

with an N/C milling machine.

CHAPTER

20

Design Studies III —Miscellaneous

Fixtures

Lathe Fixtures in General

Lathe fixtures are, for the mostpart, used on vertical and horizontaturret iathes and high-speedproduction lathes, In the past they



have not been used on engineathes to the extent they deserve; a

skillfully designed yet inexpensive

fixture can well convert an engineathe into a production machine.

However, fixtures are now being

used extensively on engine lathesequipped with N/C controls, Work-pieces are centered, located, andclamped in lathe fixtures in

essentially the same way as inconventional workholdtng devicesused with lathes: accordingly, the

fixtures can be classified as chuckswith special jaws and inserts, collettype fixtures, face-plate fixtures,pot-type fixtures, mandrels and



arbors, and special fixtures.

Since cutting forces are unbalanced

athe fixtures are always designedwith large metal thicknesses andstrong clamps. For the main fixture

body, gray cast iron is preferred tomild steel for damping vibrations;however, for high-speed operation,weight and strength considerations

may require the use of steel (inwelded or built-up construction)rather than gray cast iron. With the

exception of mandrels, which aresupported on the tailstock, lathefixtures are cantilevered. They musbe designed with as little overhang

as possible and, for operator safety,



projecting screws and pins musteither be avoided or shielded. Atmedium and high spindle speeds,

centrifugal forces becomesignificant and must be taken intoaccount. They affect clamping

forces and will cause vibration if thfixture with the part does not runtrue. Many fixtures for secondoperation cuts, taken at high

spindle speeds, are thereforedesigned so that they can beadjusted with respect to the spindle

axis. Examples of adjustablefixtures are shown in Figs. 6-43 and6-44. Irregularly shaped workpiecesmust be counterweighted. Medium



and large-size lathe fixtures forroughing work in a first

operation are usually designed onthe three-point principle. Largerfixtures for second operations and

for thin-walled parts are designedwith equalizing clamping devices(the floating principle). Examples oapplications are shown in Figs. 9-11

and 12-5.

Chuck Fixtures

The cheapest type of lathe fixture isthe standard lathe chuck (3- or 4-aws), with special jaws or inserts,

machined to fit the part. Aluminum



gray cast iron, and steel inserts arecommercially available indimensions to match standard

chucks. As a general rule, steel jawswith serrated or hardened surfacesare used for gripping on rough

parts, while soft jaws with smoothsurfaces are used on machinedparts to prevent scratching ormarring These rules also apply to

power operated chucks, found onmany turret lathes and productionathes. Designs of more elaborate

chuck fixtures are also shown inFigs. 9-11 and 12-5.

Since centrifugal force tends to

draw the jaws away from the part



(except when the part is clampedfrom the inside), and the standardchuck design is inadequate at

spindle speeds used in high-speedproduction lathes, solutions mustbe sought; i.e., using a power

operated chuck where a positiveclamping force is constantly maintained on the jaws. Anothersolution is to make use of the

centrifugal force for clamping. Anexample is shown in Fig, 20-1. Themachine is a two-spindle

production lathe and the part to beturned, in this case, is a cast-ironbrake drum. The rear chuck isshown loaded; the front chuck,



empty. The boss of the part isclamped by means of the hook clamp seen near the center of the

chuck. In addition, the part isclamped on its periphery by 10 jawsEach jaw can rotate around a

fulcrum and at the back it isprovided with an inertia block of greater mass than the forward

DESIGN STUDIES III -MISCELLANEOUS FIXTURES

Ch. 20



r~r-»

Courtesy ofHeald Machine Div.,Cincinnati Milacron inc. Fig. 20-1.Lathe fixtures (chucks) where jaws

are actuated by centrifugal force(inertia fingers).

clamping end of the jaw. The result

s that when the spindle rotates, the



nertia block is forced outward by centrifugal force, and, in turn,forces the clamping end of the jaw

against the work. Consequently, theclamping pressure increases withthe spindle speed, Uniform

pressure applied around the rim of the brake drum does not distort thepart and eliminates chatter whenthe drum is machined.

The principle of the collet chuck was shown in Fig. 9-3, with design

rules and data presented in theaccompanying text. The total travelof the jaws or fingers is determinedby elastic deformation within the

collet and is, therefore, very short.



The maximum diameter range of the spring collet is 0.002 to 0.003nch (0.05 to 0.08 mm) and it is

ntended for use on bar stock andbar-shaped parts. However, theterm "collet chuck," as applied to

fixtures in general, is now used in awider sense. Any chuck thatactuates its jaws by an axial motionrelative to a conical surface is a

collet-type chuck.

The two chucks for the centering of

gear wheels shown in Figs, 9-22 and9-23 are collet chucks. They aregrinding chucks, but that is a matteof application rather than of design

The collet principle can be applied



to chucks of any dimension andproportion, and for internal as wellas for external clamping. A collet-

type lathe chuck for interna!clamping is shown in Fig. 20-2.' It ioperated by means of the central

draw bar. The actuating cone is asolid conical plug in the center of the chuck. The sliding pads are keptn contact with the cone by springs

and each carries two pins whichconstitute the actual




(Copenhagen; G.E.C. Gad's Forlag,1930) vol. 11.

Courtesy' of E. Tlmulow Fig. 20-2. A

collet-type lathe chuck for internalclamping.

aws. As the cone is drawn in, the



aws are forced out. Dotted linesndicate buttons that can serve as

axial end stops.

The collet chuck can be combinedwith other locating and clamping

components. One example is shownn Fig. 20-3. The part is a pinion,ntegral with a long shaft, too long

to be held in any type of chuck. For

the same reason, the collet chuck provides excellent centering of thepart and is well adapted to precision

work. An application example isshown in Fig. 9-24.

Face Plate Fixtures



The face plate fixture is the naturaltype of fixture for machining largediameter parts on the vertical turre

athe. It is highly versatile and any type and combination of locatorsand clamps can be built up on it. A

typical example is shown in Fig. 20-4. The work A is a cast-iron bracketwhich has previously beenmachined along the face D and has

had the tongued portion cutapproximately central with thecored hole at Y. Four holes have

also been drilled at /. Two sizes of these brackets are made in lots of ten or twelve. An angle plate B, istongued on the underside F, to fit



one of the table T-slotsand is helddown by screws (not shown). Thedistance E, for the two sizes of

brackets, is determined by placing astud G in the center hole of thetable and locating angle plate B

from it. The bracket is placed inposition on the angle plate so thattongue H fits into the groove, andbolts / are passed through the holes

n the bracket and tightened by nutK. Clearance is provided in the boltholes

Ch. 20

DESIGN STUDIES HI -

MISCELLANEOUS FIXTURES



261

Courtesy of Heald Machine Div.,Cincinnati MUacron Inc. [■'ig. 20-3

collet-type lathe chuck for

external clamping of a pinion with aong shaft (inverted tailstock).



Fig. 20-4. A face plate fixture for

noncttcular bracket-type part?.



DESIGN STUDIES II! -MISCELLANEOUS FIXTURES

Ch. 20



Courtesy of Monarch Machine ToolCo. Fig. 20-5. A lathe fixture for a

pump body. a. Front and end views;b. Rear view.

to allow the finished edge of the

bracket to rest on pins C. Two



special jaws Q are fixed in positionon the table but may be adjustedradially, when necessary, to bring

them into the correct position forthe other size of bracket. The jawsare provided with set-screws O,

which are adjusted to support theoverhanging end of the bracket,after which they are locked by thecheck nuts at P. The jaws are keyed

at 5 to the sub-jaws of the table;and clamps N are used on theunfinished portion of the bracket.

The clamps are tightened by nuts atR, so that the surface to bemachined is clear of interferences.The boring bar L is used to bore the



hole, and the side head tool M facesthe pad. This fixture illustrates thatathe fixtures can be made to

accommodate workpieces of completely noncircular shapes andof more than one size.

On engine lathes and horizontalturret lathes it is often moreconvenient to have a special face

plate integral with the fixture and tomount the complete assembly onthe spindle nose, as was shown in

Figs. 6-43 and 6-44. A fixture for thfacing and internal machining of anaircraft fuel-pump housing on anumerically controlled lathe is

shown in Fig. 20-5a and h. The



fixture base is mounted on the faceplate of the lathe. The part isocated and clamped in V's, which

are carried on a projecting plate andbraced against the fixture base.

The pot type fixture is used forparts of large diameter andconsiderable axial length, which do

not require external machining. A typical example is shown in Fig. 20-6. The work A is a large castingwhich, because of its dimensions(diameter, length, and wallthickness), could not be adequatelysupported and driven by a

conventional chuck. The variations



three screws are then moderately and evenly tightened. In the verticadirection, the part is supported on

three points F, F, and G (G is fixed,while F and F are adjustable).Windows/ 3 in the fixture wall

allow for access to the adjustablesupports. The part is clamped downby means of U-clamps L and nutsand washers M on studs K. This

fixture presents several interestingdetails: the screwthreads F onclamping screws D and the

adjustable points F are shieldedagainst chips; screws D are providedwith "snubbers," heavy rubber padson their tips which prevent



distortion of the casting wallbecause of excessive pressure andalso assist in the damping of

ibration (chatter) duringmachining.

Ch. 20


263



SECTioti x-y-2 Fig. 20-6. A typical



pot type fixture.

Mandrel and Arbor Type Fixtures

Mandrel and arbor type fixtures wilcenter, locate., and grip the work from the inside and are normally used for parts that already have amachined internal surface. Themandrel is supported at both ends

as a simple beam; the arbor iscarried at one end only, as acantilever beam. In the simplestcase, the work is centered with asliding fit, located endwise against ashoulder, and clamped with a nutand washer. Commercially available

mandrels and arbors hold the work



between a fixed and a movablecone. Another type holds the work on an expanding sleeve. This design

s shown in Fig. 20-7 } The sleeve iscylindrical on the outside and fitsnside on a taper on the mandrel

body. When forced axially up ontothe taper, the sleeve expands andclamps the part. The

H

H

=3

Courtesy of E. Thautow Fig. 20-7. A

mandrel with an expanding split



sleeve.

sleeve is slit, with slots running

alternately from either end. Thearge nut on the left-hand end of

the mandrel serves the double

purpose of locating the partendwise and of releasing thepressure by forcing the sleeve downon the taper. These mandrels are

made with tapers varying, ingeneral, from 1:50 up to 1:15; andoccasionally up to 1:6. A 1:50 taper

represents the limit for whatconveniently can be operated with arelease nut. This type of mandrelfixture is perhaps the most

satisfactory of all. It locates and



centers well and the sleeve remainspractically cylindrical as it expands,exerting a uniform pressure on the

part. Sleeves of different sizes canbe used on the same mandrel body.For satisfactory results, the number

of slots must be related to thesleeve diameter as follows:

recent development, now

commercially available, is amandrel, or arbor, with one orseveral hydraulically expanded

sleeves. An example is shown inFig. 20-8, where two sleeves areused to clamp a part with a steppedhole. Hydraulic pressure is

generated by means of a piston



forced against the hydraulic fluid byan actuating screw, which ismanually rotated with a socket-

head screw wrench. For high-production work, the piston can beactuated by a push rod mounted

centraOy in the lathe spindle.Mandrels and arbors can bedimensioned by the same rules asboring bars. Details concerning the

design and dimensioning of theexpanding sleeve and the data forthe hydraulic system are presented

n Chapter 21, Universal andutomatic Fixtures.

nother recent development is the

mandrel with an expanding sleeve



made of an elastomer. It wasdeveloped by AEC-NASA and itsdescription is published in a Tech

Brief. 3 The purpose is to support

E. Thaulow, Maskmarbe/de

(Copenhagen: G.E.C. Gad\ Forlag,1928) vol. I.

3 An AEC-NASA Tech Brief from

Cutting Tool Engineering, April1969, p. 17.

DESIGN STUDIES III -


Ch. 20



-ACTUATING SCREW HYDRAULICPISTON

Courtesy of Hydra-Lock Corp. Fig,

20-8, An arbor with two hydraulically expanded clamping sleeves.

rough, hollow castings during



grinding and turning. The device isshown in Fig. 20-9. The elastomersleeve is supported on a mandrel

threaded at one end. The part isslipped over the sleeve, a heavy washer and nut are put in place, and

when the nut is tightened, thesleeve expands on its diameter

IzBB

NUT WASHER ELASTOMER SLEEVE W0RKPIEC6



Fig. 20-9. A mandrel with anexpanding elastomer sleeve.

while maintaining constant volumeand centers, supports, and clampsthe part. The elastomer must be

nearly completely enclosed so thatl cannot escape from the pressure.Under these conditions it behavesmuch like a rigid body after it has

filled the cavity inside the part. Itocates and centers the part on its

average inside diameter or contour.

The elastomer can be cast inaluminum molds. The diameter of the sleeve is made 0.010 to 0.015nch (0.25 to 0.38 mm) smaller

than the cast hole and the length



approxiamtely 1/8 inch (3 mm)arger. Elastomers suitable for this

application are silicone RTV, PVC

(polyvinyl chloride) plastisols,ulcanized rubber, and

polyurethane. Generally,

elastomers can be Teused.

Courtesy ofE, Thaulow Fig, 20-10. Asimple type of arbor for holding

threaded work.

Workpieces with unmachinednterior surfaces, such as cored

holes, can be mounted on mandrels



or arbors with movable internalaws.

Parts with an internal thread can bemounted on a mandrel. Thesimplest case is that shown in Fig.

20-10. The screw thread serves tocenter, clamp, and drive the part,and alignment is provided by theflat shoulder. A plain arrangement

such as this

Courtesy ofE. Thauiow Fig. 20-11.



n improved type of arbor forthreaded work, with a separateclamping and release nut.

E. Thaulow, Mttskinarbejde(Copenhagen: G.E.C, Gad's Forlag,

1928) vol, I.

Ch. 20


265

causes the part lo bind in the threadafter machining. An improvement,shown in Fig. 20-11, uses a large



clamping nut with a coarse left-hand screw thread. The nutprovides the means for end-locating

the shoulder and for aligning thepart. After machining, the nut isbacked off a fraction of a turn and

the binding pressure on the part'sscrew thread is eliminated.

Miscellaneous Fixtures

Lathe operations on parts that areunusual because of their shape ordimensions offer many opportunities for the successfulapplication of fixtures. At times thefixtures are complicated and

expensive, at other times they are



simple, if not primitive, yet they arealways efficient with respect to thesaving of time and the

mprovement of quality. A systematic classification will not beattempted here but a few examples

will indicate the possibilities.

crankshaft has one geometricalaxis defined by the main bearing

ournals, and one or moreadditional axes, each defined by awrist pin or a set of wrist pins. Each

of these axes requires a turningoperation, but only the axis throughthe main journals terminates insolid steel with surfaces that can be

center drilled. The other axes



primarily run through air. Twodifferent sets of fixtures for theturning of crankshafts are shown in

Fig. 20-12, a and b. In example a,each end of the crankshaft isprovided with a block that carries

the two sets of centers required forthe two wrist pins. The center blockat the tailstock end can also carry acounterweight, as indicated by the

dotted lines. Other fixturecomponents needed are struts foTtaking the axial thrust between the

tailstock and the spindle center, andspacers between two parallel arms.The fixtures shown in example bare comprised of two brackets; the



one to the left is clamped to thespindle, and the one to the right isprovided with a center drilled plug

that is supported by the center inthe tailstock. The fixture shown inFig. 20-13 carries, supports, and

guides the free end of a tank for aspace exploration rocket engine.The visible portion is the so-called"Y-joint," which is seen being

machined in preparation forwelding to the end closure and theadjacent tank. The fixture is

provided with adjustable bearingblocks all the way around to allow the tank to rotate accurately, just asan axle rotates in an ordinary stead



rest.

Tbid.

5 Ibid., vol. 11.

Courtesy of t\ Tkautow Fig. 20-12.



Fixtures for turning crankshafts.

Boring Fixtures

Boring is an operation whereby anexisting hole is machined to a largesize. Boring fixtures differ from driligs in that they are to be used with

boring bars. Drill jigs, however, canalso be designed for combined

drilling and boring operations.While the twist drill is supportedand guided by the hole that is beingdrilled, the cutter in a boring barreceives its support entirely fromthe boring bar itself, producingholes of greater accuracy with

respect to diameter, roundness,



position, and alignment.

Boring bars differ in design and can

be made in a rather wide range of sizes. Some boring bars, called"line" boring bars, are supported at

both ends. Others, called "stub"boring bars, are supported only atone end by the spindle of themachine. Line boring bars are used

to bore long, deep holes and holesof very large diameter. Stub boringbars are used more frequently than

ine boring bars and must be usedfor boring blind holes.

Boring operations are performed on

many types of conventional or



numerically controlled machinetools. Conventional or N/C lathes,milling machines, drilling

machines, vertical boring mills orertical turret lathes, and horizonta

boring mills (HBM) are used

extensively to perform boringoperations. The horizontal boringmill is a remarkable machine,practically a one-man machine

shop. With different accessories,this machine can perform almostany


Ch. 20



Courtesy of Aerojet-Genera! Corp.Mg. 20-13. "Missile Maker Lathe,"for machining laigc missile

components.

conventional machining operation.Its spindle assembly is designed



with the precision and rigidity required for work with boring bars,and it has the necessary power for

the boring of large holes. By itsdesign, and with experienced andcareful operation, it produces holes

that are exactly parallel to thesurface of its table. Boring, in thetoolroom, is performed on jigborers to obtain very precise hole

ocations on tools, dies, jigs, andfixtures. Jig borers sometimes areused to obtain precise hole location

tolerances on machine parts wherethe number of parts is relatively small. Boring fixtures occasionally are used on jig borers to speed-up



the location of the part on themachine or where the part cannotbe conveniently held by any other

means.

High-production boring is often

done on special machine tools.Some are special purpose machinessuch as those found in theautomotive industry. Often the part

s automatically moved to and fromthe boring station by machinescalled "transfer machines." A

different class of boring machine iscomprised of the highly automatedhigh-speed production boringmachines, known as "Bore-Ma

tics,"® or equivalent trade names,



which operate at high cuttingspeeds and fine feeds (borizing).These machine tools may be

adapted to do a variety of

obs; however, they are usually set

up to do a

particular job for a prolonged periodof time.

Boring fixtures are always usedwith these production machines.

Design of Boring Fixtures

Case 14. Design a boring fixture fora box-shaped part with a bearing



hole at each end.

The part is schematically

represented by the rectangularoutline A in Fig, 20-14. The distancebetween the holes requires a line

boring bar; consequently, this typeof fixture is called a "line boringfixture." The usual five design stepsapply here again, with some

modifications and simplifications,that are characteristic for mostboring fixtures.

1. Usually, a part to be bored isalready machined on one or severalmajor flat surfaces. Here, the base

surface of the part is machined, and



s now located and supported on amatching flat surface on the jigbase. In the side direction it is

ocated by means of pins, stops, orwith keys or dowel pins, if suchcomponents are provided for in the

part, for use in the final assembly.Endwise, it is located to providesufficient clearance at each end(dimensions B).

Ch. 20


267



2. The part may be clamped by means of clamp straps, but oftenthe part is already provided with

bolt holes which can then be usedfor clamping it to the fixture.

3. Intermediate supports areusually not needed, since the part iswell supported on its base.

4. The cutter guides serve asbushings for the boring bar. Thefixture is provided with fixedbushings mounted in brackets K,and the boring bar has slip bushingof sufficient outer diameter to allowthe bar with its cutting tools to be

nserted and withdrawn endwise.



5. The fixture body consistsessentially of a base with bracketsK, for bushings. If needed, the

brackets may be provided withstiffening ribs C. Side lugs D aremachined exactly parallel to the

boring axis and serve for the fixturealignment on the machine table,Bosses E arc for measuring andchecking the clearance B, to the end

surfaces G, of the work. B must bearge enough to allow for variationsn the size of the raw part, for the

escape of chips, and, if necessary,for the insertion of facing cutters orthe mounting of shell reamers onthe bar. The fixture is bolted to the



machine table by means of threeugs //. The use of three lugs

provides for statically determinate

support and minimizes thepossibility of elastic deformation(springing) in the fixture. To reduce

weight, the fixture base is cored outfrom below, and the bushing bossesare tapered. The fixture is a one-piece casting. Gray cast iron is

preferred over weldments becauseof its excellent damping properties.



also provides

for lengthwise adjustment of the

brackets to accommodateworkpieces of different lengths, forthe insertion of an intermediate

bracket in the case of very longworkpieces, or for the use of different brackets on-the samefixture base. For multiple boring

operations the fixture can beprovided with a multiple spindlegearbox which rotates and feeds the

boring bars. It should beremembered that mating gearwheels rotate in opposite directionsand some cutters may have to be

designed for left-hand cutting. A



boring fixture can have bushings foholes in more than one directionand is then placed on a revolving or

ndexing table.

somewhat different technique can

be used when the boring operationsaffect only a small area of a mucharger machine part or assembly. In

that case, the boring fixtures are

small and are carried by the largerunit. This method is frequently used in the manufacture of

machine tools. One example isgiven in Fig, 20-15, which shows amachine tool bed with someaccessories. The hole B which

signifies the hole for the main



bearing in the headstock E, isshown bored with a boring barsupported in two fixtures C and D

which are located on the inverted's of the machine bed. This

ensures alignment

Fig. 20-15. Example illustrating theuse of the workpiece as a guide forthe boring bar.

between bed and spindle. Next it is



assumed that hole B should bealigned with holes F and G in twoalready existing carriages or

brackets, and in this case thesesame holes serve as guides for theboring bar, if necessary by the use

of liner bushings. Finally, the frontelevation shows how hole ./ isboTed in the carriage and apron / busing the three bearings K, L, and

f as guides for the boring bar. A tapered hole is bored by means of aboring bar mounted at the required

angle in a bushing in such a way that it is fed through the bushing,rotating the bushing by means of akey and key seat. The




Ch. 20



Courtesy of E. Thaulow Fig. 20-16. boring bar with bushing for

boring a tapered hole.

The workpiece is a casting for aathe headstock, is located on

nverted V's / and 2, and clampsagainst end stop 4 by means of screw 3. When located, it is finally clamped down by screws 5. The

fixture has boring bar bushings 6for the spindle, 7 for the back gearshaft, and 8 for the rocker shaft.

The fixture body is designed like abox-type drill jig, and all operationsare performed in a radial drill.

Design of Boring Bars



The strength and rigidity of a boringbar is determined by its diameter Dand free length /.. The diameter

should always be taken as large aspossible, allowing a chip clearancebetween the bar and the raw hole o

not less than the machiningallowance in the hole. The freeength L can be taken as follows: foine boring bars

L < IQD

for stub boring bars

L <6D

The length of a bushing should



never be less than/), and forsmaller bars, the bushing lengthmay be taken up to 2 time's D. This

rule is for boring bars



Courtesy of E, Thaulow

Fig. 20-17. The boring fixture for

the operation shown in Fig. 20-16.

arrangement is shown in Fig, 20-166 The boring bar is driven through auniversal joint. This detail is part ofthe boring fixture shown in Fig. 20-17.*

E. Thaulow, Maskinarbejde(Copenhagen: G.E.C. Gad's Forlag,1930) vol. II,

to be made of steel. Solid cementedcarbide boring bars may be

dimensioned by comparison with



previously used boring bars. Thecritical property is the transverserupture strength which can vary

greatly (up to 200 to 300 percent)n any given carbide grade.

Ch. 20


269

Fixtures for Production Boring

Machines

High-speed production boringmachines use stub boring bars and



other cutting tools (fox facing,countcrboring, etc., and sometimesfor external turning) that do not

require bushings in the fixture. Thefixtures are designed for quick precision clamping of the part. As

an example, Fig. 20-18 shows thefixture for machining a die-cast-aluminum engine front cover. Withfour spindles, the machine bores

and faces pockets and shaft bores toa finish of 120 to 125 RMS at a rateof 57 pieces per hour. The part is

positioned from its rear side and isclamped by eight clamps, all air-operated for fast opening andclosing. Shown in Fig. 20-19 is an



ndexing fixture that holds twocompressor crankcases forsimultaneous machining (finish

boring and chamfering of both endsof the cylinder bores).

Grinding Fixtures

The grinding operation ischaracterized by small cutting

forces, high accuracy, and, ingeneral, a large flow of coolant.Grinding fixtures must allow for thunrestricted access of coolant to thework, as well as free drainage of theused coolant, with no sumps orpockets where sludge can

accumulate. Magnetic face plates



and chucks, supplemented by electrostatic and vacuum operateddevices for nonmagnetic

workpieces, aTe widely used asstandard work-holding devices forsurface grinding, but are

Courtesy ofHeald Machine Div.,Cincinnati Miiacron Inc. Fig. 20-19.

n indexing boring fixture for two



parts.

not usually part of the fixture. The

magnetic face plate can be used,however, as a fixture base thatoffers a fast and convenient method

of removing and replacing thefixture. Typical examples of grinding fixtures of the chuck typewere shown in Chapter 9, Figs. 9-22

through 9-24. Clamping is mainly done with hand or finger operatedmechanical elements, designed for

ight duty only, and is not apt tocause distortion of the part, norrestrain its natural thermalexpansion.



The structural design of grindingfixtures is very similar to that of other fixtures, except that they are

ighter. A fixture that structurally ends itself

Courtesy of Heald Machine Div..Cincinnati Miiacron Inc. Fig, 20-18 boring fixture with air-operated

damps for high production.


Ch. 20



Courtesy of LeBlond Inc. Fig. 20-20 jig grinding fixture designed as a



90 degree, til table angle plate.

change the setup to the position for

the second set of bores, the angleplate is placed on its back, thereby tilting it 90 degrees.

ngle Plate Fixtures

The internal grinder with planetary

spindle motion is used for internalgrinding operations on cylinderblocks and other similar type work.

fixture for this kind of operation

s shown in Fig. 20-21. 7 Thegrinder spindle is horizontal andthe fixture is formed as an angle

plate with stiffening end walls. The



cylinder block is clamped by its bassurface on the angle plate whichhas openings for the entrance of the

grinder spindle. This fixture alsohas two diamonds for the truing of the grinding wheel, a feature that is

characteristic of "many grindingfixtures. There are two reasons whya grinding fixture cannot use cutterguides of the usual type for setting

the grinding wheel; one is that thesetting would soon be lost becauseof wheel wear; the other is that an

ordinary cutter guide would soon beground down and destroyed by accidental contact with the grindingwheel. Therefore, instead of cutter



guides, grinding fixtures areequipped with diamonds for thetruing of the grinding wheel. The

diamonds are preset for the finalwork dimension and the grindingwheel is run back past the diamond

and trued before it starts thefinishing cut. The angle plate is alsoa characteristic feature of many grinding fixtures. An angle plate ha

sufficient



Courtesy ofE. Thaulow

Fig. 20-21. An angle plate fixture fonternal grinding.

to a milling fixture is shown in Fig.20-20. The part requires thenternal grinding of bores with two

perpendicular axis directions. The

machine is a jig grinder, and thespindle is positioned from hole tohole by means of the positioning

mechanism of the machine table, in



the same manner as on a jig borer.The fixture is an angle plate with asloping front. To

rigidity to withstand the weak grinding pressures, and is

frequently used to raise the work toa convenient position above thegrinder table.

E. Thaulow, Maskinarbejde(Copenhagen: 1'orlag, 1930) vol. II.

G.E.C. Gad's

Ch. 20





271

utomotive Grinding Fixtures

Of the grinding fixtures used in the

automotive industry, two areparticularly significant because of their special design features which

may well be utilized in otherapplications. They are the fixturesfor the cylindrical grinding of crankpins and for the contour

grinding of camshafts.

The principle of a crankpin grinding

fixture is shown schematically, and



somewhat simplified, in Fig. 20-22.The work spindle of the grinder hasa large face plate A, which carries a

ocator B, C for the main bearingournal at the end of the crankshaft

and an index plate D with an

ndexing mechanism, here shownsimplified as an index pin E.



Fig. 20-22. Schematic view of afixture for the grinding of four

crankpins.

The locator consists of a 1/3 bearing

shell B, and a movable pressure foo



C. A similar locator is provided atthe opposite end of the crankshaft.The locators are mounted in such a

way that the crankshaft center Ones offset from the woik spindle

center line a distance equal to the

radius to the crankpin centers. Thisdistance is adjustable so that it canbe changed, when needed, toaccommodate other crankshafts. In

the case shown, the adjustment isdone by an exchange of bearing B.The

crankshaft is nested in bearings B,and one crankpin is brought intocontact with a retractable locator F,

which brings one crankpin center



ine to coincide with the work spindle center line. Theseoperations define the starting

position for the crankshaft andwhen that position is reached thepressure feet C close; the end of the

crankshaft is clamped to the indexplate /J; and the crankpin locator Fretracts. The grinding wheel Gadvances and grinds the crankpin.

fter grinding is completed, thendex pin E is withdrawn, the index

plate with the crankshaft is indexed

to the next position, the grindingwheel is aligned with the nextcrankpin, and grinding cancontinue.



t-'ig. 20-23. A crankpin grindingfixture with work positioner.

On most crankpin grindersavailable, the fixture is integral with

the machine. The operations:oading and positioning the

crankshaft, clamping, indexing,start and stop, feeding, retracting,



and repositioning the grindingwheel, are all mechanically controlled. An example of such a

mechanized and automatedcrankpin grinder is shown in Fig.20-23.

Cam grinding is a copyingoperation. The cam contours arecopied from a set of master cams;

contact between a master cam andthe master roller is maintainedmechanically by means of a spring

or a hydraulic cylinder with piston.The principle of cam grinding isshown schematically in Fig. 20-24.

common base A, attached to the

bed of the grinder, carries fixed



bearings B and C for, respectively,the shaft D for the cradle E and theshaft for the master roller F. The

cradle, which is a casting of substantial dimensions, carriesbearings G for the work spindle on

which are mounted the


Ch. 20



Fig, 20-24. Schematic view of afixture for cam grinding.

master cam set H and a chuck /,with a live center for locating,

supporting, and clamping one endof the camshaft K. The opposite endof the camshaft is carried by atailstock, also known as the "foot-



stock." The free part of thecamshaft is supported against thegrinding pressure by a "work rest."

s the work spindle with the mastecam and the camshaft rotates, thecradle is oscillated around its shaft

D, and with the grinding wheel L inoperating position, the master camcontour is transferred to the camthat is being ground. The master

roller is moved lengthwise from ondisc to the other, on the mastercam, when the grinding wheel is

moved into position for thegrinding of the next cam on thecamshaft. A camshaft grindingoperation is seen in Fig. 20-25. The



llustration shows the cradle withts inverted-V guide, the footstock,

the work rest on the middle of the

camshaft, and the driving chuck.The master cam mechanism iscovered within the housing at the

eft in the photograph.

Fig. 20-25. A camshaft grinding

fixture.



Fixtures for Planing and RelatedOperations

Planing, shaping, slotting, andbroaching have many features incommon which are reflected in the

design of their fixtures. They arestraight-line operations which, withthe exception of broaching, areperformed by a single-point tool

and a reciprocating motion. Inplaning, the work moves, while inthe other operations, the cutter

moves. In planing and shaping, themotion is horizontal; in slotting, thmotion is vertical. Broaching isdone, horizontally (pull broaching)

and vertically (surface broaching).



Because of inertia forces at strokereversal, the cutting speeds arerather moderate. Vibration is not a

problem, but fixtures must bedesigned to withstand the impactfrom the tool each time it enters th

work. Structurally, these fixturesare related to milling fixtures. Theyrequire substantial metaldimensions, positive and strong end

stops to withstand the impact, solidbolting down upon the machinetable, and strong clamps. The

reciprocating operations requireample end clearance relative to thecut surface at each end of thestroke. The run-out distance at the



exit end is relatively small (about1/2 inch [13 mm] for shaping, and 1to 2 inches [25 to 50 mm] for

planing) to allow the tool to clearthe work and be lifted before thestart of the return stroke, and at

east 1 inch (25 mm) for slotting(vertical) to allow for theaccumulation of chips. The run-indistance at the entry end must be

ong enough to allow fordeceleration, stroke reversal, rc-acceleration, and some time for the

side-wise feed motion to becompleted. For planers this willrequire several inches, dependingon the size, type, and condition of



the machine, but for the othermachines, 1 to 2 inches (25 to 50mm) will suffice. Tool setting

blocks are often used. For themachining (planing and shaping) ofcomposite shapes, such as a

dovetail or the inverted V's on aathe bed, the setting block is

formed as a template of thecontour. (See Fig. 13-6.)

Planing Fixtures

While planing is the naturaloperation for long parts, it can alsobe economically applied to smallerparts when they are clamped in a

gang fixture. An example is shown



n Fig. 20-26. Inexpensive shapingfixtures can be made by thenstallation of special inserts on the

aws of the standard .machine vise.In the vertical machiningoperations (slotting, surface

broaching) the cutter moves in aertical path (see Fig, 20-27) and

the main cutting force F^,

Ch, 20


273



Kig. 20-26. A gang planing fixturefor twenty-three steel forgings.

s directed downwards. Since theslotting machine table is horizontalthis cutting force assists instabilizing the work. However, thethrust force Ff is horizontal andquite significant. It acts with its full

alue right from the beginning of



the cut and generates anoverturning moment on the work.It is therefore essential that slotting

fixtures for tall workpieces aredesigned with a wide base. Similarconsiderations apply to fixtures for

surface broaching. The thrust forceFj on the average, is equal to 1/2 of Fq and may in-



Fig. 20-27. A slotting fixture.

crease to the same value as Fq as

the cutting edges become dulled bywear.

Broaching Fixtures

Broaching is characterized by ashort machining cycle and, for

economical production, alsorequires a short loading andunloading time. Broaching fixturesfor production work are, therefore,

almost exclusively provided withautomatic clamping devices. They can take many forms, even for

almost identical workpieces. As an



example of the variety in the designof broaching fixtures, Fig, 20-28 aand b show two different designs of

a fixture for the broaching of thearge end of an automotive

connecting rod. In each design, the

closing and clamping motion of theclamping arm is controlled by acontoured slot in the arm, while, ina, the clamp is double and is

actuated by a crank arm on arotating shaft. In b, there arc twondividual clamps, each one

actuated by a power cylinder.

The indexing principle is alsofrequently used for the purpose of

reducing loading and unloading



time. An application is shown inFig. 20-29. The machine is not aduplex, but carries two identical

broaches on the ram and machinestwo parts with each stroke. Theparts are manually loaded into

nests on the periphery of thendexing plate and are indexed into

the machining station where they are hydraulically clamped. After

machining, they are indexed to the



Ch. 20



Courtesy of Cintimatic Division of Cincinnati Milacron Inc. Fig. 20-28a and b. Two different designs of broaching fixtures for anautomotive connecting rod.



Courtesy of Cincinnati Milacron

Inc. Fig. 20-29. An indexingbroaching fixture with two parts ineaeh station.



unloading station where they aregravity unloaded onto a ramp.

Fixtures for Internal Broaching

fixture for internal broachingconsists essentially of a work support, known as an adapter, thattransmits the main cutting force tothe (vertical or

horizontal) machine table, and annternal part (a plug) to guide the

broach relative to the work and to

provide support against the thrustforce. A thin-walled or otherwiseflexible workpiece requires a

substantial work support (Figs. 20-



30 and 20-31). A broach with anasymmetrical cut, such as thekeyseat broach, is supported by a

plug, and on a horizontal machinethe plug has an extension (a horn,see Fig, 20-32) to prevent sagging o

the free end of the broach. Thethrust force and its reaction arenow internal forces within the part.By means of parallel or tapered

nserts, the keyseat depth can bearied, or a plug can accommodate

broaches of different heights. For

tapered cuts, the face of the work support and the bottom of the slotn the plug are machined to the

appropriate angle so that the work



s tilted with respect to the broach.

Fig. 20-30. A broaching fixture forpull broaching.

Ch. 20




275

Fig. 20-31. A broaching fixture for athin-walled part.



Fig, 20-32. A broaching fixture witha hom for keyseat broaching.

Survey of Assembly Fixtures

ssembly operations are performedwith the use of fasteners (screws,

rivets, pins, stakes, stables, etc.) by permanent deformation of parts(crimping, swaging, bending of tabsetc.), and by bonding or joining

primarily through a thermal proces



(welding, brazing, or soldering).Most of the nonthermal assembly operations Lire used in the mass

production of small- and medium-size articles and require highly specialized machinery where the

fixture is closely associated orntegral with the machine. Onemportant exception is the very arge assembly fixtures used in the

aircraft industry where thecomponents for wings, fuselages,spars, or control surfaces, are

positioned find held while they areriveted or screwed together. Insome cases, drilling is done in thesame fixture, prior to the actual



assembly.

ircraft Tooling

No other industry exceeds theaircraft and aerospace industries inthe use of fixtures of large

dimensions and in the integrationof different fixtures into one maste

fixture system. The followingdescription of a typical aircraftfixture system refers to the toolingfor the aft wing section of a delta

wing of a medium-size bomber. Thesection is 612 inches wide, 261nches long, and 21 inches deep in

the landing gear wheel well area.



Each assembly consists of skinpanels and spars, bulkheads, andbox sections which form the under-

structure. The assembly workingarea in the plant is occupied by aarge steel structure, shown in Fig.

20-33, which supports thendividual fixtures, provides work

platforms in three levels, andcontains services for electricity and

compressed air, with overheadtracks for electric hoists. For eachof the wing sections, a vertical

assembly fixture carrying pinocators, stops, and clamps is

mounted on the main structure.Stops and locators, including



ocators for the fuel lines, arenstalled by means of tooling gages

which simulate the items to be

nstalled. With the tooling gages inposition, skin panel tooling samplesare located and fabricated to

minimum gap clearances at planentersection joints. Hole locationsn the tooling samples are checked

against the tooling gages and

corrected as necessary. The setupfor a typical operation is shown inFig. 20-34. By means of the locators

n the fixture, the under-structure iassembled, checked with gages, and

riveted. The prcdrilled skin panels

are located to the



bulkheads and spars. Holes aretransferred to the understructureand step drilled to size. Attachment

holes in the skin are counterboredand countersunk, as required,attachment holes in the

understructure are tapped, skinpanels are removed, and theunderstructure is deburred andcleaned. Master tooling gages

simulating the main landing gearare used to establish the holepatterns for attaching the landing

gear to the integral fittings of thecorresponding bulkheads. Mastertooling gages simulating the fittingsfor the elevon hinge, are used to



control the installation of theocators for these fittings.

Design of Are Welding Fixtures

The type of assembly tooling mostwidely used throughout themetalworking industry is tooling foarc welding. In welding shopterminology they are called jigs

when they are stationary, andfixtures when they are movable.Their purpose is to locate and holdthe parts in correct relative positionfor joining, to reduce distortion, andto orient the part so that each weldcan be laid in the most convenient

position: i.e., downhand and



horizontal. For this purpose, thefixture usually is carried by apositioner


Ch. 20



Positioners can be commercially available machines, usually operated with a worm wheel drive

to permit a wide range of positions,however, they operate ratherslowly. An example is shown in Fig

20-35. The fixture can be designedto incorporate its own positioner,preferably designed as an indexingpositioner, with which each

operating position is secured by aocking device that enters a hole or

a notch in the index plate. The

system's center of gravity should beplaced in the axis of rotation, if necessary by the addition of counterweights. The balancing of



the fixture is facilitated by the useof light alloys for the moving parts.Even large fixtures can be so well

balanced that they can be operatedmanually and with one hand.Positioning is fast, and operating

positions are accurately defined andsafely maintained.

Distortion is caused by thermal

expansion of parts during welding,and by subsequent shrinkage of deposited %veld material. Plain

thermal distortion is

transient; it disappears as thematerial cools down, and is

harmless. To allow for thermal



distortion, the parts may be firmly clamped at one place (anchored),and allowed to slide against the

friction under the clamps indirections away from the anchorpoint. Shrinkage distortion is of a

different nature. Since the weldmaterial is deposited from one sideat a time, the initial shrinkage isessentially asymmetrical and tends

to misalign the parts. It iscalculated that the distortion is 1degree per pass. Minimum lateral

shrinkage is obtained by weldingwith large electrodes in as few passes as possible. The distortion iscounteracted by the use of clamps,



fixed locators, and stops in suchplaces that they prevent orsubstantially minimize the

anticipated distortion, and also by dimensioning the fixture body withadequate rigidity and strength. The

active stresses to be encounteredequal the yield stress of the weldedmaterial at the elevatedtemperature that prevails at the

beginning of the cooling periodafter completed solidification.

Ch. 20




277

Filing Matter Gage

Courtesy of General Dynamics, ForWorth Div. Fig. 20-34. Locating andfitting of skin panel tooling samples

for an aft wing section.



fixture is designed around ornside the completed workpiece,

and must allow the finished piece to

"get out" again. For this purpose,nternal fixtures may be collapsible

and external fixtures may be

split or have a large hinged orotherwise detachable door. Rams,bumpers, or other types of ejectors

may be added for the removal of binding work-pieces.



Courtesy of Aronson Machine Co.Fig. 20-3S. An are-welding fixture

mounted on a boom typepositioner.




widely used for resistance weldingelectrodes. Stainless steel is usedwhere preheating or postheating is

required as it resists oxidation. Toprevent contamination, some exoticmaterials, notably titanium, are not

permitted to contact othermaterials. In such cases the backingbar is provided with a groove,behind and somewhat wider than

the gap between the plates. Thegroove

different technique is the use of subassembly fixtures, followed by the main assembly fixture. Duringsubassembly welding, the parts are

allowed to distort as needed, but th



pieces are cut with over-length, andfree ends are trimmed back andfitted together when installed in the

main fixture. Figure 20-36 shows afixture of this category, used for thefinal welding of subassemblies for

the engine frame of a Titan missile.

The flow of electric current must becontrolled. The fix ture or the parts

must be grounded to provide areturn path for the current. Besidesthe current carries a magnetic field

here called the "magnetic flux," Itcan affect the direction of the arcand disturb the welding (arc blow).The flux in the part and in the

fixture is controlled by the path of



the return current, and it isessential that the magnetic

Courtesy of Aerojet-General Corp.

Fig. 20-36. Main assembly weldingfixture for the engine frame of aTitan missile.



s purged with an inert gas undersufficient pressure to carry thepenetration, which then forms a

bead without contacting the backinbar. For large parts, two fixturesmay be used. The first fixture is the

"tacking" fixture and is for tack welding only; the second fixture,known as the "holding" fixture, isfor the completion of the welding.

flux has no opportunity to cross orconcentrate near the path of the arc

rule-of-thumb recommends thatn the vicinity of the joint, steelmembers should be one inch (25mm) below the part or two inches

(50 mm) above the path of the arc.



n effective means is to ground thepart at the starting end of a longweld. If clamps and fixture are

made

Ch. 2C


279

of nonmagnetic material, which wilnot provide a path for the magnetic

flux, the magnetic field will beweak.

Welding fixtures are of simple and



nexpensive design with liberaltolerances and little or nomachining. As a general rule, no

castings are used, but all thestandard structural shapes, plates,angles, channels, and I-beams, can

be employed. Preference is given toclosed sections, such as circular,rectangular, and square tubes,because they combine high

torsional strength and rigidity withow weight. Toggle clamps are used

extensively; they are inexpensive

and allow fast operation.

The screw threads on clampingbolts must be shielded against weld

splatter.



Welding fixtures within some sizeimitations can be made from

castable epoxy and phenolic tooting

resins. Since they are cast to shape,they frequently permit the work tobe located in the fixture by simple

nesting without the need of additional locators and clamps. Theplastic materials are light, havesatisfactory dimensional stability,

minimum deterioration, and they do not actively support combustion

Case 15, Design a fixture for the arcwelding of the structure shown inFig. 20-37.

The structure is a rectangular frame



with two cross bars. The frameconsists of 6 by 2 1/2-inch (150 by 65-mm) angles, bent from 1/2-inch

(13-mm) -thick plate. The cross barare 6 by 7/16-inch (150 by 11-mm)flats. The length of the structure is

6 feet (1.8 m), equal to the height oa person. The weight is 325 pounds(147 kg); in comparison, the fixtureas shown in Fig. 20-38, weighs,

empty, 428 pounds (194 kg). Withthe exception of the four shortwelds across the narrow angle

flanges, all welds are 90 degreecorner welds and they are parallel.Therefore, the fixture is designed tobe



«O0

4100 RCf

-7ZOO-

JOO

— 2.SO TYP UNaMENSlCNEORADII: CORNERS lOOiJi

binds, msise so net

TOLERANCES ON' THKKNMSStS

Z0I5

M!>TH!t)tAN£LEL£.eS -*J*

-0i



WSTAHCM

c

AKLOMS

- sij-,tt>Lotte 1 p-*»



IQQQ— f—

SECTION ANU yiewa-a

PUSH-PULL TOGCLi CLAMP tS*HANDLE,

* \lTOf I ft A PI FUR MOUNTINGKoKl

m Btmms! SSa-taio!

MATERIALS: SQTUBIHSANUPLATE, LOW

CARBON STEEL

OTHER ITEMS, COLD ROILEOSTEEL DIMENSIONS ARE fi£F



IUNTOLERANCELHANO TfR NUHBFBS IH PARENTHESIS A BEMANUFACTURER'S SERIAL

NUMBERS

Fig. 20-37. An arc welded

rectangular structure with crossbars.

Fig. 20-38. A welding fixture for the

structure shown in Fig. 20-37.

rotated primarily around one axiswhich will successively bring each

of these welds into a convenientposition. As for the short welds onthe narrow angle flanges, if the

fixture is mounted on a commercia



positioner, there may be at leastone additional axis of rotationwhereby the fixture can be laid

down flat, or the part can simply beremoved from the fixture andplaced on a bench for these welds.

Thus, it is a matter of availableequipment. In the design shown infull lines, the fixture has a circularbase with 6 holes for clamping it on

the positioner table. If corner weldsonly are required to be done in thefixture, it can be provided with a

shaft to be mounted horizontally ints own bearing bracket. The

circular plate is retained and servesas an index plate for indexing the



fixture to four positions 90 degreesapart. This alternate solution isndicated by chain-dotted lines.

The fixture is a rectangular framewith diagonals, provided with

standing lugs that function asocators for the parts, and withpush-pull toggle clamps forsecuring the four frame sides in

position. The locators for the crossbars are welded on the diagonals inpairs, with sufficient space between

for receiving and locating the parts.In this way, their position is fully defined, and no clamping is neededEach clamp acts in a point, halfway

up on the locator,




Ch. 20

so that the correct orientation of the frame members is securedwhen the parts are installed. Theend members and the cross bars arepositively positioned between the

side members; the side memberscan slide longitudinally and be

isually lined up, relative to the endmembers, at the corners. Thermaldistortion is not significant, as thewelds are small in relation to themass of metal, and are located far

apart. The effect of the heat is



expansion, and the result isessentially a bending of the framemembers away from the locators

over the free spans between, andoutside of, the clamped points. Thisdistortion is slight, transient, and

not harmful. The shrinkagedistortion which tends to pull theparts together is effectively resistedby the wide and massive locators.

ll locators are rectangular blocksof substantial thickness. They couldhave been designed as T-section

brackets, saving some steel, butwould cost significantly more incutting, fitting, and welding. Thefixture body consists of square



tubing, 4 by 4 inches (100 X 100mm), 11 gage (. 120 inch or 3.05mm). With the weight of the part

and the fixture, the requiredstrength and rigidity might well beobtained with smaller dimensions,

or with round tubing. However, theuse of this size of square tubingprovides large, flat areas forsupporting the pans and permits

assembly without the use of gussetplates. The width of the tubesprovides areas for the mounting

and welding of the locators. The twoongitudinal tubes are made onench longer than the part, to provid

backing for the welding across the



narrow angle flanges. The totalweight of the tubes is 70 pounds(32 kg), in other words, only a smal

fraction of the total weight of thefixture. The pads for the clamps areflat plates. The width may seem

somewhat excessive, relative to thebase of the clamp, but again, theextra width provides the rigidity which otherwise would have

required a bracket with a rib; amore expensive design. In the

design of welding fixtures, it isoften possible to economize by trading off labor cost for cutting andwelding, against some additional

material.



ll dimensions shown in Fig. 20-38are REF (untoleranced)dimensions. As an exercise,

calculate the dimensions with theirproper tolerances so that the fixturewill produce the part with the

tolerances shown in Fig, 20-37,

Some positioners, particularly thosefor automatic welding, have

developed into full-fledged machinetools, usually of the lathe type. A welding lathe carrying an internal

fixture is shown in Fig. 20-39. Onesegmented and collapsed backingring is seen*in the photograph. Thepart, a thin-walled cylinder, has

runner rings on the outside,



supported in the four large frames,which function as steady rests.

There is no physical upper limit tothe size of welding equipment.Probably the largest existing

welding positioner is shown in Fig.20-40. It weighs 200,000 pounds(91,000 kg) and is designed for useat the U.S. Naval Shipyard on Mare

Island. It rotates the work at speedsranging from 0.052 RPM (19,2minutes for one revolution) to

0.0052 RPM and in four minutescan tilt the table 60 degrees fromthe horizontal. The table diameters 33'o" (10 m); the height,

measured to the table in the



horizontal position, is 20'l0" (6.35m), and its work capacity is 150,000 pounds (68,000 kg).

Courtesy of Aerojet-General Corp.

Fig. 20-39. A welding lathe with



external steady rests.

Courtesy ofPandfiris Weldment Co

t'ig. 20-40. A 3 3-foot-diameterwelding positioner.

CHAPTER

21

Universal and Automatic Fixtures

Definition of "Universal"

The term "universal fixtures" covertwo different types of equipment.The first type consists of a drill jigbody with a quick-acting clamping



and locking mechanism, and whichcan be provided withnterchangeable drill bushing top

plates and sub-bases (adapters) tosupport the work. The second typecomprises sets of building elements

which can be temporarily assembled to a fixture anddismantled after use. Both types areavailable from commercial sources,

but they can also be designed andbuilt "in-house" to advantage.Examples of simplified designs for

this purpose are included in thefollowing sections.

UNIVERSAL DRILL JIGS



Custom-made Jigs

The merits of the machine tool vise

(and other vises) for use as fixturebases have been described at lengthn other chapters. Examples of how

arious types of vises, not only machine tool vises, can beconverted to universal drill jigswere shown in Figs. 18-35 through

18-37. The vise drill jig shown inFig. 8-36 includes, in addition, two

-blocks and demonstrates a

principle used in universal drill jigsof a simple type for cylindricalparts. It differs from the morecommonly used type by having the

-block installed with horizontal



s. The usual type consists of a V-block with a vertical V, one or morebrackets, each with a drill bushing

centered in the axis of symmetry ofthe V, a clamp, and an end stop. It iused for drilling holes along a

diameter of cylindrical parts withinthe full range of diameters that canbe accommodated in the V-block.

Pump Jigs

The most common type of universadrill jig is the pump jig, so namedbecause it is operated by a pump-

ng movement of the operating

handle. A pump jig, designed and



built "in-house" is shown in Fig. 1842. In the clamping position the topplate A with the drill bushing is

held against the work by springpressure. When the handle D isifted, it lifts the top plate and

releases the work.

commercial drill jig with threeposts is shown in Fig. 21-la. The

construction of universal drill jigs iquite simple. The outer style andsome details may vary, but the

principle remains the same. The topplate is secured to either one, two,or three vertical posts. The posts arraised and lowered through a lever

arm, with the top plate maintaining



a horizontal position at all times.The length of travel (the clampingrange) of the top plate is quite

imited, normally about 25 percent,or even less, of the maximumopening height. For most

workpieces it is therefore necessaryto provide a sub-base, known as anadapter, to lift them up so that thetop of the work comes within the

clamping range, as shown in Fig. 212. Locators, attached to the topplates or to the adapters, are also

used. A few rules can be formulatedfor the design of the adapters andocators. Assuming that the part ha

one machined and one unmachined



side, horizontal alignment isestablished from the machinedsurface. When the holes must be

drilled from the machined surface,this side must be up, and to ensurefull alignment with the bottom side

of the top plate, the adapter must bmade much smaller than thesurface of the part. This condition ishown in the illustration. When the

holes must be, or can be, drilledfrom the unmachined side, themachined side is down and the

adapter is made large enough toalign and support it on its entirewidth.

Internal location is preferred to



external location. A concentricocator is attached to that member

from which horizontal alignment is

established. Preferably, a concentricnternal locator is attached to the

top plate, but a concentric external

ocator is attached to or madentegral with the adapter.

UNIVERSAL AND AUTOMATIC

FIXTURES

Ch. 21

Locators are designed with a shortocating surface to preventamming, and with conical or

otherwise tapered lead surfaces



(pilots). Locators on the top platehave long lead surfaces so that theoperator can see that they catch the

part. It may be necessary tomachine a clearance space in theadapter if the lead surface is longer

than the height of the part. Locatoron the adapter have short leadsbecause they are visible to theoperator when the jig is open. A

ong lead would also require thatthe part be lifted higher whennserted and removed. In either

case, there must be space enoughover or under the locator in theopen position to bring the part inand out.



Parts having fully concentricconfigurations (contours and holesdo not require radial location, i.e.,

components that prevent rotationaway from the correct position.Most noncircular parts require

radial locators, usually hardenedsteel blocks, fastened to the topplate or the adapter.

opposite end is for locking the topplate in the fully open position.

iew c shows a unit consisting of a

pinion and rack set mounted in abearing bush-

TOP PLATE WORK



a c

Courtesy ofJergens Inc. Fig. 21-1. a.Sectional view of a pump jig withthree posts, showing the rack and

pinion movement, b, A pinion shaft



with integral braking cone. c. A rackand pinion locking unit withbearing bushing and operating

handle.

There is generally a locking

mechanism connected with theoperating mechanism. This devicemaintains the clamping pressureand prevents the work from shakin

oose when it is drilled. One widely used type of operating and lockingmechanism is shown in Fig. 21-la, b

and c. As seen in the sectioned viewa, the jig contains a helical gearpinion and a mating rack which isntegral with the post. The pinion

shaft (see b) carries the operating



handle and at one end an integralbrake cone. A counter cone is fittedon the opposite end, and mating

conical seats are machined into thebase of the jig. Operating the handlcloses the jig, and as clamping

pressure builds up, an axiai thrusts developed which locks the brake

cone into its seat. The cone at the

ng which contains the conicalseats. Such units are commercially available for installation in custom

designed jigs.

Other types of pump jigs employ braking devices based on the

principle of the overrunning clutch



as in a bicycle wheel, or a pair of cam operated brake shoes. Lockingunits of these types are also


The jig shown in Fig. U-3 has

several refinements. The topbushing plate is interchangeableand adjustable, the adapter for thework is a V-block, and an adjustable

end stop is provided for locating thework. A drill jig with an air-operating clamping mechanism is

shown in Fig. 21-4. Some drill jigshave a fixed top plate, and theadapter is mounted on

Ch. 21



UNIVERSAL AND AUTOMATICFIXTURES

283

Courtesy of Anton Ruckert, Berlin,



Germany Fig. 21-3. Pump jig with-block and interchangeable and

adjustable bushing plate.

a post that can be raised forclamping the work. Some drill jigs

have the clamping area locatedbetween the posts while still othersprovide the feature of improvedaccess to the clamping area by

allowing the top plate to swing 180degrees out of the way in



Courtesy of Heinrich Tools Inc.,Racine, Wis. Fig. 21-4. An air-operated drill jig.

the horizontal plane or tilt 45degrees in the vertical plane. Some

igs have the rear side of the body precision-machined square with thebottom surface. In this way the jigbecomes a tumbling jig; it can be

aid



down on its back to permit thedrilling of holes in two directions,90 degrees apart.

Cast iron top plates, fitted to theposts, but without bushing holes,

are supplied by jig manufacturers.Blanks for top plates can also beeconomically produced by torchcutting them out of steel plate and

drilling and reaming the post holeswith a simple drill jig, A commerciapunch holder, i.e., the upper half of

a postless die set, makes asatisfactory and inexpensiveadapter blank.

dvantages



Speed of operation is the greatestadvantage of the universal drill jig.It is so significant that the use of

this type of jig as a permanentcomponent of a single-purpose toolcan be economically justified in

highly repetitive work.

The jig operation is fast since it isoperated by a single sweep of the

ever handle, which eliminates theneed for loose tools and clampingparts. This manual operation is

always the same, regardless of thepart configuration, and a line of dissimilar parts is drilled as if they were all alike. The rate setting can

be done without individual time



studies, by determining andrecording the handling times, onceand for all, and calculating drilling

time from speed and feed. The jig isoaded and unloaded in the upright

and open position, and does not

have to be turned over as do mostother drill jigs. When one hole onlys to be drilled, the jig can be

secured to the drill press table;

therefore, the drill will enter thebushing practically withouttouching, which results in

prolonging drill and bushing life.Top plates and adapters can oftenbe so designed lhat chip cleaning isgreatly simplified if not completely



eliminated.

The cost of a fop plate with adapter

s generally less than the cost of acomplete single-purpose drill jig,and top plates and adapters are

nterchangeable so that the mainbody and operating mechanism canbe used for a variety of jobs. A topplate is usually more expensive

than an adapter, although thematerial costs are a minorconsideration. The greatest expense

tem is the precision boring of thepost and bushing holes. A top platemay be equipped with bushings formore than one hole configuration,

and an existing top plate may be



modified by the addition of morebushings. Different parts with thesame hole configuration can be

accommodated by changingadapters and locators. If a top plates made with integral locators, then

t can be turned over and the otherside used. In such cases it may benecessary to use headless drillbushings.


Ch. 21

Chips and Coolant Considerations



When coolant is needed it isdirected onto the top plate.Commercially available cast top

plates are formed as trays andprovide a reservoir from which thecoolant flows down along the drills

For use with a flat top plate, a ringarge enough to encompass all the

drills used is cut from 1/2-inch (13-mm) steel plate, and is laid on the

top plate to hold the coolant. Chipsare swept off by simply sliding thering over the plate.

UNIVERSAL FIXTURES

OjOsf



ooo ^ ■ o ex© ojf ©

Commercial Universal Fixtures

simple, yet quite versatile andefficient fixture is shown in Fig. 21-5. It is in essence a glorified V-block. With the clamping screwsshown, it can hold parts of any configuration within its own

dimensional limitations. It can berotated (like a tumble jig) 45degrees and 90 degrees in its own

ertical plane, and rotated 90degrees to either side. A swivel bases available by which it can be

rotated at an arbitrary angle. It can

be used in any machine tool,



ncluding the lathe, where it isclamped on the face plate.

Courtesy ofSchwenzer Tool & DieCo., Inc., Buffalo, N.Y. Fig. 21-5.Simple and versatile universalfixtures.

different and more representativetype of fixture is shown in Fig. 21-6

The principal component is the bas



plate. Edge strips are bolted on, sothat a lying V-block is formed, and apart is clamped in this V by means

of clamping screws of the same typeas those shown in Fig. 21-5. It isused here for precision drilling. The

drill bushing, mounted in a largeboss, is located from the sides of the V by means of gage blocks.



Courtesy of Montgomery and Co.Fig. 21-6. A base plate type of universal fixture.

The backbone of every universalfixture is the sub-base; the various

systems differ in the types andnumber of components. Theelements in general are of steel,hardened and ground to tolerances

daptor Block Thrust Element

(Stop Element



Courtesy of "Machinery " MagazineLondon, June 20, 1946 Fig. 21-7.Universal fixture components; stop

and-thrust elements.

Ch. 21


285



Right Angle Sliding Plate

Eccentric Pin

Stop Elements

Courtesy of "Machinery" Magazine,London, June 20, 1946 Fig. 21-8.Universal fixture components;

adjustable location pins.



of the order of 0,0003 inch (0.008mm) on the significant dimensionsThe basic elements are rectangular

blocks with T-slots, called stopelements {manufacturer'sterminology), thrust elements, and

adapter blocks with bolt holes forbuttressing the stop elements, fixedand adjustable height elements,angular elements with fixed angles

of 30, 45, and 60 degrees,adjustable angular elements(including sine bars), special

elements for the attachment of



Courtesy of "Machinery" Magazine,London, December 27,

1951 Fig. 21-9. Universal fixturecomponents; miscellaneous details

ocating pins (also adjustable by

means of an eccentric), V-blocks,



ack screws, clamps, holders fordrill bushings, and bearings forboring bars. Straight and angle

straps are provided for the joiningof two sub-bases, and for bracingstop elements, for example, for

forming rigid corners. Sub-bases,with T-slots of standard dimensionsand spacing, are made of nickel caststeel and are available in square,

rectangular, and round shapes.Typical elements are shown in Figs21-7, 21-8, and 21-9, and a

completed milling fixture is shownn Fig. 21-10.

Serrated Pads Double Swivel Clamp



Bled and Screw-.

Element

Height Element Stop Element

Sub-Base

Courtesy of "Machinery" Magazine,London, December 27,

1951 Fig. 21-10. A milling fixture



built with universal fixturecomponents.

Such a fixture is not designed inadvance but is built up with dummyblocks made of a castable plastic

material around an actualproduction part,



Courtesy ofMultijig Ltd.; Tynealley Tool and Gage Co.,

Northumberland, England Fig. 21-11. A drill jig built with universalfixture components.


Ch. 21

or a replica of a part. When thedummy fixture is completed, it is

photographed in detail. Thephotographs are used in thetoolroom for assembling the actual

fixture, and provide a permanent



record for filing.

nother system uses holes instead

of T-slots for the assembly. Theholes are alternately straightprecision holes and tapped holes

and are closely spaced in a modularpattern. A drill jig built withcomponents from this set is shownn Fig. 21-11. The jig was built in

two hours and is used for drillingand reaming holes with 0.003-inch(0.08 mm) tolerance on the center

distances.

Custom-made Universal Fixtures

n experienced fixture designer in



cooperation with a good toolmakercan make any desired type of universal jig or fixture. An example

of a universal jig construction isshown in Fig. 21-12. It is known as atool maker's universal drill jig and

consists of a heavy plate A,containing adjustable locating rodsB with locking screws, and a boringfor interchange-



■-' . ; £

B B

Pig. 21-12. The toolmaker'suniversal drill jig.

Fig. 21-13, An application of thetoolmaker's jig,

able drill bushings C, for different



hole sizes. The application of thisig for drilling and reaming the

holes in a large die block is shown

n Fig. 21-13. Parallels are clampedto the edges of the die block, andthe jig is positioned against the

parallels with the locating rods;measurements are taken withmicrometers and gage hlocks.When the bushing C, is correctly

positioned, the jig is clamped to thedie block and the hole is spotted,drilled, and reamed. The procedure

s repeated for each hole.

drill press can be converted to amakeshift jig borer by installing a

compound table with slides at right



angles, on the drill press table. Anupright with a bracket carrying ainer bushing for the insertion of

different size slip bushings isnstalled with the bushing axis inine with the drill spindle. The

slides arc positioned from fixedstops by means of micrometergages, gage blocks, or gage bars.

In developing and building auniversal fixture set, the first task ito design the sub-base. Any base

plate with parallel T-slots ormounting holes will serve thepurpose, but a design with partly diagonal



¥T

irr-

~s

&

Courtesy ofE, Tfiaulow

Fig. 21-14. A universal fixture sub-base.



Ch. 21


FIXTURES

287

ribs and T-slots, such as that shownn Fig. 21-14," has advantages over

the conventional type. The T-slot

pattern is more versatile, anddiagonal ribs provide additionalrigidity against torsion.

Some angle plates with single (Fig.21-15)* and multiple T-sIots andsome smaller and larger tooling



Courtesy ofE. Thaulow Kig. 21-15.

ngle plate with a single T-slot foruse in a universal fixture.

blocks (Fig. 21-16) are added to thebase. There is no rule that forbidsthe use of T-slots and mountingholes within the same set. Each

system has its advantages andselection is made according to whats needed. Finally, an assortment of

bushings, clamps, bolts, and sundrytems is selected from fixturecomponent catalogs, and theuniversal fixture set is ready for its

first assignment.



<g^-

■ w

-^

•*



_i3

Courtesy of Challenge Machinery

Co., Grand Haven, Mich. Fig. 21-16. large tooling block for use in a

universal fixture.

--——. - !*irs——- C--

1 E. Thaulow, Maskinarbefde

(Copenhagen: G.E.C. Gad's Forlag,1930) vol. 11.

2 E. Thaulow, Maskinarbefde

(Copenhagen: G.E.C. Gad's Forlag,1928) vol. 1.

UTOMATIC FIXTURES



Definitions and Principles

utomatic fixtures are those in

which the part is clamped andundamped by the use of a powermedium, usually compressed air

(pneumatic fixtures) or oil underhigh pressure (hydraulic fixtures).These devices are used for fivepurposes:

1. To apply a greater and moreconsistent clamping force than ispossible by manual operation

2. To reduce operating time andoperator fatigue



3. To operate the fixture by remotecontrol (including foot operation)

4. To clamp simultaneously anduniformly in multiple fixtures

5. To be able to incorporate thefixture into an automated program(transfer machines, conveyor-izedproduction lines, or numerical

control [N/C] machine tools).

The actuating member is always acylinder with a piston or a plunger

(referred to in the following as thepower cylinder). The actuating forces applied directly or indirectly;

direct actuation means that the



force from the power cylinder actsdirectly on the part or on a clampthat is in contact with the part;

ndirect actuation means that theforce from the power cylinder actson the clamping element through a

kinematic chain, which can be ainkage system or a combination of

cams and links.

Common Features and Advantagesof Pneumatic and HydraulicFixtures

There is no general preference forselecting one system over the otherIt is not even possible to predict

relative costs without making



comparative estimates. For theselection of a system, the followingguidelines can be applied:

Hydraulic fixtures utilizesignificantly higher working

pressures than pneumatic fixtures.Hydraulic fixtures are preferred,therefore, where large clampingforces and short strokes are

required and where available designspace is limited. Permissible flow

elocities in conduits are 15 feet per

second (4.6 m per sec) in pressureines and 4 feet persecond (1.2m pesec) in other lines. The maximumpiston speed is 2 feet per second

(0.6 m per sec). Hydraulic fixtures



operate satisfactorily under averagecycling conditions and at normaltemperature levels. The oil

temperature must not exceed 140F(60C). At higher temperatures oilsose viscosity, oxidize, and, in the

ong run, break down. Mating partswithin the equipment that expanddifferently, may bind or


Ch. 21

eak. Hydraulic cylinders are self-ubricating; air cylinders are not.



Pneumatic fixtures are used wheremedium clamping forces withalmost any length of stroke are re-

Quired, and where ample designspace is available. Permissible flow

elocities in conduits are

significantly higher for air than foroil; pistons in air cylinders operateat speeds ranging from 1/4 inch persecond (6 mm per second) to 10

feet per second (3 m per sec) with 2to 3 feet per second (0.6 to 0.9 mper sec) as a commonly used

average; this means that theclamping operation is practically nstantaneous. Pneumatic fixtures

operate satisfactorily under



conditions of high cycling andelevated temperatures.

The use of a power medium (air oroil) under constant pressurepermits close control of the

clamping pressure which, first,ensures that the part is sufficiently gripped and, second, reduces therisk of distortion or even breakage

of the part. The initial cost of pneumatic and hydraulic clamps ishigher than the cost of manually

operated clamps; they also incursome operating expenses(compressed air, power for thehydraulic pump), but these are

nsignificant. Air is cheap. A



representative figure for the cost ofcompressed air in a factory is $0.12to SO. 15 per 1000 cubic feet ($0.42

to $0.55 per 100 m 3 ), and it takesmany piston strokes for a smallcylinder to consume one cubic foot

The dominating, if not decisivefactor, in favor of using automaticfixtures is the saving in labor costs.

The saving is about 80 percent onmanual clamping operations of upto 1/2 minute duration, and 85

percent on longer operations. Inaddition, operator fatigue isirtually eliminated.

Each of the two systems can be



manually or automatically actuatedby electrical controls (solenoid-operated).

Normally, the power medium isapplied to the clamping operation.

The release of the clamp and thereturn to the open position can beaccomplished by application of thepower medium in the opposite

direction, by a return spring, or by acombination of the two. Whereseveral clamps are used in the same

fixture, they can be timed tofunction simultaneously or in apredetermined sequence. The forceF (pounds) exerted by a power

cylinder of diameter Dp (inches)



with a piston rod diameter Dr(inches), and an operating pressureP pounds per square inch is

calculated by

F = 0.7854 (Dp 2 ~D R 2 )XPX

(0.85 . .. 0.90)

For that side of the piston wherethere is no piston rod, Dft = 0. The

factor (0.85 . . . 0.90) is themechanical efficiency.

In Metric units P is in newtons, Dp

and Dr are in millimeters, and P isn newtons per square millimeter.

Pneumatic Fixtures



Pneumatic fixtures are operatedwith air from the compressed airsupply system in the plant. The

operating pressure is nominally 100pounds per square inch (0.69N permm 2 ). It is usually assumed to

range between 80 and 100 poundsper square inch (0.55 to 0.69N permm 2 ), but may well drop to 40 to50 pounds per square inch (0.28 to

0.34N per mm 2 ) at points at agreat distance from the source, or incases where the compressor or the

distribution lines are overloaded.ir cylinders are available with

diameters up to 14 inches (350mm) and lengths up to 3 to 4 feet



(0,9 to 1.2 m). The larger sizes arenot for clamping purposes, but areused for moving the part into or ou

of the fixture, for rotating the part,or for moving the fixture from onestation to another, etc.

To ensure constant output from thepower cylinder and constantclamping pressure in the fixture,

the input pressure must bemaintained constant andndependent of pressure fluctuation

n the air supply. This is done by means of a pressure reducing valven the supply line to the fixture.

With constant power output, the



power cylinder is essentially anelastic system. It maintains its ownforce as the clamping force on the

clamped part and it holds theclamped part in a position of stableequilibrium as long as the clamping

force is superior to any opposingforce. However, if the opposingforce temporarily or permanently equals the clamping force, the

equilibrium is no longer stable, andf the opposing force, even only

temporarily, exceeds the clamping

force, the clamping element ispushed back and the part is likely tobe thrown or pulled out.

For the stability of the operation,



there must be provided somemeans by which release of theclamp is prevented if the air supply

s cut off or the supply pressurefalls below the pressure at whichthe reducing valve can maintain

constant operating pressure. Onemethod is to insert a pressureswitch in the supply line which willstop the machine if the pressure

drops below a safe limit or thesupply is cut off. A different andfrequently used method is to

transmit the force from the powercylinder to the clamps

Ch. 21




289

through a mechanical device, akinematic chain, which is self-ocking when it is in the clamping

position. Wedges, cams, and toggleoints are used. Once fully activated

they hold the clamps engaged, evenf the air pressure vanishes. Thentroduction of a kinematic chain

for the transmission of the forceeliminates the elasticity in thesystem and has the additionaladvantage that it is now possible to

ncrease the applied clamping force



n mechanical language this meansto introduce a mechanicaladvantage greater than one. For a

given required clamping force thispermits the use of a smaller powercylinder or the use of a lower

operating pressure. A self-lockingdevice normally has a mechanicaladvantage that is significantly greater than one.

ir Cylinders

Rotating power cylinders for theactuation of various types of chucksare commercial items used on mansemiautomatic and automatic

athes. Non-rotating air cylinders



are used on commercially availableises. These devices are

workholders, not fixtures.

universal drill jig equipped withan air cylinder is shown in Fig. 21^.

Many commercial clamps are builtwith an attached or integral powercylinder, mostly for air operation,some of them capable of air

operation and oil operation in thesame cylinder (dual pressureclamps).

ir cylinders are available frommany sources and with a variety of rod end and rear end accessories as

shown in Fig. 21-17, for attachment



to the fixture

Courtesy of Parker-Hannifin Corp,Fig. 21-17. Air cylinder withdifferent types of connecting

components (end accessories).

and the clamp. A power cylinder canperform the following functions;push, pull, raise, and lower.Cylinders are single-acting anddouble-acting. They can be supplied

with single-end and double-end



piston rods, protruding from oneend only, or from both ends of thecylinder. For cylinders with single-

end piston rods, where possible,clamping with pressure on thepiston-rod side of the piston, and

releasing with pressure on the fullpiston area are recommended. Theadditional release force may beneeded to overcome any jamming

or sticking in the clamps. Whendimensioning air cylinders a lengththat is two times the net calculated

ength of travel plus the length of the piston is recommended tocompensate in advance for futurechanges in work dimensions and fo



other unforeseen changes.

n air-operated clamping device

with integral power cylinder isshown in Fig. 21-18. It illustratesmost of the previously described

principles. The

^



■Z77TZ

~Zt

=z

Courtesy of Cincinnati Mitacron

Inc. Fig. 21-18. An air-operatedclamping device with integral powecylinder and a sloping cam.


Ch. 21

actuating element is the largehorizontal plunger which carries



the piston at its right end and asloping double-acting cam at itsmiddle portion. This cam engages a

small vertical plunger whichactuates the clamp throughspherical equalizing washers. The

operation is controlled by an airalve. For clamping, air is admitted

to the small area on the left side of the piston, and moves the large

plunger to the right. The slopingcam pushes the small plunger downand clamps the part. The inclination

angle of the sloping cam surfacesequals the friction angle for dry surfaces so that the clamp is lockedn position even without the help of



cylinder pressure. In order torelease the clamp, air is admitted tothe full piston area on the right side

of the piston, which moves the largpiston to the left. The initialmovement simultaneously releases

the clamping pressure and thefrictional forces, and furthermovement to the left causes theupper sloping cam surfaces to

engage the vertical piston and lift itfrom the work.

Hydraulic Fixtures

The acceptable working pressure foa hydraulic fixture is determined by

the fixture, the conduits, and the



hydraulic power source. The designpressure for hydraulic fixtures is, ina sense, arbitrary, because a fixture

can theoretically be designed forany desired pressure level.However, higher pressures require

not only larger materialdimensions, but also tighter fits,closer tolerances, and smoother(precision lapped) surfaces to

ensure against leakage. Asexplained in Chapter 11, thepractical upper limit for the

working pressure is 15,000 poundsper square inch (103N per mm 2 )when a plastic (PVC) is used as thepressure medium, and 10,000



pounds per square inch (69N permm 2 ) when oil is used. Many hydraulic fixtures and fixture

components are designed forsignificantly lower pressures. Thedual pressure clamps referred to in

the previous section are designedfor operation with air pressures of 100 to 250 pounds per square inch(0.7 to 1.7N per mm 2 ) or oil

pressures up to 500 pounds persquare inch (3.4N per mm 2 ).

The maximum working pressure fostandard hydraulic tubing andfittings is about 6000 pounds persquare inch (4IN per mm 2 ) for

stationary pressure lines and about



3500 pounds per square inch {24Nper mm 2 ) for flexible tubing.Special types and qualities are

available for higher pressures.

Hydraulic fixtures can be powered

from the machine tool, if it has ahydraulic drive, from a separatehydraulic pump, and from apneumatic-hydraulic booster

(pressure intensifier). A machinetool with

a hydraulic drive is a convenientand inexpensive power source forthe fixture. The operating pressuresused in machine tools are low

(1000 pounds per -square inch [7N



per mm 2 ] or less) compared to thepreviously quoted values, and thefixture designer must always

carefully check the availablepressure. Separate and individualhydraulic pumps are available with

almost any desired pressure, butthe cost is frequently too high forconsideration. The preferred powersource, which is now widely used

for fixtures, is the booster. It issimple, small, inexpensive, and

ersatile and is commercially

available. The booster consists of two coaxial cylinders with differentdiameters and a common piston. A ow-pressure medium is applied to



the large piston area and a highpressure is developed on themedium in the small diameter

cylinder. The low-pressure mediums primarily air from the

compressed air line (at around 100

pounds per square inch (0.7N permm 2 ]) or medium pressure oil, foexample, supplied from thehydraulic system in the machine

tool, A booster is characterized by the area ratio, the maximum outpupressure, and the volume of high-

pressure medium supplied perstroke. Representative ratios are7:1, 15:1, 28:1, and 30:1. Outputpressures from commercial units



are 1500, 3000, and 7000 poundsper square inch (10, 20, and 50Nper mm 2 ). They supply from 1 to

14 cubic inches (16 to 230 cm 3 ) ofhigh-pressure oil per stroke. Somemodels are provided with an

adjustable relief valve and providenfinitely variable pressures of from

1500 to 3000 pounds per squarench (10 to 20N per mm 2 ).

Boosters are smalt and since they



Proprietary to Tomco, Inc. Fig. 21-19. A fixture with a hydraulic clampand with the air/hydraulic boostermounted on the machine tool.

Ch. 21




291

require only an air connection, they

can be placed close to the fixture. Its often found convenient to mount

the booster on the machine tool as

shown in Fig. 21-19,

Power cylinders are designed forand rated at maximum working

pressures of 3000, 5000, and10,000 pounds per square inch (2035, and 70N per mm 2 ) and arecommercially available with up toapproximately 8000 pounds (36kN) maximum capacity. Matchingholding brackets and complete

clamping sets are also available.



The most recent design trend ischaracterized by small dimensionsand low profiles. The smallest

hydraulic power cylinders arethreaded on the outside with threaddimensions down to 1/2-13 UNC

(see Fig. 21-20) and provide 150pounds (0.7 kN) of force at 3000pounds per square inch (2IN permm 2 ) rating with 1/8 inch (3 mm)

plunger travel. A typical low profileclamping unit is shown in Fig, 21-21. When power cylinders are

ncorporated in individual fixtures,utilizing only 75 percent of therated maximum stroke to provide areserve for workpiece variations,



etc., is recommended.

n entirely different principle for

hydraulic clamping consists of using a thin-walled sleeve expandedby hydraulic pressure into the part

that is being clamped. Anapplication to a lathe fixture isshown in Fig, 21-22. The fixture iscompletely self-contained and

works only with the pressuremedium (oil or plastic) that isconfined in the cavities inside the

sleeve. Pressure is applied from therear end of the lathe spindle by means of the actuating rod locatedn the axis of the fixture. The

principle is the same as that used in



the fixture shown in Fig. 11-16. Thesleeve expands very uniformly,except near the ends, and the

pressure on the part is also very evenly distributed. The unitpressure is relatively low, and the

part is not distorted. Accuracy onthe clamping surface can bemaintained within a toierance of 0.0002 to 0.0004 inch (0.005 to

0.010 mm) TIR,



Fig. 21-21

drill jig with hydraulic clamps.

Proprietary to Tomco, Inc. two law-profile swinging

depending on the expansion. With adesign stress of 60,000 pounds persquare inch (400N per mm ! ) in

the sleeve, the maximum expansion



s 0.002 times the sleeve diameter.This type of fixture is built in sizesfrom 1/2-inch (13-mm) diameter

minimum and can be used on thenside and outside of cylindrical and

slightly tapered surfaces (up to a 6

degree taper angle), on steppeddiameter cylinders, and for theclamping of several shorter rings onor in one fixture, as indicated in the

llustration. It may be noted thatwhen larger expansion is needed,the sleeve can be made of nylon

with steel inserts.

I s y



O^

»-v EXPANDED EJUHETER

X JUNIUL REFILL

UTOIUTIC »EFILL

I

Proprietary to Tomco, Inc. Fig, 21-20. Small (miniaturized) hydraulicpower cylinders.



Proprietary to Hydra-lock Corp.(Pat. Pend.J Fig. 21-22. An arbor-type lathe fixture with hydraulically

expanded sleeve.

FIXTURES FOR TRANSFER

MACHINES AND N/C MACHINETOOLS

Fixtures for N/C Machining

Because N/C machining ischaracterized by a high hourly burden (overhead rate), several

times higher


FIXTURES



Ch. 21

than that used for conventional

machine tools, a reduction in non-cutting time is of first importance.One way to reduce the non-cutting

time is by improved fixturing.

The principle of N/C machining hasdrastically affected the fixture

design. A common type of N/Cmachine, the N/C machiningcenter, performs milling, drilling,tapping, boring, and reaming in onesetting of the part and reduces thenumber of fixtures from two orthree, to one. The need for

ndividual cutter guides, except for



the starting position, is eliminatedbecause all subsequent positioningof the cutters relative to the work is

covered in the programming for theoperation. Fixtures for N/Cmachining are thereby greatly

simplified, in fact, they are reducedto locating and clamping devices forthe workpiece.

Most N/C machines have what iscalled a "floating zero" which allowsthe programmed starting point to

be adjusted to the actual startingpoint on the part. This point can bedefined by a single tool settingblock on the fixture or by a point

selected on the part itself. This



nitial operation is not alwaysnecessary. Several types of N/Cmachines work with preset tools,

and when the fixture is accurately ocated relative to the machine, it is

also positioned relative to the

cutters. Edge locators with a closefit in the slots in the machine tableare provided for this purpose.Horizontal and vertical universal

fixture bases are also available formost machines. An error in themounting of a fixture endangers the

operation, because the cutter may collide with the fixture body.Machines with more than one slidemotion have interference zones for



the protection of vital parts, as forexample, an index table. Only oneslide at a time is permitted in an

nterference zone. To fully utilizeboth slide motions on a part held ina fixture, it is sometimes necessary

to lift the fixture above thenterference zone by mounting it on

a raiser block of sufficient height.

N/C fixtures must be strong andrigid to ensure correct parttolerances and have provision for

quick loading and unloading of theworkpiece. Weight considerationsare unimportant when the fixturesare indexed from position to

position and are moved on and off



the machine by mechanical means.

One Tesult of the simplified design

s that there is easier access for thepart to the interior of the fixture,and the handling of the part is

correspondingly simplified andfacilitated.

simple and easily fabricated

universal fixture base, suitable forshort-run production, consists of aplate with key seats, T-slots,precision holes, and

tapped holes in a regular pattern.Keyseats and T-slots are spaced at

6-inch (150-mm) intervals, and



holes are located with 3-inch (75-rnm) center distances. This patternpermits side and end locators

(strips and buttons) to be installedfor any conceivable configuration othe outline of the part. It permits

easy handling of the part and itprovides the possibility for locatingall necessary clamping devices. Forfast operation, the clamps are hydra

uli-cally operated and all powercylinders are fed from the samesource of high-pressure oil. In this

way, all clamps are actuatedsimultaneously and with the samepressure, yet still independently sothat they automatically equalize for



any variations in part dimensions.The source of high-pressure oil canbe a booster, but since the total oil

olume is relatively small, it ismore practical to use a hand pump.When the oil volume required is

small enough, a screw pump willsuffice. The uniformity in thepressure distribution eliminates orminimizes distortion of the part.

The time saving obtained by thesimultaneous application of allclamps is repeated after machining

when the clamps are released andautomatically retracted, likewise,simultaneously. With automaticcontrol of the hydraulic system, the



operation of the clamps can bencluded in the program and closely

coordinated with the machining

cycle.

Proprietary to Vlier EngineeringCorp.

Fig, 21-23. A typical N/C fixturesetup with dual fixtures

and low profile swinging hydraulic



clamps.

The most drastic reduction in

oading time is realized by the useof dual fixtures. With smaller partstwo fixtures are mounted on the

machine table; one fixture isunloaded and loaded while the partn the other fixture is being

machined. With large parts and

ong machining cycles, two sets of fixtures and base plates are used.One set is in the machine, while the

other set is being unloaded andoaded in the toolroom or on thefloor. With the use of edge locatorson the table, the exchange of



Ch. 21


FIXTURES

293

the base and fixture sets takes littletime, and maximum machine "on-time" is achieved.

typical N/C setup with dualfixtures and hydraulic clamps isshown in Fig. 21-23. The clamp

straps lift and swing outautomatically as the oil pressure isreleased.



This clamp satisfies the modernrequirement of a low profile. In N/Cmachining, the machine follows,

without human supervision ornterference, the path through

which the cutter is programmed

and returns to the starting pointafter a completed cut. To preventcollision and damage it is thereforemportant that the air space above

the part is free of obstructions.

Fixtures for Transfer Machines

The highest level of development ofautomatic fixtures is found in thefixtures that are used on or

ncorporated intoeonveyorized



production lines and transfermachines as used in the automotiveand other mass production

ndustries. The design principle for[he fixture depends on its made of operation. There are two modes of

operation, the traveling fixture andthe stationary fixture.

The traveling fixture is either

moved on a conveyor, is an integralpart of the conveyor, or is pushedon a track by means of reciprocatin

transfer bars with fingers that pushthe fixture during the forwardstroke of the bar, and are retractedfrom the path of the fixture during

the return stroke of the bar. The



part is loaded into the fixture andclamped at one end station and isnot released until the fixture has

reached the other end station. Ateach work station the entire fixtures located and locked in position for

the machining operation.

With stationary fixtures, one ateach work station, the part enters a

fixture, is located, clamped,machined, and released, and is thenmoved on to the next fixture. The

part is moved by means of aconveyor or a transfer bar. Thedirect use of these transfermechanisms requires that the part

has one or several flat surfaces.



When that is not the case, the partsare nested and clamped in palletswith flat surfaces that can slide on

the track.

Regardless of the mode of

operation, these fixtures have anumber of common features. Thepart is usually provided with twotooling holes for dual cylindrical

ocation, and the fixture hasmovable locators, known as "shotpins." They are in a retracted

position as the part enters thefixture; the part is stopped in anapproximately correct position ands finally located as the shot pins

enter the tooling holes. The shot



pin has a tapered (conical

or polygonal) pilot end, or a bullet

nose end, or a combination of tapered flat surfaces and strips of the original cylindrical surface, as

shown in Fig. 21-24. The taperedportion pushes the part over andtakes the wear, and the cylinderdefines the final position. It

maintains its accuracy, because it isnot exposed to any significantsliding motion.



L. --



Fig. 21-24. The action of a taperedshot pin.

different method of locating is touse fixed locators and to move thepart in on an elevated platform

within the fixture. After the part hareached its approximately correctposition, the platform is loweredand the pins enter the tooling holes

When the machinery is completed,the platform is raised and lifts thepart clear of the pins.

Clamping devices are poweroperated, either pneumatically orhydraulically or by means of

weight-actuated cams. Complete



retraction of locating pins andclamps to clear the path of the parts mandatory. All such moving

components are therefore retractedby cam or gear wheel action, or by power


Ch. 21

(double-action hydraulic cylinders)not by springs. A broken or

otherwise inoperative spring doesnot retract the component, and theresult is serious damage as the

motion of the part is actuated.



typical cam operated work stations shown in Fig, 21-25. The cam

shaft provides the successive

movements required to locate andclamp, and later unclamp, thepallet, and to actuate the various

work slides in the station.



Courtesy of The Cross Co. Fig, 21-25. Diagram of a cam-operatedwork station in a transfer line.

The various machining operationsare performed with preset tooling

and do not require tool settingblocks. Drill jigs and boring fixturesare equipped with bushings for thesupport of the tools.

Chip disposal is mechanized.Rotating brushes are provided forthe cleaning of clamping surfaces athe part enters a fixture. A simpleway of removing chips fromoperating areas and cavities within

the parts is to provide intermediate



stations where the part is tilted sothat, the chips fall out. Otherntermediate stations are used for

turning the part over

so that new surfaces are brought

nto position for subsequentmachining operations. Intermediatestations are also provided forautomatic gaging of previously

machined surfaces. If a surface doenot gage correctly, a warning signals actuated or the machine is shut

down. Similar warning devices areused to indicate if a part isncorrectly located at a work

station.



typical detail from a productionine is shown in Fig. 21-26. The

picture shows two loaded work

stations and a section of thetransfer line which moves the partfrom station to station by means of

transfer bars. Theelectromechanical drive seen in theforeground serves to raise, transferand lower the transfer bars with the

parts.



Courtesy of The Cross Co.,Fastrattsfer.® Fig. 21-26. A transferine with two work stations and the

drive for the transfer bars.



CHAPTER

22

Economics

Classification of Fixtures by Grade

The purpose of making economicestimates and calculations for a

fixture is to justify its cost. The costs affected hy the accuracy requiredof the fixture and particularly by thevel of simplicity or complexity

embodied in its design. This appliesparticularly to the clamping devicesIn this respect, fixtures can be

classified into four grades



corresponding to four differentevels of production.

Small lot production, up to 40pieces, requires fixtures of thesimplest possible type with

manually operated screw or camactuated clamps.

Medium lot production, from 40 to

100 pieces, justifies the use of quick-acting clamping devices forsingle clamping, and multipleclamping devices, where applicableMultiple clamping is thesimultaneous actuation of severalclamps acting on a single part or the

simultaneous clamping of several



parts in one fixture.

Large lot production, from 100 to

1000 pieces, represents the areawhere well designed time-savingclamping devices are a necessity.

Multiple clamping is used wherepossible and clamps are air orhydrau-lically operated.

Mass production, over 1000 pieces,uses, in principle, the same poweroperated clamping devices hut withthe addition of such refinements aselectrical control, remote control, osemiautomatic control of theclamp-actuating components.



Estimate of Profit

n economic estimate for a fixture

shows whether or not it will beprofitable. This involves acomparison between the savings

obtained by the use of the fixtureover a fixed period and the cost of using it. The result depends on anumber of factors. They can be

expressed as mathematicalariables and written into an

equation, which, in turn, can be

solved

for any one of them. However, it isclearer to calculate separately each

of the two items, the savings and



the cost, and then compare them.

The period considered is one year.

The following symbols 1 for theariables and constants are used:

y* number of parts produced in aear

s savings in labor cost per part

produced in the fixture (dollars)

L overhead rate (burden) on laborcost

C cost of fixture (dollars)

yearly interest rate



u yearly maintenance cost rate forthe fixture

f yearly cost of taxes, insurance, etc

a number of years required orestimated for amortization of thefixture cost.

S (assuming an old, but still usable

fixture is replaced by the new fixture) the unamortized value of the old fixture less its scrap value(dollars)

The factors L, i, u, and t areexpressed as decimal fractions, not

as percents. The annual saving by



using the fixture is

X = Ns(l +£) (a)

The annual cost Y of using thefixture is

= C (i + m + t + -) (b)

a

or, if the new fixture replaces an oldone, the annual

cost Y (

f = C{i + u + t+-)+Si ' a

(c)



These symbols are the same asthosed used in/4 Treatise onMilting and Milling Machines.

(Cincinnati, Ohio: The CincinnatiMilling Machine Co., 1951.) 3rd ed.,p. 747.

ECONOMICS

Ch. 22

If the cost of setting up andremoving the fixture is substantial,t is added to (b) by a term In,

where / is the complete cost of onesetup and removal, and n is thenumber of setups in a year.



Depending on whether

(a) | (b)or(c)

the fixture is profitable, breakseven, or loses money.

Example-W\th the following values

J = $0.10, £ = 0.90, C= $600.00, i =

0.06, u = 0.04, t = 0.12, a = 2 years,and S = $200.00, how many partsmust be machined per year to breakeven?

X = NX 10.10 X{1 +0,90)= $0.1 W (a)



y=$600X (0.06 + 0.04 + 0.12 +1/2)

+ $200 X 0.06 = $444 (c)

hence $0.19 7V= S444

S444 N = $o"l9~ ~ 233? parts perear '

Example-'With N = 3000 parts perear and other values as in theprevious example, the fixture isprofitable. What is the profit?

X = 3000 X $0.10 X(l +0.90) =$570 (a)



f = $444 (c)

nnual profit: $570 — $444 = $126

Example-With JV= 2000 parts perear and other values, except C, as

before, what is the maximumallowable cost C of the fixture?

X = 2000 X SO.] OX (1 +0.90) =

$380 (a)

f = CX (0.06 + 0.04 + 0.12 + 1/2)

+ $200 X 0.06 = 0.72 C+ 12 (c)

hence 0.72 C + $12 = $380

n* ■ „ $380-512



Maximum allowable cost: C=

0.72

= $511.11

Example -With the following values

for a fixture for an N/C machinetool operation

N = 3000 parts per year; * = $0.20;L = 3.5 (350%); C= $1600.00, andother values as before, is the fixtureprofitable, and if so, what is the

profit?

X =3000 X $0.20 X (1 +3.5) =$2700 (a)



f = $1600 X (0.06 + 0.04 + 0.12 +1/2)

+ $200X0.06 = 51164 (c) Annualprofit: $2700 - $1164 = $1536

Estimate of Fixture Cost

The profit or loss estimate requiresthat the fixture cost is known or

estimated. The safest way of gettingthis figure is to make an ordinary cost estimate from the drawings.However, fixture drawings may not

be available with sufficient detailsfor a cost estimate, there may notbe enough time, or there may be

other reasons why the fixture



designer must make his ownestimate of the fixture cost.

The total cost is composed of material, labor, and overhead,ncluding the design cost. The

overhead rate is known, thematerial cost can be estimatedclosely enough from sketches. Theabor cost can be estimated with

sufficient accuracy for the presentpurpose from the formula

n/3

day technology

ff = A —— , where A = 105 with



average present-

H is machining and assembly time

n hours, W is the weight of thefixture in pounds, and V is theoverall volume of the fixture in

cubic inches. V is

calculated as length X width Xheight or — X

o

o o

t



Fig. 22-1. Overall dimensions forcost estimate.

diameter 2 X height. Thedimensions are measured

disregarded. For the fixture shownn outline in

over the main body of the fixture;

projecting flanges Fig. 22-1, theolume V is defined by

are included with one third of their



actual width; V= A X B X C

ocal projecting parts such as bosse

and feet are . ,.

where the dimensions are taken asndicated.

ppendix I

Measuring Angles in Radians

For most applications, angles aremeasured in degrees, minutes, and

seconds. However, anothermeasuring system, "CircularMeasure," is preferred for certainapplications, e.g., when the angle is



conveniently defined by the lengthof an arc of a circle. Anothercommon application of this system

s in formulas relating to revolvingbodies. The unit in the system iscalled a radian (abbreviated rad or

rad.) and is the angle for which thearc has the same length as thecorresponding radius.

For a circle of radius r the length ofa 180-degree

arc is it X r, and it measures = nradians.

Consequently,



180 degrees = n radians

ft

1 degree = tt^= 0.0175 radian, and

1 radian = =57.2958, or

rr

approximately 57.3 degrees

For small angles, the chord can besubstituted for the arc because it is

almost the same length. This leadsto a simplification in calculations,because when the length of thechord is known, the angle is readily



measured in radians, and there isno need for the use of trigonometrifunctions.

ppendix II

Transfer of Tolerances

from the ConventionalDimensioning System

to the Coordinate System

The coordinate system of

dimensioning, with the point of origin for the reference lines (thecoordinate axes) located at or nearthe upper-left-hand corner of the



workpiece, is designed to becompatible with the scale readingsavailable on most jig borers. Many

of the recently developed N/Cmachine tools are also compatiblewith this system. For tool and die

work done on the jig borer,dimensions are usually given to0001 inch without specified

tolerances, as the jig borer operator

will work to the limit of accuracy ofhis machine. In this case,dimensions are transferred from

the conventional dimensioningsystem to the coordinate system bysimply adding or subtracting. Anexample is shown in Fig. 11 -1. The



horizontal coordinate 4.7500 shownn view B is obtained, for instance,

by the addition of the individual

dimensions of 7/8 inch, 2 1/8nches, 1/2 inch, and 1 1/4 inches

shown in view A. For other work,

where tolerances are included inthe conventional dimensions, thetolerances must be transferredtogether with the dimensions when

the change is made to thecoordinate system. The tolerancesn the coordinate system will not be

the same as in the conventionalsystem. In most cases thetolerances will be reduced in thetransfer process. The new



tolerances must be carefully calculated, otherwise serious errorswill result.

The principle of the "Transfer of Tolerances" can be stated as

follows:

When transferring tolerances fromthe conventional dimensioning

system to the coordinatedimensioning system, the sum of the tolerances of any pair of dimensions on the coordinatesystem must not exceed thetolerance of the dimension thatthey replace on the conventional

system.



s the first step, all dimensionswith unilateral or with bilateral, buunequal, tolerances are changed to

make all tolerances equal. In thesecond step, the conventionaldimensions are transferred to

coordi-

REF . I000_ 2000. 3.000



UJj

a:;

4-iG-

-0

Oi Oi

O! 0<

o a

oi in.

rol KS

Fig. 11-1



B

Transfer of dimensions without

tolerances. A, Conventionaldimensions. 8, Coordinatedimensions, (Karl H. Mollrecht.

Machine Shop Practice, vol, 1, New ork: Industrial Press Inc., 1971.)

nate dimensions. In the third step,

the tolerances are transferred tocomply with the principle statedabove.

TRANSFER OF TOLERANCES

pp. II



The application of the principle andthe three steps is demonstrated by the following example. In Fig. II-2,

iews A and B show a part withconventional dimensioning. In view

some of the dimensions

have mixed (unequal and equal)tolerances; in view B the basicdimensions are changed, as needed

to make a!l tolerances equal. View Cshows the resulting coordinatedimensions with their tolerances.



REF.

±0005



1.0000

±0005

2.0000

3.0000

trjcus

0 0 0

a

e

+IO



o o

8. +i o

00

CM to

Fig. II-2, Transfer of toleranceddimensions, A. Conventional

dimensions with mixed tolerances.B. Conventional dimensions withequal, bilateral tolerances. C,Toleranced coordinate dimensions.

(Karl H. Moltrecht. Machine ShopPractice, vol. 1, New York:Industrial Press Inc., 1971.)



pp. II

TRANSFER OF TOLERANCES

301

Step 1. The __*__. tolerance on the

2.125 dimension is changed to±.002, and simultaneously, the2,125 is changed to 2,126. This does

not change the physical dimension,because

2.125

+ .003 .001

2.128 „ ±.002



2.124 = 2 " 126

Step 2. The 3.0010 dimension is

obtained by the addition

8750 + 2.126 = 3.0010

Step 3. The available tolerance on2.126 is ±.002 and must now bedivided between two coordinate

dimensions. If evenly divided, thiseaves ±.0010 for each dimension,

resulting in .8750 -0010 and 3.00O0010 .

Sometimes the tolerance of adimension on the coordinate-

system drawing is affected by more



than one dimensional requirementIn this case, the final tolerance usedmust be that which fulfills all of the

requirements. For example,consider the .502—° 02 dimensionn view B. It is replaced by the

3.0010 and the 3.5030 dimensionsn view C, If the 3.0010 and 3.5030

dimensions are each given a ±.001tolerance, the sum of their

tolerance would not exceed theoriginal ±.002 tolerance and)presumably, the requirements for

the transfer of tolerances would bemet. However, the 3.5030dimension together with the 4.6280dimension replaces the 1.125



dimension in view B, which has atolerance of only ±.001 inch. Thus,the sum of the tolerances on the

3,5030 and the 4,6280 dimensionscannot exceed ±.001 inch which,when divided equally, amounts to

±.0005 inch. The 3.5030-inchdimension must therefore be giventhe lesser tolerance of ±.0005 inch.

For this reason, when transferringthe tolerance it is usually best tostart with the smallest tolerance.

lso note that the sum of thetolerances replacing

the ,502 ±002 dimension is less

than the ±.002 inch. This is



satisfactory since the sum of thenew tolerances does not exceed theoriginal tolerances.

When a tolerance is bound by asmall tolerance, as in the case of th

502 dimension, it is sometimespossible to increase an adjacenttolerance on the

coordinate dimension drawing. Forexample, the 2.000-inch dimensionn view B, which has a tolerance of

±.005 inch is replaced by a 1.0000-and a 3.0000-inch dimension onthe coordinate drawing. Each of these dimensions could be given a

tolerance of ±.0025 inch; however,



the tolerance of the 1.0000-inchdimension on the coordinatedrawing is bound by the

requirement of the ±.001 toleranceof the 1.000 dimension shown atthe right side, in view B, Thus, the

1.0000 dimension in view C,together with the 2,0000dimension, must have a totaldimension tolerance of ±.0010 or

±.0005 inch on the 1.0000-inchdimension. Since the sum of thetolerances of the 1,0000 and the

3.0000 dimensions (view C) can be±.005 inch, the tolerance of the3.000 dimension can be increasedto ±.0045 inch.



The basis for the determination of the final tolerances for thecoordinate dimensions (view C) is

summarized below:

8 7SQ±J>010 : jhe ±.0010 tolerance

together with the ±.0010 toleranceof the 3.0010 dimension is requiredto maintain the ±.002 tolerance forthe 2.126 dimension.

3.0010 ±001 °: See requirementsfor the

g 7 50±.ooiO dimension givenabove.

3.5030 ±OOOS : The ±.0005



tolerance together with the ±.0005tolerance of the 4.6280 dimensions required to maintain the ±.001

tolerance for the 1.125+.001dimension.

4.6280 ±OOOS : See requirementsfor the

3,5030 ±OOOS dimensions given

above.

1.0000 ±0005 : The +.0005tolerance together with the ±.0005

tolerance of the 2.000 dimension isrequired to maintain the+.001 forthe 1.000 ±00) dimension given at

the right, in view B.



2.0000 ±ooos : See requirementsfor the

1.0000 ±00 ° 5 dimension givenabove.

3.0000*' 0045 : The .0045tolerance, together with the ±.0005tolerance of the 1,0000 dimensionn view C, is required to maintain

the ±.005 tolerance for the 2.000±005 dimension in view B.

ppendix III

The Dimensioning of Fixtures by Stress Analysis



The Dimensioning of Fixtures by Stress Analysis

lthough the structural design of fixtures has not been given muchconsideration in most textbooks on

stress analysis, they can bedesigned systematically by theproper application of knownformulas and calculation

procedures. An underdimensionedfixture may be damaged ordestroyed in use. An

overdimensioned and, therefore,overweight fixture is a constantsource of unnecessary expense forexcessive work in handling,

transportation, and storage, etc., of



weight is exemplified by a 3-by 3-by10-inch (75-by 75- by 250-mm)solid block of steel, or by a hollow

aluminum casting, open on oneside, with 1/2 inch (13 mm) wallthickness and 8- by 8- by 16-inch

(200- by 200- by 400-mm) overalldimensions. The cutting forces runnto hundreds, if not thousands of

pounds, and are always somewhat

approximate. There is, therefore, noneed to include the weight of thepart and the fixture in a static stress

analysis as long as these weights donot exceed 10 percent of the maincutting force.

Formulas for calculating centrifuga



forces are found in the Mechanicssections of reference books, such asMachinery's Handbook. 1 The

acceleration

of a planer table at stroke reversal i

of the order of magnitude of from0.0 lg to OAg and is insignificantexcept in special and extreme cases



To calculate the load from thecutting tool, it is resolved into itsthree components as shown in Fig.

III-l. They are:

Eg, The main cutting force or,

simply, the cutting force. It is theforce component acting in thedirection of the tool travel (thedirection of cut) relative to the

workpiece, [n a cylindrical turningoperation it is the tangential forcecomponent.

Fp, The feed force. This is the forcecomponent acting in the directionof the feed, i.e., parallel to the

surface which is being generated in



the machining operation. In acylindrical turning operation it isthe longitudinal force component,

Ff, The thrust force. This is theforce component which acts in the

direction perpendicular to thesurface being generated. In acylindrical turning operation it isthe radial force component.

Force components are in pounds ornewtons. A single-point tool hasonly one set of force components.For multiple-point tools (drills,milling cutters, broaches) there is aset of force components for each

cutting edge which is actively



cutting. Fq

Eric Oberg and F, D. Jones,

Machinery's Handbook (New York:Industrial Press Inc., 197T) 19th ed,pp. 335-337.

Fig. Hl-l. Three force components othe cutting tool.

pp. Ill



DIMENSIONING OF FIXTURES

303

s the major force component and isthe component that determines theamount of work and horsepowerabsorbed in the cutting operation.Fp and F T are significantly smallerthan F c . Average values are

1 2

F F « — F c to — F c , and

F T *^F c io-F c

Fp is maximum and F T is



minimum when the side-cutting-edge angle (SCEA) is zero;Fpdecreases and Fj increases with

ncreasing SCEA . The size of F^and the other force componentsdepends on the material, the

dimensions of the cut, and thecutting speed. Detailed data arefound in reference and text books.However, for the purpose of

dimensioning fixtures it issufficient to use the approximationthat F c equals the unit (specific)

cutting pressure p c multiplied by the area of cut A Q :

F c = p c A Q =p c fd



where.

= area of cut (square inches or

mm 2 )

/ = feed per revolution or per tooth(inches or mm)

d = depth of cut (inches or mm)

p c is essentially a material constanand can be taken as 2.5 to 3.2 timesthe tensile strength for steel andother ductile materials, and

4.5 to 5.6 times the tensile strengthfor cast iron and other brittlematerials,



where the effects of dimensions of cut and cutting speed are reflectedn the ranges quoted for the co-

efficients. The higher values are tobe used for fine feeds or shallow

depths of cut (small / and d) andower cutting speeds (as used withhigh-speed steel tools), the lower

alues are for heavy cuts and/or

higher cutting speeds (as used withcarbide and ceramic too! materials)In the final calculation of Fp and Ff

a contingency factor is introducedto allow for tool wear, cutterrunout, and local variations inmaterial dimension and hardness.

For single-point tools and drills,



this factor is 1.25. For millingcutters it is 2. For twist drills, Fp isfurther increased by a factor of 1.33

to allow for the additionalresistance caused by the chiseledge. Data for drilling forces are

found in text and reference books. 2

The clamping forces must secure

the part against being pulled out of the fixture by the cutting forces.Detailed calculations for the variou

types of clamps are given in Chapte10. The safety factor against pulloutshould be not less than 1.5,however, in most cases it will be

found that a safety factor of 2 or



better can easily be established.

With the forces calculated, the

elements of the fixture can now bedimensioned. Regardless of how complicated the fixture may appear

with a little practice on the part of the designer it can always besubdivided into simple structuralelements. These elements are

cantilever beams, simple beams,shafts and bolts (loaded in torsionand/or bending), flat or curved

plates of square, rectangular, orcircular circumference, cylinders,angles, and, occasionally, columns.Formulas for dimensioning these

are found in Machinery's



Handbook.

2 Ibid.,pp. 1743, 1744.

3 Karl H. Moltrecht, Machine ShopPractice (New York:

Industrial Press Inc., 1971) vol. 1, p.76.

"Oberg.op. eif.,pp 402-441.

ppendix IV

Metric Conversion Tables

Fractional Inch-—Millimeter andFoot—Millimeter Conversion

Tables (Based on I inch ™ 25.4 mil



I i meters b exactly)

pp. IV

METRIC CONVERSION TABLES

305

II

!

£ i

iiii iiiii iifn hip inn iMii mil smsisttii mn

***±ut- irtviwiioiD io"«o"i^r^t*

nOfflodooo th^wog PS^KS S^f!!!



m 1*51*; to n

afl ft

ft ifl

wrtrirt^ jjsjfltB SJHJE"£;*£

^s^'tErrr Er'2'2*!^ 2^?h8 S S« n «

m m n «

CJi c* vS O*

a rkseis IfMl aasaa Rpsaa ££»** a

tsssB asss* psrsr SsiSS sa^s'S

(i n n « N

rO I- f4 US £Ti



« r- nod rt

tCt^O f* "J

arj ^ N (J

5 r- WldC O

C| fi ft Tq H

X "-V

— DSDDfOH

F- m ig cn g,■

CO IM ffl^C -"

?: -x <~ st



ft Or* t»

fi i>ri eg

SB i k 3" tOAo t^.*^t^

"TS<5i %*?*■.%% %*£%%%

*&&%%% *&%%%

maioffip d d w m m

,---..-. m ft f* d n

%%%

Cft N *» f«Ol t^oj 0Q » CO

r* a f< m f*



H « H M ti

M M IN IN H

OflWMJ

JT^JfS^ rT^oScS "E.S'S'SsJ £i3££^

s5?S rT^'3'^5' «^?5^ &??3^ ff££3£cT^5 55

&tMi iiHi siis* si&ii ssm nip n*ii^s&s gti&i ^g^n

MaM ssskk ass

ft*o

Uli^ ioiOO %0 tC5 l> t-t-- I>00«

CO 00



« c*> ra r«i^

1

guS «ge 3 boo 9" m tfi $j

n n iq n n n^^rîn uv l4 wd id W^f

f-o i>st«»ffi £S»c L >c7>0'.£

MHMKH HPIWI-IJ-l « M M >*^_

M M H M H w H »«■

"< 11 l-W 11 rC«

uhj^mjtu ^^vjipus ôyj'COio-

(cuJusts^ ^S^S

ootfnîoo ortKeê rtuiooitt*i ^«5 hrt^_ «£fiQ«« rf*|«F*i| Ctf^fOf^'fl



4-9 4in Iftwi v>it> nd id *d fr-^ r-1— t-co ao CO ca O& a ^ O

^ t^ CTi « *t

88SSS

~ >b cri ■* i-^cti n

■* ^ ^ lA « n N «

%-UM ftisfts IPli OPI PIP ilMi ^^^llU IlISi =« 3 P

t*t*im*nt*i *!■■** vvvt wius^n^biotaflM^P 1 t^oaeooooo o*£^ct.O"022SJiS

-t w M M M Mi-rh'MM fiMHH—



MH_MW_H w^tnt-in ""'*"" " " CTL1 -^_

mm iHP psu? Kilt isiH iîi? iiiii misnn 11

S « H in m n M

hrh *r*? v ^ if) iflViiflnOiO <o>ci-t^J> r-coiiocoeo &

O' _■ t7' ~~ IJ

« n « H

3 âôjS 5^5^^K £Sf7«S Ba««fort««'Sft ^^m;^ cific4f0rt n^vviA

ii«iiioid S^!j"&& r-o0«'»«o



O^flivC^&O ^ S ^ «« el-"

oc O r^ua'DO

woo <-

ui (ti n N h SdmhiM S Ol i^ J^ i-»

"i N .-n "H

_ 3

P«w-f*it*5t') fW-B-^^t 1 ifliflvi*giOS'fi^'E'rt w^mwm SS'mmS S 8 t? S

n

rss&s, pall ^sgaa sas&g sgs|g !§&&£asa&a §sa|R g2|§g .



3 ia.S!^"^ Bg'^sS a-sssa S3SSS5^sa^ PSSSS SS5P8 SSSSS :

(j O D h H n m ft (I H fir^^r^^t-^fl-^iioirj

n ^ n i f^ fs m m Mfi ci w « inei__M_n awn

■O '-C J ^ O

t mk mn mi mm mi iiii^ Mta HR simi i

aaa^i hm§ iiim iihi t%m mm mM&M*. IHI| H^l ?

ooooo DOOOO odooo OQOOD



do odd ododd ooooo ooooo oooooooooo

II Is 9j R

•S3

■0-S

Is

8 1

153

s-l



si

s

I!

IB

1 * a.I

51

- i

a

iffll im ss»8i KB! pll BW WM Ipl i£ I^ «HH



t,-do'd« x'^Hiin dii'nnn ^-i«uiiuiw^io e'oi^pt. i-cooombj ^awmo

££SRH ^5325

~s5«a ««»»« e-s-aas essas IslSSll^?«?*!? H**S 'Sl^sS IrSS&H £5ils

llfl IIIII illtl Ilfff lllll ^°* s ?-. l^sS§sssfs *sb«*

nnOrtn m^

4^ruj «ujwioid «mo f^t-i> [-eoofioOEO otc7hC»c>o ogo;

MS iiiii mu am urn um ill Rpf a 4ii^«iiis

■ ^ . .l ■ " ■ ■ ! ..^.^ -_i"_l'



a>.iJLi«jJ i«^<nt>_h-r« r«tf*(I5«oiAOYOba OOOMW HMMMfl

nKtin n«nmn n-i-i-i-i wuVinviidetVhr. iM>«Biio a.o.o.o.0 SS3K

j m

1MB litis IB! Hill RH Hill iiiii P||iIIIISII!

o'dadM HHHlll cin«n« *i«*^«««w«s M«Vri t^mottM » a °"-°SSSSS SSg

sills Hill 51111 Ifpt SSlSs Wm |8f€BIiiii Pfffe k^ss



nfim pj t# t ■** w>wjwi

u-,, *' idîdrê^ m^-«:»m tc ov ^

r> e ooo>

mi ibm ma im isti nn ipi qw im «w

o d e d J wh« ci« nnn^^ r^4^*ûiiftioui* o^dd^r^ r-t-«PD«g&o.ao>o. oooog S^^5'

S a*i 3

r'

c- o a o

"si



5ss~"aasRs sssss sssrs ssss? ggs^jT3SS^| 3S3S3 S||Sf 2||£i l£f »ss?s -as^asi s'siss assss fiSsss ss^ss

ass^a 8S»5* *«»<•-'

o'i Hunaii nnmrin n»*4"t ui«ww«

>dc«t-ri r-r-aneM Mg.ao.ai oggggSSSa?

M nm urn lili 1MB DM IS® hh

WffW«

d d d i-I

nn fictrtfflffl c^-q-^*-* inin «VjiO**o

^ t^.t^ t^t»8e»» «a*o\oiai oooo>



IIIII HHI S8IS1 SIBS t§SII MM*I|I6I a*6*S 8IIII MUI

Si 6 6 6 oeded do'ded edddd 6666666666 dddoo ooooo ooooo ooooo

R

P

^^

--=

°§

Si-

J.S



fi si

ll

3f I

*3i

METRIC CONVERSION TABLES

pp. IV

Hnq Hm S fi n « k

ft'ft ft" f*) PJ

££$§§ IlliH Sglai

frfti-MA oVwi-hcus o.ft5-o*



5 « H n (in n n

tin nn ft n n ft ft

2s;

ft ft ft A H*)

Htt C 'J PI ft ft ft ft

a ^s

■ W

*0 ft ft ft ft

■IS eH

glass



oisc to I* i ior- m u)

n

ft IfSi

sss;

n nrifl n n n <

£3S&S_j?$SR3

so fi ■-" r> ■■*! fO r^ ft rn ft

IH

ftj?2_3 ffi ft

ft ft i^in fo



SBfM

psrSS

E ft_ft 3

a

I

n wA ft

ftj* O «f

ft ft ft M B

?a s s

m tiwrjn A*± ■* u « WW nfl M(1M



R'S'SUS'

CO Pi AChft

ft ft^fi *n

p ap vi ft ^~

I- HI lflff. ft

a saga

5

ft ft ft ft ft H ft ft ft ft

talti MM

(SSftSS



not-**

asRas sasss

* X- f- r - •£■

IfHf

«* o -^

US c5> ft * Q

ft ft !*) ci i^

JjLHg S r*) J> ft ft p? ft ft

ft ft ft ft A

fffli



ft ft r^ ft (^

fflfl

*e o -^a «

Oi* if) Q S

m m Oi ftJs

fMKI VIM

s^j

2KSS3

f?JT3!

r- i-r ^*cc ft



S'R'S'fi &

0>iQ flO t fnBfc M£>1

5« B v? fi

I ft ft ft p

fi*lHMM

on wioo-t

£88SS

T — K

>■': -^-. -i

saa



Hi M O OOt

uia&s

■ W g< n H

ray ggygj

fagl ci ft

c i ■- -.' — ■.■•■;

»0 P» &*£> B«5

IS O ?« ft

■B S vi B™ H

g¥i8s BsSeS



■4 s8s Ti

5 hi &*' ^

r* R ifl. gi r^

f* Sufi rn g

«d b -- ■■£ n

t- 00 HDD Ch

w w n « S

S&fefS

^wrtfnco pjirt^Sf'j wnnwti

te rt *v) m Pi



■spa

nidi-

m W fi_fi_«_5 S_5 « n

n n n n n

O I— "t H

ftsas'S

If

g'g'a^;; R'S'S^g sraftsa aaaast

cK 'it-'

illf



ffj. fj tJ> <J) pj

tsas

H »lfl(i

*an h via

flHHQft

I- k*vio ft'O

h- rt rt r>i n ft H fl ft W

■

in V) *0 'C- £

w w (i ft n

ft? ft ft c^ri



?& r^ PIS

x « -O I

pj rt ft ft fi ivir4i?jr?)f-^ t*jto ft^ tom rp r^ft Q

•5 ft QMS fO

I? el

aaag

l

SSSSg'

§



IBS5

ffVrtftftpj rj ■nj 5S- B S>

3»-i o& V) CC ft

nto" O

l

mm urn mm mm nm mu nm S£g»ynm hms §

cme thread, 125 Acorn nuts,

116,195,203-204 Actuators, plunger216 Adapters, 281-283

for vises, 129-130,235-236,272

dhesive clamping, 133 Adhesive



for bushing installation, 158-

159 Air blast for chip removal, 83-

84 Aircraft tooling, 275-277 Aircushion table, 228 Air cylinders,288-290 Air powered clamps, 1 IS,

147, 241,

269, 282, 288-290 A1SI steels, 20-21 Aligning clamps, 171-172

lignment of fixture to machinetable,

17 1-172,247,252 Allowance,

machining, 22, 32-33, 183 Alloys,ead-tin-antimony, 44

zinc base, 44 Alloy steels, 21



luminum inserts for lathe chucks,208 Aluminum tooling plate, 2 3,168, 183 American Standard Jig

Bushings, 155 Angle block, 234ngle plate, 287 Angle plate

grinding fixture, 270 Angle, side

cutting-edge (SCEA), 303 Angles inradians, measuring (App.),

298 Angle strap, 113, 120 Angular

clamps, 120-123 Angular drilling,233-2 36, 238 Angular errors(misalignment), 27-28,

54, 55, 91,93 Angular indexing, 58-61 Anneal, of castings, 179

ofweldments, 182 Arbors and



mandrels, elastomers for

expanding, 263-264 Arbors,

hydraulic, 263, 291

materials for, 22 Arc blow inwelding, 278 Archimedes spiral, 110

rc welding fixtures, 275-280ssemblies, clamp, 205 Assembly

fixtures, 275 Assembly screws, 174

materials for, 22 Automaticcentralizers, 97-101 Automatic endcam clamp assemblies,

207 Automatic fixtures, 287-2 91

Backing bars, 278 Backlash, 97, 102



104 Baffle plates for chip andcoolant control, 83-84 Ballbearings, 59

Ball knobs, plastic and steel. 201-202 Ball plunger, 217 Ball, tooling,

217 Bar knobs, 201

Base for a jig, separate, 234-235Bases fixture, 170 materials for, 23

Bayonet-lock clamp, 128, 238Bayonet type guide groove, 205Bayonet type locking device for slip

bushings, 161-162 Bayonet type T-bolt, 205 Beam clamps, 113-115

Beams,61, 113-115 Bearings, ball



and roller, 59

outboard, 60 Bell-crank, 120 Bell-

crank levers, 98-99 Bevel gears, 103

Blocking for foolproofing, 71, 73, 75Blocks, dummy 285 inertia, 260step, 207-208

tool setting, 2, 151,244,292

Blueprints and specifications forthe

part, ^ Bodies, cast, 44-45 jig or

fixture, 20, 170-184 welded, 45Bolts, eye, 195 hook, 116 index,59ig latch, 195 materials for, 22-23 T-

171, 195 Bonding agent, 195



merican LoctitetfiJ 1 S3 Booster,pneumatic-hydraulic, 290-291Boring bars, 265, 268 materials for,

22 stub, 26S, 268, 269 Boringfixtures, 265-269 Boring machines,production, 266, 269 Boring mill,

horizontal (HBM), 265-266 Boringpositioner, 208 Borizing, 266 Box-type drill jigs, 138, 170, 173, 224,

227,232-233, 238 Bracket type jigs,125,228,238 Broaching fixtures,272-275 Bronze locators, 45

Buckling instability, 141-142 Built-up fixtures, 172, 173, 175-176,

183 Bullet-nosed pins, 2 10 Burrs, 5

40, 76-77, 82-83, 95, 230



relief for, 84-86 Bushing holder forslip bushings, 238 Bushing plate,235

Bushings, American Standard Jig,155 ceramic coating on 168 circuit

board, 169 close together, 158, 163,229 conventional, 154 crimping of,169 distance from ends of, 229-230drill, 4, 154-169

ferrous titanium carbide for, 167fixed, 154, 155 floating, 9S-97

formulas for designing, 165-167guide, 1 64 headless, 154, 155 headtype, 154, 155, 15 8, 159 liner, 154,

155, 156, 158, 159 locking clamps



for renewable, 161-162 long, 165oose, 154, 155 master, 155

materials for, 22-23, 167-168 non-standard, 1S4, 163-164, 165-167potted, 168, 184 portable power

tool, 164-165 press-fit, 155, 156, 158renewable (loose), 154-156, 159-163retainer, 208

Bushings, screw, 95-97 slip, 155,159-163 special, 154 standard, 154-156 stationary press-fit, 154template, 168-169 thin-wall, 163threaded, 9S-97, 163-164, 229, 233

USA Standard, 154-156 wide, 165

Buttons, 46-48



design rules for, 48

hollow, 47, 208

nstallation of, 46-47

materials for, 46

rest, 48, 208

spherical, 208

stop, 46

threaded, 47, 201, 208

used as locators, 46, 201, 208-209

Cables, connecting 196, 2 14



Callouts, 191-193 Cam clampassemblies, 205-207 Cam clamps,108-111,205-206 Cams, 97, 99-100,

108-111, 125-129, 205-207, 290eccentric, 110-111, 125-128 materialfor, 22-23 self-locking, 106, 109-111

spiral, 110 Camshaft grindingfixture, 27 1-272 Captive locatingpins, 214-215, 22S Carbides,sintered (cemented), 23,

J67, 268 Case series, 219-227, 245-252, 266-

267, 279-280 Castable materials fornesting, 44 Castable tooling, 211, 213 Cast fixtures, 172, 173, 176-179,

183 bodies for, 44-45



rules for dimensioning of, 177-179Castings, accuracy of, 35 brokenedges in, S, 82 draft on, 34-35 effect

of machining on, 179 machiningallowances for, 32-33 minorrregularities in, 35, 95 mismatch

n, 35 normalizing of, 179 shrinkageand warpage of, 33 stabilizingtreatment of, 179 steel, 23

tolerances for, 33-35 types andapplications of, 23 uniformity of, 35Cast iron, ductile, 23 gray, 23

nodular, 23 Cast iron fixture stock sections, 217 Cemented (sintered)carbides, 23, 167,

268 Center drill, 94



Centering, single, double, and full,B7 Centralizers, 87-104, 232automatic, 97-101 for gear wheels,

101-104 linkage controlled, 89, 97-101 Centrifugal forces, 259, 302Cerrobend®, 44, 134 Chains,

kinematic, 90, 97, 287, 289 Channefixture, 173 Channel jig, 227

307

INDEX

Chip breakers, 82

Chip deflector, 84

Chip disposal, 83-84



Chip removal, 83-84, 247, 284

Chips, drilling, 82, 229-2 30

milling, 82 Chip types, 82 Chipolume. 82

Chuck fixtures for lathes, 259-260Chuck into a turning fixture,converting

a, 208 Chuck jaws, 207-208

materials for, 22 Chucks, collet, 87,

90-91, 260

drill, 90

electrostatic, 132-133, 269



nserts for lathe, 208

athe, 208

magnetic, 130-132, 269

self-centering, 87, 89, 90, 102

Chuck type grinding fixtures, 102-103,

269 Chutes for chip removal, 83-84Circuit hoards, bushings for, 169

drilling of, 241-242 Circular

ocators, 51-58, 254 Clampassemblies, removable, 205Clamping, 19



adhesive, 133

economics of, 11 5, 288

elements, 105-135,207-208

fixture to machine tool table, 171

athe fixture with floating action,67,95

multiple, 144-145

of a package, 141-142

of gas turbine compressorblades,134-135

of honeycomb with ice, 135



acuum, 133, 269

with low-melting alloys, 134-135,

147 Clamping devices, operatingtime for

(Table), 11 S Clamping forces, 303

Wm screws (Table), 108 Clampingscrews, 107, 108, 115-117,

1*7,202-205 Clamping wedges, 105-106, 122, 147-149

dual action, 64-67 Cla mps, 105

air pUWeted .115

angular, 120-123



beam, 113-115

cam, 108-111

double movement, 121, 139-140

dual pressure, 289, 290

for renewable bushings, 161-162

hydraulically powered, 11 S

materials for, 22, 23

quick-acting, 127-1 28,-239, 241, 28

safety feature for air-operated, 242,244, 288-289



Strap s for, 207-208

/Cfglerrn-l 13, 125. 128-129, 136, *

216-217,243, 279 Class II copper forbacking bars, 278 Clearance infixtures, 68, 76, 172, 227 Closed jig,

170, 224-227 Cloth, glass fiber, 24,40-41, 44 Clutch, overrunning, 282Coating on bushings, ceramic, 168Cold finished material, 22 Collet

chucks, 87, 90-91 Collets, materialsfor, 22 Color code, 170

Components, commercially available, 4, 194-218

ndexing, 215-217



ocating, 42-67, 208-209

patented, 195

propietary, 195

wear on indexing, 58-60 Conical

ocators, 93-97 Connecting cables,196-214 Construction ball,toolmaker's, 2 I 7

Contour grinding, 272

Contour milling fixtures, 252-253

Conventional bushings, 154

Conversion tables, metric (App.),

304-306



Converting a chuck into a turningfixture, 208

Conveyor, 57

Coolant control, 83-84

Coordinate dimensioning system,186-189 (App.), 299-301

Cost of fixtures, 183, 296-297

Coulomb's Law, 133

Coupling nuts, 195

Cradle, 227, 234, 272

Crank arms, materials for, 2 3



Crankpin grinding fixture, 271

Crankshaft fixtures, 265

Crimping of bushings, 169

"Cross" and crossed diamond pins,

S6-57

Cup locators, 95

Cutter guidance, 20

Cutter guides, 151-153, 246, 249,

251, 252

Cutting forces, 272-273, 302-303

C-washers, 120, 196 swing, 197



Cylinder location, dual, 29, 54-58,293

Cylinders, power, 287

Cylindrical locators, 28-30

Damping vibrations, 23

Defects in die forgings, minor, 38

Deflector, chip, 84

Deformation analysis of fixtures,

stress

and (App.), 302-303 Degrees of freedom, 18,26-28, 61,251 Depth

machining, 6 Design, of bushings,



16S-167

of drill jigs, 219-243

of fixture bodies, 18-24, 170-185

of fixtures (App.), 302-303 Design

rules, for buttons, 48

for welding, 181-182 Detent pins,

214

Determinate system, statically, 61,147 Diamond pins, 56-57, 210, 254-

256 Die forgings, draft on, 38

flash extension on, 36-38

minor defects in, 38



mismatch in, 36-38

tolerances for (Table), 36-38

Dielectric, 133 Dimensioning of casfixtures, rules

for, 177-179 Dimensioning of drawings, 186-189 Dimensioningsystems, coordinate, 1 86-

189, (App.) 299-301 Dirt, relief for,84-86

seals and shields for, 86, 210

Disposal of chips, 83-84 Distortionn weldments, 38, 275-278 Dividing

head, 59, 2S4 Dogs, gripping, 120-

121, 1 50, 152, 160 Double, (def.) 25



Double-action pins, 214 Doublecentering, 87 Double locking levers202 Double movement clamps, 121

139-

140 Double nested, 25 Double or

triple screw thread, 135 Dowel pinsapplication of, 50-51, 173, 174

materials for, 22, 174 Draft, on

castings, 34-35

on die forgings, 38

on molded plastic parts, 40Drawings, 170, 185-186

dimensioning of, 186-189 Dressers,



grinding-wheel, l 52, 270 Drillbushings, 4, 154-169

floating, 95-97

materials for, 22-23, 167-168

threaded, 95-97, 163-164, 229, 233

Drill, center, 94 Drill chuck, 90

Drilling chips, 82, 229-230 Drillingof circuit boards, 241-242 Drillingwithout bushings, 242 Drill jigs,(def.) 1,97-101, 125, 138, 170-171,

219-243, see also Jigs

design of, 219-243



design procedure for, 219

for half holes, 239-240

for heavy work, 235

ndexing, 60-61, 227, 228, 236-238

operating with, 227-229

universal, 170, 281-284 Drill press,fixed-spindle, 227-228 Drill, radial,227-228, 268 Drop forgings,machining allowance on,

33 Dual action clamping wedge, 64-67 Dual cylinder location, 29, 54-58293 Dual fixtures, 292-293 Dual



fixturing for N/C machines, 258Dual pressure clamps, 289, 290Ductile cast iron, 23 Ductliron®, 23

Dummy blocks, 285 Duplex milling245, 252, 253

F.ccenters, materials for, 2 3

Eccentric cams, 110, (Table) 111,125-

128 Eccentricity, 91, 110-111Eccentric leveling lugs, 2 10Economy of fixtures, 2, 10, 185, 242

288, 295297 Ejectors, 77-80, 217,277 Elastomers for expanding

mandrels and



arbors, 263-264 Electromagnets, 81131 Electrostatic chucks, 132-133,269 Engagement for jam-free

ocators,

ength of, 51 Epoxies, 23-24, 44, 279

Equalizers, 90, 124, 136-146, 149-150,

248 hydraulic, 145-146 plate-type,

138 Errors, angular (misalignment)27-28,

54, 55,91 in part surface geometry,

27 Estimate, of fixture cost, 296-297

of profit, 295-296 Expanding

mandrels and arbors,



elastomers for, 263-264 Eye bolts,195

Face milting cutter, 251

Face plate fixture, 64-67, 95, 260-262

Face plate, magnetic, 81, 130

Fatigue, operator, 107-108

Feeler gage, 152-153, 246,249

Feet, jig, 171.209,221, 227,233,234

FERRO-T1C®, 2 3

Ferrous titanium carbide for



bushings,

167 Field shapers, 132 Finger

handle, 202

Fire hazard in machiningmagnesium, 23 Fixed bushings, 154155 Fixed locating components,208-209 Fixed-spindle drill press,227-228 Fixture bases, 170

materials for, 23 Fixture bodies, 20172-1 84

Cast, 44, 45

design of, 170-184



materials for, 2 3

one-piece (solid), 175, 183-184,238

welded, 45, 172, 173, 179-183Fixture clamping to machine tooltable,

171 Fixture components,commercial and

standard, 4, 194-218 Fixture costestimate, 296-297 Fixture design,check list for, 10-17

309

Fixture design, general



considerations in, 9 preplanning of,5-8 procedure, 18-24, 185 Fixturematerials, 20-24 Fixtures, (def.) 1

broaching, 272, 273-275 camshaftgrinding, 27 1-272 cast, 172, 173,176-179, 183 channel, 173

classification of, 295 clearance in,68, 76, 172, 227 cost of, 183,296-297crankpin grinding, 271 crankshaft,265 dual, 292-293 economy of, 2,

10, 185, 242, 288,

295-297 estimating cost of, 296-297

faceplate, 64-67, 95, 260-262foolproofing, 69, 71-76 gearnobbing, 256-257 grinding, 102-103269 hydraulic, 287-288, 290-291,

292-



293 indexing, 58-61, 86, 254, 273-274,

279 lathe, 149, 259-265 materialsfor, 20-24 milting, 2, 244-258 NjCmachine tool, I, 187, 257-2 5 8,

259, 291-293 pallet, 58,293-294patented, 195 planing, 273pneumatic, 287-290 proprietary,

195 radial, 254

rotating, 254-255, 2 56-2 57 rulesfor dimensioning of cast, 177-

179 shaping, 272 slotting, 272-273space in, 68, 76, 172, 227 structural

design of (App.), 302-303 transfer,



58, 292-294 universal, (def.) 2,(def.) 281, 284-

287 welding, 275-280 Fixture stocksections, cast iron, 217 Flange nuts,195, 205 Flash extension on die

forgings, 36-38 Floating actionclamping, lathe fixture

with, 67, 95 Floating drill bushings,

95-97 Floating pin locators, 212-213Floating principle, 67, 95, I 36-140,

149-150 Floating screw, 139

Floating zero, 292 Flux, magnetic,132 Foam, polyurethane, 24Foolproofing, embracing fork for,

73-7 5 punched parts, 76 the fixture



69, 71-76 Force from screws,clamping (Table),

108 Force on magnetic chucks,holding

(Table), 131 Forces, clamping, 303cutting, 272-273, 302-303 inertia,260, 272, 302 manual, 107-108Fnrgings, see also Die forcings.

Drop forgings draft on die, 38 flashextension on, 36-38 machiningallowance on, 33 minor defects in,38 mismatch in, 36-38 shrinkagetolerances for, 37 tolerances for, 3538 45-degree plungers, 137, 139 4-21 locating principle, 27



Freedom, degrees of, 18, 26-28, 61,

2S1 Friction, coefficients of (Table)

106 Fulcrum pins', materials for, 23Full centering 87 Fully, (def.) 25Fully nested, 25

Gages, feeler, 152-153, 246, 249

setting, 151,253

set-up, 1 51 Gang milling, 152, 191,245, 248 Gear hohbing fixtures, 256-2 57 Gears, 102-104

Gear wheels, centralists for, 101-104Glass fibers, 24



cloth of, 24, 40-41, 44

mats of, 24

rovings of, 24 Gray cast iron,properties of, 23 Grinding, contour,272 Grinding fixtures, 102-103, 269272 Grinding-wheel dressers, 152,270 Gripping dogs, 120-12 L, 150,152, 160 Guidance, cutter, 20 Guide

bushings, 164 Guides, cutter, 151-153,246,249 251, 252

Half holes, drill jig for. 239-240

Half-turn screws, 201 Hand knobs,116-117, 199, 203-205, 209-210

four- and five-pronged, 201



materials for, 23

star, 201 Hand-knob screws, 147,

199,251 Handle, finger, 202

speed, 205 Handles, machine, 202

speed-ball, 201 Handwheels, 202Hardened washers, 23 Headlessbushings, 154, 155 Head type

bushings, 1 54, 155, 158, 159 Heattreated parts, tolerances for, 40Heavy work, drill jigs for, 235

oading, SO Helical gears, 102-104Helix angle in gears, 103 Hingeclamp assemblies, 2 05 Hobbing

fixtures, gear, 256-257 Hold-down



bolts, lugs for, 179 Holding fixture,welding, 278 Holding force onmagnetic chucks

(Table), 131 Holes, tooling, 57,170,252,293

tooling for tapered, 267-268 Hollowbuttons, 47, 2 08 Hook bolts, 116Hook clamp assemblies, 205

Horizontal boring mill (HUM), 265266 Horn for a broaching fixture,274-275 Hydraulically poweredclamps, 115,

145-146, 147,292-293 Hydraulicarbor, 263, 291 Hydraulic

equalizers, 145-146, 190 Hydraulic



fixtures, 287-288, 290-291,

292-293 Hydraulic mandrel, 263,

291

Index bolt, 59 Indexing, angular,58-61

rapid, 60

straight line, 58 Indexingcomponents, 215-217

wear on, 58-60 Indexing drill jigs,

60-61, 227, 228,

236-238 Indexing fixtures, 58-61,86, 254, 273-



274,279 Indexing plungers, 216-217Indexing table, 58-60 Inertia block,260 Inertia forces, 260, 272, 302

Inserts for lathe Chucks, 208Instability, buckling, 141-142

Integral locators, 44-45 Integralocking tabs for slip bushings,

162-163 Integrity, structural, 2

Intensifier, pressure, 290Intermediate supports, 147-150,209-

210 Internal broaching, fixtures for274 Irregularities in castings,minor, 35, 95



Jack locks, 209 Jacks, 147

spring loaded, 246 Jack screws, 117-

118, 201,224 Jam-free locators, 51-53 Jamming, 51-53 Jaws, chuck,207-208

detachable vise, 130

materials for chuck and vise jaws,

22 Jig base, 234

Jig borer, 158, 186, 191, 266,299 Jigbushings, placement of, 229-231 Jig

feet, 171, 209,221, 227,233, 234 Jiggrinder, 158, 186, 270 Jig latchbolts, 123-124, 195 Jig legs,

209,221,222, 223,226, 227 Jig or



fixture body, 20 Jig plate, 21 9-2 21,224, 22 5 Jigs, (def.) I, see also Drilligs

chanoel, 227

closed, 170, 224-227

custom-made, 281

for large work, 235

eaf, 124, 170, 226, 227, 232

open, 170, 221-224, 227, 231-232

plate, 170, 219-221, 227, 231-232

pump, 170,240, 241, 281



reversible, 231

rockers for, 235

template, 227

tumbling, 227, 228, 283, 284

universal, 208 Jigs and fixtures,origin of, t Jig stand, 234 Joints,

welded, 179-180

Keys, and keyseats for radialocating, 54

for aligning fixture to machinetable, 171-172,247,252

sine fixture, 195 Kinematic chains,



90, 97, 287, 289 Kirksite® 44

Knobs, hand, 116-117, 199,203-205,

209-210

knurled, 201

materials for hand, 23

quarter-turn, 125

quick-locking, 202

steel ball, 201-202

star hand, 201 Knob swivel screws,199-200, 205 Knurled head screws,200 Knurled knobs, 201 Knurled

ock nuts, 195



Laminated plastic sheets, 24

Laminates in plastic tooling, 23-24

Laminating resins, 24

Large work, jigs for, 235

Latch, 138

Latch bolt, jig, 123-124, 195

Lathe chucks, 208 inserts for, 208

Lathe fixtures, 149,259-265 pottype, 172, 262 with floating actionclamping, 67, 95 with recenteringdevice, 64-67 with size adjustment,

64-67



Lathe mandrel, 94

Lathes, chuck fixtures for, 259-260

welding, 280

INDEX

Leaf, 115, 123-125,205,226

swinging, 115, 123-125, 205, 226

Leaf jig, 124, 170, 226, 227, 232Legs, jig, 209,221, 222, 223, 226,227 Leveling lugs, eccentric, 210Levers, locking, 202 Line boring

bars, 265, 268 Line boring fixture,266-2 67 Liner bushings, 154, 155,156, 158,



159 Linkage controlled centralizers89, 97-

101 Linkages, pantograph systems,97, 100-101

scissor type, 97 "Liquid steel,""liquid aluminum," 24 Loading andunloading the part, 68-81 Loadingheavy parts, 80 Locating buttons,

threaded adjustable,

201 Locating by sighting, 42-43Locating components, 42-67, 208-

209 Locating error, radial, 91Locating pads (rest pads), 48-5 1,208 Locating parts (locators),

materials for,



22,45-46 Locating pins, 48, 210-215221, 222, 252,254-256

captive, 214-215, 225 Locatingplane, disadvantage of inclined,

30 Locating, preparations for, 32-41

with keys and keyseats, radial, 54Locating principle 4-2-1, 27

Locating principles, 2 5-31 Locatingthe part, 1 8, 68, 87 Locatingunmachined surfaces, 32 Location,dual cylinder, 29, 54-58, 293

Locator buttons, 46, 201, 208-209Locator pins, 48, 210-215, 22!, 222,

252,254-256 Locators, 42-67, 208-



209, 281-282

adjustable, 61-67, 209-210

circular, 51-58, 254

conical, 93-97

cup, 95

cylindrical, 28-30

floating pin, 212-213

ntegral, 44-45

am-free, 51-53

materials for, 22, 45-46



radial, 29, 54

rotational, 29

screw tvpe adjustable, 61-67, 209-210

sliding, 94-95

sliding point type adjustable, 63

slotted hole, 213

spherical, 52,90, 102,208

Split-cylinder type, 63-64

unhardened, 46



wear on, 45-46

wedge type adjustable, 62-63

with triangular relief, 52-53 Lockinclamps for renewable bushings,

161-162 Locking devices for slipbushings, 160 Locking levers, 202Locking mechanism for universal

igs,

282 Locking tabs for slipbushings.integral

162-163 Lock nuts, knurled, 195Locks, jack, 209 Loctite®,

merican, 158 Long travel cam



clamps, 205-206 Loose bushings,154, 155 L-pins, 213-215 Lugs,eccentric leveling, 2 10

for hold-down bolts, 179

Machined parts, tolerances for, 39-40 Machine handles, 202 Machinestransfer, 57-58, 266, 292-294

Machine table, keys for aligningfixture to, 171-172,247, 252 Machinetool bed, boring fixture for,

267 Machine tool table, clampingfixture

to, 171 Machine tool vises, 90, 244,



252, 281 Machining allowances, 22,32-33, 183

for castings, 32-33

for drop forgings, 33

for forgings, 33 Machining, effecton castings, 179

depth,6

surface,6 Machining fixtures,classification of, 2 Machining

magnesium, fire hazard in, 23Machining parameters, 5Machining stresses, 179 Magnesiummachining, fire hazard in,



23 Magnesium tooling plate, 2 3,168, 183 Magnetic chucks, 81, 130-132, 269

holding force on (Table), 1 31Magnetic faceplate, 81, 130

Magnetic flux, 132

n welding, 278 Magnets,permanent, 81, 131 Major burr, 82-

83 Malleable iron, 23 Mandrels,125, 263-265

gear hobbing, 256

hydraulic, 263, 291

athe, 94



materials for, 22

threaded, 264-265 Mandrels and

arbors, elastomers for

expanding, 263-264 Mandrel typefixtures, 263-265 Manipulation andoperator .criteria, 9 Manual forces,107-108 Manual work fixtures,classification

of, 2 Manufacturing operationsplan, 5 MarKs, target, 58Masonite®, 168 Master bushings,

155 Master tooling for aircraftndustry, 27S Materia! selection,

steel, 21-2 3, 167 Mats, glass fiber,

24



Measuring angles in radians (App,)298 Mechanisms, 97 Meehanite®,23 Metric conversion tables (App.),

304-

306 Milling chips, 82 Milling cutter

face, 25 ] Milling fixtures, 2,244-258 Milling, duplex, 245, 252, 253

gang, 152, 191,245,248

multiple, 245

run-in and run-out distances in, 244

string, 141, 244-245, 252-253

tracer, 255 Milling machine



dividing head, 59, 254Misalignment (angular errors), 27-28,

54, 55,91 Mismatch, in castings, 35

n die forgings, 36-38 "MissileMaker Lathe," 265-266 Mistake-proofing the fixture, 69 Module, ingears, 102 Molded plastic parts,

draft on, 40

shrinkage in, 40

tolerances for, 40-41 Mounting of press-fit bushings, 156-

159 Movable milling fixtures, 2 53-



2S4 u (coefficients of friction)(Table), 1 06 Multiple clamping,140-146 Multiple milling, 245

Multiple-spindle drill heads, 164, 236

Natural stress relief, 5

N/C drilling machines, tooling for,

240-243 N/C machine tools, dualfixturing for

fixtures for, 1, 187, 257-25 8, 259,

291-293 reference point foroperations with, 242 Nested, singledouble, fully, 25 Nesting, 25-26, 28



29, 43-44, 93

three-dimensional, 43-44 N. I, J. b\

C. M. (National Institute of Jig andFixture ComponentManufacturers), 194 Nodular cast

ron, 23 Non-standard bushings,154, 163-164,

165-167 Normalizing, of castings,

179

ofweldments, 182 Nuts, acorn, 116,195, 203-204 coupling, 195 flange.

195,205 knurled lock, 195 materialsfor, 23 speed, 117 T-, 171, 195 wing-,116, 148



Open jig, 170, 221-224, 227, 231-232Operating time for clamping device

(Table), 115 Operating with drilligs, 227-229 Operations plan,

manufacturing, 5 Operations,

sequence of, 5 Operator criteria,manipulation and, 9 Outboardbearing, 60 Overdefining, 25, 53Overrunning clutch, 282

Package clamping, 141-142

Pads, 44, 206, 208, 222, 225-226,

227

ocating, 48-51, 208



repair of worn, 45

rest, 48-51,208

swivel, 201

tooling ball, 217 Pallet fixtures, 58,

293-294 Pallets for transfermachines, 293-294 Pantographsystem linkages, 97, 100-

101 Parallels, materials for, 23Parameters, machining, 5 Part,(def.) 25

removal of, 76-80 Patented fixturesand components, 195 Permanentmagnets, 81, 131 Phenolics, 23-24,



252,254-256

materials for dowel, 22, 174

materials for fulcrum, 23

pilot, 210

retractable, 255-256

shot, 57,293

single-action, 2 14

T-, 213-215

tapered, 50 Pitch in gears, 102Placement of jig bushings, 229-231

Planing fixtures, 272 Planing, run-



n and run-out distance

n, 272 Plastic and steel ball knobs,

201-202

311

Plastic drill jigs, 24

Plastic fillings for hydraulic

equalizers,

146, 290 Plastic parts,draft onmolded, 40 shrinkage in, 40

tolerances for, 40-41 Plastics, 23-24164, 168, 172

tolerances for prefabricated shapes



41 Plastic sheets, laminated, 24Plastic tooling, 23-24, 184, 211,213,214,227.285 laminates in, 23-24

repair of, 24 Plate, jig (i.e.: jig-plate), 219-221, 224,

225 Plate jig (i.e.: plate-jig), 170,219-221,

227,231-232 Plate-type equalizer

138 PlexiglasCH 83 Plungeractuators, 2 16 Plungers, 122, 137,139, 140, 143, 14S, 147-149, 209-213215-217, 232,287 4S-degree, 137, 13ndexing, 216-217 spring, 217

Plywood, application of, 24Pneumatic fixtures, 287-290

Polyesters, 23-24 Polyuretliane



foam, 24 Polyvinyls, 23-24 Portablepower tools, bushings for,

164-165 Positioner, boring, 208

welding, 276, 280 Pot type lathefixture, 172, 262 Potted bushings,168, 184 Potting compounds, 24Power cylinder, 287 Power tools,bushings for portable,

164-165 Prefabricated shapes,tolerances for

plastic, 41 Preparation for locating,32-41 Preplanning of fixture design5-8 Prcposttioner, receiver as a, 81

Press-fit bushings, 155, 156, 158



mounting of, 156-159 Pressproducts, tolerances for, 39Pressure clamps, dual-, 289, 290

Pressure intensifier, 290 Preventingrotation, stop block for,

228 Production boring machines,266, 269 Production quantities,classification

of, 219 Profile milling fixtures, 252-2S3 Profit estimate, 295-296Proprietary fixtures andcomponents,

195 Pull broaching, fixtures for,274-275 Pump jig, 170, 240,281

Punched parts, foolproofing of, 76



Push-pull toggle clamps, 216-217

Quantities, classification of

production, 219 Quarter-turn, knob125

screw, 123-124,201 Quick-acting,clamps, 127-128, 239, 241,281

devices, 4, 206

screw components, 202 Quick ocking, knobs, 202

evers, 202 Quick operation, designfor, 11 5

Radial drill, 227-228, 268



Radial fixtures, 254

Radial locating error, 91

Radial locating, keys and keyseatsfor,

54 Radial locators, 29, 54

Radians, measuring angles In

(App.),

298 Raiser blocks, materials for, 23Rapid indexing, 60 Rate setting, 283

Receiver as a pre positioner, 81Receivers, 80-81 Recenteringdevice, lathe fixture



with, 64-67 Redundancy, 137, 147Redundant supports, 61 Referencepoint for operations with

N/C machine tools, 242 Relief fordirt and burrs, 84-86 Removable

clamp assemblies, 205 Removal of chips, 83-84, 247. 284 Removal of part, 76-80 Renewable bushings,nstallation of,

159-163 locking clamps for, 161-162Renewable (loose) bushings, 154,155 Renewable wearing bushings, )54-155,

156 Repair of plastic tooling, 24

Repair of worn pads, 45 Residual



stresses, 33, 76, 178, 179 Resins,aminating, 24 Rest buttons, 48,

208 Rest pads, 48-51, 208 Retainer

bushings, 208 Retainers, 208, 211Retractable pins, 255-256Reversible jig, 2 31 Rockers, 137-138

for jigs, 235 Roller bearings, 59

Rollers for multiple clamping, 144-

14S Rotating fixture, 254-255, 256-257 Rotational locators, 29 Rovingsglass fiber, 24 Rules fordimensioning of cast fixtures, 177-179 Run-in and run-out distances,n milling,

244 in planing, shaping, and



slotting, 272

SAE steels, 20-21

Safety feature for air-operatedclamps,

242, 244, 288-289 Sapphireocators, synthetic, 45 Scissor-typeinkages, 97 Screw bushings, 95-97

Screw clamp assemblies, 202-205Screw components, quick-acting,202 Screws, clamping, 107, 108, 115117, 147, 202-205

floating, 139

forces from (Table), 108



half-turn, 201

hand knob, 147, 199,251

ack, 117-118,201,224

knurled head, 200

materials for, 22, 116

pin-handle, 116-117

quarter-turn, 123-124,201

socket head, 11"6

swing jack, 117-118

swivel, knob, 199-200, 205



thumb-, 119, 123, 147

torque head, 117, 147, 201

wing-, 147 Screw thread, double ortriple,-135

for ejectors, 79

square, 125 Seals for dirt and dust,

86, 210 Sections, cast iron fixturestock, 217 Selection of steelmaterials, 21-23, 167 Self-centeringchucks, 87, 89, 90, 102 Self-locking

cams, 106, 109-111 Self-lockingranges for eccentric cams

(Table), 111 Self-locking wedges,



106 Sequence of operations, 5Serrated clamp straps, 207-208

Set-up gages, 1 51

Setting block, tool, 2, 1 51, 244, 292

Setting gages, 151, 253

Shapers, field (for magnets), 132

Shapes, structural, 175, 181-182,183,

218,279 Shaping fixture, 272 Sheetsaminated plastic, 24 Shields for dir

and dust, 86, 210 Shoe, swiveling,137, 147, 199 Shot pins, 57, 293



Shoulder screws, 197 Shouldertooling ball, 2 17 Shrinkage andwarpage of castings, 33 Shrinkage

n molded plastic parts, 40Shrinkage tolerances for dieforgings,

37 Shrinkage, welding, 276 Side-cutting-edge angle (SCEA), 303 Sidestops, 48 Sighting, locating by, 42-

43 Sine bars, 285 Sine fixture keys,195 Single, (def.) 25 Single-actionpins, 214 Single centering, 87

Single, double and fully nested, 25Single locking levers, 202 Sintered(cemented) carbides, 23, 167,

268 Size adjustment, lathe fixtures



with,

64-67 Sliding conical locator, 94-95

Sliding point locator, split-cylinder

type, 63-64 Sliding V-block, 92-93,101 Sliding wedges, 97-98 Slipbushings, 15 5, 159-163 bayonet-type locking device for,

161-162 integral locking tabs for,162-163 locking devices for, 160Slots, T-, 171, 195, 285-287, 292Slotted hole locators, 213

Slotted shoulder screws, 197Slotting fixtures, 272-273 Socket-

head screws, 116



Socket shoulder screws, 197

Space in fixtures, 68, 76, 172, 227

Speed-ball handles, 201

Speed handle, 205

Speed nut, 11-7

Spheres for multiple clamping, 144-145

Spherical buttons, 208

Spherical flange nuts, 195

Spherical locators, 52, 90, 102, 208



Spherical washers, 137, 196, 203,206

Spiral, Archimedes, 110

Spiral cams, 110

Split-cylinder type of sliding pointocator, 63-64

Split V-block, 97

Spring jack locks, 209-210

Springback, 39

Spring loaded jack, 246

Spring plungers, 217



Springs, 137, 138, 139, 140, 143, 147203, 205, 206, 209-210, 214, 240,249,251 materials for, 22

Spring stops, 2 1 7

Spur gear, 103

Square screw thread, 125

Stability, 60, 245

Stabilizing treatment of castings,179

Stainless steels, 21

Standard and commercial fixture

components, 4, 194-218



Standardization of fixturecomponents, 194-195

Standard steels, 20-21

Stand or base, for a jig, 234

Star hand knobs, 201

Statically determinate system, 61,

147

INDEX

Statically indeterminate system, 61Stationary press-fit bushings, 154Steel, 20-2 3, 167

alloy, 21, 67



ball knobs, 201-202

Castings, 23

nserts for lathe chucks, 208

"liquid," 24

material selection, 21-23, 167

Stainless, 21

standard, 20-2 I

tool, 21, 167, 183 Step blocks, 207-208 Stock sections, east ironfixture, 217 Stop block forpreventing rotation, 228 Stop

buttons, 41 Stop collar on a .frill,



159 Stops, 42

side, 48

spring, 217 Straight line indexing,S3 Straps, 105, 113-1 IS, 118-120,138, 202-205, 207-208, 222, 223,226, 241,248-249,251,285

angle, 113, 120

clamp, 207-208

materials for clamping, 23 Stress

and deformation analysis infixtures, (App.), 302-303 Stresses,machining, 179



ncompatible, 61

ntermediate, 147-150,209-210

redundant, 61 Surface plate,magnetic, 130-132 Swing bolts, 195Swing clamp assemblies, 205 SwingC-washers, 197 Swinging leaf, 145,123-125, 205, 226 Swing jack screws, 117-118 Swivel pads, 201

Swiveling shoe, 137, 147, 199 Swivelscrews, knob, 1 99-200, 205Symmetry considerations, 69-71Synthetic sapphire locators, 45

Tacking fixture for welding, 278Tapered holes, tooling for, 267-268

Tapered pins, 50 Target marks, 58



T-bolts, 171. 195 Template bushings168-169 Template jig, 227

Templates, 1 SI, 153, 186, 19S, 2S3,

254, 255,272 Template tooling, 168Thin-wall bushings, 163 Thread,double or triple, 135 Threadedadjustable locating buttons,

201 Threaded arbors and mandrels,264-265 Threaded buttons, 47, 201,208 Threaded drill bushings, 95-97163-164,

229, 233 Three-dimensional nestingn a castable



material, 43-44 3-2-1 Locatingprinciple, 26-27, 245,

248 Thumb-screws, 119, 123, 147Titanium carbide for bushings,ferrous,

167 T-nuts, 171, 195 Toggle clamps,111-113, 12S, 128-129,

136, 216-217, 243, 279 Tolerances,32-41, 189-1 91, 242, 247, 251,280,299-301

for castings, 33-35

for die forgings (Table), 36-38



for forgings, 35-38

for heat treated parts, 40

for machined parts, 39-40

for mill products, 39

for molded plastic parts, 40-41

for plastic prefabricated shapes, 41

for press products, 39

for torch-cut parts, 38-39

for weldments, 38

ncompatible, 27



toolroom, 191 Tooling ball, 217Tooling ball pad, 217 Tooling,castable, 211 -213

template, 1 68 Tooling for N/Cdrilling machines, 240-

243 Tooling for tapered holes, 267-268 Tooling holes, 57, 170, 252, 293Tooling plate, aluminum and

magnesium

23, 168, 183 Too [maker'sconstruction ball, 217 Toolroom

tolerances, 191 Tool setting block, 2151, 244, 292 Tool steel, 21, 167, 183Torch-cut parts, tolerances for, 38

Torque head screws, 117, 147, 201 T



pins, 213-215 Tracer milling, 25STransfer fixtures, 58, 292-294Transfer machines, 57-58, 266, 292

294

pallets for, 293-294 Transfer of

dimensions, 189, (App.)

299-301 Treatment of castings,stabilizing, 179 Triangular relief,

ocator with, 52-53 Triple screw thread, double or, 1 3S Trunnions,60, 235, 253 T-slots, 171, 195, 285-287, 292 Tubing, in welded fixtures182 Tumbling jig, 227, 22 8, 283,284 Turnbuckle principle, 97

Turning fixture, converting a chuck



t

Jig and Fixture Design Manual - Erik K. Hendriksen_3709

Documents

Transcript of Jig and Fixture Design Manual - Erik K. Hendriksen_3709