Basicuma Gd&t

8/22/2019 Basicuma Gd&t

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Training program on

Geometr ic Dimensioning andTolerancing

forHONEYWELL

TECHNOLOGIES Ltd.,

BANGALORE, 2527thAug05


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Introduction

to GD& T, Symbols, Terms


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COURSE OBJECTIVE

1. Be able to explain the main benefits ofG D & T

2. Develop a Solid foundation of GD&T

fundamentals3. Be able to properly apply all 14

geometric controls

4. Be able to demonstrate a workingknowledge of the applications of thePosition control


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What is GD&T

GD&T is a symbolic language

GD&T is a design tool

GD&T communicates design intent

GD&TAuthoritative document is

ASME Y14.5M-1994 which specifies the

proper application of GD&T

GD&T is going to stay in the industry


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When to use GD&T

When drawing and its interpretation must be same

When features are critical to function orinterchangeability

When it is important to avoid scrapping ofperfectly good parts

When it is important to reduce drawing changes

When functional gauging is required

When automated equipment is used When it is important to increase productivity

When companies want savings


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HISTORY OF GD&T

Drawings existed as far back as 6000 B.C.

Unit of measurement at that time is royal cubit.

During 4000 B.C standardized to 4000 B.C.

Manufacturing started in 1800s .

The total process was conducted under one roof,

communication among craftsmen was immediate and

constant.

Nothing less than perfection was good enough. There were variation, but back then measuring

instruments were not precise enough to identify them.


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Now Engineers understand the variation is

unavoidable.

The variation acceptable without impairing the

function of assembly is identified as tolerance.

This led to the development of Coordinate

dimensioning.


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ENGINEERING DRAWING

STANDARDS

1935- American Standards Association(ASA)published American Drawing and DraftingRoom Practices.

World war II- Stanley Parker of Royal Torpedo

Factory in Alexandria, Scotland createdpositional tolerancing with cylindricaltolerance zone rather than square tolerance

zone.1940-Draftsmens handbook by Chevrolet, U.S.

1944 &1948- British published DimensionalAnalysis of Engineering Design.


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1946- ASA published II edition ofAmerican

Standard Drawing and Drafting Room Practice.

1946- SAE published Aeronautical

Drafting Manual

1949- U.S Military published MIL-STD-8

1952- SAE Automotive Drafting Manual

1953- MIL_STDA revised.

1957- ASA Y14.5

1982- American National StandardsInstitute (ANSI) published ANSI Y14.5.

1994- ANSI Y14.5 Revised.


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I ntroduction to Geometr ic Dimensioning and

Tolerancing GD & T standards

ANSI Y 14.5M 1982

ISO 11011983

ASME Y14.5M -1994


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ISO 1101 -1983

(Technical Drawings-Geometrical Tolerancing-Tolerancing of Form, Orientation, Location and Run-out)

ANSI Y14.5

1982-American National Standards Institute (ANSI)published ANSI Y14.5

ASME Y14.5M-1994

ASME Y14.5M-1994 Revised.


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CO ORDINATE TOLERANCING

SYSTEM

Part feature is located (or defined) by means ofrectangular dimensions with given tolerances.


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THREE MAJOR SHORTCOMING OF

COORDINATE DIMENSIONING

1. Square or rectangular tolerance zones.

2. Fixed-size tolerance zones.

3. Ambiguous instruction for inspection.


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1.Square (or illogical )Tolerance Zones.

Diagonally more tolerance (0.707) than vertical and

horizontal direction (0.5)

More logical and functional approach is to allow same

tolerance on all sides, creating cylindrical tolerance

zone.


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COMPARISION BETWEEN GD&T

AND COORDINATE TOLERANCING.


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Cylindrical vs. RectangularTolerance Zones


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GEOMETRIC DIMENSIONING AND

TOLERANCING.


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Rectangular Tolerancing


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Rectangular Tolerance

Analysis


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Geometric Tolerancing


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Geometric Dimensioning

Tolerance Analysis


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2.Fixed-Size Tolerance zones

Function of a hole in assembly is , hole location iscritical when the hole is at minimum limit (MMC).


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If hole size is larger than its minimum size limit,

its location tolerance can be correspondingly

larger without affecting the part function.


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Co-ordinate tolerancing does not allow

for cylindrical tolerance zones and

tolerance hole that increase with the

hole size, lengthy notes have to be

added.

LMC

MMC


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3.AMBIGUOUS INSTRUCTION FOR

INSPECTORS


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Different ways to hold the part for inspection,

confusion for the inspector which surface to touch

the gage equipment first, second and third.

Consequence:

Good parts could be rejected or,Bad parts could be accepted.


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3 Benefits of GD & T

A. Cylindrical tolerance zones.

B. Maximum Mater ial Condition.

C. Datums specif ied in order of precedence.


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Type of dimension Appropriate

use

Poor use

Size

Chamfer

Radius

Locating part feature

Controlling angular

relationships

Defining the form of

part feature

COORDINATE DIMENSIONING USAGE


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Symbols, Terms, of GD& T


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Symbols

GD&T symbols are the essence of this graphic language.It is important not only to know each symbol, but also to

know how to apply these symbols to drawings

Terms

The names and definitions of many GD&T concepts are

very specific to this subject. In some cases they are verydifferent from general English usage


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Symbols of GD & T

Geometric characteristic symbolsare aset offourteen Symbols used in thelanguage of geometric tolerancing.

The symbols are divided into five

categories:1. Form

2. Profile

3. Orientation4. Location

5. Runout


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FEATURES

A feature is a general term applied toa physical portion of part, such as asurface, hole or slots,tabs.

An easy way to remember this termis to think of a feature as a part

surface.

S


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FEATURES

FEATURE OF SIZE


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FEATURE OF SIZE This is one cylindrical or spherical

surface, or set of two opposed elements or

parallel surfaces associated with sizedimension which has an axis, center lineor center planecontained within it.

Features of size are features, which dohave diameter or thickness.

These may be cylinders, such as shafts

and holes. They may also be slots,rectangular or flat parts, where twoparallel flat surfaces are considered toform a single feature.


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How many feature of size are there?


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FEATURE OF SIZE NON FEATURE OF SIZE


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EXTERNAL AND INTERNAL FOS

External FOS are comprised ofpartsurfaces that are external surfaces.

Like shaft diameter or width andheight of a planner surfaces.

Internal FOS is comprised of partsurfaces (or elements) that areinternal part surfaces.

like hole diameter or the width of aslot.


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Example:

FEATURE OF SIZE


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FEATURE OF SIZEDIMENSIONS

A feature of size dimension is a dimensionthat is associated with a feature of size.

ACTUAL MATING ENVELOPE


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ACTUAL MATING ENVELOPE= PERFECT FEATURE COUNTERPART.

TheActual Mating Envelope (AME) ofan external feature of size is a

similar perfect featurecounterpart ofthe smallest size that can becircumscribed about the feature so it

just contacts the surfaces at thehighest points with in the tolerancezone.


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Actual Mating Envelope (AME) of an external

FOS



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= PERFECT FEATURE COUNTERPART

The actual mating envelope (AME) ofan internal feature of sizeis a similar

perfect feature counterpart of thelargest size that can be inscribedwithin the feature so that it justcontacts the surfaces at their highest

points with in the tolerance zone.


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Actual Mating Envelope (AME) of an internal FOS

Actual Mating Envelope (AME) of an internal


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Actual Mating Envelope (AME) of an internalFOS


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MATERIAL CONDITIONS

A geometric tolerance can be specified to

apply at the largest size, smallest size oractual size of a feature of size.

Maximum Material Condition (MMC)Maximum material condition is thecondition in which a feature of sizecontains the maximumamount of material

everywhere within the stated limits ofsize.


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MMC

MMC of external Feature Of Size


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MMC

MMC of internal Feature Of Size

LEAST MATERIAL CONDITION


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LEAST MATERIAL CONDITION(LMC)

Least material condition is the condition inwhich a feature of size contains the leastamount of material everywhere within thestated limits of size .

LEASTMATERIALCONDITION

Regardless of feature size


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Regardless of feature size(RFS)

Regardless of feature size is the term thatindicates a geometric tolerance applies atany increment of size of the feature withinits size tolerance. NO Bonus tolerance

RFS applied only to size features, such ashole, shafts, pins, etc.; feature which havean axis, centerplane or centerline.

Symbol : S

Material Condition Usage


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Material Condition Usage Each material condition is used for

different functional reasons.

Geometric tolerances are often specifiedto apply at MMC when the function of aFOS is assembly.

Geometric tolerances are often specifiedto apply at LMC to insure a minimumdistance on a part.

Geometric tolerances are often specifiedto apply at RFS to insure symmetrical

relationships.

MODIFIERS


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MODIFIERS

Modifiers communicate additionalinformation about the drawing or

Tolerancing of a part.

There are nine common modifiers

used in geometric tolerancing.

Ei ht difi


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Eight modifiers

PROJECTED TOLERANCE ZONE


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PROJECTED TOLERANCE ZONE Symbol: P

The projected tolerance zone modifierchanges thelocation of the tolerance zone on the part.

It projects the tolerance zone above the part surface.

Height of the projected tolerance zone should be

equal to the max. thickness of the mating part.


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FEATURE CONTROL FRAME WITH A

PROJECTED TOLERENCE ZONE SYMBOL

U i P j t d T l Z


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Using a Projected Tolerance Zone

A projected tolerance zone is a tolerance zone

that is projected above the part surface.A projected tolerance zone modifier is specified

as P



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A projected tolerance zone is used to limit theperpendicularity of a hole to ensure assembly with

mating part.


(Contd..)



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(contd.)

TANGENT PLANE MODIFIER


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TANGENT PLANE MODIFIER

The tangent plane modifier denotes that only the

tangent plane of the toleranced surface needs to bewithin this tolerance zone.

DIAMETER MODIFIER ( )


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DIAMETER MODIFIER ( )

The diameter symbol is used two

ways: inside a feature control frame asa modifier to denote the shape of thetolerance zone, or outside the featurecontrol frame to simply replace the

word "diameter.


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Inside the featurecontrol frame

Outside the featurecontrol frame

Reference modifier ( )


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Reference modifier ( )

The modifier for reference is simply

the method of denoting thatinformation is for reference only.

The information is not to be used formanufacturing or inspection.

To designate a dimension or otherinformation as reference, thereference information is enclosed inparentheses.

Reference


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ReferenceExample:

RADIUS MODIFIER (R)


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RADIUS MODIFIER (R)

Arcs are dimensioned with radius symbol

on drawings. A radius is a straight line extending from

the center of an arc or a circle to itssurface.

The Symbol for a radius is "R.

When the "R" symbol is used, it creates azone defined by two arcs.

The part surface must lie within this zone.

The part surface may have flats orreversals within the tolerance zone.


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Radius modifier

Controlled Radius (CR)


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Controlled Radius (CR)

The symbol for a controlled radius is "CR.

it creates a tolerance zone defined by two

arcs.

The part surface must be within the

crescent-shaped tolerance zone and be anarc without flats or reversals.


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CONTROL RADIUS


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DATUM FEATURE SYMBOL

DATUM IDENTIFYING

LETTER


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DATUM FEATURE SYMBOLS ON A FEATURE

SURFACE AND AN EXTENSION LINE


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PLACEMENT OF DATUM FEATURE SYMBOLS

ON FEATURES OF SIZE


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PLACEMENT OF DATUM FEATURE SYMBOL IN

CONJUNCTION WITH A FEATURE CONTROL FRAME


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DATUM TARGET SYMBOL


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BASIC DIMESNSION SYMBOL


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SYMBOL INDICATING THE SPECIFIED TOLERANCE

IS A STATISTICAL GEOMETRIC TOLERANCE


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BETWEEN SYMBOL


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COUNTERBORE OR

SPOTFACE SYMBOL


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COUNTERSINK SYMBOL


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DIMENSION ORIGIN

SYMBOL


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DEPTH SYMBOL


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SQUARE SYMBOL


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SYMBOL FOR ALL AROUND


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FEATURE CONTROL FRAME WITH FREE

STATE SYMBOL


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FEATURE CONTROL FRAME

Feature Control Frame


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Geometric tolerances are specified

on a drawing through the use of afeature control frame.

Symbol of

Geometric Tol.

Zone of

ToleranceP.D S.D T.D

W or w/o zone Modifier


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FEATURE CONTROL FRAME

INCORPORATING A DATUM REFERENCE

SYMBOL



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The Feature Control Frame


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Feature Control Frames Attached to Features


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Datum Features Symbols Attached to Features


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Datum Features Symbols Attached to Features


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ORDER OF PRECEDENCE OF DATUM

REFERENCE


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MULTIPLE FEATURE CONTROL FRAMES


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COMBINED FEATURE CONTROL FRAME

AND DATUM FEATURE SYMBOL


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FEATURE CONTROL FRAME PLACEMENT


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RULES, BONUS TOLERANCE,

VIRTUAL CONDI TION


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1. Understand Rules of GD &T

Rule #1 and Rule #2.

2. Understand the concepts of basicdimensions, virtual condition, inner and

outer boundary, worst-case boundary

and bonus tolerance.

Rules


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There are four rules that apply to drawings in

general, and to GD&T in particular. They specify

some relationships that occur on drawing

Symbols, Terms and rules are the basics of GD&T.They are the alphabet, the definitions and the syntax

of this language

RULE # 1


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When no geometr ic tolerance isspeci f ied, the dimensional tolerance

con t ro ls thegeometr ic formas well as

the size. No element of the featureshal l extend beyond the MMC

boundary of perfect form. The form

tolerance increases as the actual size

of the feature departs from MMC

towards LMC

RULE # 1

R l #1


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Rule #1

Rule #1 is referred to as the "Individual Featureof Size Rule."

In industry the Rule #1 is paraphrased as

perfect form at MMC or the envelope rule.

Where only a tolerance of size is specified, the

limits of size of an individual feature prescribe

the extent to which variations in its form as well

as in its size are allowed.


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An example of effects of Rule #1 on a planar


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p p

FOS.

In Rule #1 the words perfect form mean


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In Rule #1, the words perfect form mean

perfect flatness, straightness, circularity and

cylindricity. In other words if the feature of sizeis produced at MMC, it is required to have

perfect form.

TECHNOTE For features of size, where

only a tolerance of size is specified, the

surfaces shall not extend beyond a boundary

(envelope) of perfect form at MMC.

INSPECTING A FEATURE OF SIZE


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When inspecting a FOS that is controlled by Rule

#1, both its size and form need to be verified. The

MMC size and the Rule #1 envelope can be

verified with a Go gage. A Go gage is made to

the MMC limit of the FOS and has perfect form.

Go gage must be at least as long as the FOS it isverifying.

The minimum size (LMC) of a FOS can be

measured with a No-Go gage.A No-Go gage is made to the LMC limit of the

FOS.


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Rule #1


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1. What straightness tolerance is implied in the drawing above?______________

2. If the pin is produced at a diameter of 1.010, it must be straight within whattolerance?

______________


______________4. If the pin is produced at a diameter of 1.020, it must be straight within whattolerance?

______________


______________


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RULE # 2 (1994 standard)

RFS automatical ly applies to individual

tolerances and to datum feature of size.

MMC & LMC must be specif ied where

Required.

Rule #2a is an alternative practice of Rule #2 according


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to which RFS may be specified as a symbol in feature

control frames if desired and applicable.


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RULE # 3(1982 Standard)

For al l other geometr ic controls, RFS

automatically applies

RULE # 4


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RULE # 4

All geometr ic tolerances specif ied for screw

threads apply to the axis of the thread

der ived from the pitch diameter.

Exceptions must be specif ied by a note (such as

Major Dia or Minor Dia).

Al l geometr ic tolerances specif ied for gears and

splines must designate the specif ic feature(such

as Major Dia or M inor Dia) at which each

applies.

RULE # 5


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RULE # 5

(Vi r tual Condition Rule)

Where a datum feature of size is

controlled by a geometr ic tolerance and

is specified as a secondary or tertiarydatum, the datum applies at virtual

condition with respect to

ORIENTATION.

INTRODUCTION TO: VIRTUAL CONDITION

AND BOUNDARY CONDITIONS


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AND BOUNDARY CONDITIONS

Definition

Virtual Condition (VC): is a worst-case boundary

generated by the collective effects of a feature ofsize at MMC or at LMC and the geometric

tolerance for that material condition.

The VC of a FOS includes effects of the size,orientation, and location for the FOS.

Inner Boundary (IB) is a worst-case boundary

f f


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generated by the smallest feature of size minus

the stated geometric tolerance (and any additional

tolerance, if applicable).

Outer Boundary (OB) is a worst-case boundary

t d b th l t f t f i l th


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generated by the largest feature of size plus the

stated geometric tolerance (and any additional

tolerance, if applicable).

Worst-Case Boundary (WCB) is a general term to

refer to the extreme boundary of a FOS that is the


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refer to the extreme boundary of a FOS that is the

worst-case for assembly. Depending upon the part

dimensioning, a worst-case boundary can be avirtual condition, inner boundary, or outer

boundary.

Worst-Case Boundary when no Geometric Tolerances are specified.

TECHNOTEIf a feature control frame is appliedto a feature (a surface) it does not affect its WCB If


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to a feature (a surface), it does not affect its WCB. If

a feature control frame is applied to a FOS (an axis

or centerplane), it does affect its WCB.


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MMC Virtual Condition

The virtual condition (or WCB) is the extreme

boundary that represents the worst-case for

functional requirements, such as clearance or

assembly with a mating part.

In the case of an external FOS, such as a pin

or a shaft the VC (or WCB) is determined by


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VC = MMC + Geometric Tol.

or a shaft, the VC (or WCB) is determined by

formula:

In the case of an internal FOS, such as a hole,

the VC (or WCB) is determined by formula:


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VC = MMC

Geometric Tol.

the VC (or WCB) is determined by formula:


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RFS inner and outer boundary

When a geometric tolerance that contains no

modifiers (RFS default per Rule #2) in the

tolerance portion of the feature control frame is

applied to a FOS, the inner or outer boundary (or

worst-case boundary) of the FOS is affected.

In the case of an external FOS, such as a pin or

a shaft the OB (or WCB) is determined by the


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OB = MMC + Geometric Tol.

a shaft, the OB (or WCB) is determined by the

formula:

In case of an internal FOS, such as a hole, the

IB (or WCB) is determined by the formula:


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IB (or WCB) is determined by the formula:

IB = MMC Geometric Tol.

Multiple virtual conditions


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p

On complex industrial drawings, it is common to

have multiple geometric controls applied to a

FOS. When this happens, the feature of sizemay have several virtual conditions.


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Panel A shows the size tolerance requirements of Rule #1.


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Panel B shows the virtual condition those results

from the perpendicularity control. This control

produces a 10.3 dia. boundary relative to thedatum plane A.


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Panel C shows the virtual condition that results

from positional control. This control produces a

10.4 dia. boundary relative to datums A, B and C.


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Multiple Virtual

Conditions.

INTRODUCTION TO BONUS


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TOLERANCE

When the actual mating size of the FOS departs

from MMC (towards LMC) an increase in the

stated tolerance- equal to the amount of the

departure- is permitted. This increase or extra

tolerance is called the bonus tolerance.


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The bonus tolerance concept applies to any geometric

control that uses the MMC (or LMC) modifiers in the

tolerance portion of the feature control frame.


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The maximum amount of bonus tolerancepermissible is equal to the difference between

the MMC and the LMC of the tolerance FOS.


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TECHNOTE-BONUS TOLERANCE


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Maximum Material Condition

MMC E i


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MMC Exercise


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Virtual Condition Rule


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Virtual Condition Calculation


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DATUM REFERENCE

FRAMES

DATUM SYSTEMS


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DATUM SYSTEMS

(PLANAR DATUM )

Set of symbols and rules that communicates to the

drawing user how dimensional measurements are to

be made.


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Datum Plane

WHY DATUM SYSTEM?


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WHY DATUM SYSTEM?

First, it allows the designer to specify which part

surfaces are to contact the inspection equipment

for the measurement of a dimension.

Second, the datum system allows the designer to

specify, in which sequence the part is to contact

the inspection equipment for the measurement of adimension.

BENEFITS OF DATUM SYSTEM


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BENEFITS OF DATUM SYSTEM

-It aids in making repeatable dimensionalmeasurements.

-It aids in communicating part functional relationships.-It aids in making the dimensional measurement as

intended by the designer.

CONSEQUENCES


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CONSEQUENCES

-Good parts are rejected

-Bad parts are accepted

DATUMS(PLANAR)


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DATUMS(PLANAR)

DATUM

DATUM FEATURE

DATUM FEATURE SIMULATOR

SIMULATED DATUM


DATUM SELECTION

DATUM


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DATUM

A datumis a theoretically exact plane, point or axisfrom which a dimensional measurement is made.

A Datum is the true geometric counter partof a datum

feature

A true geometric counter part is the theoretical perfectboundary or best fit tangent plane of a datum feature.


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DATUM FEATURE

A datum feature is a part feature that exists on the

part and contacts a datum.

SIMULATED DATUM


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SIMULATED DATUM

A simulated datum is the plane established by the

inspection equipment.

DATUM FEATURE SIMULATOR


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A datum feature simulator is the inspection equipment

that includes the gage elements used to establish the

simulated datum.


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The symbol used to specify a datum feature on adrawing is called the datum feature symbol.

FOUR WAYS OF REPRESENTING PLANAR DATUMS


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DATUM REFERENCE IN FEATURE


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CONTROL FRAME

The drawing must communicate when and how

the datums should be used. This is typically donethrough the use of feature control frames.

DATUM REFERENCE IN FEATURE CONTROL

FRAME


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DATUM SELECTION


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DATUM SELECTION

Datum features are selected on the basis of part function andassembly requirements.

Datum features often orient (stabilize) and locate the part in its

assembly.

DATUM SELECTION


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DATUM REFRENCE FRAME


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A datum reference frame is a set of three mutually

perpendicular datum planes.

The datum reference frame provides direction as

well as an origin of dimensional measurements.



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DATUM REFRENCE FRAME(contd)


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The planes of a datumreference frame havezero perpendicularitytolerance to each other

by definition.

The 90angle betweendatum planes arebasic.



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( )

When making a location measurement on a part

feature, the six degrees of freedom are restricted

by using a datum reference frame.

The method of bringing a part into contactwith the planes of the datum reference frame

has a significant impact on the measurement of

the part dimensions.



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Primary datum: This establishes the orientation of

the part(stablise the part )to the datum reference

frame.

The part contacts the datum plane with at least

three points of contact.

The primary datum restricts three degree of

freedom



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Secondary datum: This locates the part(restricts

part movement) within the datum reference frame.

Requires a minimum of two points of contact with

the secondary datum.

The Secondary datum restricts two additionaldegree of freedom



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Tertiary datum: This locates the part(restricts part

movement) within the datum reference frame.

Requires a minimum of one points of contact with

the secondary datum.

The tertiary datum restricts the last remainingdegree of freedom

Primary, Secondary and Tertiary Datums


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THE 3-2-1 RULE


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The 3-2-1 rule defines the minimum number

of points of contact required.

The 3-2-1 rule only applies on a part with

all planar datums.

Datum-related versus FOS dimensions


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Only dimensions that are related to a datum

reference frame through geometric tolerances

should measure in a datum reference frame.

If a dimension is not associated to a datum

reference frame with a geometric tolerance, then

there is no specification on how to locate the partin the datum frame.



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Datum-related versus FOS dimensions(contd)


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INCLINED DATUM FEATURES

An inclined datum feature is a datum feature that


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is at an angle other than 90, relative to the other

datum features.

MULTIPLE DATUM REFERENCE FRAMES

A part may have as many datum reference frames as needed to


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define its functional relationships.

COPLANAR DATUM FEATURES


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COPLANAR SURFACES.

COPLANAR DATUM FEATURES.

-In this case, a datum feature symbol is attachedto a profile control.

-The profile control limits the flatness and

co planarity of the surfaces.

COPLANAR DATUM

FEATURES(contd )


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FEATURES(contd)


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DATUM AXIS&

DATUM CENTER PLANE

INTRODUCTION


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Here Feature of Size

is used as a datum

features

When a diameter is

used as a datum

feature, It results in adatum axis

When a planar is used

as a datum feature, it

results in a datum

center planeDescribe the datum that results from a FOS datum feature

3 Ways for representing an axis as

datum


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Datum identification symbol can be touching the surface of a

diameter to specify axis as the datum

Describe the ways to specify an axis as a datum.

3 Ways for representing an axis asdatum (Contd.)


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Datum identification symbol can be touching the beginning

of a leader line of FOS to specify an datum axis

3 Ways for representing an axis as

datum (Contd.)


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Datum identification symbol can be touching the feature

control frame to specify an axis or centre plane as datum

Datum identification symbol can be inline with

2 Ways for representing a centre plane asdatum


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Datum identification symbol can be inline with

dimension line to specify on axis or centre plane asdatum

Describe the ways to specify an centre plane as a datum.

2 Ways for representing a centre plane as

datum (Contd.)


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Datum identification symbol can replace one side of thedimension line and arrow head

Datum Terminology


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Datum feature A

Datum feature

simulator / Gauge

element

Simulated datum

axis A

Simulated datum

Feature A

FOS datum feature referenced at

MMC


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MMC (Contd)


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MMC (Contd)

The gauging equipment that serves as the datumfeature simulatoris a fixed size

The datum axis or center plane is the axis or center

plane of the gage element

The size of the true geometric counterpart of the

datum feature is determined by the specified MMC

limit of size or, in certain cases, its MMCvirtual

condition


MMC (Contd)


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MMC (Contd)

Referencing a FOS datum at MMC has two effects

on the part gaging :

The gage is fixed in size

The part may be loose (shift) in the gage

List two effects of referencing a FOS datum at MMC

Datum axis MMC primary


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Draw the datum feature simulator for an external and

internal FOS datum feature (MMC primary).

Datum centre plane MMC primary


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Datum axis MMC secondary


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Draw the datum feature simulator for an FOS datum

feature (MMC secondary with virtual condition)

Datum axis secondary (MMC) ,

Datum centre plane tertiary (MMC)


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(Contd )


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(Contd)

When referencing the datums with the face primary,

diameter secondary (MMC), and slot tertiary (MMC),

the following conditions apply:

The part will have a minimum of three points of

contact with the primary datum plane

The datum feature simulators will be fixed size gage

elements.

The datum axis is the axis of the datum feature

simulator




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(Contd) The datum axis is perpendicular to the primary datum

plane

Depending upon the datum feature's actual matingsize, a datum shift may be available.

Second and third datum planes are to be associated

with the datum axis

The tertiary datum center plane is the center plane of

the tertiary datum feature simulator

Datum sequence


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Panel-A

Explain how changing the datum reference sequence in a feature

control frame affects the part and gauge

Datum sequence (contd) Panel A

An adjustable gauge is required


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An adjustable gauge is required.

No datum shift is permissible on datum feature A The part is oriented in the gage by datum feature A

Datum feature B will have a minimum of one pointcontact with its datum feature simulator

The orientation of the holes will be relative todatum axis A


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Panel B

Datum feature

simulator for

datum plane B

Panel B Datum feature B will have 3- point contact with its

d l


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datum plane

The part is oriented in the gauge by datum feature B

The orientation of holes will be relative to datum

plane B

An adjustable gauge is required and no datum shift is

permissible on datum feature A


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Panel C

Virtual

condition=10.2


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Cylindrical Datum Features


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Establishing Datums


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Multiple Datum Features

Datum Quiz

True or False (Circle one)

T F 1. A datum is a theoretically exact geometric reference.

T F 2 Primary datums provide feature orientation


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T F 2. Primary datums provide feature orientation.

T F 3. Datums exist on the part itself.

T F 4. Simulated Datums are established by processing or inspection

equipment.

T F 5. Surface plates and V-blocks may be used to establish simulateddatums.

T F 6. Datum features are theoretically exact surfaces.

T F 7. In certain cases, the 1982 & 1994 standards allow implied datums.

T F 8. A datum reference frame consists of three mutually perpendicular

planes

T F 9. Multiple datum features are shown like this

T F 10. The letters I,O and Q are not used for datum symbols

A-B

DATUM EXERCISE


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Establishing Datums

Datum Exercise


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DATUM TARGETS

Datum targets are symbols that describe the shape,size and location of gauge elements that used to


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size and location of gauge elements that used to

establish datum planes or axes. Datum targets are shown on the part surfaces on a

drawing, but they actually do not exist on a part.

Datum targets can be specified to simulate apoint,

line or area contact on a part. The use of datum targets allows a stable and

repeatable relationship for a part with its gauge.

Datum targets should be specified on parts where it is

not practical (or possible) to use an entire surface as adatum feature.

DATUM TARGETS SYMBOLS

A datum target application uses two of symbols:


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A datum target application uses two of symbols:

1.A datum target identification symbol

2.Symbols that denote which type ofgauge elements are to be used.

The leader line from the symbol specifies whetherthe datum target exists on the surface shown or onthe hidden surface side of the part.

Three symbols used to denote the type of gauge

element in a datum target application are thesymbols for a target point, a target line, and atarget area.

DATUM TARGETS SYMBOLS(contd.)


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A datum target point is specified by an X shaped

symbol, consisting of a pair of lines intersecting at

90.

Basic dimensions should used be used to locatedatum target points relative each other and the

other datums on the part.



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Datum target point


Datum target line


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Datum target line


Datum target areas


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http://localhost/var/www/apps/conversion/HAND_OUTS_HONEYWELL/gdnt4.swf


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Creating a partial reference frame from

offset surfaces(contd)
http://localhost/var/www/apps/conversion/HAND_OUTS_HONEYWELL/gdnt4.swf


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FEATURE CONTROL FRAME PLACEMENT


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In terp ret the f latness contro l .


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Interpret the straigh tness con trol .

Interpret the circular ity con trol .

Interpret the cyl ind r ic i ty contro l.

FORM CONTROLS


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Flatness. c

Straightness.

Circularity.

Cylindricity. g

FLATNESSSYMBOL :-


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ZONE OF TOLERANCE :- TWO PARALLEL PLANES

STRAIGHTNESSSYMBOL :-


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ZONE OF TOLERANCE :- CYLINDER

CIRCULARITYSYMBOL :-


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ZONE OF TOLERANCE :- TWO COPLANAR

CONCENTRIC CIRCLES

CYLINDRICITYSYMBOL :-


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ZONE OF TOLERANCE :- TWO COAXIAL CYLINDERS

FLATNESS

Definition : Flatnessis the condition of a

surface having all of its elements in one plane


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surface having all of its elements in one plane.

The tolerance zone for a flatness control is

three-dimensional.

General representation

Interpretation of Flatness tolerance :

It consists of two parallel planes within which


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all the surface elements must lie. Thedistance between the parallel planes is equal

to the flatness control tolerance value.

Rule #1 Effect on FlatnessWhenever Rule #1 applies to a feature of size that

consists of two parallel planes, an automatic


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indirect flatness control exists for both surfaces.

Rule #1 Effect on FlatnessWhen the feature of size is at MMC, both surfaces

must be perfectly flat.


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As the feature departs from MMC, a flatness errorequal to the amount of the departure is allowed.

Flatness Control Application

Some examples of when a designer usesflatness control on a drawing are to provide a


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flat surface: For a gasket or seal.

To attach a mating part.

For better contact with a datum plane.

When these types of applications areinvolved, the indirect flatness control that resultsfrom Rule #1 is often not sufficient to satisfy thefunctional requirements of the part surface.

This is when a flatness control is specified ona drawing:


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Inspecting FlatnessEstablish the first plane of the tolerance zone

by placing the part surface on a surface plate


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that has a small hole.

The surface plate becomes the true counterpart

of the controlled feature. A dial indicator is set in

the small hole.


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The tip of the dial indicator traces a path acrossthe entire part surface.

Then the part is moved over the hole at random.

If the FIM (full indicator movement) is larger

than the flatness tolerance value at any point on

the path, then the surface flatness is not within its


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specification.

STRAIGHTNESS :

Definition : Straightness of a line elementis

the condition where each line element (or


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axis or center plane) is a straight line.The tolerance zone for a straightness control

(as a surface line element control) is two-

dimensional.

General Representation :

General Representation


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Interpretation (Straightness applied to the

surface element)


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Rule#1s Effects on Surface Straightness

Whenever Rule #1 is in effect, an automatic

indirect straightens control exists for the surface


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line elements.

Rule#1s Effects on Surface Straightness When the feature of size is at MMC, the line

elements must be perfectly straight. As

the FOS departs from MMC a straightness error


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p g

equal to the amount of the departure is allowed.

Interpretation (Straightness applied to the

axis)


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0.2

0.2mm

Straightness at MMC Application A common reason for

applying a straightness


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control at MMC to aFOS on a drawing is to

insure the function of

assembly.

Whenever the MMCmodifier is used in a

straightness control, it

means the stated

tolerance applies whenthe FOS is produced at

MMC.

Straightness at MMC Application An important benefit

becomes available


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when straightness isapplied at MMC: extra

tolerance is

permissible.

As the FOS departsfrom MMC towards

LMC, a bonus

tolerance becomes

available.

Inspecting a Straightness Control (Appliedto a FOS at MMC)


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Definition: Circularity is a condition where all

CIRCULARITY


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points of a surface of revolution, at any Section

perpendicular to a common axis, are equidistant

from that axis.

General representation:

0.2

39.0

38.5

Example :


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A circularity controlis a geometric tolerance that

Circularity control :


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limits the amount of circularity on a part surface.

It specifies that each circular element of a

features surface must lie within a tolerance zone

of two coaxial circles.It also applies independently at each cross

section element and at a right angle to the feature

axis.

The radial distance between the circles is equal to

the circularity control tolerance value.

INTERPRETATION


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0.294.2

94.6

0.2

79.4 79.8

0.2

Two imaginary and concentric circles with their

radii 0.2mm apart.

Part

surface

Circularity application :

I li i h l bi ( f d) f h f


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Is to limit the lobing (out of round) of a shaftdiameter.

In certain cases, lobing of a shaft diameter will

cause bearings or bushings to fail prematurely.

Circularity application :


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The diameter must be within its size tolerance.

The circularity control does not override Rule #1.

The circularity control tolerance must be less than the

size tolerance.

The circularity control does not affect the outer

boundary of the FOS.

INSPECTION OF CIRCULARITY


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Cylindricity

Definition :Cylindricityis a condition of a

f f l ti i hi h ll i t f th

g


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surface of revolution in which all points of thesurface are equidistant from a common axis.

General Representation :

0.2

39.038.5

Example & Interpretation:


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A cylindricity controlis a geometric tolerance that limits

the amount of cylindricity error permitted on a part

surface.

Cylindricity control :


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It specifies a tolerance zone of two coaxial cylinders

within which all points of the surface must lie. A

cylindricity control applies simultaneously to the entire

surface.

The radial distance between the two coaxialcylinders is

equal to the cylindricity control tolerance value.

A cylindricity control is a composite control that limits

the circularity, straightness, and taper of a diameter

simultaneously.

Cylindricity application :

Is to limit the surface conditions (out of round,

t d t i ht ) f h ft di t


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taper, and straightness) of a shaft diameter.

In certain cases, surface conditions of a shaft

diameter will cause bearings or bushings to fail

prematurely.

Cylindricity application :


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The diameter must also be within its size tolerance.

The cylindricity control does not override Rule #1.

The cylindricity control tolerance must be less than thetotal size tolerance.

The cylindricity control does not affect the outer boundary

of the FOS.

INSPECTION OF CYLINDRICITY


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Unit Flatness


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Flatness


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Straightness of Surface Elements


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Straightness of an Axis (RFS and MMC)

Circularity


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Basicuma Gd&t

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