JIGS and Fixture.pdf

Metal Cutting and Tool Design (ETME-310)

Abhishek Tevatia, Mechanical & Automation Engineering Department 1

Topic - Tool Design

Lecture

onJigs and Fixtures for Machine Shop

By

Abhishek Tevatia

MAE, MAIT, Delhi



Topic - Tool Design

This lecture will cover

(i) Definition of Fixture and Jig (that aid machining)

(ii) Conceive the purposes of use of jigs and fixtures

(iii) State the design considerations for jigs and fixtures

(iv) illustrate the methods of

(a)locating

(b)supporting and

(c) clamping of jobs in jigs and fixtures

(d) guidance of cutting tools by bushes in jigs



Topic - Tool Design

DEFINITIONFixtures Mechanical (strong and

rigid) devices and used (in machine

shop) to enable easy, quick and

consistently accurate locating,

supporting and clamping, blanks

against the tool for higher productivity

and product quality in batch

production

Jig is a fixture having an additional feature of tool guidance.



Topic - Tool Design

Steps usually followed in machining without fixture

Marking & Punching Mount and fix the blank on the

machine bed or in a vise with

proper alignment, support and

clamp

Adjust the tool-work position byshifting job, vice or tool

Initiate machining andcheck/correct tool work

position,

Complete machining Inspect / measure the

machined features



Topic - Tool Design

BASIC PURPOSES OF USING JIGS & FIXTURES

To eliminate tedious marking, punching, mounting trial - basepositioning and repeated checking

Easy, quick and consistently accurate locating, mountingand clamping blank against the tool

Guidance of cutting tool (drill, reamer etc)

To raise productivity and maintain quality consistently

To reduce operators labour and skill requirement

To attain close tolerances and interchangeability

To reduce rejection and overall cost



Topic - Tool Design

Economic application of jigs and fixtures

Mode of machining

in ordinary machine without fixtures

in ordinary machines using jigs and fixtures

In automatic machine without or with special

fixturing



Topic - Tool Design

DESIGN OF JIGS & FIXTURE

Salient elements to be designed Base and body Locating elements/ system Supporting system Clamping system/ elements Tool guides framing & bushing Indexing system, if needed Auxiliary elements (fasteners etc)



Topic - Tool Design

Factors essentially considered during design of

jigs and fixtures

Easy, quick & consistently accurate locating Provide strong, rigid and stable supporting Quick , strong and rigid clamping Tool guidance for accuracy and safety Easy, quick and uninterrupted loading and unloading Use of minimum and standard parts Adjustable locating for part size variation Chip removal Simple but accurate indexing, if required Ergonomics and safety Manufacturability Service life and cost



Topic - Tool Design

DESIGN OF FIXTURE AND JIGS

Major functions requiring proper design

(a)

Locating

Supporting

Clamping of blank

(b) Tool guidance in jigs

There may be several methods of locating etc depending upon

job-shape, size, material

machining requirement

cutting tools-type, size & shape

But some basic rules are followed during design



Topic - Tool Design

Fundamental requirements for locating,

supporting and clamping



Topic - Tool Design

Locating Blanks in Jigs or Fixture Design Principles or Rules

o one or more pre machined surface holes are to be used for

reference

o The reference surface or holes should be important features

based on which other dimensions are laid

o Locating should be easy, quick and accurate

o Locating pins should be strong, rigid and hard.

o Locating pin should be wide apart

o V-block and cones are preferable for cylindrical jobs.



Topic - Tool Design

Design Locating cont

o Sight location for jobs of irregular surfaces

o Pivoted arms may be used for locating and supporting

large jobs

o adjustable pins or screws are to be used to accommodate

the part size variation



Topic - Tool Design

General methods of locating

Locating work piece for machining in lathes

o By fitting into self centering chuck

o By fitting into collets

o Mounting in between centers

o Fitting in a fixture mounted on faceplate



Topic - Tool Design

General methods of locating cont..

Locating work piece for machining in other than lathes



Topic - Tool Design


Locating by holes

Locating by holes and V-block



Topic - Tool Design


Locating by mandrel or plug



Topic - Tool Design

SUPPORTING PRINCIPALS AND METHOD

Design principals or rules for supporting

o Minimum 3 point contact required to support at flat surface

o Supporting pins or plugs should be strong, rigid and hard

o Unsupported span-should not be much wide



Topic - Tool Design

SUPPORTING Cont..

o supporting should keep job stable under forces

o Recess to be provided within wide contact area.



Topic - Tool Design

SUPPORTING Cont..

o cylindrical jobs are to be supported (and located) on V block

o additional (over 3) pins are to be usedto compensate part size variationto support large and heavy jobsto prevent deformation of job by forces

o additional pins should be adjustable

o Mandrel may be used to support land (and locate) ring

type blanks



Topic - Tool Design

SUPPORTING Cont..

Common methods of supporting

Supporting on vices Supporting at flat surfaces Supporting by fixed pins or plugs



Topic - Tool Design

SUPPORTING Cont..Common methods of supporting

Extra supporting by additional adjustable pins.

Supporting (& locating) on V-block and mandrels



Topic - Tool Design

Design for Clamping

Design principles or rules of clamping

Clamping must be strong, rigid and stable

Should be easy, quick and adequate.

Should not hinder loading and unloading work

Clamping force should act against rigid part and not damage the job

Minimum and standard parts are to be used for clamping

Wear parts have to be hard and replaceable

Reliability, safely and low cost.



Topic - Tool Design

Methods of clamping in fixtures

(a) Ordinary (b) Quick acting

Ordinary (Slow) Methods



Topic - Tool Design

Ordinary Methods Cont..

Clamping from side for through machine of top surface



Topic - Tool Design

Ordinary Methods Cont..

Clamping using swing plates

Other common clamping methods

Vice, magnetic chucks, chucks and collets



Topic - Tool Design

Quick Clamping Method

Quick acting nut



Topic - Tool Design

Quick Clamping Cont..

Clamping using Cam



Topic - Tool Design


Multiple clamping of cylindrical jobs



Topic - Tool Design


Quick acting screw (bayonet type)



Topic - Tool Design

DRILL JIG BUSHINGPurposes of using bush

To reduce or eliminate runoutTo locate the tool in positionTo prevent buckling and breaking of toolsTo attain and maintain accuracy, safety and economy

Desired characteristics

Material - hard or hardened steel (Rc >= 60)Finishing inside and outside by grindingBushes length >= 2d and diameter d+ (d is drill diameter)Quick fitting, removal and replacementDo not slip unwantedly



Topic - Tool Design

Types of bushes

According to fittingpress fit

slip type with linerscrew type with/ without liner

According to headwithout head

with head

with flange



Topic - Tool Design

Types of bushes Cont

Frequently replaceable bushes



Topic - Tool Design

Types of bushes Cont

Special types of bushes

JIGS and Fixture.pdf

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