Jigs & Fixture Design

JIG

AND

_FIXTURE

DESIGN

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JIG

AND

FIXTURE

DESIGN

A TREATISE COVERING

THE

PRINCIPLES OF

JIG

AND

FIXTURE

DESIGN,

THE IMPORTANT

CONSTRUCTIONAL

DETAILS,

AND

MANY

DIF-FERENT

TYPES OF

WORK-HOLDING

DEVICES

USED

IN

INTERCHANGEABLE

MANUFACTURE

EDITED BY

FRANKLIN

Df JONES

ASSOCIATE EDITOR OFMACHINERY

AUTHOR OF "TURNING AND BORING," "PLANING AND MILLING,

"_{MECHANISMSAND MECHANICALMOVEMENTS,"}

"THREAD-CUTTING METHODS," ETC.

FIRST

EDITION

NEW

YORK

THE

INDUSTRIAL PRESS

LONDON:

THE MACHINERY

PUBLISHING CO., LTD.

BY

THE

INDUSTRIAL

PRESS

NEW

YORK

PREFACE

The

_development of machine tools has been _accompanied

by

a corresponding development of auxiliary equipment for

in-creasing the quantity

and

improving the qualityof_{the products}

of these machines.

Whenever

_{duplicate parts require}

some

operation such as _{drilling, planing,} or _milling, the selection

of a suitable _type of machine is often followed

_by

_{the design}

of whatever _{special tools} or attachments are needed to _adapt

the machine to _{the operation}

required.

The

tool-guiding

and

work-holding jigs

and

fixtures which are

now

used in

prac-tically all machine shops represent the

most

_important class

of _{special equipment,}

and

this

book

deals _exclusively with

their _design

and

construction.

As

most

_jigs are used for _{drilling operations,} a

book

was

previously published entitled "Drilling Practice

and

Jig

De-sign," covering different _types of _drilling machines

and

their

use, the design of drill _{jigs, and,} to

some

extent, the design of

fixtures _such, for _example, as are used on _milling machines.

While the _{subjects of drilling}

and

_{jig design are closely allied,}

it is no _{longer possible to} cover

them

both in a _single volume, owing to the extensive changes in _{drilling practice}

and

the

increasing use of jigs

and

fixtures of various types on different

classes of machine tools. _{Therefore, the}

book

referred to has

been _replaced

_by

two

_volumes, of which this is one.

The

other book,

"Modern

Drilling Practice," is _already well

known

to

many

designers, shop foremen,

and

machinists interested in

the latest _types of _drilling machines

and

their use.

This

new

book, "Jig

and

Fixture Design," contains that part of the

volume

on"Drilling Practice

and

Jig Design" which

dealt with _jigs

and

fixtures. This material

was

used because it

is a treatise on the _principles of _jig

and

fixture _design which

VI

kind. These original chapters which explain the general

pro-cedure in _{designing jigs}

and

fixtures

and

how

work

should be

located, clamped, etc.,' have been supplemented

by

a large

amount

of

new

_{matter, thus}

_making

_{the present}

book

_unusually

complete.

A

great variety of jig

and

fixture designs have been described

and

illustrated in orderto

show

just

how

theprinciples

and

_important details referred to in the _{forepart of} the

book

are

applied under

many

different conditions

and

to jigs

and

fixtures

used on _{various types} of machine tools.

Most

of _{the designs} illustrated in this

book

have been sent to

MACHINERY

from

men

in the _{machine-building field,}

be-cause the designs were considered unusual

and

worth _placing

on

record. While it

would

not be _possible to _give credit to

each individual contributor,

we

are indebted to all

who

have

assisted indirectly in preparing this treatise,

and

especially to

Einar

Morin and

Albert A.

_Dowd,

_recognized tool _experts

and

production engineers,

who

have supplied valuable material for

several of _{the chapters} on jig design.

THE

EDITOR.

CONTENTS

CHAPTER

I

PAGES

PRINCIPLES

OF

_JIG

DESIGN

Objects of Jigs and Fixtures Difference between _Jigs

and Fixtures Fundamental _Principles of _{Jig Design}

Locating Points Clamping Devices _{Weight of Jigs}

Jigs provided with Feet Materials for

Jigs General

Remarks on _{Jig Design}

_Summary

of

Principles of Jig

Design Types of _Jigs

_Open

_Jigs Box _Jigs

De-tails of Jig Design

1-20

CHAPTER

II

DESIGN

OF

OPEN

DRILL

_JIGS

Jig Drawings Designing

Open

Jigs Improving the

Simple

Form

of _{Jig by Adding Locating} Screws

Pro-viding Clamps and Feet for the _{Jig Examples} of _Open

Drill Jigs 21-44

CHAPTER

III

DESIGN OF CLOSED

OR BOX

JIG

General Procedure inthe_Designof ClosedorBox_Jigs

-Jigs for Rapid Production Special Features of Box _Jigs

Examples of Closed or

Box

_{Jigs 45-6?}

CHAPTER IV

JIG

BUSHINGS

Removable Bushings Material for _{Jig Bushings}

-Dimensions of Stationary Jig Bushings Miscellaneous Types of _{Jig Bushings}

Means

for _Preventing Loose Bushings from Turning Dimensionsof Removable

Bush-ings Screw Bushings Special Designs of Guide

Bush-ings Methods of making Jig Bushings Hardening Jig

Bushings Grinding and Lapping... 68-91

CHAPTER

V

LOCATING

POINTS

AND

ADJUST-ABLE

STOPS

PAGES

Pins and _Stops used as _Locating

Means

Locating

by

Means

of V-blocks

Cup

and Cone _Locating Points

Screw _Bushings and _Sliding Bushings used as _Locating

Means

_Adjustable Locating Points _{Special Types} of

Adjustable Stops Locating from Finished Holes

Lo-cating by

Keyways

in the

Work

Common

Defects in

Jig Design 92-109

CHAPTER

VI

JIG

CLAMPING

DEVICES

Types of _Clamps Hook-bolts Screw-tightening

De-vices Swinging Leaves

Wedge

or _Taper Gibs

Ec-centric _{Clamping Arrangements} Applications to Jig

Design 110-150

CHAPTER

VII

EXAMPLES

OF DRILL

_JIG

DESIGN

Different _Types of _{Indexing Jigs} Jig for _Deep-hole

Drilling Jig of Simple Design for _{drilling Straight} and

Angular Holes Drill _{Jig equipped with Milling}

Attach-mentJig

for _{Cross-drilling} Pistons and _Facing

Wrist-pin Bosses Universal Jigs Machine Vises with Drill

Jig Attachments Miscellaneous Designs 151-194

CHAPTER

VIII

BORING

JIGS

BoringJigofSimple Design Adjustable BoringJigs

-Boring Jig supported on

Work

Jigs designed for

Sup-porting Bar on

One

Side ofHole Only Jigs for Multiple

ix

CHAPTER

IX

MILLING

AND

PLANING

FIXTURES

PAGES

Fixturefor_milling to a Given _{Length Duplex}Fixture

-Adjustable Fixture for _Angular

_Work

Fixture

ar-ranged for Lateral and Angular Adjustment Indexing

Milling Fixtures Various _Designs of Radial _Milling

Fixtures _Examples of Planer Fixture _{Design 211-241}

CHAPTER

X

ADJUSTABLE

FIXTURES

FOR

TURRET

LATHES

AND

VERTICAL BORING

MILLS

Important Points in the _{Design Adjustable} Fixture

for _{Holding Castings} of Different Diameters _Adjustable

Fixture for _Special Bevel Gear Blanks Provision for

maintaining Accuracy in _Adjustable Fixture Various

Designsof _{Adjustable Fixtures} for Vertical_BoringMills... _242-256

CHAPTER

XI

THE

FLOATING

PRINCIPLE

AS

APPLIED

TO

FIXTURE

WORK

ImportantPointsin_{the Application}of_{FloatingPrinciple}

Piston Drill _Jig with _{Floating Clamps} Drill _{Jig for}

Rough

Collar Drill _Jig with Floating Bushings and Locating Vees Milling Fixture with Floating Clamps andLocator Various other _Designsof _{Locating Devices}

illustrating the Applicationof the FloatingPrinciple 257-275

CHAPTER XII

APPLICATION OF

THE

THREE-POINT

PRINCI-PLE

IN

FIXTURES

Three-point Locating and Clamping Devices

Three-point Supportfor _Flywheel Fixture Three-point Fixture

for_{Pot Casting}

_Two

Methodsof_{Obtaining aThree-point}

Supportona

Hub

Casting Fixturehaving Three Clamp-ing Jaws and Three Locating Pads DoubleThree-point

,

CHAPTER

XIII

SPECIAL

JIG

AND

FIXTURE

MECHANISMS

PAGES

Equalizing the Pressure of Clamping Devices _Clamps

that draw the

Work

down

Firmly on the _Locating

Means

_{Multiple-clamping Devices Clamping} Devices

for Fixtures that do not interfere with the Tools used

Three-point Clamping Devices 288-305

CHAPTER

XIV

PROVIDING FOR UPKEEP

IN

DESIGNING

JIGS

AND

FIXTURES

Points Pertaining to _{Upkeep Drill Jig for} a Receiver

Forging Drilling and Reaming Jig Indexing Fixture

for a Clutch Gear Fixture with Inserted _Jaws Bevel

Gear Fixture with _Adjustable Features Fixture for a

JIG

AND

FIXTURE

DESIGN

CHAPTER

I

PRINCIPLES

OF

_JIG

DESIGN

Jigs

and

fixtures

_may

be _{defined as devices} used in the

manu-facture of _{duplicate parts} of machines

and

intended to

make

possible interchangeable

work

at a reduced _cost, as

_compared

with the cost of _producing each machine detail _{individually.}

Jigs

and

fixturesservethepurposeof_holding

and

_properly

locat-ing apiece of

work

whilemachined,

and

are_{provided with}

neces-sary,appliances for guiding, supporting, setting,

and

gaging the

tools in such a

manner

that all the

work

_produced in the

same

jig or fixture willbe alike in all _respects, even with the

employ-ment

of unskilled labor.

When

_{using the expression "alike,"}

it _{implies, of course, simply that the}

pieces will be near _enough

alike for _{the purposes} for which the

work

_being machined is

intended. _Thus, for certain classes of _work, wider limits of

variation will be _permissible without _affecting the _{proper use}

of the _{piece machined,} while in other cases the limits of

varia-tion willbe so smallas to

make

the_{expression "perfectly alike"}

literally true.

Objects of Jigs

and

Fixtures.

The

main

_{object of using}jigs

and

fixturesis the reduction of the cost ofmachines ormachine

details

made

in _great numbers. This reduction of cost is

ob-tained in _consequence of the increased _rapidity with which the machines

_may

be built

and

the

_employment

of _{cheaper labor,} which is _possible

when

_using tools for _{interchangeable}

manu-facturing. Another object, not less _important, is _{the accuracy}

with which the

work

can be _produced,

making

it _possible to assemblethe_{piecesproduced}in_jigswithout

_any

_great

amount

of

fitting in the assembling department, thus also effecting agreat

saving in this _respect.

The

use of _jigs and fixtures practically

does

_away

with the fitting, as this expression

was

understood in

2 JIG

away

withso-called _"patch-

work"

in _{the production} of

machin-ery. It

makes

it possible to have all the machines builtin the

shop according to the drawings, a thing which is rather difficult

to do if each individual machine in a _large lot is built without

reference to the other machines in the

same

lot.

The

_{interchangeability obtained}

_by

theuse of_jigs

and

fixtures

makes

it alsoan _easy matter to _{quickly replace}broken or

worn-out parts without _great additional cost

and

trouble.

When

machines are built on the _{individual plan,} it is _necessary to fit

the partreplacingthebrokenorworn-outpiece, in place,

involv-ing considerableextra expense, nottomention the delay

and

the

difficulties _{occasioned thereby.}

As

mentioned, jigs

and

fixtures permit the

_employment

of

practically unskilled labor. There are

many

operations in the

building of a machine, which, if each machine were built

indi-vidually, withouttheuse of specialtools,

would

require the

work

of _{expert machinists}

and

toolmakers. _Special tools, in theform

ofjigs

and

fixtures, permitequally good, or, in

some

cases, even

better results to be obtained

_by

a

much

cheaper class _{of labor,}

provided the jigs

and

fixtures are properly designed

and

cor-rectly made. Another possibility for saving, particularly in the

case of drill

and

_{boringjigsprovided with guide bushings} inthe

same

plane, is

met

within thefact thatsuch_jigs are _adapted to

be used in _{multiple-spindle} drills, thereby still

more

_increasing

the _rapidity with which the

work

_may

be produced. In shops

where _{a great}

_many

_{duplicate parts} are _{made, containing} a

number

of drilled _{holes, multiple-spindle} drills of _complicated

design, which

may

be rather expensive as regards first _cost, are

really cheaper,

by

far, thanordinary simple drill_presses.

Another advantage which has been gained

by

the use of _jigs

and

_fixtures,

and

which should not be lost sight of in the enu-merationof _{the points} infavorof _{building machinery}

_by

theuse

of special tools, is that the details of a machine that has been

provided with a complete equipment of accurate

and

durable

jigs

and

fixtures can all be finished simultaneously in different

departmentsofa_{large factory,}withoutinconvenience, thus

JIG ₃

the parts from the different _departments;

and

there is no need

of _waitingfor _{the completion} of one _part into which another is

required to fit, before

making

this latter part. This gain in

time

means

a great deal in _{manufacturing,}

and was

entirely

impossibleunder the old-time systemof machine _building,

when

each part

had

to be

made

in the order in which it

went

to

the finished _machine,

and

_{each consecutive part}

had

tobe lined

up

with each one of _{the previously}

made

and

assembled details.

Brackets, bearings, etc.,

had

to be drilled in place, often with

ratchetdrills, whichisa slow

and

alwaysinconvenient_operation. Difference

between

Jigs

and

Fixtures.

To

exactly define the

word

_"jig," as _{considered apart} from the

word

"fixture,"

is difficult, as the differencebetween a _jig

and

afixture is

often-times _{not very} easy to decide.

The

word

jig is _frequently,

al-though incorrectly, applied to

any

kind of _{a work-holding} appli-ance used in _{the building} of _{machinery, the}

same

as, in

some

shops, the

word

fixture is applied to all kinds _{of special} tools.

As

_{a general}rule, however, a_jigisa _special tool,which, while it

holds the work, or is held _{onto the work,} also _{contains guides} for _{the respective} tools to be used; whereas a fixture is _only

holding the

work

while the cuttingtoolsareperforming the

oper-ation on the_piece, without _containing

_any

_{special arrangements} for _guiding these tools.

The

_{fixture, therefore, must, itself,} be

securely held or fixed to the machine on which the operation is

performed; hencethename.

A

fixture,however,

may

sometimes be provided with a

number

of _gages

and

_stops, althoughit does not contain

_any

_specialdevicesfor _{the guiding}ofthe tools.

The

definition given, in a general way, would therefore

clas-sify jigs as special tools used particularly in drilling

and

boring

operations, while fixtures, in particular, would be those special

tools used on milling machines, and, in

some

cases, on planers,

shapers,

and

slotting machines. Special tools used on the lathe

may

beeither ofthe natureof_jigsor_fixtures,

and

sometimes the

special tool is _actuallya combination of _both, in which case the

term _{drilling fixture,} boringfixture, etc., is suitable.

Fundamental

Principles of Jig Design. Before entering

4 JIG

fixtures, the fundamental principles of jig

and

fixture design

will be brieflyoutlined.

Whenever

a jig is

made

for a

compo-nent _partof a_machine,it is almost _{alwaysrequired that a} corre-sponding jigbe

made

up

for the place on the machine, or other

part, where the first-mentioned detail is to be attached. It _is,

of course, absolutely necessary that these

two

jigs be perfectly

alike as to the location of guides

and

gage points. In order'to

havetheholes

and

_guidesinthetwojigsinalignment, it is

advis-able,

and

almost always cheaper

and

quicker, to transfer the

holes or the_gagepoints fromthefirst_jig

made

to theother. In

many

instances, it is_{possible to} use the

same

jig for both parts.

Caseswhere theone or theother of _{these principles}is_applicable will be

shown

in _{the following chapters}in the _detailed

descrip-tions of drill

and

_{boring jigs.}

There are

some

cases where it is not advisable to

make

two

jigs, one for each of the

two

partswhich are tofit _together. It

may

be_impossible to _properlylocate the _jig on one of _{the parts} to be drilled, or, if the _jigwere made, it

_may

be so _complicated

that it

would

not be economical.

Under

such conditions the

component

part itself

_may

be used as a _jig,

and

the _respective

holes in this _part used as _guides for the tools

when

_machining

the machine details into which it fits. Guide _bushings for the

drills

and

_{boring bars}

_may

then be _placed in the holes in the

component

partitself. In

_many

cases, drilling

and

boring

opera-tions are also _done, to _{great advantage,}

_by

_{using the brackets}

and

_{bearings already} assembled

and

fastened to the machine

body

as guides.

One

ofthe

most

_{importantquestions}tobedecided before

mak-ing a jig is the

amount

of

money

which can be expended on a

special tool for the operation required. In

many

cases, it is

possible to get a highly efficient tool

_{by making}

it

more

compli-cated

and more

_expensive, whereas a less efficient tool

_may

be produced at _{very small expense.}

To

decide which of these

two

types of jigs

and

fixtures should be designed in each individual

case _{depends entirely}

_upon

the circumstances. There should be

acareful _comparison of_{the present}cost of _{carrying out a}certain

JIG ₅

with an efficient _tool,

and

the cost of _{building that} tool itself.

Unless thisis _done, it is _likelythat the _shopis burdened witha great

number

of _special tools

and

fixtures _which, while _they

may

be veryuseful for_{the production}of_{the parts}forwhich_they are intended, actually involve a loss. It is _readily seen

how

uneconomical it

would

be to

make

an _expensive jig

and

fixture for a machine or _{a part} of a machinethat

would

_only have to be _{duplicated a}few times. In

some

_{cases, of course, there}

_may

be _{a gain} in_{using special}devicesin order to_{get extremelygood}

and

accurate results.

Locating Points.

The

most

_{important requirements} in the designofjigsare thatgoodfacilities _{be providedfor locating} the

work,

and

that the_{piece to} be

machined

_may

be easily inserted

and

_quickly taken out of the _jig, so that no time is wasted in

placing the

work

in positiononthemachine performingthework.

In

some

cases, a longer time is _{required for locating}

and

clamp-ing the piece tobe

worked upon

thanis _required for the actual

machine _operation itself. In all such cases the machine per-formingthe

work

is_actuallyidlethe_greaterpartofthetime,and,

added tothe lossof the _{operator's time,} is the_{increased expense}

formachinecostincurred

_by

such a condition. Forthisreason,

the _locating

and

_clamping of the

work

in _{place quickly}

and

accurately should be carefully studied

by

the designer before

any

attemptis

made

to _{design the} tool. In _{choosing the}

locat-ingsurface or points of the piece or part, consideration

must

be

given to the facilities _{for locating the corresponding part} of the

machine in a similar manner. It is _{highly important} that this

be done, as otherwise, although the jigs

may

be alike, as far as their _{guiding appliances} are concerned, there

_may

be no _facility

for _{locating the corresponding part} in the

same

manner

as the

one _already drilled,

and

while the holes drilled

may

coincide,

other surfaces, also _required to _coincide,

_may

be _considerably

out of line.

One

of the

main

principles of location, therefore,

is that

_{two component}

_parts of the machine should be located

from _{corresponding points}

and

surfaces.

If_{possible, special arrangements} shouldbe

made

in _{the design}

6 JIG

the correct _way. Mistakes are often

made

on this account

in _shops where _{a great} deal of _{cheap help} is _{used, piecesbeing}

placed in jigsupside down, or in

some

_way

other than the

cor-rect one,

and

work

that has been _previously

machined

at the

expenditure of a great deal of time is _{entirely spoiled.}

There-fore, wheneverpossible, a jigshouldbe

made

"

fool-proof ."

When

the

work

to be

machined

varies in _shape

and

size, as,

for instance, in thecase of _{roughcastings,}it is_necessary tohave

at least

some

of the _{locating points adjustable}

and

_placed so

that they canbe_easily reached for_{adjustment, but, at} the

same

time, sofastenedthat they are, to acertain extent, positive. In

the _{following chapters} different kinds of _{adjustable locating}

pointswillbedescribedindetail.

ClampingDevices.

The

strappingorclamping arrangements

should be as _{simple as possible,} without_sacrificing effectiveness,

and

the strengthofthe _clampsshouldbesuch astonot_{only hold}

thepiecefirmlyin place, butalsototake thestrain of_{the cutting}

tools without _springing or "_giving."

When

_{designing the jig,}

the direction inwhich the strain of the tool or cutters acts

_upon

the

work

should _{always be considered,}

and

the _clampsso_placed

that they will have _{the highest degree} of _strength to resist the pressure of the cut.

The

main

principles in the application of clamps to a _jig or

fixture are tha _they should be convenient for the _operator,

quickly operated, and,

when

detached from the work, still con-nected with the jig or fixture itself, so as to prevent the

oper-ator from losing them.

Many

a time, looking for lost _straps,

clamps, screws, etc., causes

more

delay in _shops than the extra

cost incurred in _{designing a jig} or fixture

somewhat more

com-plicated, in order to

make

the binding arrangement an integral

part of the fixture itself. Great complication in the clamping

arrangements, however, is not advisable. _{Usually clamping} arrangementsofthiskind

work

well

when

thefixtureis_new,but,

as _{the various parts}

become

_worn, complicated arrangements are

more

likelyto getoutof order,

and

the extracostincurredin

repairing often outweighs the temporary gain in quickness of

JIG 7

The

_judgment

of _{the designer is,} in _{every case,} the

most

im-portant point in _{the design} of _jigs

and

fixtures. Definite rules

forall cases cannot be_given. General _principlescanbe_studied,

butthe_efficiencyof the individualtoolwill_{dependentirely}

_upon

the

_judgment

of the tool _designer in _{applying the general}

prin-ciplesof tool designto thecase inhand.

When

_{designing the} jig or fixture, the locating

and

bearing points for the

work and

thelocation of the _clamps

must

also be

so selected that thereisas little _{liabilityas possible of springing}

the_{piece or jig,}or _{both, outof shape,}

when

applying the clamps.

The

_springing of either the one or _{the other part} will cause

in-correct_results, asthe

work

surfaceswillbeoutof_{alignment with}

the holes drilled or the faces milled.

The

_clamps or _straps

should _{therefore, as} far _{as possible,} be so _{placed that they} are

exactly opposite

some

bearing point or surface on the work.

Weight

of _Jigs.

The

_designer

must

use his

_judgment

in

re-gard to the

amount

of _{metal put} into the _jig or fixture. It is

desirable to

make

these tools as _{lightas possible, in} order that

they

may

be _{easily handled,} be of smaller size,

and

cost less in

regard to the

amount

of material used for their _making, but, at

the

same

time,it is _poor

_economy

tosacrifice

_any

ofthe _rigidity

and

stiffnessof thetool, as thisisone ofthe

main

considerations

in _obtaining efficient results.

On

_{large-sized jigs}

and

_fixtures,

it is_{possible to core}out themetalina

number

of places,without

decreasing, in the least, the strength of the jig itself.

The

corners of _jigs

and

fixtures should _{always be} well _rounded,

and

allburrs

and

_{sharp edges}filed_off,so as to

make

them

convenient

and

_pleasant for _handling. Smaller _jigs should also be

made

withhandles in _{properplaces, so that they}

_may

beheld _in

posi-tion _{while working,} as in the case of _drilling jigs,

and

also for

conveniencein

_moving

the _jigabout.

Jigs Provided with Feet. Ordinary drill jigs should always be provided with feet or _legs on all sides whichare _{opposite the}

holesfor _{the bushings,} so that the_jigcan be _placedlevelon the

table of the machine. These feet also _greatly facilitate the

making

ofthe_jig,

_making

it_{easier to lay}out

and

_{plane the}

differ-ent finished surfaces.

On

the sides of thejig where

no

feet are

8 JIG

required, if the

_body

is

made

from a _casting, it is of _advantage

to have _{small projecting lugs for bearing} surfaces

when

_laying

out

and

_planing. While _jigs are

most

_commonly

_{provided with}

fourfeet on eachside, in

some

casesit is sufficient to_{provide the}

toolwith _only threefeet, but careshould be takenin either case

thatall_bushings

and

_places where_pressurewillbe_applied tothe

tool are _placed inside of _{the geometrical figure} obtained

_by

con-necting,

by

lines, the pointsof location for the feet.

Whileit

_may

seem

thatthreefeet_{are preferable to use,}because the _{jig will} then _always obtain _{a bearing} on all the three feet,

which it

would

not with four _feet, if the table of the machine were not _absolutely plane, it is not _{quite safe to}use the smaller

number

_{of supports,because a chip}or

some

_{other object}isliable

to

come

under one foot

and

throw the jig

and

the piece out of

line, without this being noticed

by

the _operator. If the

same

thinghappens to a_jigwith four_feet, itwillrock

and

_invariably

cause the operator to notice the defect. If the table is out of

true, this defect, too, willbe noticed for the

same

reason.

Jigfeetare generally cast solidwiththe jigframe.

When

the

jigframeis

made

from machine steel,

and

sometimesinthe case

of cast-iron_jigs, detachable feetare used.

Materialsfor _{Jigs. Opinions} differ as to the relative merits

of cast iron

and

steel as materials from which to construct the

jig

and

fixturebodies.

The

decisiononthispoint shoulddepend

to_{a great extent}

_upon

_{the usage} towhich thefixtureisto_{be put}

and

the character of the

work

which it is to handle. For small

and

medium

sized _work, such as _{typewriter, sewing machine,}

gun, adding machine, cash register, phonograph,

and

similar

parts,thesteel jig offersdecided advantages,butfor largerwork,

suchasthat encounteredin _{automobile,engine,}

and

machinetool

fixtures, the cast-iron jig is _{undoubtedly the cheaper}

and

more

advisable to use.

The

_{steel jig} should be left softin order that

at

_any

future timeadditional holes

_may

beadded, orthe_existing bushings changed as required.

With

a cast-iron_{jig this adding} of _bushings is a difficult _matter, as the frame is _{usually bossed}

and

_"spotfinished" at _{the point}where _{the bushings}are located,

9

locate or _{change the bushings.}

When

_{designing the jig,} these

points should be

remembered and

provision

made

for _them, where _possible.

General

Remarks

on Jig Design.

One

mistake, quite

fre-quently made, is that of _{giving too} little clearance _between the

piece to be

machined and

thewalls or sides of thejigusedforit.

Plenty ofclearance should _{always be allowed, particularly}

when

roughcastings are beingdrilledor

machined

inthe_{jigs; besides,} those surfaces in the _jig which do not _actually bear

_upon

the

work

_{do not always}

come

_exactlyto thedimensions indicated on

the drawing, particularly in a cast-iron_jig,

and

allowance _ought

to be

made

for such differences.

Inregardtothe locating points,it_ought tobe remarked that,

in all _instances, these should be visible to _{the operator}

when

placing the

work

in position, so that he

may

be enabled to see

that the

work

reallyisinits_{right place.}

At

times the

construc-tion of the_{piece to} be

_{worked upon}

_may

_{prevent a} full view of

the_{locating points.} In such acase a coredordrilled hole inthe

jig, near thelocatingseat, willenable aviewofsame, so that the operator

may

either see that the

work

rests

_upon

the _locating

point, or so thathe can place a feeler or thickness gage between

the

work and

the locating surface, to

make

sure that he has the

work

in its _{correct position.} Another _{point that should not} be

overlookedisthatjigs

and

fixturesshouldbedesigned with aview

of

_{making them}

_easily cleaned from the _chips,

and

_provision

should also be

made

sothat the_{chips, as}far_{as possible,}

_may

fall

outofthe_jig

and

notaccumulate onorabout_{the locating points,} where theyareliable tothrowthe

work

outof its _{correct position}

and

_{consequently spoil} the _piece.

The

principles so far referred to have all been in relation to

the holding of the

work

in thejig,

and

thegeneral design of the

jig for producing accurate work. Provisions, however, should

also be

made

for _clamping the jig or fixture to the table of the

machine, in cases where it is _necessary to have the tool fixed

while _{in operation.} Small _{drilling jigs} are not _clamped to the

table, but boringjigs andmillingand planingfixtures _invariably

JIG

Plain _{lugs, projecting out} in the

same

plane as the

bottom

of

the_{jig, or lugs} with aslotin

them

tofitthe

_body

_{of T-bolts, are}

the

common

means

for _clamping fixtures to the table. For

boring jigs, it is unnecessary to _provide

more

than three such

clamping points, as a greater

number

is _likely to cause

some

springing actioninthefixture.

A

slightspringingeffectisalmost

unavoidable,

no

matter

how

strong

and heavy

thejigis, but,

by

properlyapplyingthe clamps, it is_{possible to confine} this

spring-ingwithincommercial limits.

Jigs should always be tested before _{they are used, so as to}

make

sure _{that the guiding provisions} are _placed in the right

relation to the _{locating points}

and

in _proper relation to each

other.

Summary

of _Principles of _{Jig Design.}

_Summarizing

the

principles referredto,the followingrules

may

begivenasthe

main

pointstobeconsideredinthe designing ofjigs

and

fixtures: 1. _{Before planning the design} of a tool,

compare

the cost of

productionof the

work

withpresent toolswith _{the expected}cost of_{production,using the}tool to_{be made,}

and

seethat the cost of

buildingis notin excess of _{expectedgain.}

2. _{Before laying out thejig}or_fixture, decide

_upon

the

locat-ing points

and

outline a clamping arrangement.

3.

Make

all clamping

and

binding devices as quick-acting

as possible.

4. Inselectinglocating points, seethat two

component

parts

of a machine can be located from _{corresponding points}

and

sur-faces.

5.

Make

thejig

"

fool-proof "; that is,arrange itso that the

work

cannot be inserted_exceptin the correct_way.

6. For rough _castings,

make

some

of the _{locating points}

adjustable.

7. Locate clamps so that they will bein the best position to

resist _{the pressure} of _{the cutting} tool

when

at work.

8.

_Make,

if _possible, all _{clamps integral parts} of the _jigor

fixture.

9.

Avoid

complicated clamping arrangements, which are

JIG II

10. Place all _clamps as _nearly as _{possible opposite}

some

bearing point of the work, to avoid _springing.

11. Core out all_{unnecessary metal,}

_making

the toolsas _light

as possible, consistent with _rigidity

and

stiffness.

12.

Round

all corners.

13. Provide handles wherever these will

make

the handling

of the jig

more

convenient.

14. Provide feet, preferably four, opposite all surfaces

con-taining guide bushings in _drilling

and

_{boring jigs.}

15. Place allbushings inside of _{the geometrical figure} formed

by

connecting the points of location of the feet.

16. Provide

abundant

_{clearance, particularly} for _rough castings.

17.

Make,

if_possible, all_{locatingpoints}visibleto_{the operator}

when

_{placing the}

work

_{in position.}

18. Provide holes or _escapes for the _chips.

19. Provide clamping lugs, located so as to prevent

spring-ing of the fixture, on all tools which

must

be held to the table of the machine while in _use,

and

_tongues for the slots in the

tables in all milling

and

planing fixtures.

20. _{Before using} in the _{shop, for} commercial _purposes, test all _jigs as soon as made.

Types of Jigs.

The

two

principal classes of jigs are drill jigs

and

boring jigs. Fixtures

may

be grouped as milling,

planing,

and

splining fixtures, although there are a

number

of

special fixtures which could not be classified under

any

special

head.

Drill _jigs are intended _{exclusively for} drilling, reaming,

tap-ping,

and

facing.

Whenever

these four operations are required

on

a _pieceof work, it _is, as arule, possible to provide the

neces-sary arrangements for _performing all these _operations in one

and

the

same

jig. Sometimes separate jigs are

made

for each

one of these operations, but it is doubtless

more

convenient

and

_cheaper to have one jig do for all, as the design of the jig will not be

much

more

_{complicated. Although} it

_may

be

pos-sible to

make

a distinction between a

number

of different _types

JIG

names

for the various classes, owing to the great variety of

shapesofthe

work

tobedrilled. Thereare,however,

two

general

types that are

most

commonly

used, the difference between

them

_being very marked. These types

may

be classified as

openjigs

and

closed jigs, or boxjigs. Sometimes the open jigs

are calledclampingjigs.

The

openjigsusually haveallthe drill

bushings in the

same

plane, parallel with one another,

and

are

not _{provided with} loose or removable walls or leaves, thereby

making

it _{possible to insert} the piece to be drilled without

any

manipulation of _{the parts} of the jig. These jigs are often of

such a construction that _they are _applied to the

work

to be

drilled, the jig being placed on the work, rather than the

work

being placed in the jig.

The

jig

may

be held to the

work by

straps, bolts, or clamps, but in

_many

cases the _jig fits into or

over

some

finished _part of the

work and

in this

_way

the jig is

located

and

held _{in position.}

The

closed drill _jigs, or box _{jigs, frequently resemble}

some

form of a box

and

are intended _{for pieces} where the holes are to be drilled at _{various angles} to one another.

As

a rule, the

piece to be drilled can be inserted in the jig only after one or

more

leaves or covers have been

_swung

out of the _way.

Some-times it is _necessary to

remove

a loose wall, which is held

_by

bolts

and

dowel pins, in order to locate the piece in the jig.

The

work

in the closed drill _jig

_may

be held in _place

_by

set-screws, screw bushings, straps, or hook-bolts.

The

combination drilling

and

boring jig is another type of

closed _{jig designed} to serve both for _drilling

and

_boring

opera-tions. Before _designing a combination drill

and

_{boring jig,}

the relation _between,

and

number

of, the drilled

and

bored

holes

must

be takeninto consideration,

and

also the size of the

piece to be machined. In case thereis a great

number

of holes,

it

_may

be of _advantage to have

two

or even

more

jigs for the

same

_piece, because it

makes

it easier to _design

and

make

the

jig,

and

very likely will give a better result.

The

holes drilled

orbored in the firstjig

may

be used as a

means

for locatingthe

piece in the jigs used later on. Combination drill

and

boring

JIG

Open

Jigs.

Open

jigs of the simpler forms are simply

plates provided with bushed holes which are located to

cor-respond with the required locations for the drilled holes. While

holes are sometimes drilled

_by

first _{laying out the holes directly}

upon

the work, it is _quite evident that this

method

of _drilling

would

not be efficient if a _large

number

of _{duplicate parts}

had

to be drilled _accurately, as there is

likely to be

more

or less

variation in the location of the _holes,

and

considerable loss of

time. In the first _place, a certain

amount

of time is _required

for_{laying out} these holes_preparatoryto _drilling.

The

_operator,

Fig. 1. Jig forCylinderFlange andHead, andits_Application

when

starting the drill,

must

also be careful to

make

it cut

concentric with the scribed _circle, which _requires extra time,

and

there will _necessarily be

more

or less variation.

To

over-come

_{these objections,} jigs arealmost universally usedfor

hold-ing the

work and

guiding the drill,

when

drilling duplicate parts,

especially

when

quite a large

number

of duplicate pieces

must

be drilled.

The

_ring-shaped jig

shown

at

A

in Fig. i is used _{for drilling}

the stud bolt holes in a _{cylinder flange}

and

also for _drilling the

JIG

the jig

when

the cylinder flange is _being drilled is

shown

at

B.

An

annular _projection on the _jig fits _{closely in} the _cylinder counterbore,astheillustration_shows, to locatethejigconcentric

with the bore.

As

the holes inthe _{cylinder are to be tapped} or

threaded for studs, a "tap drill," which is smaller in diameter

than the bolt _body, is used

and

the drill is _guided

_by

a

remov-able _bushing b of _{the proper} size. _{Jigs of} this _type are often

held _{in position}

_by

_inserting

an

_{accurately fitting plug through}

the _jig

and

into the first hole drilled, which prevents the jig

from _turning with relation to the _cylinder,

when

_drilling the other holes.

When

the _jig is used for _{drilling the head, the}

opposite side is _placed

next to the work, as

shown

at C. This side

has a circular recess or

counterbore, which fits

the _projection on the

head to _properly locate

the _jig.

As

the holes in

thehead

must

be_slightly

larger in diameter than

the _studs, another sized

drill

and

_{a guide bushing}

of _{corresponding} size are

used.

The

_cylinder is, of

course, bored

and

the

head turned before the _drilling is done.

Jigs of the open class, as well as those of other types, are

made

in _{a great variety of shapes, and,}

when

in _{use, they} are

either _applied to the

work

or the latter is _placed in the jig.

When

the

work

is _quite

large,the jig is frequently placed on it,

whereas _{small parts are}

more

often held in the _jig, which is so

designed that the

work

can be clamped in the proper position.

The

form of

_any

_{jig depends,} to _{a great} extent, onthe shape of

the

work

for which it is intended

and

also on the location of

the holes to be drilled.

As

the

number

of _{differently shaped}

pieceswhich go to

make

up

even a single machine is _{often very}

JIG 15

great,

and

as

most

parts require

more

or less _{drilling, jigs} are

made

inanalmost endless _variety of sizes

and

forms.

When

all

the holes to be drilled in a certain _part are _parallel,

and

es-pecially if _they are allin the

same

plane, a very simple form of

jig can ordinarily be used.

Box

Jigs.

A

great

many

machine parts

must

be drilled on

different sides

and

_{frequently castings} or _forgings are _very

irregular in shape, so that a jig which is

made

somewhat

in

J _L

Fig.3. BoxJigfor DrillingBallshownenlargedatA

the form of a box,

and

encloses _{the work,} is _{very essential,} as

it _{enables the guide bushings} to be _placed on all sides

and

also

makes

it _{comparatively easy} to locate

and

_{securely clamp} the part in the proper position fordrilling. This type of jig, which, because of its _form, is

known

as a closed or

"box

jig," is used

very extensively.

A

box _jig of _{simple design}is

shown

in _Fig. 2. This

particu-lar jig is used for _drilling four smallholes in _{a part (not shown)} which is located with reference to _{the guide bushings}

B

_by

a

central _pin

A

attached to the _{jig body.} This pin enters a hole

JIG

with _{a previous operation.} After the

work

is inserted in the

jig, it is clamped

by

closing the cover C, whichis _hinged at one end

and

has a _{cam-shaped clamping} latch

D

at the other, that

engages a pin

E

in the jig body.

The

four holes are drilled

_by

passing the drill through the guide bushings

B

in the cover. Another _jig of the

same

_kind, but _designed for _drilling a hole _having

two

diameters _through the center of a steel _ball,

Fig. 4. BoxJigsfor DrillingPartsshown by HeavyDot-and-dash Lines is

shown

_{in Fig. 3.}

The

_work, which is

shown

_enlarged at _A,

is inserted while the cover is thrown back as indicated

_by

the

dotted lines.

The

cover is then closed

and

_tightened

_by

the cam-latch Z),

and

the large part of the hole is drilled with

the _jig in the _position shown.

The

_jigis then turned over

and

a smaller drill of the correct size is fed _{through guide bushing}

B

on _{the opposite} side.

The

_depth of the large hole could be gaged for each _{ball drilled,}

_by

_{feeding the} drill _spindle

down

to

JIG

if the _spindle has an _{adjustable stop,} this should be used.

The

work

is located in line with the

two

_{guide bushings}

_by

_spherical

seats formedin the_jig

_{body and}

inthe _{upperbushing,}asshown.

As

the

work

can beinserted

and removed

quickly, a large

num-ber of _{balls, which, practically speaking,} are _duplicates, can

be drilled in a _{comparatively} short time

_by

_{using a jig} of this

type.

A

box jig that differs

somewhat

in construction from the

design just referred to isillustrated at

A

_{in Fig. 4,} which shows

Fig. 5. JigshownatA,Fig. 4,inTwoDifferentDrillingPositions

a side

and

top view.

The

work, in this case, is a small _casting

the form of which is indicated

_by

the

_heavy

dot-and-dash lines.

This _casting is drilled at a, b,

and

c,

and

the

two

larger holes a

and

b are finished

_by

reaming.

The

hinged cover of this jig

is _{opened for inserting} the

work

_by

unscrewing the T-shaped clamping screw s _one-quarter of a turn, which brings the head

inlinewith aslotinthecover.

The

castingis_clamped

_by

tighten-ing this screw, which forces an _{adjustable screw} bushingg

down

against the work.

By

having this bushing adjustable, it can

cast-JIG

ings should vary, the position of the clamping bushing could

easily be changed.

The

work

is _{properly located}

_by

the inner ends of the three

guide bushings ai, bi,

and

ci,

and

also

_by

the _locating screws I

against which the casting is held

_by

knurled thumb-screws

m

and

n.

When

the holes a

and

b _{are being drilled,} the jig is

placed with the cover side down, as

shown

at

A

_{in Fig. 5,}

and

the drill is_guided

_by

removable_bushings, one ofwhichis

shown

at r.

When

the _drilling is _completed, the drill _bushings are

replaced

by

reamer bushings

and

each hole is finished

_by

ream-ing.

The

small hole c, Fig. 4, is drilled in the end of the

cast-ing

by

simply placing the jig on end as

shown

at B, Fig. 5.

Box

jigs which have to be placed in

more

than one _position

for _drilling the different holes are _{usually provided} with feet

or extensions, as shown, which are _accurately finished _{to align}

the guide bushings properly with the drill. These feet extend

beyond any

clamping screws, bolts, or bushings which

may

protrude from thesides of the jigs,

and

provide asolid _support.

When

inserting

work

in a jig, care should be taken to

remove

all _{chips which might have} fallen

_upon

those surfaces _against

which the

work

is _clamped

and

which determine its location.

Still another _jig of the box type, which is _quite similar to the one

shown

at A, Fig. 4, but is _arranged

differently, owing to the _shape of the

work and

location of the holes, is

shown

at

B

in the

same

illustration.

The

work

has three holes in

the base h,

and

a hole at i which is at an angle of 5 degrees

with the base.

The

three holes are drilled with the jig

stand-ing on the opposite end y,

and

the angular hole is drilled while

the jig rests onthe fourfeet k, the ends ofwhich are at such an

anglewiththe jig

body

thatthe guide bushingforholei is

prop-erly aligned with the drill.

The

_casting is located in this _jig

by

the inner ends of the

two

_{guide bushings}

w

and

_{the bushing}

o

and

also

_{by two}

_locating screws

_{p and}

a side locatingscrew q.

Adjustable screws t

and

t\ in the cover hold the casting down,

and

it is held _laterally

_by

the

two

knurled thumb-screws

u

and

v. If

an

_{attempt were}

made

to drill this _{particular part}

JIG 19

needed) it

would

havetobe setwithconsiderable _{care, provided}

the angle between hole i

and

those in the base

had

to be at

all _accurate,

and

it

would

be rather difficult to drill a

number

of these _castings

and

have

them

all _duplicates.

_By

the use of a jig, however, designed for

drilling this particular casting,

the _{relative positions of} the holes in

_{any number}

of _{parts are}

practically the

same

and

the

work

can be done

much

more

quickly than

would

be possible if it were held to the _drill-press

table

_by

_{ordinaryclamping appliances. Various designs of jigs}

will be described in _Chapter VII.

Details _{of Jig Design.}

The

_{general principles of the design}

and

use of _jigs have been _explained.

The

details of _{jig design}

will

now

be considered. _{Generally speaking,} the

most

im-portant parts of a _jig are _{the guide bushings} for the drills

and

other tools, the clamping devices,

and

the locating points,

against which the

work

is _placed to insure an accurate

posi-tion in the jig.

The

guides for the _{cutting tools in} a drill _jig

take the form of concentric steel _bushings, which are _placed in

the _jig

_body

in _{proper positions.}

The

drill _{bushings are generally}

made

of tool _steel, hardened

and

_lapped, and, where convenient, should be ground inside

and

out.

_They

should also be _{long enough} to _support the

drill on each _{side regardless of} the _fluting,

and

_{they should be}

so located that the lower end of _{the bushings} will _stop about

the

same

distance above the

work

as the diameter of the drill,

so that _chips will clear _{the bushings readily.}

Where

holes are

drilled

on

the side of a convex or a concave surface, the end of

the _bushing

must

be cut on a bevel

and

come

closer to _{the part}

being drilled, to insure the drill having adequate support while

starting into the work.

The

bushings should have heads of

sufficient diameter.

_Long

_{bushings should} be relieved

_by

in-creasing the hole diameter at the upper end.

The

lower end

of the _bushingshouldhaveits_{edges rounded,} in orderto _permit

some

of the _{chips being shed} from the drill _easily, instead of

all of

them

being forced

up

through the bushing. It is also

good practice to cut a groove under the head for clearance for

com-JIG

plete treatise covering dimensions

and

design is _given in the

chapter on "Jig Bushings."

In order to hold the

work

_{rigidly in} the _jig, so that it

_may

be held _against the _{locating points} while the _cutting tools

operate

upon

the _{work, jigs}

and

fixtures are provided with

clamping devices.

Sometimes

a _clamping device serves the

purpose of _{holding the jig} to the _work, in a case where the

work

is a very _{large piece}

and

the _jig is attached to the

work

in

some

suitable _way.

The

_purpose of the _clamping device,

however, remains the same, namely, that of _preventing

_any

shifting of the guiding bushings while the operation on the

work

is _performed.

The

_clamping device should _{always be an}

integral part of the jig

body

in orderto prevent its _gettinglost.

Different _types of _clamping devices are

shown and

described

in _{the chapter} on "Jig

Clamping

Devices.

"

The

_{locating points}

_may

consist of _{screws, pins,} finished

pads, bosses, ends of bushings, seats, or lugs cast solid with

the jig body, etc.

The

various _types used are described in

detail in the _chapter

on

_"Locating Points

and

_Adjustable

CHAPTER

II

DESIGN OF

OPEN

DRILL

_JIGS

To

_give

_any

rational rules or methods for _{the design} of drill

jigs

would

be almost impossible, as _{almost every jig}

must

be

designed in a

somewhat

different

_way

from _{every other jig,} to

suit

and

conform to_{the requirements} of thework. All that can

be done is _{to lay}

down

the _principles.

The

main

_{principles for}

jigs as well as fixtures were treated at length in Chapter I.

It is

proposed in the present chapter to dwell

more

in detail on

the carrying out of the actual

work

of _{designingjigs.}

Jig Drawings. Before

_{making any}

_attempt to _put the lay-out of the _jig on _{paper, the designer} should _{carefully consider}

what

the jig will be required to do, the limits of _{accuracy, etc.,}

and

to _form, in his _{imagination, a} certain idea of the kind of a

jig that

would

be suitable for the purpose. In doing so, if a

model

or _sample of the

work

to be

made

is at _hand, it will be foundtobe_{a great help}to_studytheactualmodel. If the

draw-ing, as is

most

often the case, is _{the only thing} thatis at _hand,

then the outline of the

work

should be

drawn

in red _(or other

colored) ink

on

the drawing paper, on which the jig is

subse-quently tobe laid out,

and

the jig built up, so to speak, around

this outline.

The

_designing of the _jig will be _{greatly simplified}

by

doing this, as the relationbetween the

work and

the jig will

always be plainly before the designer, and it will be

more

_easily

decided where the locating points

and

clamping arrangements

may

be _properly placed.

When

drawing

and

projecting the

differentviewsofthe_jigonthe paper, the redoutlineofthe

work

willnotin

_any

_way

_interfere,

and when

the_jig is

made

from the

drawing, the red lines are simply ignored, except to the extent

to which the outline of the pieces

may

help the toolmaker to

understandthe_drawing

and

the_purposeof_{certain locating points}

and

_clamping devices.