Jigs and fixture
Jigs and FixturesBy S K Mondal1IntroductionJigs and fixtures are
devices that are used for production of repeated parts essentially
for mass production.Functionality of Jigs or
fixtures:LocatingClampingSupportingResistance to cutting
forcesSafetyJigs and fixtures are the production devices that are
used for the accurate production of repeated parts essentially for
mass production. The required accuracy is achieved by maintaining
the precise relationship between the various surfaces of the
fixture and the part to be manufactured. A jig or fixture needs to
provide the following functionality to be an effective production
device:LocationClampingSupportResistance to cutting
forcesSafety
2JigBoth jigs and fixtures hold, support, and locate the work
piece.A jig also guides the cutting tool.
3FixturesBoth jigs and fixtures hold, support, and locate the
work piece.A fixture has a reference point for setting the cutting
tool with reference to the work piece.
4GATE - 1999Choose the correct statement:(a)A fixture is used to
guide the tool as well as to locate and clamp the workpiece(b)A Jig
is used to guide the tool as well as to locate and clamp the
workpiece(c)Jigs arc used on CNC machines to locate and clamp the
workpiece and also to guide the tool(d)No arrangement to guide the
tool is provided in a jig.Ans. (b)5Purpose of using Fixtures and
JigsEliminate marking, punching, positioning, alignments etc.Easy,
quick and accurate locating, supporting and clamping the
blank.Guide the cutting tool like drill, reamer etc.Increase
productivity and maintain product quality.Reduce operators labour
and skill requirementEnhancing technological capacity of the
machine toolsReduce overall machining cost and increase
interchangeability.The basic purposes of developing and using
suitable jigs and fixtures for batch production in machine shops
are :to eliminate marking, punching, positioning, alignments
etc.easy, quick and consistently accurate locating, supporting and
clamping the blank in alignment of the cutting toolguidance to the
cutting tool like drill, reamer etc.increase in productivity and
maintain product quality consistentlyto reduce operators labour and
skill requirementto reduce measurement and its costenhancing
technological capacity of the machine toolsreduction of overall
machining cost and also increase in interchangeability.
6Design considerations for Jigs and FixturesJigs and fixtures
are manually or partially power operated devices. comprised of
several elements :Base and body or frame with clamping
featuresLocating elements for proper positioning and orientation of
the blankSupporting surfaces and baseClamping elementsTool guiding
frame and bushes (for jig)Auxiliary elementsFastening parts7
Fig. Major elements of jig and fixtures83-2-1 Locating
PrincipleA workpiece, just like any free solid body, has six
degrees of freedom (some researchers have referred this to the
twelve degrees of freedom by considering the +/- movements in each
category)For locating it is necessary to arrest all these six
degrees of freedom to ensure the mechanical stability.A single
locator in Plane 1 would arrest the linear motion along the
X-axis.A second locator in the same plane would arrest the rotary
motion about the Z-axis. Another locator placed in the same plane
would arrest the rotary motion about the Y-axis.Any free body has
six degrees of freedom (3 linear and 3 rotary) for a simple cube.
Whenever , location is planned, it is necessary to plan the
arresting of all these six degrees of freedom to ensure the
mechanical stability of the component in the fixture.A single
locator in Plane 1 would arrest the linear motion along the X-axis
as shown in Fig. 14.7. A second locator in the same plane would
arrest the rotary motion about the Z-axis. Anotherlocator placed in
the same plane would arrest the rotary motion about the Y-axis.
9 Adding one more locator in Plane 1 would not serve any
purpose. So fourth locator is placed in Plane 2 which is
perpendicular to Plane 1. This would restrict the linear motion
along the Y-axis. The fifth locator is placed in the Plane 2 which
can arrest the rotational motion about the X-axis.The sixth locator
placed in Plane 2 would not serve any purpose.So, sixth locator is
placed in Plane 3 which is perpendicular both the planes 1 and 2.
This would arrest the linear motion along the Z-axis.Adding one
more locator in Plane 1 would not serve any purpose. Also, to
locate a plane, only three locators are required. The fourth
locator in any plane thus would be redundant and should not be
placed on any single plane.Hence, the fourth locator can be placed
in Plane 2 which is perpendicular to Plane 1. This would restrict
the linear motion along the Y-axis. The fifth locator can also be
placed in the Plane 2 which can arrest the rotational motion about
the Y-axis.The sixth locator placed in Plane 2 would not serve any
purpose.Hence, the sixth locator would have to be placed in Plane 3
which is perpendicular both the planes 1 and 2. This would arrest
the linear motion along the Z-axis.
10
Fig. A component with six locators11IES - 2007According to the
principle of location in jigs and fixtures, how many degrees of
freedom are to be eliminated to have a body fixed in
space?(a)3(b)4(c)5(d)6
Ans. (d) 3- 2- 1 fixing Principle we eliminate (3+2+1=6) six
degrees of freedom.12Considering 12 DOFYou must fix all the 12
degrees of freedom except the three transitional degrees of freedom
(-X, -Y and -Z) in order to locate the work piece in the fixture.
So, 9 degrees of freedom of the work piece need to be fixed.Rest
the work piece onthreenon-collinear points of the bottom surface
(XY), and you will be able to fix
the+Z,CROT-X,ACROT-X,CROT-YandACROT-Ydegrees of freedom.Now, rest
the work piece attwopoints of side surface (XZ), and you will be
able to fix the+YandACROT-Zdegrees of freedom.Now, rest the work
piece atonepoint of the adjacent surface (YZ), and you will be able
to fix the+XandCROT-Zdegrees of freedom.Any free body has six
degrees of freedom (3 linear and 3 rotary) for a simple cube.
Whenever , location is planned, it is necessary to plan the
arresting of all these six degrees of freedom to ensure the
mechanical stability of the component in the fixture.A single
locator in Plane 1 would arrest the linear motion along the X-axis
as shown in Fig. 14.7. A second locator in the same plane would
arrest the rotary motion about the Z-axis. Anotherlocator placed in
the same plane would arrest the rotary motion about the Y-axis.
1314Points to ponder When more than one locator is placed on a
surface (plane), they should be distributed as far apart as
possible on the surface.While selecting the surface for the largest
locators, consideration should be given to the largest area of the
workpiece.Any free body has six degrees of freedom (3 linear and 3
rotary) for a simple cube. Whenever , location is planned, it is
necessary to plan the arresting of all these six degrees of freedom
to ensure the mechanical stability of the component in the
fixture.A single locator in Plane 1 would arrest the linear motion
along the X-axis as shown in Fig. 14.7. A second locator in the
same plane would arrest the rotary motion about the Z-axis.
Anotherlocator placed in the same plane would arrest the rotary
motion about the Y-axis.
15GATE - 2005When 3-2-1 principle is used to support and locate
a three dimensional work-piece during machining, the number of
degrees of freedom that are restricted is(a)7(b)8(c)9(d)10
Ans. (c)3-2-1 principle is also known as six point location of a
three dimensional body. The bottom is supported against 3 point,
the rear face against 2 point and the side of the block rest
against single (1) point. So 3-2-1 principle.16GATE - 20013-2-1
method of location in a jig or fixture would collectively restrict
the workpiece in n degrees of freedom, where the value of n
is(a)6(b)8(c)9(d)12
Ans. (c)17GATE-2013 (PI)In the 3-2-1 principle of fixture
design, 3 refers to the number of(a) Clamps equired(b) Locators on
the primary datum face(c) Degrees of freedom of the workpiece(d)
Operations carried out on the primary datum face
Ans. (b)18IES 2011In the 3-2-1 principle of fixture 3 refers to
number of (a) Setups possible(b) Clamps required(c) Positions on
primary face(d) Locating positions
Ans. (d) Locations on primary face not positions on the primary
face.
19IES 1999Assertion (A): Spherical washers are used to locate
the job in the fixtures. Reason (R): 3-2-1 principle should be
adopted to locate the job.(a)Both A and R are individually true and
R is the correct explanation of A(b)Both A and R are individually
true but R is not the correct explanation of A (c)A is true but R
is false(d)A is false but R is trueAns. (d)20Duplex FixtureIt is a
type of multi-station fixtures used primarily for high speed, high
volume production runs where the machining cycle must be
continuous.It uses only two stations. Once the machining operation
is complete at station one, the fixture is revolved and the
machining is started at station two. During this period, the
machined part is unloaded from station one and a fresh part is
loaded there, and so onIFS-2011What are the functions of jig ? Draw
a jig to machine four holes in a plate. What are two reasons for
not having drill bushings actually touching the workpiece ? What is
a duplex fixture ? [10-marks]
Different methods used for LocationFlat Locator : Used for
location of flat machined surfaces of the component.
Flat Locator :Flat locators are used for location of flat
machined surfaces of the component. Three different examples which
can be served as a general principle of location are described here
for flat locators.
23Cylindrical Locators: Used for locating components having
drilled holes. The cylindrical component to be located is gripped
by a cylindrical locator fitted to the jigs body and inserted in
the drilled hole of the component.
Cylindrical Locators: It is used for locating components having
drilled holes. The cylindrical component to be located is gripped
by a cylindrical locator fitted to the jigs body and inserted in
the drilled hole of the component. The face of the jigs body around
the locator is undercut to provide space for swarf clearance.
24Conical Locator : Used for locating the workpieces having
cylindrical hole.
It is superior as it has a capacity to accommodate a slight
variation in the hole diameter of the component without affecting
the accuracy of location.
Conical Locator : This is used for locating the workpieces
having cylindrical hole in the workpiece. The workpiece is found
located by supporting it over the conical locator inserted into the
drilled hole of the workpiece. A conical locator is considered as
superior as it has a capacity to accommodate a slight variation in
the hole diameter of the component without affecting the accuracy
of location.
25Jack Pin Locator : Used for supporting rough workpieces.A
suitable method to accommodate the components which are rough and
un-machined.
Jack Pin Locator : Jack pin locator is used for supporting rough
workpieces from the button as shown in Figure. Height of the jack
pin is adjustable to accommodate the workpieces having variation in
their surface texture. So this is a suitable method to accommodate
the components which are rough and un-machined.
26Drill Bush Locator : Used for holding and locating cylindrical
workpieces. The bush has conical opening for locating purpose and
it is sometimes screwed on the jigs body for the adjustment of
height of the work.
Drill Bush Locator : It is used for holding and locating the
cylindrical workpieces. The bush has conical opening for locating
purpose and it is sometimes screwed on the jigs body for the
adjustment of height of the work.27Vee Locators: Quick and
effective method of locating the workpiece. Used for locating the
circular and semi-circular type of workpieces.
Vee Locators:This is quick and effective method of locating the
workpiece with desired level of accuracy. This is used for locating
the circular and semi-circular type of workpieces. The main part of
locating device is Vee shaped block which is normally fixed to the
jig. This locator can be of two types fixed Vee locator and
adjustable Vee locator. The fixed type locator is normally fixed on
the jig and adjustable locator can be moved axially to provide
proper grip of Vee band to the workpiece.
28ISRO-2010Cylindrical parts are held on planer by(a) V-blocks
and arrestors (b) Angle plates(c) V-block, T-bolts and clamps (d)
T-bolt and clamps
Ans. (a or c)??????????????????????? *****29Diamond Pin
LocatorDiamond pins are often used for radial location .One
cylindrical locator (Pin A) arrests five degrees of freedom, second
cylindrical locator at the position B will arrest the sixth degree
of freedom.If the two holes are identical in size then any pin can
be made the principal locator. However, if one of the holes is
larger then the principal locator will be placed in the larger
hole. The second locator is made slightly smaller than the hole and
relieved from both sides to take care of the variation in the X
direction. The cylindrical surfaces will locate the part in the Y
direction.Diamond pins are often used for radial location .One
cylindrical locator (Pin A) arrests five degrees of freedom, which
is termed as the principal locator. The second cylindrical locator
at the position B will arrest the sixth degree of freedom-The pin A
will be slightly longer than the other pin such that the part is
located on it and then rotated till it is engaged with the second
locator. If the two holes are identical in size then any pin can be
made the principal locator. However, if one of the holes is larger
then the principal locator will be placed in the larger hole. The
second locator is made slightly smaller than the hole and relieved
from both sides to take care of the variation in the X direction.
The cylindrical surfaces will locate the part in the Y
direction.
30
31IES 1998, 1999Diamond pin location is used in a fixture
because(a)It does not wear out (b)It takes care of any variation in
centre distance between two holes(c)It is easy to clamp the part on
diamond pins(d)It is easy to manufactureAns. (b)32IES - 2009A lever
having two precisely drilled holes, one smaller than the other, has
to be located in a fixture using hardened and ground plugs for
further machining in relation to the holes. Select the correct
method of locating the lever from the given alternatives.(a)Using
two hardened and ground plugs, the smaller one having flats
machined on each side(b)Using two hardened and ground plugs(c)Using
one hardened and ground plug and one V-block(d)Using two
V-blocksAns. (a)33IES - 1995If the diameter of the hole is subject
to considerable variation, then for locating in jigs and fixtures,
the pressure type of locator used is(a)Conical
locator(b)Cylindrical locator(c)Diamond pin locator(d)Vee
locator
Ans. (a)34Setting BlocksAfter the fixture has been securely
clamped to the machine table , the work piece which is correctly
located in the fixture , has to be set in correct relationship to
the cutters. This is achieved by the use of setting blocks and
feeler gauges. The setting blocks is fixed to the fixture. Feeler
gauges are placed between the cutter and reference planes on the
setting block so that the correct depth of the cut and correct
lateral setting is obtained.
IES - 2005Match List I (An Element of Jigs and Fixtures) with
List II (Associating System) and select the correct answer using
the code given below the Lists:List I List IIABush 1.Milling
fixtureB.Setting block 2.Turning fixtureC.Diamond pin 3.Radial
locationD.V-block 4.Cylindrical location5.Drill jigsCodes:ABC DAB C
D(a) 5 4 3 1 (b) 3 1 2 4(c) 5 1 3 4 (d) 3 4 2 1
Ans. (c)37IES-2011With the help of a neat sketch, explain the
working of the diamond pin locator.[5-marks]
EjectorsUsed to remove work from close-fitting locators, such as
full nests or ring nests. These devices speed up the unloading of
the part from the tool, which reduces the in-tool time and
increases the production rate.
Ejectors are used to remove work from close-fitting locators,
such as full nests or ring nests. These devices speed up the
unloading of the part from the tool, which reduces the in-tool time
and increases the production rate.
39IES - 2000Match List I (Components used in jigs and fixtures)
with List II (Their functions) and select the correct answer using
the codes given below the Lists:List IList IIA.Jack pin1.To guide
the drill bit during machiningB.V-locator2.For easy removal of the
work piece from the jig or fixture after the machining operation is
overC.Bushes3.To locate the circular or semicircular objects in a
jig or fixtureD.Ejectors4.To locate work piece whose dimensions
aresubject to
variationsCode:ABCDABCD(a)3412(b)4312(c)3421(d)4321Ans. (b)
40Clamping To restrain the workpiece completely a clamping device
is required.Holds the workpiece securely in a jig or fixture
against the forces applied over it during on operation. Device
should be incorporated into the fixture, proper clamp in a fixture
directly influence the accuracy and quality of the work done and
production cycle time. To restrain the workpiece completely a
clamping device is required in addition to locating device and jigs
and fixtures. A clamping device holds the workpiece securely in a
jig or fixture against the forces applied over it during on
operation. Clamping device should be incorporated into the fixture,
proper clamp in a fixture directly influence the accuracy and
quality of the work done and production cycle time. 41Strap
ClampsBased upon the lever principles to amplify the clamping force
required. By tightening the stud the clamping force is transferred
to the part.Heel pin is the fulcrum about which the lever acts
while the clamping force is applied at the stud by tightening the
screw.The actual amplification of the applied force depends upon
the distance between the stud and the heel pin (B), and that
between the stud and the part (A).Strap Clamps are the simplest
type of clamps used in jigs and fixtures. Most of these clamps are
based upon the lever principles to amplify the clamping force
required. By tightening the stud the clamping force is transferred
to the part.Heel pin is the fulcrum abut which the lever acts while
the clamping force is applied at the stud by tightening the
screw.The actual amplification of the applied force depends upon
the distance between the stud and the heel pin (B), and that
between the stud and the part (A).
42
Screw ClampsA much faster way of applying clamping is to make
use of either a swing washer or a cee-washer if the workpiece has a
bore for clamping. A swing washer can be used to clamp a part
having a hole. This helps in loading and unloading the part
quickly. The only condition is that the hole used for clapping
should be larger than the nut used for clamping.A cee-washer is
similar to a swing washer, which remains loose unlike a swing
washer. Other-wise, application is very similar.In the strap clamps
discussed earlier, screws are used to apply the clamping force.
However, these clamps require considerable time to fasten. A much
faster way of applying clamping is to make use of either a swing
washer or a cee-washer if the workpiece has a bore for clamping. A
swing washer can be used to clamp a part having a hole. In order to
release the part, the nut needs to be opened slightly so that the
swing washer becomes loose, at which time it can be swung to the
side thereby releasing the part. This helps in loading and
unloading the part quickly. The only condition is that the hole
used for clapping should be larger than the nut used for clamping.A
cee-washer as shown in Fig. is similar to a swing washer, which
remains loose unlike a swing washer. Other-wise, application is
very similar.
44
Cam ClampsProvide clamping force because of the contour of the
cam surface that comes into contact with the plate used for the
clamping.Plate is pushed down by the cam against the spring
pressure to hold the part in place.Cam clamps are quick in
operation. Cam clamps are of three types, eccentric cam, flat
spiral cam and cylindrical cam. The design shown in Fig. is flat
spiral and is the most commonly used clamp.Fig. A cam clamp used
for quick and easy clamping a partCam clamps provide clamping force
because of the contour of the cam surface that comes into contact
with the plate used for the clamping.Notice that a plate is pushed
down by the cam against the spring pressure to hold the part in
place.Cam clamps are quick in operation. Cam clamps are of three
types, eccentric cam, flat spiral cam and cylindrical cam. The
design shown in Fig. is flat spiral and is the most commonly used
clamp.
46The design shown is indirect pressure clamping where the
pressure is transmit to the part through the plate. This is more
stable and the vibrations during machining do not affect the a part
clamping.
Fig. An example of a fixture held by a cam clampToggle ClampsA
toggle clamp is a quick acting mechanical linkage where two of the
elements make up a toggle action. Toggle clamps are mainly used
because of their fast action for clamping and unclamping, their
ability to completely clear the work piece and the force Fixture
amplification possible for clamping.
Fig. A push-pull type toggle clampEqualizersWhen the clamping
force is to be applied at more than one location then an equalizing
clamp is useful. In this type of clamp the link arm system is being
used to apply an equally divided clamping force to a pair of clamps
acting on the same component. It is also possible to use this
system of clamping to clamp two parts. This is particularly useful
in a condition where the operator may be denied easy access to one
or other of the clamps.
Fig. An equalizing clampIES - 1995Match List I with List Ii and
select the correct answer using the codes given below the
lists:List I (Task)List II (Recommendation) A.Three components in a
straight 1.Clamp with a floating pad.line should worked in one
loadingB.Unloading of clamp element from jig is essential2.Quick
action nutC. Clamping of rough surfaces3.Cam clampD. Need for heavy
clamping force4.Equalising clamp5.Strap
clampCode:ABCDABCD(a)5234(b)4215(c)1423(d)4153
Ans. (d)51IES - 2005Which one of the following is the most
significant property to be considered in the selection of material
for the manufacture of locating pins and drill jig buses used in
jigs and fixtures?(a)Wear resistance(b)Elasticity(c)Shear
strength(d)Tensile strength
Ans. (a)52IES - 1996Assertion (A): A workpiece with rough
un-machined surface can be located in a jig or fixture on three
supporting points.Reason (R): Indexing is made accurate by
supporting on three points.(a)Both A and R are individually true
and R is the correct explanation of A(b)Both A and R are
individually true but R is not the correct explanation of A (c)A is
true but R is false(d)A is false but R is trueAns. (c)53IES -
1996Consider the following statements:The cutter setting block in a
milling fixtureSets the cutting tool with respect of two of its
surfaces.Limits the total travel required by the cutter during
machining.Takes location from the location scheme of the
component.(a)1,2 and 3 are correct (b)1 and 2 are correct (c)2 and
3 are correct(d)1 and 3 are correct
Ans. (d)54IES - 1993The floating position of the holding fixture
in a rotary transfer device is used to(a)Improve the accuracy of
location(b)Reduce the tendency to over index(c)Reduce the cycle
time(d)Improve upon the acceleration and deceleration
characteristics
Ans. (d)55GATE 2009 (PI)Match the following:
(a) P-1, Q-3, R-1, S-2(b) P-1, Q-2, R-3, S-4(c) P-1, Q-4, R-3,
S-2(d) P-4, Q-3, R-2, S-1DeviceFunctionP. Jig1. Helps to place the
workpiece in the same position cycle after cycleQ. Fixture2. Holds
the workpiece onlyR. Clamp3. Holds and positions the workpieceS.
Locator4. Holds and positions the workpiece and guides the cutting
tool during a machining operationAns. (d)56
Jigs and FixturesQ. NoOptionQ. NoOption1C6C2D7D3B8D4B9C5C
