Jigs and
Fixtures
By S K
Introduction
Jigs and fixtures are devices that are used for
production of repeated parts essentially for
mass production.
Functionality of Jigs or fixtures:
Locating
Clamping
Supporting
Resistance to cutting forces
Safety
Jig
Both jigs and fixtures hold, support, and locate
the work piece.
A jig also guides the cutting tool.
Fixtures
Both
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.
GATE - 1999
Choose 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.
Purpose of using Fixtures and Jigs
Eliminate
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
requirement
Enhancing technological capacity of the
machine tools
Design considerations
for Jigs and Fixtures
Jigs and fixtures are manually or partially power
operated devices. comprised of several
elements :
Base and body or frame with clamping
features
Locating elements for proper positioning and
orientation of the blank
Supporting surfaces and base
Clamping elements
Tool guiding frame and bushes (for jig)
Auxiliary elements
Fig. Major elements of jig and
fixtures
3-2-1 Locating Principle
A 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.
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.
Fig. A
component
with six
locators
IES - 2007
According 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
Considering 12 DOF
You 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-YandACROTYdegrees of freedom.
Now, rest the work piece attwopoints of side surface
(XZ), and you will be able to fix the+YandACROTZdegrees 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.
Points 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.
GATE - 2005
When 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
GATE - 2001
3-2-1 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
GATE-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
IES 2011
In 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
Duplex Fixture
It 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
complete
machiningis is
startedat station one, the fixture
is
and the
atrevolved
station two.
During
this
period,
the
machined
part
is
unloaded from station
one and a fresh part is
loaded there, and so
Different methods used for Location
Flat Locator : Used for location of flat
machined surfaces of the component.
Cylindrical
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.
Conical 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.
Jack Pin Locator : Used for supporting rough
workpieces.
suitable
method
to
accommodate
the
components which are rough and un-machined.
Drill 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.
Vee Locators: Quick and effective method of
locating the workpiece.
Used for locating the circular and semi-circular
type of workpieces.
Diamond Pin Locator
Diamond 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.
IES 1998, 1999
Diamond 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 manufacture
IES - 2009
A 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-blocks
Setting Blocks
After 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.
Ejectors
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.
Clamping
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.
Strap Clamps
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 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).
Screw Clamps
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.
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.
Cam Clamps
Provide 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
part
The 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 clamp
Toggle Clamps
A 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 pushpull type
toggle clamp
Equalizers
When 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 clamp
IES - 1996
Assertion (A): A workpiece with rough unmachined 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 true
IES - 1996
Consider the following statements:
The cutter setting block in a milling fixture
1. Sets the cutting tool with respect of two of its
surfaces.
2. Limits the total travel required by the cutter
during machining.
3. 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
Jigs and Fixtures
Q. No
Option
Q. No
Option
