Location and locating devices used in jigs and fixtures

Location
&
Locating Devices
Prof. Ms. Amruta A. Rane
Assistant Professor,
DJSCE, University of Mumbai,
Mumbai.

Location System
• The location refers to the establishment of a desired relationship between the
workpiece and the jigs or fixture.
• Correctness of location directly influences the accuracy of the finished product.
• The location system in conjunction with the clamping system should completely
constrain the workpiece or eliminate as many of the six degrees of freedom as in
necessary for operation to be completed with required accuracy.

The Six Degrees of Freedom
• Figure shows a body that is free in space.
• A body in this condition had six degrees of freedom, three of these are freedoms
of translation and three are freedoms of rotation.

The Choice of Location System
• The requirements of the location system depend upon workpiece before
operation and the operation being performed.

The Choice of Location System
• When there is choice of location points the most effective location system must
be selected.

Location Principles
• Workpiece Requisites: Dimensional requirements of the workpiece stated on
component drawing.
• Face A should be used as datum, so that hole will always be at distance D from
face A irrespective of the variation in length L.
• Location should be w.r.t. face A.
Location and Workpiece Dimensions

Location Principles
• Accuracy: Location should be done on most accurate surface of the workpiece.
• A machined surface is preferable to an unmachined one.
• When more than one machined surfaces are available, locate on the most
accurate surface.
• The φ80 has tolerance of 0.05 and φ40 has tolerance of 0.1 so the workpiece can
be located most accurately from outside diameter φ110.
• Location from φ40 bore would be less accurate than location from φ80.
Accuracy of Location surfaces

Location Principles
• Constraints: Location should prevent linear and rotary motion of the workpiece
along and around the three major axes X, Y and Z.
Six Degrees of Freedom

Location Principles
• Motion economy: Location should facilitate easy and quick loading of the
workpiece in the fixture.
• Parallel locators are preferable to those placed at right angles.
• If we drill hole B first and use it for location while drilling hole C, it is possible to
load the workpiece on both the locators in hole A and B in one motion as both the
locators would be parallel.
Motion Economy in Location

Location Principles
• Redundant Location: A redundant location is said to exist when two locators are
attempting to constrain one freedom from two location points.
• Redundant locators must be avoided.
• The distance between surface A and B would vary according to the process
capability.
• We can only locate effectively on surface A or B.
Redundant Location

Location Principles: Redundant Location
Redundant Location
• We can only location workpiece effectively on one machined surface.
• Inaccurate location can result in bending, dimensional error, flatness error,
distortion.
• We can only locate effectively on surface A or B.
• The clamping force would distort the workpiece and distorted part would spring
back to its original position when the workpiece is unclamped.
• Redundant location will also occur if the workpiece is located from two concentric
cylinders or between two fixed V-locators.
• Redundant location can be prevented by providing an adjustable support.
Workpiece Distortion due to Redundant Location
Adjustable Support

Location Principles: Redundant Location
• The workpiece is located over two pins.
• The purpose of pin 2 is to prevent rotation about pin 1 but the system is such that
both the pins are attempting to constrain the workpiece along X-X.
Redundant Location

Location Principles: Foolproofing
• Foolproofing: The location system should positively prevent wrong loading of the
workpiece by foolproofing.
• It would be impossible to load the workpiece wrongly due to the foolproofing pin.
Foolproofing

Locating Methods
• Locators are made separate from the fixture or jig body.
• Made from casehardened steel accurately ground to size.
• A workpiece can be located from:
i. Plane surface
ii. Profile
iii.Cylindrical surface

Location from Plane surface
There are three most common types of locating points
• Buttons
• Pins
• Pads

Location from Plane surface: Buttons
• Buttons are round and have either a flat head or
a crowned head.
• They are made of steel.
• Usually alloy steel, low grade tool steel
hardened to 40-45 RC, low carbon steel (AISI
1113) hardened to 53-57 RC.
• Ground after heat treatment to have precise
dimensions
• Flat buttons are used against machined surfaces
only.
• When the plane is defined by three buttons, they
are surface ground across their faces after
installation.
• Crowned buttons are used for unmachined
surfaces. However they do not provide a well-
defined bearing area.

Buttons are termed as –
• Rest buttons
• Stop buttons

Location from Plane surface: Button Positioning
• Installation of the button in the fixture body is
done with press fit in a cylindrical bore
(reamed or precision bored).
• The shank ends with 30o chamfer.
• Spot facing is done around the hole to provide
the seating for the head of the buttons
• Hole is chamfered to ensure good seating.
• Undercut is provided under the head.

• The buttons have interference fit / press fit with
the hole in the fixture.
• The shanks on the buttons are supplied with
standardised tolerances, resulting in oversize
ranging from 0.03 to 0.04mm.
• Rest and stop buttons are commercially available
in standardised dimensions.

• For the flat buttons, H can be
selected from 1/3 D to 4/3 D (5 to
25mm).
• L = ½ (D + H)
• B = ¾ (D – 3)
• For the crowned buttons, H can be
selected from1/3 D to D.
• R = 3/2 D
• B = ¾ D
• L = ¾ D

Location from Plane surface: Threaded Buttons
• A threaded shank buttons are also used.
• They are installed in a tapped hole.
• This practice is not recommended as screw thread requires clearance and is less
accurate with respect to location and direction.

Location from Plane surface: Hollow Buttons
• Hollow buttons are fastened by separate screws.
• The screw head is countersunk safely below the face of the button.

Location from Plane surface: Hollow Buttons

Location from Plane surface: Pins
• A pin is a cylindrical component that is contacted on its side.
• Height of the pin is not a critical dimension.
• Buttons can be substituted for pins, but pins can not be substituted for buttons.
• Installed by press fit.
• With or without a shank of a reduced diameter.
• Used for a nest.
• They are used as side stops and for locating in holes.
• Pins can be used on unmachined surfaces.
• Pins as side stop should be used only on shallow parts with light side loads.

Location from Plane surface: Pins

Location from Plane surface: Pads
• Pads are usually flat components made from steel and heat treated to similar
hardness levels.
• They are ground flat and parallel.
• They are used as base locators in cases where rest buttons do not provide
sufficient bearing area.
• Pads are placed at locations not easily accessible by the operator. Hence, the
edges and corners of a pad are usually not rounded or chamfered as the edges on
rest buttons. They are lightly polishes to make them smooth to touch.
• Pads are fastened by means of screws with countersunk heads.

Location from Plane surface: Pads
• Dowel pins are used to secure the position of the pads. Since screws are not
capable of precision location.

• A machined surface can be better located by pads having a flat surface.
• For large components the pads can be screwed to the body of the fixture as
shown in figure.
• Due to use of locating pads only seats for the pads need to be machined instead
of entire body of the large fixture which saves machining time.
Location Pads for Large Fixtures

Location from Cylinder: Dowel Pins
• Dowels are used for permanent assembly
of two parts with significant precision.
• Two dowel pins are required for locating a
component and they are placed as far as
possible.
Standard Dowel Pins

• Dowel pin holes are drilled through so
that the pin can be taken out.
• The recommended bearing length of a
dowel pin in each part is 1 ½ to 2 times
the diameter of the pin.
• Dowel pins are cylindrical or tapered.
• The straight type is available in
unhardened and hardened form.
• The fit of the dowel pin can be a press fit
in each part.
• Tapered pins are easily taken out by the
application of light pressure or a blow on
the small end.
• Dowel pins are extensively used in the
construction of built-up fixture bodies.
Standard Dowel Pins

Location from Cylinder
• It is necessary to use two dowel pins to ensure that the workpiece is completely
constrained.
• If we use only one dowel pin P, plate B can pivot around P.
• The dowels P and Q should be placed as far a possible.
Use of Dowels

• A plane surface can be located from 3 points
on the plane surface.
• A rough, unmachined surface can be located
with three location pads having point contact.
• Three location pins having spherical surfaces
at locating points can be used.
• The pins should be placed as widely as
possible for more accurate location.
• The height is adjusted to make the workpiece
surface parallel to the baseplate.
• An adjustable support with locking nut can be
used.
Adjustable Support for Rectangular Workpieces

• 3 pins can define the plane but they can’t provide adequate support to the
workpiece during machining operation.
• Additional adjustable supports are necessary to prevent distortion and vibrations
in workpiece during clamping and machining.
• The number of additional supports would depend on the shape, size and strength
of the workpiece.
• The bolt with hex nut
Adjustable Support

Adjustable Support

• For locating very rough, uneven surfaces adjustable locating pads are used.
• Castings and forgings are located by adjustable screw pads.
Adjustable Locators

• Sometimes adjustable support is provided at inaccessible place since it is difficult
to reach a support which is in recess or is distant from an operator.
• inclined surface on wedge pin raises the pad when the wedge pin is pushed
forward by height adjusting screw.
• When the adjusting screw is withdrawn the return spring pushes the wedge pin
towards the right and the pad slides down by gravity.
• Pad is locked in position by clamping screw.
• The retaining screw prevents the pad from being pushed out of the housing by the
spring.

• Square and rectangular workpieces can be located by replacing one of the
locating pads by an equalising rocker.
• The rocker provides support at two points “R”. It provides itself to suit the surface
to be located.
• It provides contact at four points without contradicting the three-point location of
the plane.
• The pivot pin of the rocker acts as a single point complementing the pother two
points “F” in defining the plane.
Equalising Rocker Location

• For small workpieces location pads are not necessary.
• The fixture body itself is machined suitably to provide locating surface.

• Ample recess is provided in the corners so that burr on the workpiece corners,
dirt or swarf do not obstruct proper location through positive contact of the
workpiece with locating surface.

Location from Profile
• A sighting plate is provided for simple components where appearance is
important.
• It is slightly bigger than the workpiece.
• The workpiece is placed on the plate in such a way that there is equal margin on
all the sides.
Sighting location

• The profile of a workpiece can also be located by confining the profile with
cylindrical locating pins.
Profile location by pins

• The profile of the workpiece can be located
by providing pocket or nest around the
profile of the workpiece.
• The inside profile of the nest matches with
the outside of the workpiece.
• The height of the nest should be lesser than
the workpiece to ease unloading.
• A partial nest can be used.
Location nests

• It is most common and convenient form of
location.
• When the cylinder is located on its axis and
base, it can only rotate about its axis. All
other motions are constrained.
• Clamping from top prevents linear motion
along Y-Y axis.
Cylindrical locator

• The seating surface of the locator should be
recessed to provide space for dirt or
workpiece burr.
• There should be ample chamfer or radius at
the entry point so that the components can
be loaded quickly.
• The chamfer is called “lead”.
• The locator itself is located in fixture by a
concentric diameter generally made press fit
in the fixture body.
• The cylinder is best location shape because a
cylindrical locator is least difficult to
produce and eliminates five of the six
degrees of freedom.
Cylindrical locator

Circular locators types –
• Inside locators
• Outside locators
Problems with cylindrical locators -
• Jamming
• Clearance

Location from Cylinder: Jamming
• Jamming is result of a friction.
• The amount of clearance, the length of the engagement and steadiness of the
hand of the operator are reason for jamming.
• Jamming occurs when part has entered short distance into the inside locator or
around an inside locator.
• A workpiece is likely to be tilted to the axis of the location post during quick
loading.

• The part has entered the locator over a short length “L”, the length of
engagement.
• If the part is slightly tilted, then one side of the leading edge comes in contact
with the inside of the locator and is caught up by the friction resulting in jamming.
• The locator has dia. W.
• The workpiece has dia. (W-C), where “C” is clearance.

• The length of the locator should be small to prevent jamming of the workpiece.
• A long lead aligns the axis of the workpiece with locating post.
• If the locator is long and lead is short, the workpiece would get jammed on the
post.
• Long location posts are used for fragile workpieces since locator provides support
to the workpieces.
• Long locators are relieved i.e. made undersize in central portion.
Prevention of jamming

• For the length of engagement L, there exist two critical values L1 and L2, which can
be calculated.
• The distance between L1 and L2 is where jamming possible and likely to occur.
• This area can be completely eliminated by providing a relief-groove on the locator
over a length of at least from L1 to L2.
• L1 = 0.02 D
• L2 = 0.12 D
• L3 = 1.7 (D)1/2
• d = 0.97 D

Location Post

Locating post
• They are used for anchoring clamping studs.
• Location post is secured to the base otherwise it will be pulled out by clamping
force.
• The post is secured by retainer nut or a grub screw.
• C washer is used.
Locators subjected to
axial pull

• The jamming of the workpiece on the locating post can also be prevented by
providing special lead at the entry point.
Jamming prevention lead

• Location pot is used for locating a cylinder on outside diameter.
• They have lead at entry point and central portion is relieved in case of long pots.

• When we use two holes in workpiece for location, we must take into account variation in
centre distance of the two holes.
• The variation is taken care of by making one of the two location pins diamond shaped.
• Out of the two holes, the accurate one should be used for principal cylindrical location
with full pin.
• The diamond shaped pin is used to constrain pivoting of the workpiece around the
principal locator.
• The locating surface of a diamond pin is usually less than 8% of a full cylindrical pin.
Diamond Pin Application

• A location pin is positioned as far away as possible from the principal locator in
order to minimise the angular error.

Conical Location
• Used to locate rough machined surfaces of castings and forgings.
• They locate the workpiece from hole or shaft.
• Similar to location posts and pots.
• Centralization
• 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.
Conical Locators

Vee (V) Location:
• Used to locate cylindrical surfaces from
outside.
• 2 “V” locators are used for accurate
location.
• A “V” locator can be adjusted quickly by
using a cam for adjustment.
• Return spring is provided to bring back
the V locator to its original position.
• Fixed V blocks are used for
approximate location.
• Fixed V Blocks are attached to fixture
body by screws and dowelled to
prevent shifting during operation.
Fixed V locator

• Adjustable V Block – for more accurate location, to take care of size of workpiece.
• Adjustable along the axis of “V”.
• Guide plate is provided to guide and to constrain movement along the axis of V.
• The side of V face is sometimes inclined slightly to provide downward clamping
force.
Screw adjusted V locator

Inclined V:
• When a sliding V locator is used, a small downward clamping force can be
introduced by inclining the sides of the V.
Inclined V

• A swinging eyebolt is used when a V plate has to be withdrawn quickly.
Quick action V locator

• When a cylindrical workpiece is located by a V block, the centre would always lie
on the centre line of V.
• For drilling vertical holes in round bars, the V block should be placed in such a way
that its centre line is vertical.
Suitable position of V for drilling vertical hole

• If V block axis is horizontal, the variation in the diameter of the workpiece would
lead to errors.
• Hole would be eccentric or offset in undersize and oversize bar.
Unsuitable position of V for drilling vertical hole

Summary
1. Location must be related to the dimensional requirements stated on the
component / workpiece drawing.
2. It is preferable to use a more accurately machined surface than a less accurate
surface for location.
3. The workpiece should be prevented from moving along and rotating around the
X, Y and Z axes.
4. Location system should facilitate easy and quick loading and unloading of the
workpiece and aim at motion economy.
5. Redundant locators must be avoided.
6. Location system should positively prevent wrong loading of the workpiece by
foolproofing.

Summary
Locating Methods: Plane surfaces
1. A reasonably flat surface can be located by three pins of equal height having
spherical surfaces at the location points.
2. A rough, uneven or tapered plane surface should be located by adjustable
location pins having spherical ends.
3. Additional adjustable supports are necessary to prevents vibrations or distortion
of the workpiece machining operation.
4. A machined surface can be located by pads having flat surface.
5. There should be ample clearance for burr or dirt to ensure proper seating of the
workpiece surfaces.
6. A cube can be prevented from linear movement and rotation around axes X, Y
and Z by six location pads.

Summary
Locating Methods: Profile
1. A profile can be located approximately by aligning it with a slightly bigger sighting
plate.
2. Locating pins can also be used to locate a profile or cylindrical workpieces.
3. Variations in workpiece sizes from batch to batch can be taken care of by using
eccentric locators whose eccentricity can be set to suit the batch.
4. Workpiece with little variation can be located precisely with nesting plates with
suitable provision for unloading or ejection.

Summary
Locating Methods: Cylinder
1. Spigots used for locating bores should have ample lead for easy entry and their
length should be short to prevent jamming of the workpiece.
2. Long locators for fragile workpieces should be relieved at the centre.
3. Location posts which are also use for clamping should be retained a nut or a grub
screw.
4. When two location pins are used, less important one should be made diamond-
shaped. The main cylindrical pin should be longer than the diamond pin in order
to facilitate easy loading of the workpiece.
5. Rough cored holes and bosses are located on conical locators which often have
integral clamping arrangement and drill bush.
6. Fixed V blocks are used to locate approximately the outside surface of the
cylinder.
7. For precise location, adjustable guided V block is necessary. The V block can be
adjusted by a screw or a cam.
8. V blocks should be positioned in such a way that the variation in the workpiece
would not affect the location for the operation.

References
1. Jig and Fixture Design Manual, Erik K. Henrikson, Industrail Press.
2. Jigs and Fixture, P.H. Joshi, THM.
3. An introduction to jig and tool Design, M.H.A. – Kempster, III Ed.Pub ELBS.

Location and locating devices used in jigs and fixtures

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