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 completelyconstrain the workpiece or eliminate as many of the six degrees of freedom as innecessary 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 freedomsof 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 fromface 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 mostaccurate surface.

• The φ80 has tolerance of 0.05 and φ40 has tolerance of 0.1 so the workpiece canbe 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 toload the workpiece on both the locators in hole A and B in one motion as both thelocators 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 processcapability.

• We can only locate effectively on surface A or B.

Redundant Location

Location Principles: Redundant LocationRedundant 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 springback to its original position when the workpiece is unclamped.

• Redundant location will also occur if the workpiece is located from two concentriccylinders or between two fixed V-locators.

• Redundant location can be prevented by providing an adjustable support.

Workpiece Distortion due to Redundant LocationAdjustable 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 thatboth 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

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

Location from Plane surfaceThere 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 steelhardened to 40-45 RC, low carbon steel (AISI1113) hardened to 53-57 RC.

• Ground after heat treatment to have precisedimensions

• Flat buttons are used against machined surfacesonly.

• When the plane is defined by three buttons, theyare surface ground across their faces afterinstallation.

• Crowned buttons are used for unmachinedsurfaces. However they do not provide a well-defined bearing area.

Location from Plane surface: ButtonsButtons are termed as –

• Rest buttons

• Stop buttons

Location from Plane surface: Button Positioning

• Installation of the button in the fixture body isdone 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 providethe seating for the head of the buttons

• Hole is chamfered to ensure good seating.

• Undercut is provided under the head.

Location from Plane surface: Buttons• The buttons have interference fit / press fit with

the hole in the fixture.

• The shanks on the buttons are supplied withstandardised tolerances, resulting in oversizeranging from 0.03 to 0.04mm.

• Rest and stop buttons are commercially availablein standardised dimensions.

Location from Plane surface: Buttons• For the flat buttons, H can be

selected from 1/3 D to 4/3 D (5 to25mm).

• L = ½ (D + H)

• B = ¾ (D – 3)

• For the crowned buttons, H can beselected from1/3 D to D.

• R = 3/2 D

• B = ¾ D

• L = ¾ D

Location from Plane surface: Buttons

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 lessaccurate 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 providesufficient bearing area.

• Pads are placed at locations not easily accessible by the operator. Hence, theedges and corners of a pad are usually not rounded or chamfered as the edges onrest 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.

Location from Plane surface• 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 asshown in figure.

• Due to use of locating pads only seats for the pads need to be machined insteadof 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 acomponent and they are placed as far aspossible.

Standard Dowel Pins

Location from Cylinder: Dowel Pins

Location from Cylinder: Dowel Pins• Dowel pin holes are drilled through so

that the pin can be taken out.

• The recommended bearing length of adowel pin in each part is 1 ½ to 2 timesthe diameter of the pin.

• Dowel pins are cylindrical or tapered.

• The straight type is available inunhardened and hardened form.

• The fit of the dowel pin can be a press fitin each part.

• Tapered pins are easily taken out by theapplication of light pressure or a blow onthe small end.

• Dowel pins are extensively used in theconstruction of built-up fixture bodies.

Standard Dowel Pins

Location from Cylinder: 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

Location from Plane surface• A plane surface can be located from 3 points

on the plane surface.

• A rough, unmachined surface can be locatedwith three location pads having point contact.

• Three location pins having spherical surfacesat locating points can be used.

• The pins should be placed as widely aspossible for more accurate location.

• The height is adjusted to make the workpiecesurface parallel to the baseplate.

• An adjustable support with locking nut can beused.

Adjustable Support for Rectangular Workpieces

Location from Plane surface• 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 vibrationsin workpiece during clamping and machining.

• The number of additional supports would depend on the shape, size and strengthof the workpiece.

• The bolt with hex nut

Adjustable Support

Location from Plane surface

Adjustable Support

Location from Plane surface• For locating very rough, uneven surfaces adjustable locating pads are used.

• Castings and forgings are located by adjustable screw pads.

Adjustable Locators

Location from Plane surface• 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 pushedforward by height adjusting screw.

• When the adjusting screw is withdrawn the return spring pushes the wedge pintowards 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 thespring.

Location from Plane surface• 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 surfaceto be located.

• It provides contact at four points without contradicting the three-point location ofthe plane.

• The pivot pin of the rocker acts as a single point complementing the pother twopoints “F” in defining the plane.

Equalising Rocker Location

Location from Plane surface• For small workpieces location pads are not necessary.

• The fixture body itself is machined suitably to provide locating surface.

Location from Plane 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 theworkpiece with locating surface.

Location from Profile

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 onall the sides.

Sighting location

Location from Profile• The profile of a workpiece can also be located by confining the profile with

cylindrical locating pins.

Profile location by pins

Location from Profile• The profile of the workpiece can be located

by providing pocket or nest around theprofile of the workpiece.

• The inside profile of the nest matches withthe outside of the workpiece.

• The height of the nest should be lesser thanthe workpiece to ease unloading.

• A partial nest can be used.

Location nests

Location from Cylinder

Location from Cylinder• It is most common and convenient form of

location.

• When the cylinder is located on its axis andbase, it can only rotate about its axis. Allother motions are constrained.

• Clamping from top prevents linear motionalong Y-Y axis.

Cylindrical locator

Location from Cylinder• The seating surface of the locator should be

recessed to provide space for dirt orworkpiece burr.

• There should be ample chamfer or radius atthe entry point so that the components canbe loaded quickly.

• The chamfer is called “lead”.

• The locator itself is located in fixture by aconcentric diameter generally made press fitin the fixture body.

• The cylinder is best location shape because acylindrical locator is least difficult toproduce and eliminates five of the sixdegrees of freedom.

Cylindrical locator

Location from CylinderCircular 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 thehand of the operator are reason for jamming.

• Jamming occurs when part has entered short distance into the inside locator oraround an inside locator.

• A workpiece is likely to be tilted to the axis of the location post during quickloading.

Location from Cylinder: Jamming• 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 contactwith 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.

Location from Cylinder: Jamming• 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 thepost.

• Long location posts are used for fragile workpieces since locator provides supportto the workpieces.

• Long locators are relieved i.e. made undersize in central portion.

Prevention of jamming

Location from Cylinder: 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 locatorover 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 from Cylinder

Location Post

Location from CylinderLocating post

• They are used for anchoring clamping studs.

• Location post is secured to the base otherwise it will be pulled out by clampingforce.

• The post is secured by retainer nut or a grub screw.

• C washer is used.

Locators subjected to axial pull

Location from Cylinder: Jamming• 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 from Cylinder• 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.

Location from Cylinder• 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 locationwith full pin.

• The diamond shaped pin is used to constrain pivoting of the workpiece around theprincipal locator.

• The locating surface of a diamond pin is usually less than 8% of a full cylindrical pin.

Diamond Pin Application

Location from Cylinder• A location pin is positioned as far away as possible from the principal locator in

order to minimise the angular error.

Location from CylinderConical 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 thehole diameter of the component without affecting the accuracy of location.

Conical Locators

Location from CylinderVee (V) Location:

• Used to locate cylindrical surfaces fromoutside.

• 2 “V” locators are used for accuratelocation.

• A “V” locator can be adjusted quickly byusing a cam for adjustment.

• Return spring is provided to bring backthe V locator to its original position.

• Fixed V blocks are used forapproximate location.

• Fixed V Blocks are attached to fixturebody by screws and dowelled toprevent shifting during operation.

Fixed V locator

Location from Cylinder• 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 clampingforce.

Screw adjusted V locator

Location from CylinderInclined V:

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

Inclined V

Location from Cylinder• A swinging eyebolt is used when a V plate has to be withdrawn quickly.

Quick action V locator

Location from Cylinder• 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 waythat its centre line is vertical.

Suitable position of V for drilling vertical hole

Location from Cylinder• 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

Summary1. 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 accuratesurface for location.

3. The workpiece should be prevented from moving along and rotating around theX, Y and Z axes.

4. Location system should facilitate easy and quick loading and unloading of theworkpiece and aim at motion economy.

5. Redundant locators must be avoided.

6. Location system should positively prevent wrong loading of the workpiece byfoolproofing.

SummaryLocating Methods: Plane surfaces

1. A reasonably flat surface can be located by three pins of equal height havingspherical surfaces at the location points.

2. A rough, uneven or tapered plane surface should be located by adjustablelocation pins having spherical ends.

3. Additional adjustable supports are necessary to prevents vibrations or distortionof 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 theworkpiece surfaces.

6. A cube can be prevented from linear movement and rotation around axes X, Yand Z by six location pads.

SummaryLocating Methods: Profile

1. A profile can be located approximately by aligning it with a slightly bigger sightingplate.

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 usingeccentric locators whose eccentricity can be set to suit the batch.

4. Workpiece with little variation can be located precisely with nesting plates withsuitable provision for unloading or ejection.

SummaryLocating Methods: Cylinder

1. Spigots used for locating bores should have ample lead for easy entry and theirlength 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 grubscrew.

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 orderto facilitate easy loading of the workpiece.

5. Rough cored holes and bosses are located on conical locators which often haveintegral clamping arrangement and drill bush.

6. Fixed V blocks are used to locate approximately the outside surface of thecylinder.

7. For precise location, adjustable guided V block is necessary. The V block can beadjusted by a screw or a cam.

8. V blocks should be positioned in such a way that the variation in the workpiecewould not affect the location for the operation.

References1. 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.

Thank You!