Chapter 4

JIGS AND FIXTURES

Rahul A. PasaleAsst. ProfessorAMGOI Vathar Maharashtrarhlpasale@gmail.com+91-9823131203

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Most common machining processes used are Turning Drilling Reaming Milling Boring Internal and External threadingWhen these operations are used for mass production, there is need of toolings to reduce the operational time and enhance the production.e.g. Machining of two holes at 40 mm center distance.

Jigs and Fixtures Jigs are used for operations like drilling, reaming, tapping, spot facing etc. Fixtures are used for operations like milling, boring, turning, broaching, slotting grinding etc. Jigs and fixtures are used for a specific operation and for a specific job only.

Basic elements of Jigs and Fixtures: A locating system: For accommodating the component at the desired place, A clamping system:For holding the component firmly at the located place, A skeleton or body:To contain locating and clamping elements and other elements as well as the component.

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Locating SystemThe purpose of the locating system is to receive and accommodate the component at the specific desired place with respect to the cutting tool and assure consistency.e.g. As shown in fig. if M12 hole is to be at specific position from side AB and side AC, then the component has to be located under the drilling spindle to assure the position.

3-2-1 Principle

The degrees of freedom of a free body are 6.Linear motion parallel to +/- X axis.2. Linear motion parallel to +/- Y axis.3. Linear motion parallel to +/- Z axis.Rotational motion around X axis.Rotational motion around Y axis.Rotational motion around Z axis.

If a locating system is able to curtail these six degrees of freedom, it shall be called as an ideal locating system.

The group of 3 pads (A, B and C) restrict the freedom for linear motion along Z axis and rotational freedom around X and Y axis.The group of 2 pads (D and E) restrict the freedom for linear motion along Y axis and rotational freedom around Z axis.The single pad F restricts the remaining freedom for linear motion along X axis.Hence to curtail these 6 degrees of freedom optimum requirement shall be a group of 3+2+1= 6 local surfaces. This principle provision is known as 3-2-1 principle for location, or Six point principle for location

Surface Locators

Axis Location : Pin Locators

Components with drilled / reamed / bored holes are preferred pin locators. With clamping, it is capable of restricting 4 degrees of freedoms. Able to prevent the component to move along X and Y axis and to rotate around X and y axis. Larger pins having diameter like 30 mm, 35 mm are called as posts. A suitable relief is provided in order to reduce the contacting area.

The pins or posts should have their own integral resting pads offering appropriate resting area. The junction of the pad and post diameter is always provided with an undercut to avoid fouling of the sharp corners / burr at the mouth of the burred hole.

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Two Pin Location

A single pin with a pad is able to restrict five degrees of freedom except the permission for the component of revolve around the axis of the pin. Sixth degree of freedom is curtailed by adding one more pin i.e. diamond pin. When two pin locator is used, the smaller pin is converted in to a diamond pin. To fit the job on two pins, when center distance between two holes is not accurate.

Edge Locators

In this case, the line contacts provided by the pin surfaces are more reliable than the contact provided by a surface locator due to minimization of contacting area.

V blocks location offer two parallel line contacts for cylindrical or convex components. V block ensures self aligning of the work piece parallel to the V axis.

Adjustable Pin Locators

Sometimes due to space limitations it is prepared to insert the locating pin in the component hole rather than mounting the component over the pin.

The actuation can be using rack and pinion arrangement, cam arrangement of compression spring.

Adjustable locators are preferred for heavy large size components.

Conical Locators

The conical location is of self aligning type. Minor variations at the component size (bore) can be accepted at the location without disturbing the alignment of component.

Nest Locators

Nesting provides locating surface from all sides of component, is unable to move in any direction.

e.g. connecting rods, base bracket, flanged components with hexagonal, octagonal flanges, valve body ends.

Adjustable V Locator

Components like raw castings / forgings are required to be sometimes located on their as cast/as forged circular part profiles. An adjustable or sliding V locator is used which accommodate the size variation at the profile. The actuation can be cam operated, screw operated or rack pinion operated.

Characteristics of a Good Locating System Rest pads should have their contacting area as small as possible but strong enough against warpage and wear. Locating elements should be as far as possible. While locating elements like raw material of cast surface, care should be taken to see that location bears no contact with the zones having flash lines, fins or burrs etc. To locate mutually perpendicular surfaces, locating pin diameter with its own rest pads etc. should provide undercut reliefs to accommodate component sharp corners / burrs.

Characteristics of a Good Locating System Locating elements should be visible to the operator in absence of the workpiece. Locating elements should not make loading / unloading difficult. Redundency in location must be avoided. Locating elements used as resting surfaces shoud be symmetrical at the arrangement. The locating system should not create accumulation of chips and swarf.

Clamping SystemA clamping system is supposed to hold or retain the component at the located place.

Simple Screw Clamp

It blocks one side of the jig / fixture which makes loading and unloading difficult. Due to direct contact with the workpiece, it tries to rotate the workpiece.

Swinging Screw Clamp

During loading and unloading the swinging clamp is switch to off position, so it will not affect the loading and unloading of the component.Due care is taken at the fitment of fulcrum pin to see that the pin itself dose not come out while clamping.

Screw Clamp with Float Pad

By using screw clamp with floating pad, tendency of the workpiece rotating with clamp is avoided.

Post Clamp

The post clamp is usable for components carrying through, central, precise bores.

Strap Clamp

Bridge Clamp

Bridge clamp is able to provide clamping force horizontally, when clamping area is not available at the top surface.

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Latch Clamp

For central vertical downward clamping force.

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Two Way Clamp

This clamp can provide two mutually perpendicular clamping forces in a single clamping action.

Equalizer Clamp

This clamp is helpful in providing two clamping force of equal magnitude at two different points over a component. This clamp is suitable for components with uneven surfaces exposed for clamping.

Clamps Operated using Eccentric

Cam Clamp

Toggle Clamp

Other Clamping Aids C Washer

For design of C washer: If the bolt / stud size is d mm, thenSlot width 1.1d to 1.2d, Outer diameter 3.5d to 4d, Thickness 0.75d to 1.25 d

Spherical Washer

A set of Spherical washers is able to restore the surface contact and ensure proper clamping.

Quick Acting Nut

This is the device which ensures quick action of the nut while clamping and de-clamping.

Characteristics of Good Clamping System It should secure the work piece against the influences of cutting and other forces The clamping force should not be opposite to the cutting force of self weight of the component. It should be easily accessible to the operator. It should not be eccentric of offset. Clamping force should not inbalance the component. The clamping system should not dislocate the component. The clamping action should not deform of deflect the component. It should be simple and quick to operate.

Automated Clamping

Fool Proofing ArrangementsWhile locating a workpiece within a locating system, it is sometimes the work-piece does not have a unique single orientation.To locate workpiece at its predefined position fool proofing arrangement is made in the jigs or fixtures.Examples :

Fool Proofing Arrangements

Fool Proofing Arrangements

Design of Drilling JigsDrilling jigs are used for operations like drilling, tapping, reaming, counter boring etc.The jig contains A body or skeleton A locating system A clamping system A system for guiding cutting tools

Guide Bushes The guide bushes are cylindrical passages to guide a drill towards the component at the desired place and in the desired direction.The guide bush is made of high wear resistant alloy steel and case hardened.The plate on which bushes are fit is known as jig plate or bush plate.

The upper portion is known as Head and lower one is Shank.The tolerance on the size of the shank O.D. and passage I.D. can be order of IT 6 or IT 7 grade.Fundamental deviation to the bore size is of the F type.e.g. If drilling size 15 mm diameter, then I.D. specification as 15.00 F 7 i.e. 15.017 / 15.035

Fixed BushesThese bushes are fixed at the jig plate. The jig plate carries bore having nominal size that of the shank of the bush. There is interference fit between shank O.D. and bore. O.D. of the bush shank is ground to the fundamental deviation of n or p type, while the bore is of H type.These type of bushes are suitable for small or medium quantity.

Renewable Bushes

These bushes are used where the replacement of the bushes is expected to be frequent due to quantum of production and relatively faster wear and tear. e.g. A production programme of around 4000-5000 components/month may required to change the bush every after 45 to 60 days.

Liner is press fitted in the jig plate where bush is press fitted in the liner.Liner maintained its size as H6 or H7 fitment.Bush maintained its size as f6 or f7 or g7 fitment.

Slip Bushes

The slip bushes are used for paired operations like Drilling + Reaming or Drilling + Tapping.

Recommendations for Bush sizesParticularsSizeDrill Bush I.D.For reamed hole 1 mm less than d i.e. 15 mmFor tapped hole 0.8 times di.e. 16 X 0.8 = 12.8 mm ~ 13 mmBush O.D. (assuming 4 to 5 mm wall thickness)For tapping bush 16 + 8=24 mmFor drilling bush 13 + 8= 21 mmSo common bush OD is 24 mm Shank lengthIn the steps of 15, 20, 25 and 30 mm depends on size of holes.Head diameter2.0 to 2.5 times d Head thicknessAround 4 to 5 mmLiner OD (assuming 4 to 5 mm wall thickness)24 + 8 = 32 mm

e.g. For 16H8 reamed hole or M16 tapped hole

Special BushesExtra Long BushesBush with tapered faceBushes for closed center distance holes

Types of JigsJigs are tailor made to suit to the specific component processing requirements.Template JigThis is the simplest type of jig; It is simply a plate made to the shape and size of the work piece; with the require number of holes made it. It is placed on the work piece and the hole will be made by the drill; which will be guided through the holes in the template plate should be hardened to avoid its frequent replacement This type of jig is suitable if only a few part are to be made.

2. Box jigWhen the holes are to drill more than one plane of the work piece , the jig has to be provided with equivalent number of bush plates. For positioning jig on the machine table feet have to be provided opposite each drilling bush plate. One side of the jig will be provided with a swinging leaf for loading and unloading the work piece, such a jig would take the form of a box.

3. Leaf JigSwinging jig plate is provided to accommodate the component form top,Locating is provided on bottom plate of the jig,Loaded and unloaded in vertical direction,The processing axis is parallel to the loading direction,

4. Sandwich Jig Suitable for the components with through, machined bore which can be used for location.

5. Turnover Jig Suitable for light weight components Loaded / unloaded and located in vertical direction, Locating surface and processing surface is same

Types of Jigs Contd.6. Indexing JigApplicable for those components where holes more than one in number, having symmetrical peripheral placement,

7. Angular Hole JigApplicable for the components with hole axis inclined to normal component axis,

8. Tumble JigApplicable for small sized components with number of similar and dissimilar holes on different sides,

9. Pot JigUsed for larger size external machined diameter components,Processing axis is parallel to the external diameter,

Indexing Mechanism

6 basic steps of Jig designAssumptions regarding earlier performed operations,Selection of type of jig with justifications,Selection and conceptual finalization of the location system, Selection and conceptual finalization of the clamping system, Designing the liner bush system,Any other specific features to be adopted,

Design and draw a jig for 2 holes M14 at 60 +/- 0.1 C.D. for the component shown in figure.

Operations performed earlierFacing of 150 baseFacing of opposite face maintaining 70 mm dia.Drilling of 16 dia. Holes at the base.

2. Selection of type of jigFor this component, base is suitable for the location , so it shall be required to be loaded/unloaded in vertical direction. The processing of the component is at opposite side of the locating surface. Therefore a Leaf Jig is suitable for this component.

Selection of locating systemThe component can be given a two pin location at a any pair of opposite holes at the base. One pin shall be cylindrical with a diameter 16f7 and clear length of 18 mm. Other pin shall be a diamond shaped. For resting purpose 4 pads are preferred of size 30 mm dia. and 15 mm height. These pads can be placed on the same PCD, but in between a pair of holes.

4. Selection of clamping systemThe clamping of work piece shall be preferred from upper side centrally. Hence an adjustable clamping pad can be provided at the swinging jig plate. Hence latch clamp is preferred.

5. Design of Bush / Liner system.

6. Specific FeaturesThe height of the fulcrum pin shall be responsible for maintaining the horizontally of the jig plate. So it is selected asH = Rest pad thickness (15 mm) + Component Height (70 mm) + Chip removal gap (10 mm) + X Jig plate thickness= 105 mm

ItemI.D.O.D.QuantityComponentM 14-2 tapped holesTapping bush14 F724 g61 slip bushDrilling bush12 F724 g61 slip bushLiner24 H734 p62 nos.Jig plate34 H7-2 bored holes

b. On the other side, there shall be a similar clamping post with its clamping surface height = 15 + 70 + 10 = 95 mmc. The clamping pad preferably softer than Mild steel, shall be adjustable to impart required clamping pressure.All items of the jig shall be of mild steel expect the liner bushes.The clamping pad shall be of semicircular shape.

Design of FixtureMilling fixture consists of Locating systemClamping systemBody of the fixtureSetting block to set the cutter

Setting Block height = H + t (without feeler gauge)Metal Feeler strips of size available 0.1 mm, 0.25 mm, 0.5 mm, 1.0 mm etc.Setting Block height = H + t feeler gauge size ( with feeler gauge)

Design and draw a milling fixture for producing the 16 +/- 0.05 wide slot at the component shown in the fig.

Operations performed earlierPlain milling of the baseDrilling of the 12 dia. holes at the baseBoring of 16 +/- 0.1 dia. Hole

2. Selection of the type of fixturea slot milling fixture to be used on a horizontal milling machine, (with a side and face cutter with 16 mm width) can be selected for design.3. Selection of location systemthe component has to rest on its base on 4 pads of equal height 15 mm, out of 4 pads 2 opposite pads have locating pins of 12f6 size. One should be diamond pin out of two pins.

4. Selection of clamping systemtwo strap clamp can be provided along the 140 mm sides. Maximum height of the stud has to be less than (Rest pads height 15 mm ) + (Slot height from component base 45 mm) = 60 mm

5. Design of setting blockthe setting block shall need two setting surfaces. The horizontal setting surface shall be at a height ofH = Rest pad height 15 mm + Slot height 45 mm paper feeler 0.08 mm = 59.92 mm from fixtureThe vertical setting surface shall be at a distance ofS = (0.5 X slot width 16 mm) + paper feeler 0.08 = 8.08 mm from fixture center lineFor effective cutting, the required minimum cutter radiusR = slot depth 50 mm + arbor sleeve radius 20 mm + relief for component 10 mm = 80 mm

Assuming cutter diameter 160 mm, the setting block shall be required to be 160 mm away from nearest component edge.

6. Design of tenonstwo tenons shall be required at the two colinear slots at the bottom side of the fixture. Tenon thickness should match with the machine T slot i.e. 16 f6.