104452989 Jigs and Fixtures Design

7/22/2019 104452989 Jigs and Fixtures Design

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JIGS AND FIXTURES DESIGN


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JIG

A Jig may be defined as a device which holds and locates

a work piece, guides and controls one or more cutting

tools.

FIXTURE

A Fixture may be defined as a device which holds and

locates a work piece during an inspection or for a

manufacturing operation. The Fixture does not guide thetool.

DEFINITIONS


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The following are the advantages of employing Jigs and

Fixtures in mass production work.

(1) It eliminates the marking out, measuring, and other

setting methods before machining.

(2) It increases the machining accuracy, because the work

piece is automatically located and the tool is guided

without making any manual adjustment.

(3) It enables production of identical parts which are

interchangeable. This facilitates the assembly operation.

ADVANTAGES OF EMPLOYING JIGS AND FIXTURES


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(4) It increases the production capacity by enabling a number

of work pieces to be machined in the single set up

(5) It reduces the operator's labour and consequent fatigue

as the handling operations are minimised and simplified.

(6) It enables semi-skilled operator to perform the operationsas the setting operation of the tool and the works are

mechanised. This saves labour cost.

(7) It reduces the expenditure on the quality control of the

finished products.

(8) It reduces the overall cost of machining by fully or partly

automatising the process.


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The following are the fundamental differences between a

Fixture with a Jig

(1) A Fixture holds and position the work but does not

guide the cutting tool, whereas a Jig holds, locates and as

well as guides the tool.

(2) The Fixtures are generally heavier in construction andare bolted rigidly on the machine table, whereas the Jigs

are made lighter for quicker handling, and clamping with

the table is often unnecessary.

(3) The fixtures are employed for holding work in Milling,Grinding, Planning, or Turning operations, whereas the

Jigs are used for holding the work and guiding the tool

particularly in Drilling, Reaming or Tapping operations.

DIFFERENCE BETWEEN FIXTURE WITH JIG


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PRINCIPLES OF JIGS AND FIXTURES DESIGN

The following are the essential factors which must be considered in

designing a Jig or a Fixture Study of the

(1) Study of the component/work piece.

(2) Type and Capacity of the machine.

(3) Locating elements.

(4) Loading and unloading arrangement.

(5) Clamping arrangement.

(6) Power devices for operating the clamping elements.

(7) Clearance required between the Jig and the component.

(8) Indexing devices.

(9) Tool guiding and cutter setting elements.

(10 Fool-proofing arrangement.

(11) Ejecting devices.

(12) Swarf removal arrangement.

(13) Rigidity and vibration problem.

(14) Table fixing arrangement.

(15) Safety devices.

(16) Methods of manufacture of the jig base, body or frame.


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Generally, all the jigs and fixtures consist of

LOCATING ELEMENTS: These position the work piece accurately with

respect to the tool guiding or setting elements in the fixture.

CLAMPING ELEMENTS: These hold the work piece securely in the

located position during the operation.

TOOL GUIDING AND SETTING ELEMENTS: These aids are used in

guiding or setting of the tools in correct position with respect to the workpieces, e.g. drill bushes guide the drills accurately onto the work piece.

ELEMENTS OF JIGS AND FIXTURES


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Generally, all Jigs / fixtures consist of the following elements

LOCATORS

A locator is usually a fixed component of a fixture. It is used to establish

and maintain the position of a part in the fixture by constraining the

movement of the part. For work pieces of greater variability in shapesand surface conditions, a locator can also be adjustable.

CLAMPS

A clamp is a force-actuating mechanism of a fixture. The forces exerted

by the clamps hold a part securely in the fixture against all other externalforces.



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SUPPORTS

A support is a fixed or adjustable element of a fixture. When severe part

displacement/deflection is expected under the action of imposed clamping

and processing forces, supports are added and placed below the work

piece so as to prevent or constrain deformation. Supports in excess of

what is required for the determination of the location of the part should becompatible with the locators and clamps.

FIXTURE / BASE BODY

Fixture body, or tool body, is the major structural element of a fixture. It

maintains the spatial relationship between the fixturing elementsmentioned above, viz., locators, clamps, supports, and the machine tool

on which the part is to be processed.



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FIXTURE / JIG DESIGN

The following outputs are included in the jig / fixture plan

Fixture / Jig type and Complexity

Number of work pieces per fixture

Orientation of work piece within Jig / fixture

Locating datum faces

Clamping surfaces

Support surfaces, if any

The following outputs are included in the fixture / Jig layout

Positions of locators

Positions of clamps

Positions of supports, if any

Type of locators

Type of clamps

Type of supports

Clamping forces and sequence


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Detailed design of locators

Detailed design of clamps

Detailed design of supports, if any

FIXTURE DESIGN


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JIGS AND FIXTURE

DESIGN FLOWCHART


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DEGREES OF FREEDOM OF A WORK PIECE

Three rectilineardisplacements along the mutually orthogonal

co-ordinate axes

Three angulardisplacements with respect to the same axes


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3-2-1 PRINCIPLE OF LOCATION


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Three supports restricting five degrees of freedom.

As shown in figure, three locators, or supports, are placed under the

work piece. The three locators are usually positioned on the primarylocating surface. This restricts axial movement downward, along the z -

axis (+) and radially about the x (+ and -) and y axes (+ and -).


Z +

RotationY + -

Rotation X + -

X

Y


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Two supports restricting three degrees of freedom.

The next two locators are normally placed on the secondary locating

surface, They restrict an additional three degrees of freedom by arrestingthe axial movement along the y-axis (+) and the radial movement about

the z-axis (+ and -).


X

Y

Y + Rotation Z + -


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Total Six locators restricts nine degrees of freedom,

completing 3-2-1 locations.

The final locator, is positioned at the end of the part. It restricts the axial

movement in one direction along the x-axis (+). Together, these six locators

restrict a total of nine degrees of freedom. The remaining three degrees of

freedom (-x, -y, and -z) will be restricted by the clamps.


X

Y

X +

Final One support restricting one degree of freedom.


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Based on the surface from which the work piece is located, the locators

are classified in to three groups

CYLINDRICAL SURFACE

Conical locator.

Cylindrical locator.

Female locator.

Diamond locator.

Mandrel locator.

Collet locator.

Vee locator- Screw adjusted V locator.Cam operated V locator.

Quick action V locator.

PLANE SURFACE

Adjustable locator.

Spring loaded pad.

Equalizer rocker locator.

PROFILE SURFACE

Six point locator.

Profile location by pins.

Eccentric locator.

Nest & cavity locator.

TYPES OF LOCATORS


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ADJUSTABLE LOCATORS

Adjustable locators require less precision to mount on the tool body. For a typical work

holder, they are mounted at approximately the correct position and then adjusted to

the exact location.

In the design of an adjustable locator, four factors should be considered :

1) the stability and precision of the location;

2) the ability to compensate for variance in work piece sizes and locating areas

3) the ability to compensate for wear on the locating element

4) the ability to maintain the fixed location points.

The main disadvantage of adjustable locators is that the operator has the most

responsibility of ensuring accuracy and precision, not the tool builder.


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SPRING-LOADED LOCATORS

Spring-loaded locating help ensure the correct by

pushing the work piece against the fixed locating

SIX POINT LOCATOR


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Equalizing supports are a form of adjustable support used when acompensating support is required. Although these supports can be fixed in

position, in most cases equalizing supports float to accommodate work piece

variations. As one side of the equalizing support is depressed, the other side

raises the same amount to maintain part contact. In most cases adjustable

and equalizing supports are used along with solid supports.

EQUALIZING SUPPORTS


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PROFILE LOCATION BY PINS


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For regular or complex shapes, a

nesting locator can also be cast fromplastic compounds or low-melting-

point alloys.

Nesting locator completely locates

the outside surface of a work piece.

it can be machined if the part shape

is simple.

NESTING


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CONICAL LOCATORS

The main problem with using this type of locator is in the position of the work piece

relative to the work holder. The locating hole can contact the locator at any point onits conical surface. So slight differences in the hole diameter affect the work piece

height, as illustrated in the figure This vertical height variation must be considered in

the design of the work holder.


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Round pins can be used for both internal and external work piece location. For internal

location, the diameter of the pin must match the size of the locating hole. Theselocators come in many standard sizes, and are readily available,

For external location, the size of the locating pin is not as critical as internal. Here, a

standard pin size strong enough to resist machining forces is the best choice.

These pins have a shank larger than the head. The purpose of the shoulder is to

prevent the pin from being pushed into the tooling plate

Unlike the plain pins, shoulder-type pins are made in two styles Press-fit type and lock

screw type,

ROUND LOCATING PINS


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Vee locators are a specialized form of locating element. They are used mainly for

round or cylindrical work pieces. The two basic styles of Vee locators are the Pad

and the Vee Block as shown in the figure.

Vee locators can be used to locate

a variety of part shapes

Vee pads are well suited for corner

mounting a square or rectangular work

piece.

VEE LOCATORS


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DIAMOND LOCATING PINS

Another style of locating pin frequently seen in Jig-and-Fixture design is the diamond,

or relieved locating pin. Like round locating pins, diamond pins are available in either

the plain or shoulder-type, as shown in figure. These locating pins are the most-common form of relieved locating pin in work holders. To limit the pin's contact area,

the diamond locating pin is made with four machined flats. The exact width of the

contact area varies with the size of the pin, and is usually equal to one-third of the

diameter on each side.

Diamond pins are generally used as shown in figure.

Here the diamond pin acts as a radial locator to restrictmovement of the work piece around the concentric

locator, shown by the round-pin locator. The diamond

pin is positioned to restrict the radial movement of the

part. Since a diamond pin locates in only one axis, the

contact areas of the pin must be positioned as shown in

the figure. Positioning the pin any other way would allow

the part to move about the concentric locator.


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DIAMOND PIN CALCULLATION

G

B

H

F -0.008


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Another condition to avoid in work holder

design is redundant or duplicate location.

Redundant locators restrict the same

degree of freedom more than once.

The work pieces in figure, show several

examples. The part at (a) shows how a flatsurface can be redundantly located. The

part should be located on only one not both

side surfaces. Since the sizes of parts can

vary, within their tolerances, the likelihood

of all parts resting simultaneously on both

surfaces is remote. The example at (b)points out the same problem with

concentric diameters. Either diameter can

locate the part, but not both.

REDUNDANT LOCATION


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Fool proofing prevents improper

loading of a work piece. The problem

is most prevalent with parts that are

symmetrical or located concentrically.

The simplest way to foolproof a work

holder is to position one or two pins in

a location that ensures correct

orientation. With some work pieces,

however, more-creative approaches to

fool proofing must be taken as shownin figure

Fool proofing the locating preventsimproper work piece loading

FOOL PROOFING


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CLAMPING

BASIC PRINCIPLE OF CLAMPING

Once a work piece is located, it is necessary to press it against the

locating surfaces and hold it there against the forces acting upon it. The

tool designer refers to this action as Clamping and the mechanisms

used for this action are known as Clamps.

However, simple or complex, all clamps must fulfill four essential

requirements:

1) The work piece must be held rigidly while the cutting tools are in

operation.

2) The time required for loading and unloading the tool must be as short

as possible, which means the clamping device must be quick-acting.

3) when subjected to vibration, chatter, or heavy pressure, the clamping

must be positive.

4) The clamp must not damage the work piece.


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TYPES OF CLAMPS:

STRAP CLAMPS


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Eccentric cams should usually not be used for

clamping because they do not provide positive

locking.

CAM CLAMPS :

Cam clamps are also based on the principle of the inclined plane. The most

common forms of Cam Clamps are the Eccentric Cam and Spiral Cam. Depending

on their arrangement, these cams can be used as Direct pressure Clamps orIndirect pressure Clamps.

Spiral cams have a continious rise to lock positively at any point in the

clamping range


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A toggle action locks positively because one of the pivots moves

past the center line of two other pivots, against a stop.


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hold-down

push/pull,

latch

squeeze action.

The four basic Toggle actions are as shown below


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LATCH CLAMPS :

The Latch Clamp is employed to clamp a job by a Latch. A pivoted latch and leaf is

used for this. The leaf is closed on the job and is kept in position by the Latch. The

work is gripped by the spring fitted to the leaf. To unload the work piece the laid end ofthe latch is pushed by the hand to cause the leaf to swing open as shown in the

Figure.

ADVANTAGE :

The main advantage is the ease and

speed of manipulation .

DISADVANTAGE :

1.Construction is limited to relatively

light work.

2.Difficult to secure rigid clamping.


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SCREW CLAMPS :

Screw clamps are among the simplest and least-expensive. Screw clamps offer the

designer more clamping options than many other clamps. For clamping effectiveness

and size/force ratio, screw clamps make excellent work holders as shown in the figure-23a. But they also have a drawback; their inherently slow clamping speed limit their use

in high-production Jigs and Fixtures.

QUICK ACTING SCREWS AND KNOBS :

Quick-acting knobs are another

variation of quick-acting

threaded fastener.


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TOOL GUIDING ELEMENTS

The cutter is set relative to the work in a fixture by adjusting the machine or by

using the cutter Setting Block. The tools are guided in Jigs by Drill Bushes which

are fitted on the jig plates. There are three different types of Jig bushes;

Slip Bush, (Fixed and Renewable) and LinerBushFIXED BUSH :

Fixed bushes are most common and

least expensive, permanent

bushes.These are press fitted and are

designed for one step operation such

as Drilling or Reaming. The bushingsare pressed directly into the jig plate.

They are held in plates by the force of

press fit as shown in the figure

FIXED RENEWABLE BUSH :

Fixed renewable bushings are typically employed in long

production runs where bushing changes are needed. The fixedrenewable bush is intended for single step applications such as

Drilling or Reaming. These bushings are changed only when the

bushings wear. Fixed renewable bushings are held in space with a

lock screw which prevents any movement of the bush during

machining cycle as shown in the figure.


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SLIP RENEWABLE BUSH:

These are convenient for applications when

multiple operations are performed in same

hole. One example is Drilling and Reaming

the same hole.The first Slip Renewable

bush is installed and the hole is drilled. The

drill bush is removed. Then the reaming

bush is installed and the hole reamed to

size. The Slip Renewable side allows rapid

change over. The bush is rotated clockwise

to lock it in place and rotatedcounterclockwise for removal as shown in

figure


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DRILL JIGS

1.TEMPLATE JIG :

Template Jigs are normally used for accuracy

rather than speed. This type of Jig fits over,

on, or into the work and is not usually

clamped. Templates are the least expensive

and simplest type of Jig to use. They may or

may not have bushings, when bushings are

not used, the whole jig plate is normally

hardened.


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2. PLATE JIG :

Plate Jigs are similar to Templates. The only difference is that Plate Jigs have

built-in-drive clamps to hold the work. These Jigs can also be made with or

without bushings, depending on the number of parts to be made. Plate Jigsare sometimes made with legs to raise the jig off the table for large work. This

style is called a Table Jig.


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3. SANDWICH JIGS :

Sandwich Jigs are a form of plate jig with a back plate. This type of jig is ideal

for thin or soft parts which could bend or warp, in other type of Jig. Here again,

the use of bushings is determined by the number of parts to be made.


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4. ANGLE-PLATE JIGS :

Angle-Plate Jigs are used to hold parts which are machined at right angles to their

mounting locators. Pulleys, collars and gears are some of the parts which use this

type of Jig as shown in the figure.

A variation of this jig is the modified Angle-Plate Jig. This jig is used for machining

angles other than 90 degrees as shown in the figure.


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05. BOX JIG or TUMBLE JIGS:

These usually totally surround the part. This style of jig allows the part to be

completely machined on every surface without repositioning the work in the Jig.

06. CHANNEL JIGS :

Channel Jigs are the simplest form ofBox Jig. The work is held between two

sides and machined from the third. In some cases, where jig feet are used, the

work can be machined on three sides.


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07. LEAF JIGS :

Leaf Jigs are small box jigs with a hinged leaf to

allow for easier loading and unloading. The main

differences between Leaf and Box Jigs are size and

part location. Leaf Jigs are normally smaller than

Box Jigs. Sometimes they are made so that they do

not completely surround the part. Leaf Jigs are

usually equipped with a handle for easier movement.01.Work

03.Drill bush

10.Button

11.Hinge pin

12.Set screw

13.Leaf.

14.Leaf clamping screw.

08. INDEXING JIGS :

Indexing Jigs are used for circular hole patternsin which the part is indexed successively to the

different positions under a single bushing. The

location of the holes may be taken from the first

hole drilled, or from other holes in the part.

Sometimes an indexing plate or device is

incorporated in the Jig.


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09. TRUNION JIGS :

Trunion Jigs are a form of rotary Jig for very large or odd-

shaped parts. The part is first put into a box-type carrier

and then loaded on precision bearing mounts calledTrunnions. This Jig is well suited for large, heavy parts

which must be machined with several separate Plate-Type

Jigs.

Trunnion jigs allow easily turning a large part to work on all sides.

10. PUMP JIGS:

Pump Jigs are commercially made Jigs which must be adopted by the user. Thelever activated plate makes this tool very fast to load and unload. Since the tool is

already made and only needs to be modified, a great deal of time is saved by

using this Jig.


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11. MULTISTATION JIGS :

Multistation Jigs, are for repetitive simultaneous

operations on several identical parts. In most

cases, almost any Jig may be used with amultistation arrangement. The unique feature of

a Multistation Jig is the way the Jigs are

mounted and arranged with respect to the

machining stations. In this example, the jig has

four stations: #1 is the load/unload station; #2 is

for Drilling; #3 is the Reaming station; #4 is

where the work piece is counter bored. Anindexing arrangement is also included with this

Jig to accurately position the jigs at each

station.


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FIXTURE

PLATE FIXTURE :

The Plate Fixture is the most-basic and most-

common fixture. The Plate Fixture is built with a millfixture base, cast flat section, tooling plate, or similar

plate material. All locators, supports and clamps are

mounted directly to the plate. A complete plate fixture

can be built using only standard, off-shelf

components.

Plate fixtures usually hold a work piece parallel tothe machine table

ANGLE-PLATE FIXTURES :

Angle-Plate Fixtures are a variation of the basic Plate

Fixture. They are useful when the locating surface is at

an angle to the machine table. The two main variationsof Angle-Plate Fixtures are the right angle and modified-

angle plate fixtures. Right-angle Plate Fixtures, are

constructed at 90 degrees to the base. The right-angle

plate fixtures can be built with tooling blocks, T-cast

sections, L-cast sections, angle brackets, or any

comparable material. Adjustable angles or sine plates

may be used to build the modified-angle plate fixtures.


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TURNING FIXTURE:

Commonly used work holding devices as

fixtures in Turning are Soft Jaws, Chucks,(as

Three Jaw and Four Jaw Chuck), Face Plate

etc.

Turning Fixtures are used for complicated work

pieces and are in effect simplification of the

technique of butting the work piece to a Face

Plate.

The Fixture body is located on the machine

spindle and bolted in position. It carries the

work piece location and clamping system.

Sometimes the work piece is located and

clamped to a separate shaft that projects from

the fixture body.

When the weight of the work piece and the

fixture are imbalanced, the balancing weightmay be incorporated.

A setting face machined relative to the location

system and a typical hardened setting piece is

used to set the depth of the tool.

A pilot bush may be used to guide the boring

bar.


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MILLING FIXTURE :

A Milling Fixture is located accurately on the machine table and then bolted in position;

the table is positioned relative to the cutter or cutters with the aid of the Setting Block.

The work piece is located on the Fixture base and then clamped in position. The cutter

is not guided during cutting.

The location and clamping systems are similar to those used for drill jigs, but as the

cutting forces are high, interrupted, and tend to lift the work piece, the clamping forces

must be big; hexagonal nuts are usually used to clamp the work piece rather than hand

nuts. The details that are peculiar to Milling Fixtures are the Setting Block and the

Location Tenons.

Setting Block :

The setting block as shown in the figure, is located relative to the fixture location

system and retained in position by screws; it has two hardened setting faces, so that

the table can be positioned both horizontally and vertically (note that the table is

located from one side of the cutter only). The table setting is done with a 0.25 mm

feeler between the cutter and the setting face so that the block will not be damagedby the cutter during machining. The setting block is positioned so that the cutter is

between the vertical face of the block and the operator during setting; this

arrangement gives maximum convenience for the operator during setting.


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Tenons :

The two Tenons (as shown in the figure) are made from case hardened steel and are

located on the underside of the fixture base; these two tenons sit in one of the tee

slots that run along the length of the machine table so that the fixture is locatedrelative to the table feed; the two Tenons should be as far apart as possible, to

produce maximum accuracy. The Fixture is bolted to the table by two or four tee bolts

that are placed in the tee slots (these bolts are not called up as part of the fixture).


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Example - 1


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Documents

104452989 Jigs and Fixtures Design