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1. Introduction:

Fixture is the work holding or support devices used to securely

locate and support the work. This ensures that all parts produced using

the fixture will aid in repeatability and interchangeability. Fixtures help

to smoothen the operation by reducing the requirement for skilled

labour by simplifying how work pieces are mounted, quick transition

from part to part and increasing conformity across production a run.

2. Purpose of Fixtures:

A fixture’s primary purpose is to create secure mounting point for

a work piece, allowing for support during operation, increased

accuracy, precision, reliability and interchangeability in finished parts.

It reduces working time by reducing set up time. It reduces complexity

of the process by allowing unskilled labours to perform effectively.

3. Design:

Fixtures must always be designed with the purpose to reduce cost

of production. They must be designed to facilitate two or more process

or operations. It should also aid in manufacturing of multiple small or

medium sized component.

Clamps are moveable components which allow work pieces to be

easily placed in the machine or remove from the machine. Clamps are

also adjustable in type which facilitate work pieces of different sized

for different operations. Fixtures must be designed such that the

pressure are the motion of the machining operation is directed primarily

against the solid component of the fixtures this reduces the probability

of failure of the fixture.

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4. TYPES OF FIXTURES

4.1Milling fixtures:

Milling operations tend to involve large, straight cuts that produce

lots of chips and involve varying force. Locating and supporting areas

must usually be large and very sturdy in order to accommodate milling

operations; strong clamps are also a requirement. Due to the vibration

of the machine, positive stops are preferred over friction for securing

the work piece. For high-volume automated processes, milling fixtures

usually involve hydraulic or pneumatic clamps.

4.2 Drilling fixtures:

Drilling fixtures cover a wider range of different designs and

procedures than milling fixtures. Two common elements of drilling

fixtures are the hole and bushing. Holes are often designed into drilling

fixtures, to allow space for the drill bit itself to continue through the

work piece without damaging the fixture or drill, or to guide the drill

bit to the appropriate point on the work piece.

Bushings are simple bearing sleeves inserted into these holes to

protect them and guide the drill bit. Because drills tend to apply force

in only one direction, support components for drilling fixtures may be

simpler. If the drill is aligned pointing down, the same support

components may compensate for the forces of both the drill and gravity

at once. However, though mono directional, the force applied by drills

tends to be concentrated on a very small area. Drilling fixtures must be

designed carefully to prevent the work piece from bending under the

force of the drill.

5. Locating Methods

5.1Locating from plane surfaces:

The proper location of work piece requires use minimum number

of locating points. Redundant locations is not desired in a jig or fixture

design. For external plain surface, 3-2-1 principle of location is

generally employed. In order to understand 3-2-1 principles of location,

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let us consider a rectangular block with all the plains are perpendicular

to each other. The rectangular blocks has 12 degrees of freedom such

as 6 rotation and 6 axial degrees of freedom. In order to properly locate

the block, all the 12 degrees of freedom need to be restricted with

suitable locating points. Location of the block can be done using 6

locating points; 3 locating points in the primary locating surfaces, 2

locating points in the secondary locating surface and one locating point

in the ternary locating surface. Placing the primary locating surface of

the block on the 3 locating pins restricts 5 degrees of freedom. The 2

locators placed on the secondary locating restricts 3 more degrees of

freedom. The 6th locator positioned on the trinary surface of the block

restricts 1 linear movement. Therefore, 6 locators restricts 9 degrees of

freedom. Remaining 3 degrees of freedom is restricted by clamping the

block.

5.2 Locating from circular surfaces

The basic reference for location is the flat plane of the machine-

tool table surface. It is necessary to locate the axis of the circular work

piece. One of the come methods of locating from a circular surface is

by using cones, a method commonly referred to as conical location and

usually employed when locating is done from a hole. The same system

may be used when locating on the outside of the circle, except that the

cones are inverted to form cups.

V method is used primarily to locate round work pieces or

workpieces with convex circular surfaces. Smaller included angles hold

a round workpiece more securely but are more susceptible to location

errors case by burrs, chips, dirt, and workpiece inaccuracies. The V

should be directed in such a way that variations in workpiece size will

not affect location on the workpiece.

5.3 Locating from irregular surfaces

Irregular surfaces are neither flat nor circular the degree of

roughness may determine whether a surface would be considered flat,

circular or irregular. V locating method may be used to locate certain

parabolic surfaces and button locators may be used to locate certain

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elliptical surfaces. It is necessary to locate from an irregular surface

only during the first machining operation, as it should produce holes or

surfaces that can used as a reference or locating points for subsequent

operations.

The unevenness of the surface of a casting will allow a maximum

of three contact points. More than three points will allow the casting

teeter. Moreover, if more than three points are used, the workpiece will

reform when clamping pressure is applied. It is therefore necessary to

used adjustable rest pins are equalizer to compensate for the

unevenness of the workpiece surface. One of the simplest type of

adjustable rest pins is a fixture jack the workpiece is positioned on three

non-adjustable locators, and the jack or jacks are adjusted until they

touch the workpiece locators. The constant pressure between the jacks

and the workpiece depends upon the judgment of the operator and is a

disadvantage when a number of jacks are used.

The Basic Principles of Clamping

Once a workpiece is located, it is necessary to press it against the

locating surface and hold it there against the forces acting upon it.

Clamps is largely determined by the workpieces and the kind of

operation to which it is applied.

However, clamps must fulfil four essential requirements:

The workpiece must be held rigidly while the cutting tools are

in operation.

The time required for loading or unloading the tool must be as

short as possible, which means it must be quick-acting.

When subjected to vibration, chatter, or heavy pressure, the

clamping must be positive.

The clamp must not damage the workpiece.

The clamp design should ensure that the clamping forces should be

directed toward the locating surfaces. They should be arranged in such

a manner that the thrust of the cutting tool is away from the clamp.

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Clamping forces should be applied over a heavy part of the workpiece

whenever possible, preferably directly over a rest pad in order to

prevent distortion of the workpiece. Clamping mechanisms should be

designed in such a manner that they cannot be applied in any way

except the correct one.

6. Types of clamps

While the types of clamps are numerous, they can be classified in

three basic groups namely:

Mechanical clamping

Hydraulic clamping

Pneumatic clamping

Commonly used clamps are readily available from tool specialty

companies. They are often referred to as standard clamps and should

be used whenever possible.

6.1Mechanical clamping

6.1.1 Screw Clamping:

Screw clamps are simple clamps where torque developed in a

screw thread is used to hold the workpiece. They have the advantage of

exerting adequate force besides resisting loosening tendencies set up

by vibration. It can be direct or indirect. There are different holding

forces for different screw sizes. However, they are relatively slow and

may not suitable for high production.

Fig 1: Screw clamping

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6.1.2 Strap clamping:

Strap clamps are simplest and most commonly used clamp. All

strap clamps employ the principles of levers. Numerous shapes are

available as standard parts to suits different applications. Strap clamps

can be easily slid onto and off of the workpiece for easy workpiece

removal. Essential parts of a standard strap clamps are clamping bar,

clamping knob, spherical washer, compression spring and pillar pin.

Fig 2: Strap clamping

6.1.3 Swing clamping:

Swing clamps are swinging type of strap clamp. It is a

combination of strap clamp and swinging clamp. It is used in cases

where the forces acting against the workpiece are light and heavy

clamping pressure is not required.

Fig 3: Swing clamping

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6.1.4 Cam clamping:

Cam clamps are used when it is desirable to push the workpiece

against stops or into a nest. Cam clamps provide an effective and rapid

means of clamping but they are not a positive clamping method and

may loosen under vibrating forces. They should not be used where

heavy clamping forces are required. There are two types of cam clamps

namely spiral and eccentric.

Fig 4: Cam-clamp assembly

6.1.5 Latch clamping:

Latch clamp is known for its advantage in the ease and speed of

manipulation. It is limited to light work as it is difficult to secure

rigidly. The types of latch clamps are Simple latch clamp, Cam type

latch, Self -locking clamp, Thumb screw latch.

Fig 5: Simple latch clamp

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6.1.6 Wedge clamping:

A plain wedge clamp consists of a movable inclined plane which

forces the work piece against a fixed stop. These clamps tend to loosen

under vibration. The taper end of the plain wedge ranges from 6 to 18

degree, depending upon the coefficient of friction of the metal. A plain

wedge clamp can be improved by adding levers and links, so that it is

quick-acting and provides lock.

Fig 6: Wedge clamping

6.1.7 Hinged clamping:

Hinged clamp provides rapid clearance for loading and unloading

the workpiece. It has a hinged bolt and hinged strap/plate which when

swung apart gives space to mount the workpiece.

Fig 7: Hinged clamping

6.1.8 Pivoted clamping:

Pivoted clamps are often pivoted at the centre to simplify their

operation. Tightening and loosening is done using knurled head screw.

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7. Hydraulic clamping

Hydraulically clamped fixtures have many advantages over

manually clamped fixtures. In most cases, these benefits reduce costs

for manufacturers allowing them to justify the initial investment for a

hydraulic clamping system.

7.1Advantages of Hydraulic Clamping:

More productivity: More parts will fit within machine envelope

due to the high clamp forces generated with small hydraulic

components.

Consistent clamping forces: Every cycle, your parts are clamped

with the same clamping force, eliminating variables and

improving process stability.

Repeatable clamp location: Every cycle, your parts are clamped

in the same location eliminating the variability in part deflection

from clamping forces.

Eliminates human error: Assurance that every clamp will be

actuated with every cycle, eliminating human error and missed

steps.

Eliminates human error: Assurance that your proven clamping

sequence was followed on every cycle.

Faster: load and unload times and more productivity when cycle

times are operator dependant.

Ergonomic efficiency: Allows operators to be consistently more

productive with less effort.

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Improved part stability: Hydraulic work supports can be used to

support the part and or dampen machining forces without

distorting the work piece. Manual work supports are easily

ignored, distort the part and cause miss-loads.

Flexibility: Sophisticated clamping sequences can be developed

with “live” hydraulic systems. Clamping can be sequenced

automatically during the machining cycle to provide clearance for

cutting tools, to remove forces for finish cuts of close tolerance

features, retain parts for robotic loading, reduce cycle times and

improve productivity.

8. Pneumatic clamping:

Pneumatic operated clamps differ from hydraulically operated

ones in the size of the cylinder, which is smaller with hydraulics

because of the higher pressures. Pneumatically operated clamps are not

used where heavy clamping pressures are required because extremely

large cylinders would be required. Pneumatically operated clamping

devices are convenient because most manufactures have a supply of

compressed air available. However, the line pressure may be too low

to provide adequate pressure to cylinders for clamping purposes. This

condition can be remedied by the installation of an air-to-hydraulic

booster, which is used to convert low pressure air into higher hydraulic

pressure for operating a hydraulic work cylinder. The booster operates

from regular shop-line pressure without pumps or high pressure valve.

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9. Locating pins:

Pins are used to locate the components and firmly hold the

workpiece in position. Button locators are also similar. Buttons are

generally shorter than pins and used for vertical location. Pins on the

other hand are used for horizontal location. Some of the common pins

used are cylindrical pins and Diamond pins. Diamond pins are relieved

on two sides to allow variations in the centre to centre distance of holes.

The locators must properly position the work piece and then maintain

its location against primary cutting forces throughout the machining

cycle. As it is a locating element it should have high hardness so the

locator is made with single piece of EN353 Case harden &Tempered

with 58 To 62 HRC. Threads are kept soft during hardening process.

Fig 8: Cylindrical and Diamond locating pins

10. Rough Guide:

Rough guide are used to guide component while loading and

unloading of the component. In this fixture two types of rough guides

were used one is conventional type of rough guide and another one is

bottom rough guide which is used to support the bottom side of the

component while loading. Distance between the component and rough

guide is 2mm. Material used for rough guide is C45, polystyrene.

Fig 9: Rough Guide

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11. Rest pad:

Rest pad is made by En-353 material. Component is rest on this

rest pads. Mainly where ever air holes are located air seat check holes

are given to the rest plate to check for proper resting of the component.

This rest pad is fixed to the base structure by using M6 socket head cap

screws. O-rings are placed at the connection of base structure and rest

pad for avoiding leakage of air. It works on pneumatics i.e. if there is

any leakage of air through it, it sends signals to the machine and the

machine stops.

Fig 10: Rest pad

Conclusion:

The above report is an account of various processes associated

with the industry that were imbibed into me by the exposure to such

activities. Specifically I was astonished to know the existence of many

pre and post machining and assembly processes and had a thorough

learning of the same. I profusely thank the industry for giving me the

opportunity to appreciate the processes in the engineering world.