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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.
ELEMENTS OF JIGS AND FIXTURES
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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.
ELEMENTS OF JIGS AND FIXTURES
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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 -).
3-2-1 PRINCIPLE OF LOCATION
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 -).
3-2-1 PRINCIPLE OF LOCATION
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.
3-2-1 PRINCIPLE OF LOCATION
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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