Chapter 27: Workholding
Devices for Machine Tools
DeGarmo’s Materials and Processes in
Manufacturing
27.1 Introduction
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Workholding Devices are call Jigs and
Fixtures.
Jigs and Fixtures are critical to repeated
manufacturing to with high degrees of
accuracy and precision.
Jigs and Fixtures hold one or multiple parts in
one or multiple machine centers to provide
stability and repeatable alignment of the part.
27.2 Conventional Fixture Designs
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Workholding devices provide to fundamental
functions, locating and clamping.
Locating refers to orienting and positioning
the part relative to the cutting tool.
Clamping refers to holding the part in its
proper orientation with enough force to resist
the force of cutting but not deform the part.
Example of a workholder
FIGURE 27-2 A CNC turning
center with two chucks, turrets for
cutting tools, and C-axis control for
the main spindle. The C-axis
control, on the spindle, can stop it
in any orientation so the powered
tools can operate on the
workpiece.
Jigs and Fixtures
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A Jig is a special workholding device that,
through built-in features, determines location
dimensions that are produced by machining
or fastening operations.
A Fixture is a special workholding device that
holds work during machining or assembly
operations and establishes size dimensions
General purpose clamps and chucks are not
fixtures or jigs.
Location versus Sizing
FIGURE 27-3 Drawing of a
plate showing locating
dimensions (a, b, c, d) versus
sizing dimensions (e, f, g, h).
27.3 Tool Design Steps
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The classical design of a workholder (e.g., a drill jig)
involves the following steps:
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1. Analyze the drawing of the workpiece and determine
(visualize) the machining operations
required to machine it. Note the critical (size and location)
dimensions and tolerances.
2. Determine the orientations of the workpiece in relation to
the cutting tools and the movements of the tools and
tables.
3. Perform an analysis to estimate the magnitude and
direction of the cutting forces (see Chapter 21).
4. Study the standard devices available for workholders
and for the clamping functions. Can an off-the-shelf device
be modified? What standard elements can be used?
Tool Design Steps, cont.
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5. Form a mental picture of the workpiece in position in the
workholder in the machine tool with the cutting tools
performing the required operation(s). See the figures in
chapters on machining for examples.
6. Make a three-dimensional sketch of the workpiece in the
workholder in its required position to determine the location
of all the elements: clamps, locator buttons, bushings, and
so on.
7. Make a sketch of the workholder and workpiece in the
machine tool to show the orientation of these elements with
respect to the cutting tool in the machine tool.
3-2-1 Location Principle
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The 3-2-1 location principle is used to ensure
that every part placed in the device occupies
the same position with respect to the cutting
tools
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The principle is based on first establishing a
plane, locating the part on three fixed points.
Then location the part to a second plane,
perpendicular to the first by using two points.
And finally locating the part relative to the first two
planes by establishing a third plane perpendicular
to the first two planes using a single point.
3-2-1 Principle
FIGURE 27-4 Workpiece location is based on the 3-2-1 principle. Three points will define a
base surface, two points in a vertical plane will establish an end reference, and one point
in a third plane will positively locate most parts.
27.4 Clamping Considerations
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Clamping forces do produce stresses in the part,
excess clamping forces can cause distortion
Clamping force should be in the direction of cutting
forces
Clamping should be designed such that the cutting
forces work against the fixed portion of the clamp,
not the movable portion.
Clamping forces should be as near in alignment with
the cutting forces to minimized torsional moment.
Distortion During Clamping
FIGURE 27-5 Exaggerated
illustration of the manner in
which excessive clamping forces
can affect the final dimensions of
a workpiece.
Clamping Examples
FIGURE 27-6 In (a) and (b), proper work
support to resist the forces imposed by
cutting tools is demonstrated. In (c), three
buttons form a triangle for the work to rest
on.
27.5 Chip Disposal
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Jigs and Fixture need to accommodate chip
removal
Proper clearances need to be made to
ensure chips do build up, increasing heat in
the tool.
Chips must also be easy to remove after
machining so that they do not interfere with
the alignment of the next workpiece.
Proper Chip Clearance
FIGURE 27-7 Proper clearance
between drill bushing and tool
of workpiece is important.
Chip Clearance
FIGURE 27-8 Methods of
providing chip clearance to
ensure proper seating of the
work.
27.6 Unloading and Loading Time
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Time to clamp and unclamp a workpiece can
reduce the rate of production.
Clamp design should minimize the motion
needed to remove a part.
Cams latches are faster mechanisms than
screw mechanisms.
27.7 Examples of Jig Design
FIGURE 27-9 (Lower left) Part to be
drilled; (lower right) box drill jig for drilling
two holes; (upper left) jig in
drill press; (upper right) drill being guided
by drill bushing.
27.8 Types of Jigs
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There are several basic forms for jigs, some
of the basic types are:
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Plate Jig
Channel Jig
Ring Jig
Leaf Jig
Box Jig
Universal Jigs
Common Jigs
FIGURE 27-10 Examples of
some common types of
workholders—jigs.
Universal Jigs
FIGURE 27-11 Two types of universal jigs are manual
(bottom) and power-actuated (center). A completed jig
(on the top) made from unit right below.
27.9 Conventional Fixtures
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Conventional Fixtures
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A Vise are general purpose fixtures mounted on
subplates and can have their jaws interchanged
base on part geometry.
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Lathe Chucks are general purpose fixtures for
rotational parts
Conventional
Vises
FIGURE 27-12 The conventional or
standard vise (top left and right) can
be modified with removable jaw
plates to adapt to different part
geometries. These vices can be
integrated into milling fixtures (right
middle and bottom).
Conventional Chucks
FIGURE 27-13 Quick-changing of the top jaws on a three-jaw chuck.
27.10 Modular Fixturing
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Modular Fixtures are similar to conventional
fixture, except they are more versatile.
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Modular systems use dowel pins and T-slots to
provide a rigid, adjustable fixture.
Standard elements are positioned to fit the part
needs, such as
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Riser blocks
Angle plates
Box parallels
Locator pins
Vee blocks
Cubes
Supports
Clamps
Modular Fixtures
FIGURE 27-14 Modular
fixturing begins with a subplate
(grid base) and adds locators
and clamps.
Modular Fixture
FIGURE 27-15 Dedicated fixture on the left versus modular fixture on the right.
27.11 Setup and Changeover
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To speed up changeover, master jigs or
intermediate jigs can be used.
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A Master Jig, is a jig that can be used to make a
number of similar parts.
An Intermediate jig is a jig that is designed hold
another jig that can be quickly changed out for
each part.
Master Jig
FIGURE 27-18 Master jig
designed for a family of similar
components. (a) Part family of
rounds plates (six parts, A–F);
(b) group jig for drilling,
showing adapter and part A.
Intermediate Jig
FIGURE 27-19 Example of the intermediate jig concept applied to lathe chucks.
The actuator is mounted on the lathe and can quickly adapt to three different chuck
types. (Courtesy of ITW Workholding)
27.12 Clamps
FIGURE 27-20 Examples of basic types
of clamps used for workholding. The clamp
elements come in a wide variety of sizes.
Power Actuated Clamps
FIGURE 27-21 Examples of
power-clamping devices:
(a) extending clamp;
(b) edge clamp.
27.13 Other Workholding Devices
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Other workholding devices include
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Assembly jigs
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Magnetic workholders
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Limited in holding force, but ensures that there is no distortion
of a steel workpiece
Electrostatic workholders
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Used to keep ensure the final assembly meets the location
and fit
Similar to magnetic chucks, but used on electrically coductive
non-ferromagnetic materials, limited clamping force
Vacuum Chucks
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Works with any material, initial set up more time consuming.
Assembly Jig
FIGURE 27-22 Example of large assembly jig for an airplane wing. The
body of the wing and flap are held in the correct location with each other
and then the flap is mechanically attached.
Electrostatic Chuck
FIGURE 27-23 Principle of
electrostatic chuck.
Vacuum Chuck
FIGURE 27-24 Cutaway view
of a vacuum chuck. (Courtesy of
Dunham Tool Company, Inc.)
27.14 Economic Justification of Jigs and
Fixtures
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To determine the economic justification of any
special tooling, the following factors must be
considered:
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1. The cost of the tooling
2. Interest or profit charges on the tooling cost
3. The savings resulting from the use of the tooling; can
result from reduced cycle times or improved quality or
lower-cost labor
4. The savings in machine cost due to increased
productivity
5. The number of units that will be produced using the
tooling
Economic Justification
Economic Justification