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Chapter 27: Workholding Devices for Machine Tools

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Chapter 27: Workholding Devices for Machine Tools DeGarmo s Materials and Processes in Manufacturing Power Actuated Clamps FIGURE 27-21 Examples of power-clamping ... – PowerPoint PPT presentation

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Title: Chapter 27: Workholding Devices for Machine Tools


1
Chapter 27 Workholding Devices for Machine Tools
  • DeGarmos Materials and Processes in
    Manufacturing

2
27.1 Introduction
  • 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.

3
27.2 Conventional Fixture Designs
  • 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.

4
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.
5
Jigs and Fixtures
  • 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.

6
Location versus Sizing
FIGURE 27-3 Drawing of a plate showing
locating dimensions (a, b, c, d) versus sizing
dimensions (e, f, g, h).
7
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8
27.3 Tool Design Steps
  • The classical design of a workholder (e.g., a
    drill jig) involves the following steps
  • 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?

9
Tool Design Steps, cont.
  • 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.

10
3-2-1 Location Principle
  • 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
  • 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.

11
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.
12
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13
27.4 Clamping Considerations
  • 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.

14
Distortion During Clamping
FIGURE 27-5 Exaggerated illustration of the
manner in which excessive clamping forces can
affect the final dimensions of a workpiece.
15
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.
16
27.5 Chip Disposal
  • 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.

17
Proper Chip Clearance
FIGURE 27-7 Proper clearance between drill
bushing and tool of workpiece is important.
18
Chip Clearance
FIGURE 27-8 Methods of providing chip clearance
to ensure proper seating of the work.
19
27.6 Unloading and Loading Time
  • 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.

20
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.
21
27.8 Types of Jigs
  • There are several basic forms for jigs, some of
    the basic types are
  • Plate Jig
  • Channel Jig
  • Ring Jig
  • Leaf Jig
  • Box Jig
  • Universal Jigs

22
Common Jigs
FIGURE 27-10 Examples of some common types of
workholdersjigs.
23
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.
24
27.9 Conventional Fixtures
  • Conventional Fixtures
  • A Vise are general purpose fixtures mounted on
    subplates and can have their jaws interchanged
    base on part geometry.
  • Lathe Chucks are general purpose fixtures for
    rotational parts

25
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).
26
Conventional Chucks
FIGURE 27-13 Quick-changing of the top jaws on a
three-jaw chuck.
27
27.10 Modular Fixturing
  • Modular Fixtures are similar to conventional
    fixture, except they are more versatile.
  • 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
  • Riser blocks Vee blocks
  • Angle plates Cubes
  • Box parallels Supports
  • Locator pins Clamps

28
Modular Fixtures
FIGURE 27-14 Modular fixturing begins with a
subplate (grid base) and adds locators and clamps.
29
Modular Fixture
FIGURE 27-15 Dedicated fixture on the left versus
modular fixture on the right.
30
27.11 Setup and Changeover
  • To speed up changeover, master jigs or
    intermediate jigs can be used.
  • 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.

31
Master Jig
FIGURE 27-18 Master jig designed for a family of
similar components. (a) Part family of rounds
plates (six parts, AF) (b) group jig for
drilling, showing adapter and part A.
32
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)
33
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.
34
Power Actuated Clamps
FIGURE 27-21 Examples of power-clamping
devices (a) extending clamp (b) edge clamp.
35
27.13 Other Workholding Devices
  • Other workholding devices include
  • Assembly jigs
  • Used to keep ensure the final assembly meets the
    location and fit
  • Magnetic workholders
  • Limited in holding force, but ensures that there
    is no distortion of a steel workpiece
  • Electrostatic workholders
  • Similar to magnetic chucks, but used on
    electrically coductive non-ferromagnetic
    materials, limited clamping force
  • Vacuum Chucks
  • Works with any material, initial set up more time
    consuming.

36
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.
37
Electrostatic Chuck
FIGURE 27-23 Principle of electrostatic chuck.
38
Vacuum Chuck
FIGURE 27-24 Cutaway view of a vacuum chuck.
(Courtesy of Dunham Tool Company, Inc.)
39
27.14 Economic Justification of Jigs and Fixtures
  • To determine the economic justification of any
    special tooling, the following factors must be
    considered
  • 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

40
Economic Justification
41
Economic Justification
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