Transcript ch27 - lecture
Chapter 27: Workholding Devices for Machine Tools
DeGarmo’s Materials and Processes in Manufacturing
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.
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.
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
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
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?
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.
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.
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
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
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
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
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
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
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
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
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 Angle plates Vee blocks Cubes Box parallels Locator pins 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
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.
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
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.
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
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