chapter 27: workholding devices for machine tools degarmo’s materials and processes in...

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 turningcenter with two chucks, turrets forcutting tools, and C-axis control forthe main spindle. The C-axiscontrol, on the spindle, can stop itin any orientation so the poweredtools 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 aplate showing locatingdimensions (a, b, c, d) versussizing 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 Exaggeratedillustration of the manner inwhich excessive clamping forcescan affect the final dimensions ofa 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 clearancebetween drill bushing and toolof workpiece is important.

Chip Clearance

FIGURE 27-8 Methods ofproviding chip clearance toensure proper seating of thework.

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 indrill 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

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 Vises

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 Vee blocks Angle plates Cubes Box parallels Supports Locator pins Clamps

Modular Fixtures

FIGURE 27-14 Modularfixturing begins with a subplate(grid base) and adds locatorsand 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 jigdesigned for a family of similarcomponents. (a) Part family ofrounds 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 typesof clamps used for workholding. The clampelements come in a wide variety of sizes.

Power Actuated Clamps

FIGURE 27-21 Examples ofpower-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 ofelectrostatic chuck.

Vacuum Chuck

FIGURE 27-24 Cutaway viewof a vacuum chuck. (Courtesy ofDunham 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

Economic Justification

Economic Justification