DeGarmo Chapter 25 – Workholding Devices for Machine Tools
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.
- Jigs hold and or guide a tool (usually a cutting tool)
- Fixtures hold and or guide a workpiece.
Conventional Fixture Designs
Workholding devices provide 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.
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.
The classical design of a workholder (e.g., a drill jig) involves the following steps:
- Understand the work piece and tolerances
- Visualize Machining Steps
- Determine orientation of work in relation to the cutting tools
- Study standard devices available
- Form a mental (or CAD) picture of the workipece in position.
- Locate Clamp, buttons, bushings
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.
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
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
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.
Types of Jigs
There are several basic forms for jigs, some of the basic types are:
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
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
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.
Power Actuated Clamps
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
- Magnetic Chuck
- Vacuum Chuck
- Pallet Systems
- T-Slot Table
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