LECTURE PRESENTATION

Reading Assignment:

DeGarmo Chapter 25 – Workholding Devices for Machine Tools

Optional Reading:

• Setup Reduction through Effective Workholding

Outline

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.

• 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.

 

Design Steps

The classical design of a workholder (e.g., a drill jig) involves the following steps:

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

 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

 

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

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:

Plate Jig

Channel Jig

Ring Jig

Leaf Jig

Box Jig

Universal Jigs

Common Jigs

Universal Jigs

 

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

Softjaws

Other Jaws

Kurt Hydramax

Conventional Chucks

Tombstones

 

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

 

See also

• Carr-Lane (machining fixtures)
• Bluco (mostly welding fixtures)

 

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.

Clamps

Power Actuated Clamps

Manual Clamps

Swing Clamps

Strap 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