Jigs and Fixtures in Manufacturing

Introduction to Jigs and Fixtures

• Jigs and fixtures are devices used for mass production of repeated parts.
• Key functions include locating, clamping, supporting, and resisting cutting forces. Safety is also crucial.

Jig Overview

• Both jigs and fixtures hold, support, and locate workpieces.
• Jigs also guide the cutting tool.

Fixture Overview

• Fixtures hold, support, and locate workpieces.
• Fixtures have a reference point for setting the cutting tool, relative to the workpiece.

Purpose of Jigs and Fixtures

• Eliminate marking, punching, positioning, and alignments.
• Provide easy, quick, and accurate locating, supporting, and clamping of workpieces.
• Guiding cutting tools like drills and reamers.
• Increase productivity and maintain product quality.
• Reduce operator effort and skill requirements.
• Improve machine tool technological capacity.
• Reduce machining cost and increase interchangeability.

Design Considerations for Jigs and Fixtures

• Jigs and fixtures are manually or partially power-operated devices.
• They are comprised of various elements: base, body/frame (with clamping features), locating elements, supporting surfaces/base, clamping elements, tool guiding frame/bushes (for jigs), auxiliary elements, and fastening parts.

Locating Principle

• Workpieces have six degrees of freedom (or twelve, considering positive and negative movement in each category).
• Locating requires arresting all six degrees of freedom for mechanical stability.
• A single locator in plane 1 arrests linear motion along the X-axis.
• A second locator in the same plane arrests rotary motion around the Z-axis.
• Another in the same plane arrests rotary motion about the Y-axis.

Additional Locations

• Placing a fourth locator in plane 2, perpendicular to plane 1, restricts linear motion along the Y-axis.
• A fifth locator in plane 2 arrests rotational motion about the X-axis.
• The sixth locator in plane 3 (perpendicular to both planes 1 and 2) arrests linear motion along the Z-axis.

Considering 12 Degrees of Freedom

• To locate a workpiece, fix all 12 degrees of freedom (except three transitional degrees of freedom (–X, –Y, and –Z).
• Resting a workpiece on three non-collinear points on the bottom surface (XY plane ) will fix +Z, clockwise rotation-X, and counter-clockwise rotation-X, C Rot-Y, and AC Rot-Y degrees of freedom.
• Fixing the workpiece by two points on the side surface (XZ plane) fixes +Y, C Rot-Z, and AC Rot-Z degrees of freedom.
• Lastly, fixing the workpiece at one point on an adjoining surface (YZ plane) fixes the +X degree of freedom.

Points to Ponder

• If using multiple locators on a surface/plane, distribute them as far apart as possible.
• Choose a surface for large locators, considering the workpiece's largest area.

Duplex Fixtures

• Used in high-speed, high-volume production, where machining cycles must be continuous.
• These use only two stations, revolving to continuously machine parts.

Milling Fixtures

• Most common jigs in use today due to workpieces' geometric complexity.
• Simplest type is a milling vise mounted on a machine table.
• Fixtures become more complex as workpiece size, shape, or complexity increase.

Tool Changes in Milling Fixtures

• Changing tools is preferable to moving parts for efficient operations.
• Locators should resist all tool forces; clamps should not be used for this purpose.
• Sufficient clearance is required for tool changes and part loading/unloading.
• Fixtures should be low-profile to prevent twisting or springing during operation.

Setting Blocks

• Used to accurately position workpieces relative to cutters, after fixtures are clamped.
• Achieved through setting blocks and feeler gauges.
• Setting blocks are fixed to the fixture.
• Feeler gauges position cutters relative to workpiece reference surfaces/planes.

Ejectors

• Used to remove workpieces from close-fitting locators (e.g., full nests, ring nests).
• Speed up unloading, reducing in-tool time and increasing production.

Clamping

• Essential for firmly restraining workpieces in jigs and fixtures to resist applied forces during operation.
• Proper clamping directly affects the accuracy and quality of work and production time.

Strap Clamps

• Utilize lever principles to amplify clamping force.
• Tightening a stud transfers clamping force to the part.
• The fulcrum of the lever is the heel pin.
• Clamping force amplification depends on distances between the stud, part, and heel pin.

Screw Clamps

• Faster clamping methods use swing/cee washers, especially when workpieces have bores.
• Clamped components are quickly loaded/unloaded.
• The hole for clamping should be larger than the clamping nut.

Cam Clamps

• Provide clamping force based on the cam's contour.
• The plate is pressed down against a spring, holding the part.
• Cam clamps are quick and come in different designs (eccentric, flat spiral, cylindrical).
• Indirect pressure clamping ensures stability and resistance to machining vibrations.

Toggle Clamps

• Quick-acting mechanical linkages.
• Utilized for their fast clamping/unclamping actions, and for work pieces clearances.
• Provide force amplification suitable for clamping operations.

Equalizers

• Useful when clamping force is needed at multiple locations.
• Link arm systems are used to distribute clamping force evenly to multiple-clamp points.
• Convenient when operator access to one clamp is limited.

Test your knowledge on the use of jigs and fixtures in manufacturing processes. This quiz covers the fundamental principles, benefits, and design considerations involved in the effective use of jigs and fixtures. Explore concepts like the 3-2-1 locating principle and the role of clamping elements.