title = "Machining Fixtures and Jig Design"

content = """
Fixtures and jigs are workholding devices that locate, support, and clamp a workpiece during machining to ensure consistent, accurate, and repeatable part production. At Ridgecrest Metalworks, proper fixture design and use is fundamental to achieving dimensional consistency across production lots.

The 3-2-1 (six-point) locating principle governs fixture design. A workpiece has six degrees of freedom (three translations, three rotations) that must be constrained to position it uniquely and repeatably. Three locating points on the primary datum surface (typically the largest flat surface) constrain three degrees of freedom. Two locating points on a secondary datum surface constrain two more. One locating point on a tertiary datum constrains the last. Points are implemented as precision pins, pads, or buttons. Over-constraining (redundant locators) must be avoided because it creates ambiguous and inconsistent part location.

Clamping applies force to hold the workpiece against the locating elements without distorting the part or causing it to lift or shift during cutting. Clamping force direction should always be toward primary locating surfaces. Clamp force must be sufficient to resist cutting forces but not so great as to distort thin sections. Strap clamps, toggle clamps, and hydraulic clamps are common types. Clamp placement away from unsupported areas prevents bending.

Jigs are a specialized type of workholding that also guides the cutting tool—drill jigs have hardened drill bushings that guide the drill to the correct position and prevent drill wander. This eliminates the need for center drilling or spot drilling and is essential for hole patterns that must be held to close positional tolerances in production.

Fixture design considerations include: adequate chip evacuation (chips must not pack under locators or in the part), coolant access to the cutting zone, part loading and unloading ergonomics, and quick-change capability for production flexibility. Tombstone fixtures on horizontal machining centers hold multiple parts or multiple faces of one part, enabling high-volume multi-side machining with minimal repositioning.

Fixture maintenance requires regular inspection of locator wear and replacement of worn buttons, cleaning of locating surfaces, and verification of clamping mechanism function. A worn locator can shift part position by several thousandths of an inch, causing dimensional failures that are difficult to diagnose.
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