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Drill jig and fixture design considerations

Short answer

Design uses 3-2-1 location from machined datums, clamping over supports, rigid bodies, chip clearance, foolproof loading and economy. Drill jigs include template, plate, box or leaf types, with locators, clamps and hardened bushes. A straddle-milling fixture mills two parallel faces with spaced cutters and uses a set block with feeler gauge for positioning.

List the principles to be considered in the design of jigs and fixtures. Describe any three types of drill jigs and the main elements (locators, clamps, bushes) used. Briefly describe a straddle-milling fixture.

Design principles

  1. Location: use the 3-2-1 principle; locate from machined datum faces; locators should be hard and replaceable.
  2. Clamping: the clamp must hold the work firmly without distorting it; clamp directly over a support; use quick-acting clamps (cam, toggle, hinged strap). The clamping force should push the work against the locators, and the cutting force should be taken by the locators, not by the clamps.
  3. Rigidity and strength: the body should resist cutting forces without vibration; use ribs and short overhangs.
  4. Chip clearance: allow chips and coolant to escape; avoid pockets where chips collect; provide openings for cleaning.
  5. Foolproofing (poka-yoke): the part can be loaded only in the correct way.
  6. Ease of loading and unloading, operator safety, and light weight (for handling).
  7. Economy: use standard parts and cast or fabricated bodies; keep the cost low for the batch.

Types of drill jigs

JigDescriptionUse
Template jigPlate with holes laid on the work; no clampingFew parts, light work
Plate jigPlate with bushes and clampsParts with holes in one plane
Box (tumble) jigBox with bushes on several sides; the work is turned overHoles on several faces
Leaf jigHinged leaf swings open for loadingSmall parts
Channel jigChannel-shaped body holding the work between its sidesSimple, small work

Elements: locators (pins, V-blocks, pads, buttons), clamps (strap, screw, cam, toggle, quick-acting), and drill bushes - fixed (press fit), renewable (with a liner) and slip bushes, made of hardened steel, which guide the tool.

Straddle-milling fixture

In straddle milling, two side milling cutters mounted on one arbor and spaced apart by collars mill two parallel faces in one pass (e.g. the two flats of a bolt head). The fixture holds the work on the table with a V-block or a vice-like clamp, and a set block fixed on the body shows the cutter position; a feeler gauge (usually 2-3 mm) is placed between the cutter and the set block to avoid contact. Tenons on the underside locate the fixture in the T-slot.

✓ Answer checked by Claude Opus 5.5 (high effort)

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