Jigs and Fixtures for Machine Shops

A jig is a work-holding device that holds, locates and also guides the cutting tool (by drill bushes), e.g. a drilling jig. A fixture is a work-holding device that holds and locates the work but does not guide the tool; the tool is set relative to it by set blocks or feeler gauges, e.g. milling fixture.

Differences

Purposes and advantages

1. Eliminates marking out, trial and individual setting; interchangeability.
2. Higher production and lower cycle time; semi-skilled labour is enough.
3. Consistent accuracy, repeatability, and lower scrap.
4. Safe operation and reduced operator fatigue.
5. Allows multiple operations on the same setup, and permits use of automatic machines.

Economical only if the batch size is large enough to repay the tooling cost.

3-2-1 principle of location

A free body in space has 12 degrees of freedom: 6 translations (+/- along x, y, z) and 6 rotations. A rigid body is completely located by restraining all of them using only 6 locating points, in the pattern 3-2-1.

         z
         |   Plane 1: 3 points on base (XY)
         |        . . .   (restrain 1 translation
         |                 and 2 rotations)
         +----- y   Plane 2: 2 points on side
        /              (restrain 1 translation, 1 rotation)
       x     Plane 3: 1 point on end face
                       (restrain last translation)

1. Three points on the base (primary plane) fix the height and two tilts.
2. Two points on a side (secondary plane) fix the side position and rotation about the vertical axis.
3. One point on an end face (tertiary plane) fixes the last translation.

The remaining 6 freedoms (movement in the opposite directions) are removed by the clamps. For round work, a V-block and a flat or a locating pin are used instead. A locating surface should be a machined face (not a rough casting) and the points should be spread as far apart as possible.

Design principles

1. Location: use the 3-2-1 principle; locate from machined datum faces; locators hard and replaceable.
2. Clamping: clamp must resist cutting force but not distort the work; clamp near the support; quick acting (cam, toggle, hinged strap); clamping force should act against the locators, not the cutting force.
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; no 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; low cost for the batch.

Types of drill jigs

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

Straddle-milling fixture

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