Toolmaking: jigs, fixtures, dies and moulds
Toolmaking: jigs, fixtures, dies and moulds tests how work is located, clamped and guided, and how dies and moulds are constructed for repeatable production.
Core rules
- A jig locates and clamps the workpiece and also guides the cutting tool, usually a drill. A fixture locates and clamps but does not guide the tool.
- The 3-2-1 principle uses three points on the primary datum, two on the secondary and one on the tertiary to remove six degrees of freedom.
- Locators should be hardened and replaceable where wear matters. Clamps should act over supports, not spring the work.
- Drill jig bushes are fixed, slip or renewable. Slip bushes suit tools that must change size; fixed bushes give better guidance.
- Die clearance depends on material and thickness. Too little clearance causes pick-up and burrs; too much gives a poor edge and excessive rollover.
- Moulds need draft angle, shrinkage allowance, a parting line, gating, ejection and temperature control.
Worked example
A rectangular block is to be drilled in a jig. Place its largest face on three hardened pads, push two edges against two locators, and clamp over a support. This 3-2-1 arrangement fixes position and prevents lifting or rotation. Then fit the bush so the drill is guided at the correct position; the bush bore should clear the drill but not allow excessive wander.
Traps
Over-constraining a machined face can distort the part; using a fixture as a jig gives no drill guidance; forgetting draft makes mould ejection difficult; and using one clearance value for all sheet materials causes burrs or short die life.