Designing a clamping fixture: the 3-2-1 principle in practice

Many dimensional deviations in production do not come from the machine but from part location. A fixture that over-constrains or distorts the part creates scatter that nobody can compensate afterwards.

Three support points define the primary plane, two more set the orientation in the secondary plane, and a sixth acts as the stop in the third direction. Together these six points remove all six degrees of freedom. Additional supports may support but must not locate, otherwise over-constraint creates a changing contact situation.

Clamping forces belong where the part is supported and should act against a fixed support. Welding adds thermal distortion: allow degrees of freedom for expansion, otherwise the assembly moves out of tolerance.

Load, clamp, unload, every movement counts. Short distances, visible contact faces, toggle clamps instead of screw clamps and free chip or fume extraction turn a correct fixture into one that production actually accepts.