A manufacturing fixture must place a workpiece repeatably and resist process forces without overconstraining or damaging the part.

A free rigid body has six degrees of freedom: three translations and three rotations. The 3-2-1 locating principle removes them using three sets of locating contacts:

• three points on a primary locating plane;
• two points on a secondary plane;
• one point on a tertiary plane.

The three primary contacts establish a plane and remove one translation plus two rotations. The two secondary contacts remove another translation and one rotation. The final contact removes the last translation.

Locators determine position. Clamps push the part against those locators and hold it there. A clamp should not be used as an uncertain locator if repeatable geometry matters.

For example, a rectangular block can rest on three pads, contact two side pins and one end stop. A top clamp then presses it toward the primary pads. If the clamp force is directed so that it seats the part against the locators, loading improves repeatability rather than fighting the locating scheme.

Real parts are not perfect. Too many hard locating contacts can make a fixture sensitive to dimensional variation: the part may rock, jam or deform. Good fixturing therefore balances constraint, accessibility, rigidity and loading speed.

The same logic applies to machining fixtures, inspection fixtures and assembly tooling: establish a clear reference frame, constrain only the required motions, then apply holding forces that preserve that reference.