Fixture Design: Locate First, Then Clamp

A free part can move in three directions and rotate about three axes. The fixture's locating surfaces remove those six degrees of freedom in a repeatable way. Clamps should hold the part against the locators, not become the locators themselves.

1. Three points on the primary face. Removes Z translation and rotation about X and Y: three degrees of freedom. Spread the points as widely as possible; the larger the triangle, the more stable.
2. Two points on the secondary face. Removes Y translation and rotation about Z: two degrees of freedom.
3. One point on the tertiary face. Removes the last X translation.

The 3-2-1 model uses three, two and one contacts to establish a repeatable position. Extra fixed contacts can over-constrain an imperfect surface and make the part rock. If more support is needed, bring an adjustable support into contact after locating the part.

• Never clamp over unsupported material. Clamping force must be met by a support directly beneath, or the part bends, springs back when released, and the machined face is distorted.
• Never clamp on a machined face, and never on material that is about to be cut away.
• Never let cutting force push the part off its locators. Plan the tool path so the force presses the part into its datums rather than away from them.

How much clamping force

Clamping force must resist cutting loads without distorting the workpiece. Thin walls and softer materials often need the force spread over more contact area, not a harder push at one point. Consider vacuum, magnetic or shaped supports where appropriate.

The fixture should locate from the functional datums shown on the drawing whenever practical. If machining uses another face as its reference, include the resulting datum-transfer error in the tolerance plan.