The 3-2-1 principle, and three things never to clamp against

A rigid body in space has six degrees of freedom: translation along X, Y and Z, and rotation about each. Locating means removing all six one at a time. Clamping only maintains that state; it does not locate.

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.

Three plus two plus one is six, exactly fully located. One more support point is over-constraint: four points cannot all touch an imperfect plane, so the part rocks and sits differently each time. Where extra rigidity is genuinely needed, use an adjustable or floating support brought up after the part is located and clamped.

• 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 has to exceed the tendency of cutting forces to slide or tip the part, while staying below what would deform it. Thin-walled, aluminium and plastic parts have a very low ceiling, so the usual answer is more clamping points rather than more force per point, or a vacuum chuck or magnetic fixture to spread the load.

A fixture's locating datums should correspond to datums A, B and C on the drawing. If the fixture locates on a different face, the machined dimensions carry a datum transfer error, and that error routinely eats more than half the tolerance band.