Guides / Process

3-2-1 Locating: How a Fixture Holds a Part in the Same Place

November 1, 2026

A rigid part in space can move in six ways: along three axes and around three axes. A fixture has to stop all six, and it should do it with as few contact points as possible. That is the 3-2-1 principle.

The six points

Points	Surface	Stops
3	Primary datum, the largest flat face	Movement down and rotation around two axes
2	Secondary datum, a long side	Movement sideways and rotation around the vertical axis
1	Tertiary datum, an end face	The last movement along the length

Three points define a plane. Two define a line. One defines a point. Together they put the part in exactly one place.

Where to put the locators

Spread the three primary points as far apart as the part allows. A wide triangle resists rocking. Put the two secondary points on the longest straight edge, again far apart. Use the datums from the part drawing whenever possible, so the fixture measures the part the same way the drawing does.

Then clamp

Clamps hold the part against the locators. Each clamp should push directly toward a locator or a support. Clamping on an unsupported wall bends the part, and it springs back when released.

Common mistakes

1. More than three points on the primary face, so the part rocks between them
2. Locating on a rough cast surface that changes from part to part
3. Clamps that block the tool path or the operator’s hands
4. No way to remove chips from locating surfaces

Check before machining

A fixture design review takes an hour and saves a remake. Check that every locator touches a datum surface, every clamp pushes toward a support and the part can only be loaded one way.

Frequently asked questions