The Part That Wouldn’t Locate Consistently
We designed a welding fixture. The part sat on two support pads. Two side stops positioned it left-right. On the first part, it welded fine. On the tenth part, the weld was 2 mm off. The problem: the part had a casting flash on the bottom face. The two support pads were on the same plane, but the flash made the part rock between them. It wasn’t sitting flat. The locators didn’t control the part’s orientation — it could pivot. The fix: add a third support pad at the far end (a 3-2-1 layout) and a pin that locates a hole. The part sat on three points (not two), and the hole pinned its rotation. The weld was consistent. The mistake was using two supports (which leaves the part free to rock) instead of three (which defines a plane).
Datum and locator design for fixtures is about constraining the part’s six degrees of freedom. Too few locators, and the part moves. Too many, and it binds. This article covers the standard 3-2-1 principle.
The Six Degrees of Freedom
A rigid part in space has six ways to move:
- Three translations: X, Y, Z (slide along each axis).
- Three rotations: Around X, Y, Z (pitch, roll, yaw).
A fixture must locate the part so it can’t move in any of these. The locators constrain these six DOFs.
The 3-2-1 Locating Principle
The standard method to locate a part:
- Three points (3): On the primary datum surface (the part’s bottom). Three points define a plane (Z translation, and rotation around X and Y are constrained). The part sits on three support pads. Any more points on the same plane over-constrain (if the part’s bottom isn’t perfectly flat, it rocks).
- Two points (2): On a secondary datum (a side face). Two points along one edge constrain Y translation and rotation around Z. The part pushes against two side locators.
- One point (1): On a tertiary datum (another side face or a pin in a hole). One locator constrains the last translation (X). The part touches this last stop.
3 + 2 + 1 = 6 locators, constraining all six DOFs. The part is fully located. It can’t move or rotate.
| Stage | Locators | Surface | Constrains |
|---|---|---|---|
| Primary (3) | 3 support pads | Bottom face | Z translation, X rotation, Y rotation |
| Secondary (2) | 2 side pads/pins | Side face | Y translation, Z rotation |
| Tertiary (1) | 1 side stop (or pin) | End face | X translation |
Why Three Points (Not Two or Four)
Two points: The part can rock (rotate around the line between the two points). Not stable. The casting flash problem we had.
Three points: The three points define a plane. The part sits flat. It can’t rock. This is the minimum for a stable plane.
Four points: If the part’s bottom isn’t perfectly flat (and it never is, for castings or welded parts), four points means only three touch. The part rocks on three, and the fourth point either misses or forces the part to bend. Over-constraint. Use three points on the primary datum, and spring-loaded supports (or adjustable supports) for any additional contact.
Datum Selection: Use the Part’s True Datum
The locators should reference the part’s own datum features (the surfaces used in the drawing). If the part’s drawing specifies the bottom face as datum A, locate on that face. Don’t pick a random surface — use the datum the design engineer specified.
Why? Because the machining or assembly tolerances are relative to that datum. If you locate on a different surface, the tolerances stack up. The feature that was machined relative to datum A won’t align with the fixture.
Locating Pins (For Holes)
When the part has precision holes, use pins instead of surface locators.
- One round pin: Locates two translations (X and Y). The hole sits on the pin. The part can rotate around the pin.
- One diamond pin: The second pin (in a second hole) is a “diamond” (flat on two sides). It constrains the rotation around the first pin but allows for hole-to-hole distance variation (the diamond slides along one axis). This prevents binding.
Two pins (one round, one diamond) locate the part in a plane without over-constraining. The round pin takes the X and Y location. The diamond pin takes the rotation but allows the hole distance to vary.
The fixture locating rule: Use 3-2-1. Three support pads on the primary datum (the part’s bottom). Two side locators on the secondary face. One stop (or diamond pin) on the tertiary. For parts with holes, use one round pin + one diamond pin. Never use four points on the primary datum — it over-constrains and causes rocking. The part that wasn’t locating consistently had two supports instead of three.
Support vs. Locate: Different Jobs
Don’t confuse support (holding the part up) with location (positioning the part).
- Locators: Precision pads or pins that define the position. Made of hardened steel (or carbide). They contact the part’s datum features. They are the reference.
- Supports: Additional points that hold the part up (prevent sagging) but don’t define the position. Spring-loaded or adjustable. They touch the part but don’t constrain it (they give way slightly).
A large part might need three locators (defining the plane) plus two spring supports (holding up the middle so it doesn’t sag under its own weight). The spring supports are not locators — they just keep the part flat. They should not interfere with the 3-2-1 location.
Clamp Placement: Over Locators
The clamps should press the part down onto the locators. Don’t clamp in a way that lifts the part off the locators.
- Clamp directly over a support pad (the clamp force goes through the pad, not through the part’s unsupported span).
- Don’t clamp on a thin wall (it deforms). Clamp on solid material over a support.
- For a welded part, clamp near the weld (so the weld doesn’t distort the part off the locators).
Datum Feature Design (In the Part)
The fixture designer can’t fix the part’s datum features. But they can request (in the design review) that the part has good datum surfaces.
- Flat bottom face: The primary datum should be a flat, machined surface. Not a rough casting surface.
- Two datum holes: For pin location, two precision holes (with machined bosses) are better than surface locators.
- Ribs and bosses: The part should have flat pads where the locators and clamps contact. Not curved surfaces or thin walls.
If the part’s design doesn’t have good datum features, the fixture will struggle. Raise this in the design review, not after the part is made.
A Fixture Locating Checklist
- What is the part’s primary datum? (The surface on the drawing.)
- Are there three support pads on that datum? (Not two, not four.)
- What is the secondary datum? (Side face.)
- Are there two locators on that face?
- What is the tertiary datum? (End face or hole.)
- Is there one stop (or diamond pin) there?
- For hole location: one round pin + one diamond pin?
- Are the locators hardened? (Wear resistance.)
- Are there spring supports (for sag) that don’t over-constrain?
- Are clamps over the supports (not spanning gaps)?
- Can the part be loaded/unloaded without hitting the locators?
- Are the locators adjustable (for wear compensation)?
The Bottom Line
Datum and locator design for fixtures is the 3-2-1 principle. Three points on the primary datum (defining a plane), two on the secondary (constraining translation and rotation), one on the tertiary (last translation). The part that wouldn’t locate consistently had two supports (it rocked) instead of three. Use three support pads on the bottom, two side locators, one stop. For holes, use one round pin and one diamond pin. The fixture that holds every part in the same position wasn’t over-constrained with many locators — it was correctly constrained with exactly six.
