How to make a 3-2-1 alignment on a CMM

You have set the part on the table. The machine has no idea where it sits; it only knows its own axes. Alignment is the job of telling the machine where the part is. Get this wrong and every dimension afterwards is wrong — and not one at a time: they all shift the same way at once.

Without alignment a measurement means nothing

Every dimension on a drawing is taken from somewhere. That somewhere is the face marked as a datum. The machine cannot work that out by itself; it has to be shown. Alignment is exactly that: measuring the real faces on the part and seating the drawing’s coordinate system onto the machine.

So alignment is not preparation for the measurement — it is part of the measurement. If a group of dimensions in a report all drift the same way, the first place to look is not the part or the machine. It is the alignment.

What 3-2-1 means: three points, two points, one point

A rigid body has six degrees of freedom in space: translation along three axes and rotation about three axes. The 3-2-1 method locks those six in three steps, in order. That is where the name comes from.

Those counts are the mathematical minimum. In practice more than three points on the plane is worth the time: three points give you a plane, but they say nothing about how flat the face actually is. On an uneven face, which three points you pick changes the answer.

What you do at the machine, in order

The sequence below is a 3-2-1 alignment on a CMM running TouchDMIS. The software may differ but the logic does not; only the button names change.

1. Clamp the part to the table. The fixturing has to leave all three faces reachable by the probe, and the part must not move during the measurement.
2. Import the CAD model. If the alignment is going to be tied to the model, this step comes first.
3. Measure the primary plane: take at least three points on it. Do not bunch them in one corner — spread them across the face.
4. Measure the secondary line: take two points on a second face. That face should be perpendicular to the first plane.
5. Take a single point on the third face. This point sets the origin.
6. Open the coordinate system (alignment) screen from the main tools.
7. Select the plane as the primary datum and check the axis direction — see whether the normal points outwards or inwards; reversed, the signs flip.
8. Select the line as the secondary datum and confirm where the origin lands.
9. Make sure the equate-to-CAD option is switched on. With it off, the alignment you measured is not tied to the model’s coordinate system.
10. Confirm the alignment and run the program.

The mistake people make: not tying the alignment to CAD

All three measurements are correct, the points are where they should be, and the results still do not add up. More often than not the cause is this: the measured alignment was never equated to the CAD model’s coordinate system. The software then measures the part correctly but believes it sits somewhere else relative to the model. The deviations in the report look plausible, yet every one of them is systematically offset.

The second common mistake is choosing the wrong face as the primary datum. A burred, roughed or small face produces a plane that changes depending on which three points you happened to take.

Change the order and you change the result

A|B|C is not the same as A|C|B. Which face is primary and which is secondary decides how the part seats, and therefore what the measured values are. The order you align in at the machine has to match the datum order on the drawing. If the drawing says A|B|C, measure A|B|C.

Which is why whoever runs the measurement has to read the drawing. Align without looking at it and you get a result that is technically sound and has nothing to do with the print.

How we do it

Dimensional inspection is done in house: a COORD3 ARES NT coordinate measuring machine (1000×700×500 mm measuring volume, Renishaw PH20 five-axis head). We read the incoming drawing before production starts, so which face is the datum and where each dimension is taken from is settled before the part reaches a machine. If you want an inspection report, say so at quoting stage.

• CMM
• 3-2-1 alignment
• datum
• coordinate measuring
• TouchDMIS
• Ostim
• Ankara

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