How to create a CMM measurement program

You can measure one part by hand. You can measure ten by hand too, but nothing guarantees you take the same points in the same places every time. A measurement program solves exactly that: you set it up once, and every part after it is measured the same way. This is how that program gets built.

When does a program pay off?

For a single part, writing a program is wasted time — measuring by hand is quicker. A program pays off when a second, a third, a twentieth of the same part is coming. The second gain is repeatability: the same program takes the same points in the same places, so the difference between two measurements comes from the part, not from the operator.

The third gain is the record. With the program and the session stored, a batch arriving six months later can be measured under the same conditions, and the two batches compared.

The six steps

Step by step

The sequence below is how it runs on a CMM with TouchDMIS. Change the software and the button names change; the logic does not.

1. Load the CAD model. The program is built against the model, so this comes first.
2. Measure the primary plane and set it as the primary datum; check the axis direction.
3. Measure the second face and make it the secondary datum. Depending on the part this can be a second plane rather than a line.
4. Define the origin. If the part has a suitable hole, using its centre gives a steadier result than a single point.
5. Make sure the equate-to-CAD option is on; with it off the program is not tied to the model.
6. Confirm the alignment, then run a best-fit alignment to seat the model more closely onto the part.
7. Pick the planes to be measured. The software proposes a measurement path — you do not have to accept it.
8. Edit the path: drag the points or type values to move them. Keep points away from edges, chamfers and the mouths of holes.
9. Add the holes and diameters the same way, checking the point count and the height for each.
10. Enter the tolerances from the results window.
11. Save the session. On the next batch you recall the same program.

Why run a best-fit after 3-2-1?

A 3-2-1 alignment establishes the part’s coordinate system: it locks the six degrees of freedom and sets the origin. But it does so from very few points. If the surfaces carry small deviations, the result shifts slightly depending on which few points you happened to take.

A best-fit alignment compares all the measured points against the CAD model at once and finds the position that minimises the total deviation. It does not replace the 3-2-1 — it refines it. In practice the order is: rough it in with 3-2-1, then fine-tune with best-fit.

The measurement path: do not accept the default as it comes

Select a face and the software generates a path automatically. That path is geometrically correct but knows nothing about the real part. It can put a point too close to an edge, land one on a chamfer, or set one right at the mouth of a hole. Those points return wrong values, and the error gets written down against the part.

Dragging the points into sensible places is the part of program-building that repays the most time. A point placed properly once measures from the right place on every part that program ever runs.

Entering tolerances from the results window

When the measurement finishes, the results window shows the value of every feature. Enter the tolerances there and the report states pass or fail directly. Leave them out and the report only shows the measured value, and whoever reads the deviation has to go back to the drawing and decide for themselves.

Enter tolerances from the drawing, not from memory — and for untoleranced dimensions, check which general tolerance class the title block calls up.

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) and a vision measuring system. For repeat batches we keep the measurement program, so the second batch is measured at the same points from the same places. If you want an inspection report, say so at quoting stage.

• CMM
• measurement program
• best-fit
• measurement path
• TouchDMIS
• Ostim
• Ankara

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