CMM CMM inspection
CMM inspection · Inspección y ensayos · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
On the drawing, “CMM program rev. 3 · datums A|B|C · PDF report” states a cmm inspection requirement with a tolerance of 3 mm, measured from datum B.
What controls
Requirement to inspect the part on a coordinate measuring machine, following a program that must reproduce exactly the drawing's datum reference frame and its order of precedence. It is the only practical method for evaluating position, profile, orientation and patterns relative to a complete datum reference frame. Measuring on a CMM does not automatically make the measurement correct: the result depends entirely on how the datums were constructed and on the point density taken.
How to verify
Tactile probing or optical measurement as appropriate, with the probe qualified before the session and a stylus suited to the feature. Datums are constructed in drawing order, and each feature is measured with enough points to represent its form: three points on a plane are enough to orient it but say nothing about its flatness. Results are reported as deviation against the tolerance zone, including bonus tolerance when there is a material condition modifier.
Common mistakes when interpreting it
- Aligning the part by best fit when the drawing defines a datum reference frame with an order of precedence.
- Taking the minimum number of points per feature and then evaluating form tolerances with those same points.
- Accepting the measurement before the part and the machine are temperature-stabilized.
How to get it in the workshop / if it comes out of tolerance
Before accepting a CMM result, review the program: which features are used as datums, in what order, how many points are taken on each surface and with what fitting criterion. A program that aligns by best fit when the drawing calls for an A-B-C datum reference frame gives different, and usually more favorable, results. The part must be clean, deburred and temperature-stabilized before it goes in.
Results that change between repeats: check workholding, cleanliness and temperature before suspecting the part. Parts rejected on position that assemble fine: check whether the program evaluated the material condition modifier and the corresponding bonus. Results better than expected: suspect a best-fit alignment instead of the drawing's datum reference frame. And if the tolerance is very tight, consider the uncertainty of the machine itself before arguing over hundredths.
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