A CMM report is the printout or file from a coordinate measuring machine. It lists each feature measured on a part, with its nominal value, tolerance, measured value and deviation, and shows whether it passes.
A CMM touches the part with a probe, or scans it, and calculates positions in three dimensions. That lets it measure features that hand tools cannot, such as true position, concentricity, flatness and angles between surfaces.
What a good report contains
- The part number, drawing number and drawing revision.
- The supplier name, lot number, sample number and date of measurement.
- The CMM used, its calibration date and the room temperature.
- How the part was aligned, meaning which datums were used and how they were set up.
- A numbered list of features that matches balloon numbers on a copy of your drawing.
- For each feature, the nominal, upper and lower tolerance, measured value and deviation.
- A clear pass or fail for each feature, and the name of the person who measured it.
How to read it
First check that the drawing revision matches yours. A report against an old revision proves little. Then check that every dimension on the drawing has a line in the report. Missing features are a common gap.
Look at the alignment. If the datums do not match your drawing, positions and runouts may be measured from the wrong place. Ask the supplier or lab to explain any feature where the method is unclear.
Finally, look at how close values sit to the limits. A part that passes but sits on the edge of tolerance on several features tells you the process is not centred.
What to ask for
Tell the supplier in advance what you need. These requests make reports far more useful:
- A ballooned copy of your drawing, so each report line links to a feature.
- Reports on several parts, not one, so you can see the spread.
- The report as an editable file as well as a signed PDF.
- The same feature numbering on every report, so you can compare lots over time.
Limits of a CMM report
A CMM report describes the parts measured, not the whole lot. A perfect report on five hand-picked samples does not prove that production will match. Ask how samples were chosen, and repeat measurement on parts from production lots.
Brass expands with heat, so parts measured straight off the machine can read differently from parts measured at a controlled temperature. Reports should note the temperature.
A CMM report forms part of a first article inspection for new parts. Other measuring methods are covered in checking brass part dimensions.
Next step
We check CMM reports against your drawing and arrange independent measurement where needed. To set up dimensional checks for your parts, book a free sourcing review.