CMM inspection uses a coordinate measuring machine to capture accurate points on a part and compare them with a drawing or CAD model. It is one of the most important measurement methods for precision machined components, especially when parts include GD&T callouts, true position, flatness, profile, or complex relationships between features.
For high-value parts, CMM results can support first article inspection, production approval, and ongoing quality control. MTSPi includes CMM and other inspection methods as part of its quality assurance workflow when the drawing requires documented measurement.
How CMM Inspection Works
A CMM records coordinates in X, Y, and Z. A probe touches or scans the workpiece, and software uses the measured points to calculate dimensions, feature locations, and geometric tolerances. The part is usually held on a granite table with fixtures that keep it stable during inspection.
- The operator imports the CAD model or enters drawing dimensions.
- The part is aligned to a datum structure.
- The probe measures selected features.
- Software compares actual data with nominal values and tolerances.
- A report shows pass/fail results and measured deviations.
Features Commonly Checked by CMM
| Feature | What CMM Can Confirm |
|---|---|
| Hole patterns | True position, diameter, spacing, and depth references |
| Flat surfaces | Flatness, parallelism, and datum relationships |
| Profiles and contours | Surface profile against CAD geometry |
| Milled pockets | Width, depth, corner location, and wall position |
| Turned parts | Concentricity, runout, diameter, and length relationships |
CMM vs Other Inspection Tools
| Tool | Best For | Limitation |
|---|---|---|
| Caliper | Quick general checks | Limited accuracy and operator-dependent |
| Micrometer | Accurate outside or inside dimensions | Not ideal for complex feature relationships |
| CMM | GD&T and multi-feature geometry | Requires programming and setup time |
| Optical system | Small features and non-contact checks | May struggle with hidden surfaces |
When to Request a CMM Report
Request CMM inspection when a part has tight tolerances, important assembly alignment, safety-critical function, or customer documentation requirements. If the part includes machining plus coating, it is useful to decide whether inspection should happen before finishing, after finishing, or both. This is especially important for close-fit holes, bearing seats, and sealing surfaces connected to surface treatment.
Related MTSPi Resources
FAQ
Is CMM inspection required for every part?
No. It is most useful for complex geometry, tight tolerance features, GD&T, and documented inspection requirements.
Can a CMM inspect plastic parts?
Yes, but fixturing and probe force should be chosen carefully because plastics may flex.
What is included in a CMM report?
Reports typically include measured dimensions, nominal values, tolerance limits, deviations, and pass/fail status.
