Inspection method · CMM
CMM and 3D measurement
Complex geometry and geometrical tolerances need coordinate measurement. The quality of the result depends on alignment, probing strategy and programme.
01How it works
A coordinate measuring machine records points on the part surface with a probe and calculates features (planes, cylinders, circles) and their relationships. Software evaluates sizes and geometrical tolerances relative to a datum reference frame established by the alignment.
Laser trackers and articulated arms extend coordinate measurement to large parts; optical 3D scanners capture dense point clouds for comparison with CAD models.
02Typical industrial applications
- GD&T verification of machined parts
- First article inspection
- Castings and forgings layout
- Large fabrications with laser trackers
- Freeform surfaces with scanning
- Reverse engineering
03Benefits and what it detects
- Accurate evaluation of geometrical tolerances
- Comprehensive reports
- Repeatable programmes for series parts
- Comparison with CAD
04Limitations
- Wrong alignment produces precise but wrong results
- Point density affects form evaluation
- Programmes must reflect drawing intent
- Temperature and fixturing influence results
- Machine availability at suppliers
Process capability: Cp and Cpk
Specification 10.00 ±0.05 mm. Move the process mean and spread to see how capability changes.
Cp compares tolerance width with process spread; Cpk also accounts for centring.
05Specifying and reviewing the method
Request the CMM report with alignment description, probe configuration, evaluation method and actual values for all characteristics. Witness the first programme run for new parts.
Other references: ISO 10360 (CMM acceptance and verification)