How to Choose a Coordinate Measuring Machine (CMM)

Which CMM type suits the task?

The machine type determines measuring volume, stiffness and access. The bridge (portal) CMM is the standard for parts up to about 1200 mm, while cantilever, gantry and horizontal-arm machines target very small or very large workpieces.

The measuring volume must accommodate the largest workpiece plus fixturing and probe reach. Add margin in every axis so the stylus can reach deep bores and back faces without collision.

How do you configure the probe?

The probe head defines both the feature range and the throughput. Touch-trigger probes capture point by point and suit prismatic dimensions, while scanning probes gather thousands of points per second and reliably evaluate form and position tolerances such as roundness or flatness.

• Touch-trigger: robust and economical, ideal for bores, distances and angles in series inspection.
• Scanning: captures freeform surfaces and form deviation at high point density.
• Ruby stylus tip, typically 1 to 5 mm diameter - a large ball filters roughness, a small one reaches tight contours.
• Star clusters and extensions reach undercut features, but every extension raises probing uncertainty.
• Optical or multisensor systems for touch-sensitive or very fine geometry.

What does MPE accuracy per ISO 10360 mean?

CMM accuracy is stated as MPE (Maximum Permissible Error). The key figure is MPE_E for length measurement error, usually written E = A + L/K in micrometres, where A is the constant term, L the measured length in millimetres and K a divisor.

A typical lab machine reaches around MPE_E = 1.8 + L/300 µm, while a high-accuracy CMM stays below 1 µm plus the length term. Choose the MPE by the golden rule that measurement uncertainty should be roughly one fifth to one tenth of the tightest part tolerance.

Series or first-article inspection - what counts?

The inspection task shifts the priorities. Series inspection rewards short cycle time, repeatable programs and stable fixtures, while first-article inspection (FAI/PPAP) rewards complete feature capture and full CAD-based documentation.

• Series inspection: scanning probing and CNC routines for maximum throughput, often near production.
• First-article inspection: high accuracy and software with CAD comparison and inspection report.
• Repeatable fixtures and datums cut part-to-part scatter.
• Standard-compliant software delivers form and position tolerances and PPAP-ready reports.

Frequently asked questions