Metrology and Inspection
A CMM with a published error expression, in a temperature-controlled room, in the same building as the machines.
Most shops at this scale send parts out for inspection. Keeping it in the building changes the loop between cutting a feature and knowing whether it conforms — from days to the same shift.
What the CMM actually guarantees
The headline figure on a CMM specification is the constant term of a formula, not a flat accuracy. For the CRYSTA-Apex V:
E₀,MPE = (1.7 + 3L/1000) µm, evaluated to ISO 10360-2:2009
where L is the measured length in millimetres.
| Measured length | Permissible error |
|---|---|
| 100 mm | 2.0 µm |
| 250 mm | 2.5 µm |
| 500 mm | 3.2 µm |
| 1000 mm | 4.7 µm |
A 500 mm dimension is therefore permitted 3.2 µm of measurement error, not 1.7. Stating the formula rather than the constant is the honest version, and it is the version a supplier-quality engineer can check.
Full specification
| Specification | Value |
|---|---|
| Measuring range X / Y / Z | 700 / 1000 / 600 mm |
| Resolution | 0.1 µm |
| Single stylus form error PFTU,MPE | 1.7 µm |
| Maximum drive speed | 519 mm/s combined |
| Measuring speed | 1–8 mm/s |
| Granite table | 880 × 1720 mm |
| Maximum workpiece | 800 mm high, 1000 kg |
| Guaranteed-accuracy environment | 16–26 °C, 2 K/hour, 5 K/24 h, 1 K/m gradient |
From Mitutoyo's published CRYSTA-Apex V specification sheet; installation per the RAVIS profile, p. 16.
The reason a measurement drifts
The environment band above is not a footnote. A CMM quoted at its specification in an unconditioned room is not being operated at its specification — steel and aluminium move with temperature, and so does the machine's own structure. Ours runs in a temperature-controlled inspection room for that reason.
Not everything needs the CMM
- Mitutoyo LH-600 FG height gauge — 0.1 µm digital step, accuracy (1.1 + 0.6L/600) µm, programmable routines. Fast dimensional checks that do not warrant a CMM programme.
- Video measuring machine — non-contact optical measurement and GD&T analysis, for features a touch probe would deflect or cannot reach.
Choosing the right instrument for a characteristic is part of the process plan, not an afterthought — the inspection method often constrains the machining route.
Before you send a drawing
Mark which characteristics are critical and which datum scheme they reference. If a tolerance is tight enough that measurement uncertainty consumes a meaningful share of the band, we will say so and propose how to verify it, rather than discovering it at first article.
Further reading: reading a CMM specification and why in-house metrology shortens a qualification timeline.