Reading a CMM specification, and why "1.7 µm" is not an accuracy figure
Supplier capability sheets often lead with a single micron figure for their coordinate measuring machine. Read literally, it suggests the machine holds that value anywhere in its volume. It does not, and any supplier-quality engineer who has specified a CMM knows it.
The number is a formula
CMM length measurement error is published as E₀,MPE — maximum permissible error — and it is stated as an expression, not a constant. For the Mitutoyo CRYSTA-Apex V installed at RAVIS, the manufacturer's specification gives:
E₀,MPE = (1.7 + 3L/1000) µm, evaluated to ISO 10360-2:2009
L is the measured length in millimetres. The constant term is the part everyone quotes; the
second term is the part that decides whether the machine can verify your feature.
| Measured length | E₀,MPE |
|---|---|
| 100 mm | 2.0 µm |
| 250 mm | 2.5 µm |
| 500 mm | 3.2 µm |
| 1000 mm | 4.7 µm |
A 500 mm datum-to-datum dimension is therefore permitted 3.2 µm of measurement error, not 1.7.
Why this matters for your tolerance stack
Measurement uncertainty consumes tolerance. If a drawing calls a 500 mm length at ±0.01 mm and the measuring system is permitted 3.2 µm, roughly a third of the tolerance band is spent before the part is even considered. That is the calculation behind a measurement capability check, and it is the reason a bare micron figure is not enough to answer "can you verify this feature".
The other numbers on the sheet
Three more specifications do real work and are worth asking for:
- Resolution — 0.1 µm on this machine. Resolution is not accuracy; it is the smallest increment the scale reports.
- Single stylus form error, PFTU,MPE — 1.7 µm. This governs form measurements such as roundness, which length error does not describe.
- Guaranteed-accuracy environment — 16–26 °C, with a 2 K/hour and 5 K/24 hour limit and a 1 K/m gradient. A CMM quoted at its specification in an unconditioned room is not being operated at its specification.
What to ask a machining supplier
- The full E₀,MPE expression and the standard it is evaluated to, not the constant alone
- The measuring range, and whether your part fits within it in the orientation it must be measured
- Whether the inspection room is temperature controlled to the guaranteed-accuracy band
- Whether the probe configuration used for your part matches the one the specification was issued against
RAVIS runs the CRYSTA-Apex V in a temperature-controlled inspection room, alongside a Mitutoyo LH-600 height gauge and a video measuring machine for non-contact and GD&T work. Send us a drawing and we will tell you which of the three will verify it.
Specification figures are from Mitutoyo's published CRYSTA-Apex V specification sheet. Machine installation is per the RAVIS capability profile.