Coordinate Measuring Machine specifications: what the numbers mean
Coordinate Measuring Machine is a measuring machine that probes points on a part and compares them with the nominal geometry. It is used for verifying dimensions and geometric tolerances on finished parts. This entry explains what it is, what the numbers on a data sheet mean, and what to put on a drawing.
It is written from the point of view of the buyer and the drawing office: the values that are realistic in production, the ones that cost money, and the way the result is verified. Where an industry sets the requirement, food processing is used as the example: surfaces that clean down between production runs.
What the numbers mean
A data sheet for Coordinate Measuring Machine is only useful if the numbers are read against the job. The usual entries, and what they tell you, are these.
- Measuring volume – 700 x 1000 x 600 mm.
- Length error – 1.9 + L/300 µm.
- Probing error – 1.8 µm.
- Scanning – optional, contact or optical.
- Temperature range – 18 to 22 °C.
- Software – CAD based with GD&T evaluation.
Comparing offers
Two suppliers can quote the same measuring volume and mean different things, because the figures are often quoted under the most favourable conditions. Ask what the value is measured over, what the material was, and whether it is a typical or a guaranteed figure.
| Specification | Typical value |
|---|---|
| Measuring volume | 700 x 1000 x 600 mm |
| Length error | 1.9 + L/300 µm |
| Probing error | 1.8 µm |
| Scanning | optional, contact or optical |
| Temperature range | 18 to 22 °C |
| Software | CAD based with GD&T evaluation |
What to check before specifying
Make sure the value you need is the value you have called up. Let parts reach room temperature before measurement, and the difference between a typical and a worst case figure is where most disputes start.
Applications
Coordinate Measuring Machine is used for verifying dimensions and geometric tolerances on finished parts, and in practice it appears most often in first article inspection, batch sample checks, capability studies, reverse engineering checks.
How it fits into a machining route
Coordinate Measuring Machine is one station in a route rather than the whole of it. The part arrives with the previous operation finished and leaves with the features it is responsible for completed, so the datum and the clamping method are agreed before the first part rather than discovered on it.The route is fixed before the first part is cut, and it is the route rather than the individual setting that holds the result: the number of features to be evaluated. Where a drawing calls for something the route cannot hold economically, that is raised at quotation rather than after the parts are made.
Standards, documentation and what is issued
The documentation requirement is ISO 17025 calibration of the machine itself. Material is supplied against a certificate where the application calls for it, inspection results are recorded with the job, and the records are kept so that a repeat order can be compared with the first instead of being measured from scratch.
The report is the output is the verification step. Where a customer format or a standard report is required, it is agreed before the batch starts, because a report written after the event is a report nobody can use.
Frequently asked questions
What tolerance should be specified for Coordinate Measuring Machine?
±0.002 mm measurement uncertainty is realistic in production. Tightening it beyond the functional requirement increases cost without improving the part.
Which material is usually used?
any, the machine is material independent is the usual choice, supplied against ISO 17025 calibration of the machine itself.
How is the result verified?
The report is the output, with the results recorded so a repeat order can be compared with the first.
What is the most common mistake?
Temperature: a part measured straight off the machine is still warm.
How long does it take and what does it cost?
A realistic cycle is 30 to 90 minutes for a full first article, and the cost driver is the number of features to be evaluated.
Related capability
If you are specifying Coordinate Measuring Machine on a drawing and want a second opinion on the route, send the model and the quantity. The first response is a manufacturability review with the likely risk points and a quotation, so the decision can be made on facts rather than on a price alone.
Machining, inspection and documentation are handled in one place, which is why the dimensional report and the material certificate arrive with the parts rather than after them.
Related entries
These entries sit next to Coordinate Measuring Machine in the encyclopedia: CNC Lathe, Swiss-Type Lathe, CNC Turning. Reading them together gives a fuller picture of how the process, the material and the measurement affect each other.
