Coordinate Measuring Machine inspection and measurement methods
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, oil and gas is used as the example: materials that work harden and still have to be certified.
How it is measured
The report is the output. The equipment is calibrated, the uncertainty is recorded, and the part is at room temperature before the first point is taken.
| Specification | Typical value |
|---|---|
| Characteristic | Method |
| Size | gauge or CMM, to the drawing tolerance |
| Geometry | CMM evaluated against the datum frame |
| Surface | not applicable, this is a measuring machine |
| Material | ISO 17025 calibration of the machine itself |
| Tolerance | ±0.002 mm measurement uncertainty |
What the report contains
- Every feature called up on the drawing, with a measured value rather than a tick.
- The datum order used, so the report can be reproduced.
- The certificate reference for the material and the calibration dates for the equipment.
Common sources of dispute
Most disagreements are about method, not about the part: a warm part, a burr left on, or a datum taken in a different order. Let parts reach room temperature before measurement
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: Cylindrical Grinding Machine, Jig Boring Machine, Wire EDM. Reading them together gives a fuller picture of how the process, the material and the measurement affect each other.
