How to read a CMM inspection report
The parts have arrived and a few pages of inspection report came with them. You need to answer one question quickly: are they good or not? Here is what the report says and what to look at.
What is a CMM and what does it measure?
A coordinate measuring machine touches the surface of a part with a probe, collects point coordinates and calculates geometry from those points. A caliper gives you one dimension; a CMM can tell you whether a bore is actually round and whether two faces are actually perpendicular.
We run a COORD3 ARES NT: 1000×700×500 mm measuring volume, Renishaw PH20 five-axis head, TCR20 probe changer and TouchDMIS software. The five-axis head lets us measure from different angles without re-fixturing the part, which is both faster and more consistent.
What the columns mean
| Column | Meaning |
|---|---|
| Nominal | The target dimension from the drawing. What it should be. |
| Actual | What the machine measured on the part. |
| Tol + / Tol − | Upper and lower deviation allowed by the drawing. |
| Deviation | Difference between actual and nominal. Can be positive or negative. |
| Status | In tolerance or out of tolerance. |
In practice there is one column to read: deviation. If it sits inside the allowed band, the dimension is good. Most reports also flag this with colour or an OK / NOK mark.
Is a non-zero deviation bad?
No. Zero deviation is neither expected nor possible. A tolerance is precisely the statement "it may deviate this much". For a Ø20 h7 shaft what matters is not that the deviation is zero but that it stays inside the band.
The second thing worth looking at is where in the band the deviation sits. Dimensions that consistently land near the upper limit risk crossing it later in a production run. When we see that, we intervene during production.
Geometric tolerances (GD&T)
A report covers more than sizes. The geometric characteristics you meet most often are:
These are calculated against reference surfaces (datums) — the faces marked A, B, C on the drawing. If the datums are not clearly stated the measurement result is open to argument, which is why datum marking matters.
When should you ask for a report?
You do not need a report on every dimension, and asking for one raises cost without benefit. Marking the critical dimensions on the drawing and asking for those is both cheaper and more useful.
Why we keep measurement in-house
Where inspection is outsourced the part leaves the building, waits in a queue and the result comes back days later — by which time production has moved on. With measurement in-house we can check mid-production and correct a drift before it grows. The report ships with the parts.
Do you inspect every part?
Not unless you want that. The common approach is first article inspection plus a sample from the batch. Full inspection of every piece raises cost significantly; if it is genuinely required we of course do it.
How large a part can you measure?
Our COORD3 ARES NT has a measuring volume of 1000×700×500 mm. Parts that fit within it can be measured.
What format is the report?
PDF. If your quality system needs a particular format, mention it at the quotation stage and we will prepare it accordingly where possible.
My drawing has no GD&T. Is that a problem?
Not a problem, but inspection is then limited to sizes. If the part has to seat precisely in an assembly, adding flatness, perpendicularity or position tolerance helps. If you are unsure, send the drawing and we will look at it together.
Do you offer inspection as a standalone service?
Our main business is production and our measuring capacity serves the parts we make. If you have a part in hand, ask and we will see what is possible.
- CMM
- coordinate measuring
- inspection report
- GD&T
- first article inspection
- Ostim
- Ankara
Send us your technical drawing and quantity; we will prepare your quote with a manufacturability review.
Request a Quote