Guides / Quality

Reading a CMM report without a metrology background

August 2, 2026

A CMM report is a page of numbers that either reassures you or wastes your time, depending on whether you know which three lines to read.

Here is how to get what you need out of one in a few minutes.

Datums come first

Every measurement on the report is relative to a datum scheme, usually labelled A, B and C. The primary datum, A, constrains the part the most, typically a face. B and C fix the remaining freedom, often a bore and a second feature.

Two consequences:

• The report only means something if the datums match the function. If a motor mount is measured from its outside edges but assembles on its centre bore, the report measures the wrong relationship perfectly.
• Changing datums changes the numbers. The same physical part measured from a different scheme gives different readings and neither is wrong. This is the most common reason two inspection reports disagree.

Get the datums right on the drawing and the inspection follows. Get them wrong and no amount of measurement rescues it.

The four kinds of number you will see

Size. A diameter, a length, a depth, with a nominal and a tolerance. Easy to read, and the one people over specify.

Position. Usually true position, reported as a diameter of a tolerance zone. This is the number that decides whether a bolt pattern interchanges. A true position of 0.04 means the feature centre is inside a 0.04 diameter circle around the nominal point.

Form. Flatness, roundness, cylindricity. No nominal, just a number and a limit. A flatness of 0.018 against a 0.02 limit is a part passing, and it is also a warning that there is very little margin.

Orientation. Perpendicularity, parallelism, angularity. These need datums to mean anything, which is why the datum scheme matters so much.

The three lines to read first

1. The critical feature on your drawing. You called out two or three. Read those.
2. Anything near the edge of its band. Look for readings using more than about 70 percent of the tolerance. Those are the features that will fail on some future lot.
3. Form on any face that mates with something. Flatness on a sealing face or a motor face is the number that decides assembly, and it is the one that moves when the process changes.

Reading drift across lots

One report is a snapshot. Several reports are information.

If bore readings move steadily across three lots, the process is drifting, usually tool wear. That is a conversation to have before the fourth lot, not after a rejection. Ask your supplier to keep the reports and compare them, and keep your own copies, because this is the cheapest process monitoring available to a buyer.

It is also the reason to ask for actual values rather than pass and fail ticks. Ticks cannot drift.

Things that quietly invalidate a measurement

• Temperature. Aluminium moves roughly 23 microns per metre per degree. A precision part measured hot reads differently. Serious measurement happens in a controlled room after the part has settled.
• Fixturing. Clamping a thin plate to measure it can flatten it. The report then describes a shape that does not exist unclamped.
• Probe size. A large probe cannot enter a small internal radius, so it measures something adjacent instead.
• Burrs. A burr on a datum face tilts the whole part and moves every number on the page.

If a result looks impossible, one of those four is usually the reason.

What to ask your supplier for

• Actual readings, not ticks.
• The drawing revision and the datum scheme on the report.
• The instrument used per feature.
• The same report format on every lot, so you can compare.
• A sample plan you agreed in advance, rather than a number of parts nobody defined.

What we send

A first article report against your drawing revision, measured in house on a CMM and an optical comparator, with actual values and the instrument listed per feature. Lot inspection follows the sample plan we agreed at quote stage, and the readings come with the parts rather than on request.

The full list of what travels with a lot is on the quality page, and first article inspection on UAV parts covers the step before this one.

Frequently asked questions

Keep reading

• Edge breaks and deburring on drone parts: what to write on the drawing
• What a first article report should tell you before the lot runs