CMM inspection is how a precision machine shop proves a part matches the print. A coordinate measuring machine touches or scans dozens of features, records their real position in space, and compares each one to the nominal dimension and tolerance on your drawing. If you buy CNC parts and you have ever wondered what separates a shop that says it holds tolerance from one that can show it, the answer is the CMM and the report it produces.
At Wexmar we run in-house Brown & Sharpe CMM inspection alongside our turning, milling, and Swiss departments, so verification happens in the same building that cuts the metal. This article explains how the machine works, what a CMM report actually shows, and what dimensional inspection typically costs.
How a coordinate measuring machine works
A CMM is a rigid bridge or gantry structure carrying a probe that moves along three axes: X, Y, and Z. Precision glass scales on each axis report position to a resolution of roughly 0.00002 in (0.5 micron). When the probe contacts a surface, the machine captures that XYZ coordinate. Collect enough coordinates and you can reconstruct the true geometry of a feature.
Touch-trigger versus scanning probes
- Touch-trigger probing takes discrete points. A hole might be measured with 4 to 8 points, a plane with 3 to 6. It is fast and repeatable for prismatic features.
- Analog scanning drags the stylus across a surface and captures hundreds of points per second. This is how you evaluate profile of a surface, roundness, and cylindricity with real statistical confidence rather than a few sampled points.
Ruby styli in diameters from 0.5 mm to 6 mm reach into bores, slots, and counterbores. The machine's software subtracts the stylus radius automatically, so a reported hole diameter is the actual feature size, not the probe path.
Datums and alignment
Before measuring anything, the CMM builds a coordinate system from the datums called out on your print. A typical 3-2-1 alignment locks 6 degrees of freedom: a primary datum plane (3 points), a secondary (2 points), and a tertiary (1 point). Every position and profile result is then reported relative to that datum reference frame, exactly as your GD&T requires. This is why a CMM catches errors a set of calipers never will. Calipers measure size; the CMM measures size, location, orientation, and form against the datums.
What a CMM report shows
A finished CMM report is a feature-by-feature record. For each characteristic you generally see:
- Feature name and callout tied to a balloon number on the drawing
- Nominal value from the model or print
- Upper and lower tolerance (for example a bore at 0.7500 in with +/-.0005 in)
- Actual measured value
- Deviation from nominal
- Pass or fail, often with out-of-tolerance rows flagged in red
Good reports also carry a ballooned drawing so a buyer can trace characteristic 14 on the report back to the exact feature on the geometry. When we quote tight work, plan for realistic capability: general CNC tolerances run around +/-.005 in, precision features hold +/-.001 in, and critical fits hold +/-.0005 in or tighter. Surface finish callouts such as Ra 16, Ra 32, and Ra 63 uin are verified separately with a profilometer and referenced on the same package.
First article versus in-process versus final
- First article inspection verifies the first good part against every dimension before a production run continues. This is where a full CMM layout earns its keep.
- In-process inspection checks key features while the job runs so drift is caught early.
- Final and lot inspection samples or fully verifies the finished lot before it ships, often paired with material certs and a certificate of conformance.
What CMM inspection costs
Inspection is a real line item, not a rounding error. The two big drivers are programming time (a one-time cost per part number) and per-part run time. The table below shows typical illustrative ranges. These are estimates for planning, not binding quotes.
| Cost driver | What it covers | Typical range |
|---|---|---|
| CMM program setup | One-time programming, fixturing, and alignment for a new part number | $150 to $600 |
| First article layout | Full dimensional report on the first good part, all features ballooned | $120 to $450 |
| Per-part run (production) | Automated CMM cycle on each sampled or 100% part | $3 to $30 |
| Surface finish check | Profilometer readings at Ra 16 to Ra 63 uin callouts | $15 to $60 |
| Documentation package | PDF report, ballooned print, certs, C of C | $40 to $150 |
On a low-volume precision job, inspection often lands at 8 to 15 percent of the part price. On a mature production run with an automated CMM cycle, per-part inspection can drop below 3 percent. The setup cost amortizes fast across quantity, which is why buyers who plan repeat orders should ask the shop to reuse the CMM program.
Where the value shows up
The point of CMM inspection is not paperwork. It is risk reduction. A verified first article means the run is dimensionally correct before hundreds of parts are cut. A documented report means a rejected shipment can be argued with data. And a shop with in-house metrology does not have to send parts out for measurement, which protects lead time.
Wexmar performs coordinate measuring machine inspection on the same floor as our CNC turning and CNC milling cells. For programs that require formal documentation, see our inspection and quality capabilities, or send a print through our instant quote to get a part with the inspection level you need.