A part that passed inspection on your shop's CMM gets rejected by the customer's CMM for the same dimension. It happens more often than anyone admits, and most of the time neither side is “wrong”. Here are the five root causes we see repeatedly, and the engineering changes that stop them coming back.
A common scenario in precision machining: you ship a shaft with OD 19.950 mm ± 0.005 mm. Your shop’s CMM reads 19.952 mm — pass. The customer’s CMM reads 19.957 mm — reject. Both machines are calibrated. Both operators are competent. The drawing is unambiguous. So who is wrong?
In most cases like this, neither side is wrong — they are measuring subtly different things, on subtly different setups, at subtly different conditions, and the drawing doesn’t pin down which interpretation is authoritative. The five root causes below cover ~90% of these disputes.
The single biggest source of disputed rejects. “OD 19.95±0.005” can be measured by:
On a perfectly round part all three agree. On a part with 3-lobe out-of-round (common from a 3-jaw chuck), they can differ by 2–4× the lobing amplitude — easily 0.005–0.010 mm on a ground part.
| Method | What it actually measures | Sensitivity to lobing |
|---|---|---|
| Two-point micrometer | One chord at one height | Misses odd-lobe (3-jaw lobing is invisible) |
| CMM best-fit circle | Least-squares over N points | Reports average — halves the apparent lobing |
| Air gauge / ring gauge | Average diameter over a band | Strongly averages — hides lobing entirely |
| V-block + indicator | 3x out-of-round amplitude (for 60° V) | Amplifies odd-lobe (best for detecting it) |
Ø19.95 ±0.005 (CMM, 8 pts min, lsq). ASME Y14.5-2018 §5 explicitly permits this. For critical fits, cite the method on the FAI report so both sides use the same one.Steel grows ~11 µm per meter per °C. Aluminum grows ~23 µm per meter per °C. A 100 mm aluminum part measured at 28°C instead of the standard 20°C has grown by 0.018 mm — on a ±0.01 mm tolerance that’s the whole band gone before any real error.
| Material | CTE (×10⁻⁶/K) | Growth per 100 mm per °C (µm) |
|---|---|---|
| Aluminum 6061 | 23.6 | 2.36 |
| Brass / copper | 17–18 | 1.7–1.8 |
| Steel (carbon) | 11–12 | 1.1–1.2 |
| Stainless 304 | 17 | 1.7 |
| Titanium Grade 5 | 8.6 | 0.86 |
| Invar 36 | 1.3 | 0.13 |
Standards (ISO 1, ASME B89.6.2) define the reference temperature as 20°C. But shop floors sit at 22–28°C; inspection rooms are supposed to be 20°C but drift; and parts come off the machine at 30–40°C after cutting. If your shop CMM and the customer’s CMM are at different temperatures — or if either side measures a part that hasn’t soaked to ambient — you get systematic disagreement.
Two inspectors measuring “position&rdash; within 0.1” on the same bore will get different numbers if they pick different datums. The most common variants:
On a part where the bottom face isn’t perfectly perpendicular to the bores, these two setups report position errors that differ by the perpendicularity error — easily 0.05–0.1 mm on a 100 mm part. The drawing is the contract; deviating from its datum scheme is the single most common GD&T reject cause.
A CMM with a 6 mm ruby probe measuring a bore nominally with 5 hits reports one number; the same CMM with 16 hits reports another. The differences:
Customer CMMs running automated programs often default to a standard hit count that doesn’t match the shop’s FAI setup.
Sometimes neither side is wrong; the drawing is. The classic ambiguities:
Once a customer raises a reject, the order in which you investigate matters. The workflow we use:
| Stage | Check | Why |
|---|---|---|
| Drawing release | Standard cited (ASME Y14.5-2018 or ISO 1101)? | Prevents cause 5 (mixed-standard ambiguity) |
| Drawing release | Datum scheme physically realizable & unambiguous? | Prevents cause 3 (shop “optimizes”) |
| Drawing release | Tight round/cylindrical features specify measurement method? | Prevents cause 1 (lobing invisible to two-point) |
| RFQ | Inspection temperature stated (default 20°C)? | Prevents cause 2 (thermal growth) |
| FAI | Probe strategy (tip, hits, scan) documented on report? | Prevents cause 4 (probe mismatch) |
| FAI | Reject dispute clause: third-party lab specified? | Shortens cause-resolution time from weeks to days |
| Production | 8D opened for every reject, root cause recorded? | Builds institutional knowledge; stops recurrence |
Most of the time both are right — they’re measuring subtly different things. The five common causes are: (1) measurement method mismatch (two-point mic vs CMM best-fit circle vs air gauge), (2) temperature, (3) datum scheme mismatch, (4) probe strategy, and (5) drawing ambiguity. Before arguing, ask the customer for their raw CMM report with the temperature, probe tip, hit count, and datum scheme they used. Re-measure under their conditions; the cause usually reveals itself.
More than most people expect. A 100 mm aluminum part measured at 28°C instead of the ISO 1 standard 20°C has grown by 0.018 mm — enough to consume a ±0.01 mm tolerance entirely. Steel grows ~0.011 mm per 100 mm per °C, stainless ~0.017 mm, titanium ~0.009 mm. The reference temperature for dimensional measurement is 20°C / 68°F per ISO 1. Both sides must measure at the same temperature (or apply CTE correction) for any ±0.02 mm or tighter work.
Almost always because of odd-lobe out-of-round — typically 3-lobe lobing left by a 3-jaw chuck. A two-point micrometer measures a single chord and is mathematically blind to odd-lobe errors. A CMM best-fit circle (least-squares over multiple points) reports the average diameter and shows some of the lobing. The two methods can differ by 2–4× the lobing amplitude. The fix is to specify the measurement method on the drawing (e.g. Ø19.95 ±0.005 (CMM, 8 pts min, lsq)) or use a V-block + indicator which amplifies odd-lobe for detection.
Four things: (1) Cite the governing standard explicitly — ASME Y14.5-2018 or ISO 1101, not both. (2) Make the datum scheme physically realizable (a 0.5 mm wide primary datum is a recipe for disagreement). (3) On any round/cylindrical feature tighter than ±0.01 mm, specify the measurement method on the drawing. (4) State the inspection temperature if it’s tighter than ±0.02 mm. A 20-minute drawing review with the shop before release is the cheapest reject prevention there is.
Don’t argue the number; ask for the data. Request the customer’s raw CMM report with temperature, probe tip diameter, hit count (or scan parameters), and the datum scheme they used. Re-measure the same part on your CMM under their conditions. About half of disputed rejects resolve at this step (setup difference). If still disagreeing, propose an independent third-party lab with documented traceability — the ~$200–500 cost is trivial versus a stalled shipment. Whatever the outcome, open an 8D so it doesn’t repeat.
No. Per ASME Y14.5-2018 §4 and ISO 1101, the datum reference frame called out on the drawing defines the measurement. If the shop “optimizes” the datum scheme for easier fixturing (e.g. uses two best-fit bores instead of the called-out face + bore), they are measuring a different thing — and any position/profile tolerance they report is on the wrong reference frame. If the drawing’s datum scheme is genuinely impractical, raise it at the DFM review and change the drawing; never change it silently on the shop floor.
8D (Eight Disciplines) is the problem-solving workflow from AIAG, used across automotive and aerospace. For CMM rejects, 8D forces you to: identify the root cause (not just “the part is wrong”), implement an interim containment, and verify a permanent fix. The value isn’t the paperwork — it’s that each resolved reject gets its root cause recorded, building institutional knowledge that stops causes 1–5 from recurring on the next batch.
We document measurement method, temperature, probe strategy, and datum scheme on every FAI — so what we ship is what gets accepted. Send your drawing and inspection requirements for an engineering review.
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