Guides / Quality
How a Machined Bike Part Gets Inspected
September 19, 2026
Quality control on a machined part is not a department, it is a set of documents and a frequency. If you know what to ask for and when, you can tell a capable shop from a hopeful one before you place an order.
The four documents that matter
First article inspection report (FAI). The first piece from a new setup is measured against the drawing and the actual values are recorded. You approve it before the rest of the batch runs. This is the single most valuable document in the whole process, because it catches a misread drawing while it has cost one part rather than three hundred.
Insist on measured values. A report that lists dimensions and ticks them is not a report, it is a promise.
Material certificate. From the mill, stating alloy, temper, lot and mechanical properties. Needed if you will fatigue test, certify, or ever have to answer a question about a failed part.
In-process inspection record. Which dimensions are checked, how often and with what. A sensible plan measures the three to six critical dimensions at a stated frequency, for example first piece, then every tenth, then last piece.
Certificate of conformity. A statement from the shop that the batch was made to the drawing revision named. Cheap, and it pins the drawing revision to the batch, which matters more than it sounds.
The equipment, and what each is actually for
Coordinate measuring machine (CMM). A probe on a precise three axis frame. Measures position, form and relationships between features: bolt circle position, coaxiality of two bearing seats, perpendicularity of a face to a bore. This is the tool for anything with a positional or geometric tolerance.
Micrometers and bore gauges. Fast, accurate on a single dimension, and what an operator actually uses at the machine between parts. Right for a bearing bore diameter, wrong for a bolt circle position.
Height gauge and surface plate. Simple, reliable, good for flatness and step heights.
Optical comparator. Projects a magnified silhouette against an overlay. Very good for tooth forms, thread profiles and small radii, which is why it turns up in chainring inspection.
Surface roughness tester (profilometer). Measures Ra. Needed where you called out a surface finish, typically a bearing seat or a clamp face, and nowhere else.
Hardness tester. Confirms temper. Relevant when the part is heat treated after machining, or when you want to confirm the material is what the certificate says.
What a sensible inspection plan looks like
For a typical machined bike part:
| What | When | With |
|---|---|---|
| Full dimensional FAI | First piece of each setup | CMM plus micrometers |
| Critical bores | First, every tenth, last | Bore gauge |
| Bolt circle position | FAI, then one per batch | CMM |
| Mounting face flatness | FAI, then one per batch | Surface plate or CMM |
| Surface finish on called-out faces | FAI | Profilometer |
| Visual and deburr | Every piece | Operator |
| Finish color against master | Every anodize batch | Approved physical sample |
That plan costs little and catches nearly everything that actually goes wrong.
Questions that separate shops
Ask these on a first conversation and listen to how quickly the answers come:
- What does your first article report look like? Ask to see a redacted example.
- Do you have a CMM in house, or is it subcontracted?
- Can you supply mill certificates, and do you keep material traceability by lot?
- What is your process when a dimension is found out of tolerance mid-batch?
- Do you keep the fixture and program for repeat orders?
- Who deburrs, and is deburring inspected?
A shop that answers all six without hesitation has done this before. A shop that treats question 4 as hypothetical has not.
What to put on the RFQ
Name the inspection requirement explicitly: “FAI with measured values required before batch release, mill certificate required, critical dimensions marked on drawing”. It costs a little and it changes which shops take the job seriously.
Say it on the RFQ and the quotes you get back already include it, instead of it becoming a surprise line item later.
Frequently asked questions
What is a first article inspection?
A measured report on the first piece from a production setup, showing the actual value found for each dimension against the drawing. It is approved before the batch runs. A first article with real numbers is useful. One that only says pass is not.
Do I need a CMM report on every part?
No, and you should not pay for one. A first article on a CMM plus in-process checks on the critical dimensions at an agreed frequency is the normal arrangement. Full inspection of every piece belongs to aerospace budgets.
What is a material certificate and when do I need it?
A document from the mill stating the alloy, temper, heat or lot number and mechanical properties of the metal. Ask for it whenever the part is structural, whenever you plan to certify by test and whenever traceability matters. Request it on the RFQ, because it cannot be added afterwards.
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