A Supplier Audit Checklist for Uncertified Machine Shops
Most of the machine shops that can make a 200-piece aluminum housing well do not hold a management-system certificate, and many that do hold one still ship bad parts. A certificate tells you that a shop documented a process and paid an auditor. It does not tell you whether the CMM was calibrated in March. A certificate is a proxy for evidence, and proxies fail quietly.
Audit the evidence, not the wall
Walk into a shop and you will see plaques: a quality certificate, maybe a customer award. None of them are evidence about your part. What you want is the specific, current, checkable record behind a claim.
The test is simple. When a supplier says "we inspect every part," ask to see the inspection record for the last lot they shipped and count the dimensions. When they say "our machines are calibrated," ask for the sticker date on the machine's probe and the certificate behind it. When they say "we have a quality system," ask for one corrective action from the last six months and read the root cause.
A shop with no certificate but clean records is a better bet than a certified shop that cannot produce a lot file in ten minutes. The audit exists to tell them apart.
Machine list, condition and spindle time
Ask for the machine list in advance, with model, year, axes, travels and controller. Then verify three things on the call.
Capability match first. Your part needs enough travel plus fixture clearance, and enough spindle taper for the cutters the job requires. A 40-taper machine at 10,000 rpm behaves very differently from a 30-taper drill-tap center on a steel part with deep pockets.
Condition second. Ask the operator to jog the machine and show the way covers, chip conveyor and spindle taper. A taper with fretting marks means tool runout, which means your ±0.02 mm bore will drift. Look at the floor: a pool of coolant is a maintenance signal.
Load third. Ask how many jobs are on the machine this week. A shop that says "we can start Monday" while showing a full board is either very efficient or overpromising.
| Audit area | What to request | Pass criteria | Common failure |
|---|---|---|---|
| Machine list | Model, year, axes, travels, controller | Two or more machines per critical operation | Single machine, no backup |
| Machine condition | Live jog, taper and way inspection | No fretting, backlash under 0.01 mm | Visible taper wear, oil leaks |
| Calibration | Certs for CMM, height gauge, micrometers | In date, 12-month interval, traceable | Expired certs, no gauge ID |
| Gauge inventory | Pin and thread gauges with asset IDs | Every gauge has an ID and a record | Shop-made gauges, no record |
| Inspection method | Sample sheet for a similar part | Method and frequency per feature | "Check with caliper" only |
| Traceability | Material cert and heat number to lot | Heat number on cert matches lot tag | Cert for a different heat |
| Corrective action | One real report from the last 6 months | Root cause, containment, verified fix | "Operator retrained," no check |
| First article | Ballooned drawing with values | Every callout numbered and reported | 20% of callouts missing |
| Cleanliness | Chips at machine and before packing | Parts deburred and washed | Chips in tapped holes |
| Capacity | Current load and lead time by machine | Quoted lead time matches load | 7-day promise, 3-week board |
See also: CNC Capacity Planning: Reading a Supplier's Promise Date and Payment Terms With a China Supplier: What Each Structure Really Costs.
Calibration records and gauge control
Calibration is the cheapest thing to fake and the easiest to verify. Ask for the gauge list, then pick three items at random and request their certificates: the CMM, a 0 to 25 mm micrometer and a set of pin gauges.
A working shop has 60 to 200 gauges in a controlled inventory, each with an asset number, a calibration date, an interval and a sticker matching the record. Interval is usually 12 months for hand tools and 12 to 24 months for a CMM. Anything with an expired sticker is out of the system by definition.
Look for the gap that matters: a shop with a calibrated CMM but uncalibrated pin gauges will pass a bore on the CMM and fail it at the bench, and nobody notices the two numbers disagree until a customer does.
Inspection method and traceability
Escapes usually start here, so spend the most time on it. Ask for the inspection sheet from a comparable part and read the method column. A caliper with 0.02 mm resolution cannot support a ±0.01 mm tolerance, and a two-point micrometer does not catch ovality.
Ask how the shop inspects a true-position callout — the answer should mention a CMM program or a functional gauge, not a height gauge and arithmetic. Then ask the question that separates the serious from the hopeful: "What do you do when a dimension sits at 90 percent of tolerance?" The right answer describes an offset change and a re-check. A shop that manages to the middle of the band has margin for its own measurement error.
Traceability means picking a part out of a box and reaching the heat of material it came from. That needs three links: material certificate on file, heat number recorded against the work order, and lot identification on the packaged parts. The certificate should be an EN 10204 3.1 mill cert naming grade, heat, chemistry and mechanicals.
Corrective action: read one real report
Ask for a corrective action from the last six months involving a dimensional reject, then read four fields: what happened, why, what changed, and how the change was verified.
A credible report is unglamorous. "Bore measured 12.03 mm on 40 of 200 parts. Cause: reamer worn past the 0.02 mm wear limit because the tool life counter was set at 800 holes instead of 400. Containment: 200 parts sorted, 40 scrapped. Change: counter reset and reamer added to the crib check sheet. Verification: next three lots measured 12.005 to 12.012." That is a shop that closes loops.
What a walkthrough proves, and how to run the audit
A video walkthrough cannot prove that the calibration date on a certificate you have not received is current, that the lot you are shown belongs to your order, or that the inspection sheet was filled in at the time rather than that morning. For those, request documents by email before the call and spot-check afterwards.
The most useful thing about a live call is that you can ask an unprepared question. Ask the operator to measure a feature while you watch, and see whether he reaches for the gauge the inspection sheet specifies or the nearest convenient one.
Run the audit in this order: request documents 48 hours ahead, then score each line pass, conditional or fail. Two fails in calibration or inspection method is enough to walk away.
If you would rather start with a part than an audit, send a drawing through the contact page and use the quote, the DFM notes and the first article report as your first three data points. Our about page describes the equipment and process behind them.
Frequently asked questions
Can I trust a supplier with no quality certificate?
Often yes, if the evidence is there. Check calibration records, an inspection sheet with a method per feature, material traceability to heat number and one closed corrective action. A certificate is a proxy; the records are the thing itself.
How many gauges should a real machine shop have calibrated?
Typically 60 to 200 items, from micrometers and pin gauges to a CMM and a roughness tester. Each needs an asset number, a 12-month interval and a matching sticker.
What is the single biggest red flag in a shop audit?
Inspection methods that cannot support the tolerance, such as a caliper on a ±0.01 mm dimension. It means the shop does not distinguish between measuring and guessing, and escapes will follow.
How long should a remote supplier audit take?
About two hours of your time: 48 hours of document lead time, a 60 to 90 minute call, and an hour reading the lot file and corrective action. Anything shorter is a tour, not an audit.
Related reading
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