Start with certifications, not the website gallery
I have audited more than forty CNC shops across China for procurement teams in Germany and the US. The ones with the shiniest websites are rarely the ones holding tight tolerances on a Tuesday night shift. Certification is the first filter because it tells you whether a quality management system exists on paper and, more importantly, whether it is being followed on the floor.
ISO 9001:2015 is the baseline. Ask for the certificate, check the expiration date, and verify the scope covers CNC machining (not just trading or assembly). If your parts go into automotive, demand IATF 16949. For medical devices, ISO 13485 is non-negotiable. A certificate means an auditor showed up and found the system functional; no certificate means you are betting your supply chain on hope.
Audit the machine list and capacity
A shop with three 3-axis mills quoting 5-axis work is outsourcing it. That is not always bad, but it adds a middleman and removes process control. Ask for a current machine list with model numbers, axis counts, spindle speeds, and year of manufacture. A mature shop will have a mix of 3-axis, 4-axis, and 5-axis machines plus turning centers with live tooling.
- 3-axis VMC: sufficient for prismatic parts with simple geometry, lower hourly rate
- 4-axis / 5-axis: required for complex aerospace and medical parts, reduces setups
- CNC turning with live tooling: handles combined mill-turn features in one chucking
- Swiss-type lathe: needed for small-diameter shafts under 32 mm with tight length-to-diameter ratios
- Inspection equipment: Zeiss or Hexagon CMM, optical comparator, surface roughness tester, hardness tester
Capacity matters beyond the machine count. Ask about current utilization. A shop running at 95 percent capacity will not give your parts priority; a shop at 50 percent may have quality issues from idle staff turnover. Sweet spot is 70-80 percent utilization with a second shift available.
Demand evidence of inspection capability
Inspection separates production shops from precision shops. Any shop can produce a dimension sheet typed into Excel. You want to see calibration records for their CMM, traceability to national standards, and a sample first article inspection (FAI) report with actual measured values, not just pass/fail marks.
Look for a Zeiss or comparable CMM with a stated measurement uncertainty. At TruPart, the Zeiss CMM holds ±0.001 mm measurement uncertainty on critical dimensions, calibrated quarterly with traceable artifacts. If a shop cannot tell you their CMM uncertainty budget, they are not controlling their measurement process.
Test communication and response discipline
Communication quality in the first 48 hours predicts delivery performance. Send a technical question about a GD&T callout on your drawing and time the response. A competent shop answers with a specific, technical response; a trading company forwards your question to their actual supplier and waits.
- Technical questions answered by an engineer, not a salesperson forwarding messages
- DFM feedback provided within 48 hours of receiving drawings
- Clear escalation path: project engineer named, not just a generic email inbox
- Proactive communication when a problem is found, not after the delivery date passes
- English capability on the engineering side, not just the sales desk
Validate with a pilot order before committing volume
Never place a production order on a first relationship. Start with a prototype or low-volume run of 10-50 pieces. Treat it as an audit. Measure the parts yourself against the drawing. Track quote-to-delivery time. Evaluate packaging quality and part cleanliness. If the pilot goes well, scale up; if not, you learned with minimal cost.
During the pilot, watch how the shop handles problems. Every supplier runs into issues occasionally. The good ones call you immediately, explain the root cause, propose a correction, and document it. The bad ones ship non-conforming parts and hope you do not notice.
Check financial stability and long-term viability
A shop that goes out of business mid-production costs far more than the parts. Ask about how long they have been in business under the same ownership, whether they own their facility, and how many full-time staff they employ. A supplier operating for ten-plus years with owned equipment and a stable workforce is a safer bet than a new shop with leased machines and high turnover.
| Evaluation Criterion | Green Flag | Red Flag |
|---|---|---|
| Certifications | ISO 9001:2015 current, scope covers machining | Cert expired, scope wrong, or refuses to show cert |
| Inspection | Zeiss/Hexagon CMM, calibrated, FAI with actuals | Only calipers/micrometers, no CMM, FAI is pass/fail only |
| Communication | Engineer answers within 1 business day | Salesperson forwards questions, 3+ day response |
| Machines | Owns equipment, model list provided, 70-80% utilization | Leased unknown machines, no list, 95%+ or <50% utilization |
| Pilot order | Ships on spec, on time, clean packaging | Out of spec, late, damaged, no DFM feedback |
Frequently Asked Questions
What is the minimum certification I should require from a CNC shop?
ISO 9001:2015 with a scope that explicitly covers CNC machining or precision parts manufacturing. This is the floor, not the ceiling. Automotive parts require IATF 16949; medical requires ISO 13485.
Should I visit the shop in person before placing an order?
For production volumes, yes. A factory visit reveals housekeeping, organization, and equipment condition that photos hide. If travel is impractical, request a live video walkthrough focused on the machining floor and inspection room.
How important is it that a shop has 5-axis capability?
Only as important as your parts require it. Simple prismatic brackets and plates are fine on 3-axis. Complex geometries with features on multiple faces, undercuts, or tight angular relationships need 5-axis to avoid multiple setups that stack tolerance.
What does a good FAI report look like?
It lists every drawing dimension (including GD&T callouts), the nominal value, the actual measured value for each piece sampled, the measurement device used, and the inspector identification. Pass/fail without actuals is not an FAI report.
How many suppliers should I qualify for a given part family?
Two is the practical minimum for risk mitigation. Qualify a primary and a backup. Running three or more splits volume too thin and reduces leverage with each supplier.