How to Vet CNC Processing Companies in the UK
A working checklist for engineers and buyers comparing CNC processing companies in the UK. Seven checks cover tolerance, lead time, MOQ, certifications and quote quality, with the numbers to ask for and the answers that should make you walk away.
In this article
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Key takeaways
Seven checks and what a good answer looks like
Use this as a scorecard when you shortlist CNC processing companies in the UK.
| Check | Ask for | Good answer | Walk-away signal |
|---|---|---|---|
| Tolerance | Routine production tolerance | ±0.005 mm documented with inspection data | Only a brochure number, no data |
| Lead time | Quote turnaround and ship window | Quote in 12 hours, parts in 3–5 days | Vague "depends on the shop" |
| MOQ | Smallest and largest run | One prototype to 10,000+ parts | Minimum order above your forecast |
| Certifications | Current certificate scope | ISO 9001, plus IATF 16949 or ISO 13485 | Expired or out-of-scope cert |
| Inspection | In-process and final checks | 100% inspection, reports on request | Final check only, no records |
| Materials | Grades held in stock | 6061-T6, 316L, Ti-6Al-4V, PEEK | One generic "aluminium" |
| Confidentiality | NDA and file handling | NDA on request, secure uploads | Refuses to sign an NDA |
The short answer
Pick the supplier whose quote reads like a process plan and whose inspection records prove it. If a shop cannot answer the tolerance and inspection questions in writing, keep looking.
Tolerance, lead time and MOQ: the three numbers that filter the list fastest
Start with tolerance, because it decides which machines can bid at all. A shop quoting ±0.05 mm is running three-axis mills with standard fixturing. A shop quoting ±0.005 mm needs temperature control, probing and a metrology plan. Ask what tolerance they hold on a normal production day, not what the best machine can do on a single part. The gap between those two numbers is where scrap comes from.
Lead time is really two numbers: how fast they quote and how fast they ship. A 12-hour quote with free DFM feedback tells you the front office has engineers in it. A five-day quote with no questions tells you the RFQ sat in a queue. On the shop floor, production can start within 24 hours when material is on the shelf, and parts ship in 3–5 days. If your parts need outside finishing, add that to the schedule.
MOQ sorts suppliers by business model. Job shops that take one prototype and 10,000-part runs are set up for flexible batches. Shops with a 500-piece minimum are optimized for volume and will price your prototype high on purpose. Neither is wrong. Match the shop to your stage: prototype first, then move to the volume house once the design freezes.
One trap: buyers compare tolerance, lead time and price, then ignore the inspection plan. Those three numbers are meaningless without a way to prove them. Ask how many parts are measured and what happens to the data before you send a drawing.
- 1Ask for routine toleranceProduction capability, not machine capability.
- 2Split the lead timeQuote speed and ship speed are separate metrics.
- 3Match MOQ to your stagePrototype shops and volume shops price differently.
Certifications and inspection: what the paperwork actually proves
ISO 9001:2015 is the entry ticket. It shows a documented quality system, not a specific process capability. For automotive work, IATF 16949:2016 adds traceability, APQP discipline and change control. For medical devices, ISO 13485:2016 covers design transfer, clean handling and record retention. If your part touches a patient or a vehicle, a supplier without the matching certificate is a paperwork risk, even if the machining is good.
ISO 27001:2022 is the one buyers overlook. It covers information security, which matters when you send CAD files and revision-controlled drawings. A shop with ISO 27001 has a defined process for storing, sharing and deleting your data. A shop without it may be fine, but you are relying on individual habits rather than a system.
Inspection is where certificates meet reality. Ask for raw material certification, in-process monitoring at the critical dimensions, and a final inspection before shipment. A shop that inspects 100% of parts and can send reports on request is telling you the process is stable. A shop that checks the first and last part is telling you the middle is your problem.
The useful question is not "are you certified" but "what does your certificate scope cover, and who audits you." A certificate that covers a different site or an expired scope proves nothing about the parts you are buying.
- 1Match cert to industryIATF 16949 for auto, ISO 13485 for medical.
- 2Ask about data handlingISO 27001 covers your CAD files and revisions.
- 3Request inspection recordsMaterial certs, in-process data, final report.
Quote quality and confidentiality: how to read the response you get back
A good quote is a mini process plan. It names the material grade, the machine class, the tolerance on critical features, the finish, and the inspection method. It flags thin walls, deep pockets or tight true-position callouts before you ask. A quote that lists only price and a ship date means someone copied your drawing into a spreadsheet. That shop will find the hard features after the PO, not before.
Read the DFM feedback carefully. If the supplier suggests a different material, a relaxed tolerance on a non-critical face, or a fixturing change that saves setup time, they are engineering the job. If they accept everything without comment, either the part is simple or nobody looked at it. For a five-axis part with compound angles, the second case is expensive.
Confidentiality is a practical issue, not a legal formality. Your drawings carry design intent. Ask how files are uploaded, who can open them, and whether an NDA is available before you send the package. A supplier who signs an NDA on request and stores uploads securely is easier to work with than one who asks you to trust an email thread.
Price still matters, but compare quotes on the same basis. Confirm whether the number includes material certification, finishing, inspection reports and packing. A low quote that excludes anodizing is not low. Ask for the exclusions in writing so the comparison is honest.
- 1Quote as process planMaterial, machine, tolerance, finish, inspection.
- 2DFM feedback is a signalQuestions mean an engineer read the drawing.
- 3Confirm exclusionsFinishing, certs and reports change the real price.
A seven-step shortlist process you can run this week
Each step produces one document or one answer. Do not skip ahead.
- 1Write your requirement sheetList material grade, critical tolerances, surface finish, annual volume and any industry certificate you need. Keep it to one page.
- 2Send the same RFQ to every shopSame STEP file, same 2D drawing, same questions. Different inputs make quotes incomparable.
- 3Score the quote, not the emailCheck for material grade, machine class, tolerance, finish and inspection method. Missing items are questions, not disqualifications.
- 4Ask two follow-up questionsAsk about routine tolerance and about the last feature they would machine differently. Vague answers tell you a lot.
- 5Verify certificates by scopeRequest the certificate number and confirm the site and process scope. An expired or off-site certificate does not count.
- 6Run a paid sample before the production POOne or two parts, inspected and reported. This is the cheapest risk reduction in the whole process.
- 7Lock the inspection plan in the POState which dimensions get measured, at what frequency, and which reports ship with the parts.
Questions buyers ask after the shortlist
Should I only use suppliers based in the UK?
Not necessarily. The deciding factors are tolerance, lead time, certification and communication, not the postcode. A supplier that quotes in 12 hours, holds ±0.005 mm and ships in 3–5 days can serve a UK program well, especially for prototypes and bridge volumes.
What matters is that the supplier can meet your drawing and your audit requirements, and that shipping and customs time are included in your schedule.
How do I compare quotes when the prices are far apart?
Normalize first. Confirm that every quote covers the same material grade, the same tolerance on critical features, the same finish and the same inspection reports. Then compare.
A quote that excludes anodizing, material certification or final inspection is not cheaper. It is a partial quote. Ask each supplier to list exclusions in writing.
What tolerance should I specify on a CNC part?
Specify the loosest tolerance the function allows on each feature. Tightening every dimension to ±0.005 mm raises cost and scrap without adding value.
Reserve tight tolerances for mating surfaces, bearing seats and alignment features. Leave cosmetic and clearance faces at ±0.1 mm or looser.
Is a certificate enough to trust a supplier?
No. A certificate proves a system exists, not that it ran on your job. Ask for the inspection records for your parts.
Raw material certification, in-process measurements and a final report are the evidence. If a supplier cannot provide them, the certificate is decoration.
How many suppliers should I keep on the list?
Two active suppliers plus one backup is a practical balance for most programs. One supplier creates risk during capacity crunches.
Run the same RFQ package through all three so the comparison stays valid when you shift volume.
What should I do before sending production files?
Sign an NDA if the design is sensitive, and confirm how uploads are stored. Send the latest revision only, and mark superseded files clearly.
A short file-transfer note with the revision number, date and contact person prevents most manufacturing errors.
Send your drawing and get a real process plan
We quote in 12 hours with free DFM feedback, hold ±0.005 mm, and ship inspected parts in 3–5 days. No minimum order quantity, from one prototype to 10,000+ parts.
12-hour quote100% inspectionNo MOQNDA on request
More machining notes
We publish setup notes, tooling trials and inspection data from the factory floor.
