CNC processing service samples: 7 essential checks before you commit

• 1
  Key takeaways
• 2
  Machined sample vs. other sample routes
• 3
  What a CNC processing service sample actually proves
• 4
  Tolerance, finish, and the numbers you should put in the RFQ
• 5
  Material and finish choices that survive the transition to production
• 6
  How to read a sample quote and spot a low-ball number
• 7
  Lead time, MOQ, and how sample speed is measured
• 8
  Certifications and paperwork that matter for your industry
• 9
  Red flags in a CNC processing service sample
• 10
  How to run a sample order from RFQ to approval

A sample is a physical answer to a drawing. It tells you whether the supplier's interpretation of your GD&T matches yours, whether the chosen alloy machines cleanly at your wall thickness, and whether the surface finish you specified is reachable without hand work. None of that shows up in a CAD review.

The sample also sets the baseline for production. If the sample holds ±0.005 mm on a bore, the supplier has demonstrated the machine, the fixture, and the metrology to repeat it. If the sample only holds ±0.05 mm, that is the number you should expect at volume, no matter what the quote says.

GreatLight machines samples on the same equipment used for production runs: 16 simultaneous 5-axis centers, 12 four-axis mills, 27 three-axis machines, and 16 mill-turn centers. That matters because a sample made on a different machine class than the production parts is only a rough sketch. The process has to be the same.

• 1
  Fit and functionAssemble the sample with its mating parts before you approve anything.
• 2
  Material behaviorMachine the production alloy, not a free-machining substitute.
• 3
  Process proofConfirm the sample came off the same machine class as the production run.

State the tolerance band on the drawing, not in the email. A supplier reading ±0.005 mm on a Ø12 mm bore knows it needs a reamed or bored finish and a CMM check. The same bore at ±0.05 mm can be drilled and pin-gauged. That single change can move cycle time by several minutes per part.

Surface finish works the same way. Ra 0.2–0.8 μm usually means fine boring, grinding, or polishing after machining. Ra 0.8–1.6 μm is a normal turned or milled finish on aluminum and brass. Ra 1.6–3.2 μm is as-machined and rarely needs a second operation. If you write 'smooth finish' instead of an Ra value, you will get whatever the operator considers smooth.

For most samples, specify the tightest tolerance only where it functions. A mounting face at ±0.05 mm and a bearing bore at ±0.005 mm is a reasonable split. Calling out tight tolerance across the whole part multiplies inspection time and invites a higher quote.

• 1
  Call out datum featuresA tolerance without a datum is open to interpretation.
• 2
  Limit tight bandsApply ±0.005 mm only to functional features.
• 3
  Name the finishUse Ra values, not words like smooth or polished.

Sample material should match the production material unless you are testing form only. Aluminum 6061-T6 and 7075 machine differently, anodize differently, and behave differently under load. Titanium TC4 (Ti-6Al-4V) and 17-4PH stainless both cut slowly and heat up, so a sample made in 6061 will not tell you whether the production part distorts.

Finish is the second half of the same problem. If the production part will be hardcoat anodized, send the sample through hardcoat too. Hardcoat adds roughly 0.05 mm of build-up and can close a tight bore if the machinist did not leave allowance. Finding that out on a sample costs one part. Finding it out on 5,000 parts costs a rework batch.

GreatLight runs anodizing, electroless nickel, zinc, silver and gold plating, powder coating, black oxide, bead blasting, tumbling, brushing, polishing, and laser marking. Laser marking has a minimum character height of 1.5 mm, so plan part numbers and serials around that limit.

• 1
  Match the alloy6061-T6, 7075, 304, 316L, 17-4PH, TC4, and PEEK are all available.
• 2
  Finish the sampleApply the production coating so build-up and masking get tested.
• 3
  Respect marking limitsKeep laser-marked characters at 1.5 mm or taller.

A sample quote should list material, machining time, finishing, and inspection as separate lines. When those are collapsed into one number, you cannot tell what happens when the part changes. A quote that only shows a lump sum is hard to audit later, and it usually hides the inspection step.

Watch for quotes that assume a three-axis process on a part with undercuts or deep cavities. Those features need 5-axis or mill-turn work, and the second setup is where cost appears. If the quote does not mention setups or workholding, ask how the part will be held. The answer tells you whether the supplier has actually looked at the geometry.

Inspection is the line most often omitted. A sample without a dimensional report is an opinion. Ask for the report on the first article, with the dimension numbers tied to the ballooned drawing. GreatLight inspects 100% of parts before shipment and provides reports on request.

• 1
  Separate line itemsMaterial, machining, finish, and inspection should each show up.
• 2
  Ask about workholdingA vague answer usually means the geometry was not studied.
• 3
  Request the reportTie each measured value to a ballooned drawing number.

Ask what the clock covers. A 12-hour quote and free DFM analysis is not the same as a 12-hour part. At GreatLight, quotation and DFM analysis come back within 12 hours, production can start within 24 hours, and parts ship in 3–5 days. Those three numbers are separate promises, and you should ask every supplier to break them out the same way.

MOQ matters more on samples than on production. Some shops set a minimum that pushes you into a batch you do not need yet. GreatLight has no minimum order quantity, so a single prototype and a 10,000+ part run go through the same intake. That is useful when the sample is for an internal design review and you need exactly one part.

Ask about late delivery history. Any shop can quote fast. A useful answer is a tracked rate, like the historical late-delivery probability below 2% that GreatLight reports. If a supplier cannot answer that question at all, treat the lead time as a target, not a commitment.

• 1
  Separate the clocksQuote time, start time, and ship time are three different numbers.
• 2
  Check the MOQNo MOQ means you can order one part without a penalty.
• 3
  Ask for a track recordA measured on-time rate beats a verbal promise.

Certification requirements follow the end market, not the part. An aerospace bracket and a medical housing may look similar on a drawing, but the paperwork trail is not. Aerospace and automotive buyers usually want IATF 16949:2016 or ISO 9001:2015. Medical device work typically points to ISO 13485:2016. If your drawings leave the company, ISO 27001:2022 covers information security.

GreatLight holds ISO 9001:2015, IATF 16949:2016, ISO 13485:2016, and ISO 27001:2022. Ask for the certificate scope, not just the certificate. A valid certificate with a scope that excludes your process does not help your audit.

For anything under NDA, get the agreement signed before you upload CAD. Uploads are kept secure and confidential, and an NDA is available on request. That step costs a day and prevents a much longer conversation later.

• 1
  Match the standard to the marketIATF for automotive, ISO 13485 for medical, ISO 9001 elsewhere.
• 2
  Read the scopeA certificate only counts for the processes it names.
• 3
  Sign the NDA firstDo it before the CAD file leaves your network.

The first red flag is a sample that arrives without any measurement data. If the supplier cannot show which dimensions were checked and what they measured, you are approving a part based on appearance. Appearance does not hold a bearing fit.

The second is a material substitution that was never mentioned. If the quote says 7075 and the part machines like 6061, the sample tells you nothing about the production part. Ask for the material certificate with the sample. It is a one-page document and it settles the question.

The third is a schedule that keeps moving. One delay on a first sample is normal. Three revised dates without a written reason is a pattern. At that point, ask for the machine schedule and the inspection queue, not another promise.

The fourth is silence on workholding and setup. Complex parts with undercuts, thin walls, or deep cavities need a plan before the first cut. A supplier who cannot describe the fixture in a sentence has not planned the job.

• 1
  No data, no approvalInsist on measured values tied to the drawing.
• 2
  Ask for the mill certConfirm the alloy before you test the part.
• 3
  Track the datesRepeated reschedules without a reason are a warning.
• 4
  Demand a setup planWorkholding should be describable in one sentence.