Precision CNC Parts: What to Check Before You Order

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  Key takeaways
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  Which shop fits your program
• 3
  Tolerance: what ±0.005 mm really covers
• 4
  Lead time: ask when the clock starts
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  MOQ, tooling and the cost you are not seeing
• 6
  Which certifications actually apply to your part
• 7
  Inspection and the 99.99% number
• 8
  Materials and the parts that go wrong
• 9
  Traps that add a week
• 10
  How to run the supplier check in one day

Every shop can quote ±0.005 mm. Fewer can hold it on a 300 mm aluminum plate after anodizing, because the coating moves the surface by a few micrometres and the heat from machining moves it more. When a drawing says ±0.005 mm, ask which feature that applies to. On most parts, two or three dimensions actually drive function. The rest can sit at ±0.05 mm and nobody will notice.

A useful answer names the machine class and the inspection method. A 5-axis center with a Ø400 mm rotary table can hold position on a part that would need three setups on a 3-axis mill, and every extra setup adds stack-up error. If the shop cannot say how it will measure the feature, the tolerance on the drawing is a wish, not a plan.

Surface finish sits in the same conversation. Ra 0.8–1.6 μm is a normal machined finish and comes off the tool with a sensible feed rate. Ra 0.2–0.8 μm needs a finer step-over, a sharper tool and more time. Put the finish callout only where it matters, otherwise you pay for polishing on surfaces nobody touches.

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  Name the critical featuresTwo or three dimensions per part, not the whole drawing.
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  Ask for the measurement planCMM, micrometer or gauge, and who signs the report.
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  Check finish vs. functionSealing faces and bearing bores need Ra 0.2–0.8 μm. Brackets do not.
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  Watch heat and coatingAnodizing and hardcoat change dimensions; plan the allowance.

A quotation and free DFM analysis within 12 hours is a real number, and production can start within 24 hours once the drawing is released. Parts ship in 3–5 days for straightforward work. The gap between those numbers and your actual delivery date is usually material. A 7075 plate or a Ti-6Al-4V billet may not sit on the shelf, and no machine can cut metal that has not arrived.

So ask two questions. Which material is in stock today, and what is the lead time on the rest? A shop that answers both without checking with someone else is a shop that tracks its own inventory. A shop that says "should be fine" is guessing.

For repeat programs, the schedule matters more than the first shipment. Historical late-delivery probability below 2% is a process number, not a promise about your order, but it tells you the shop measures its own performance. Ask how they measure it.

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  Freeze the drawing firstAny change after material cut restarts the clock and wastes stock.
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  Separate material from machiningTwo timelines, two risks, one date you can plan around.
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  Ask about the finishing queueAnodizing and plating often add more days than machining does.

No minimum order quantity is common in contract machining, and it is genuinely useful when you need one prototype on Tuesday. But MOQ is not the number that decides your unit price. The number that decides it is how much of the cycle is spent loading, fixturing and measuring the part rather than cutting it.

A part with a 40-minute cycle on a 5-axis center will cost roughly the same for one piece as for fifty, minus material. A part with a 3-minute cycle on a lathe drops fast at volume. That is why a quote for one prototype and a quote for 10,000 pieces look unrelated even when they come from the same shop.

If you plan to scale, say so at the quoting stage. The shop will then design the first fixture so it can run in a pallet system later, and you avoid paying for tooling twice. Bring the annual volume, not just the first order.

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  Quote both volumesOne piece and your realistic annual quantity, side by side.
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  Ask what changes at volumeFixture, bar feeder, pallet or dedicated soft jaws.
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  Check the finishing minimumPlating lines often have their own batch minimum.

ISO 9001:2015 covers the general quality system. IATF 16949:2016 applies to automotive work and adds traceability and change control. ISO 13485:2016 is the medical device standard. ISO 27001:2022 covers information security, which matters when your drawings are the product. A supplier holding all four is not automatically better for your bracket, but the certificates tell you which paperwork you can expect without asking for a favor.

The certificate is a starting filter. What you actually receive is the inspection record: raw material check, in-process monitoring, final inspection, reports on request. For a medical or aerospace part, ask for the material certificate and the dimensional report with the shipment. If that request feels awkward, the relationship is already in the wrong place.

Confidentiality belongs in the same conversation. Uploads should be secure, and an NDA is available on request. For defense, medical and unreleased consumer products, sign it before the first STEP file leaves your network.

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  Match the certificate to the industryIATF for automotive, ISO 13485 for medical, ISO 9001 as the floor.
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  Ask for the report with the partsMaterial cert plus dimensional data, not a promise to send it later.
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  Sign the NDA firstBefore drawings, not after the quote.

A qualification rate of 99.99% sounds like a marketing figure until you ask what it counts. In a shop running 100% inspection before shipment, it counts parts that passed the final check, not parts that were never checked. Those are different claims. Ask which one you are buying.

The practical question is what happens at the boundary. If a bore measures 0.003 mm over nominal and the tolerance is ±0.005 mm, the part ships. If it measures 0.006 mm over, the part is scrap or rework, and someone has to decide who pays. Get that answer before the first order, because it is much harder to negotiate after a rejected lot sits on your dock.

For prototypes, a first-article report on the critical features is worth more than a general inspection claim. It shows the shop measured the dimensions you care about, on the machine or on a CMM, and recorded the numbers. That document is also your baseline when the part goes to production.

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  Define pass and fail in writingAt the tolerance limit, not at the drawing review.
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  Ask who pays for reworkRework caused by machining is normally on the shop.
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  Keep the first-article reportIt becomes the reference for every later run.

Material choice drives both the process and the failure mode. Aluminum 6061-T6 and 7075 machine cleanly and hold tight tolerances, and 7075 gives you strength at the cost of some corrosion resistance. Stainless 303 and 304 turn well; 316L and 17-4PH are for corrosion and strength, and they work-harden if the feed rate is too light. Titanium TC4 (Ti-6Al-4V) and Inconel need low cutting speeds, rigid setups and patience.

Thin walls are where precision CNC parts fail. A 0.3 mm wall on a titanium impeller will deflect under cutting force, and the second pass cuts air while the first pass cut metal. The fix is not a slower spindle. It is a support strategy: adaptive toolpaths that keep radial engagement constant, a fixture that backs the wall, and in-process probing between passes.

Plastics behave differently again. POM and PEEK hold dimensions well, but they move with temperature and absorb coolant. ABS and PC scratch easily, so the finishing step matters as much as the cut. If the part is a fixture or a cover, say so, because the shop will plan the clamping differently.

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  Match alloy to environment6061 for general work, 7075 for strength, 316L for corrosion, TC4 for weight.
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  Flag thin walls earlyAnything under 1 mm needs a support plan in the quote.
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  Separate cosmetic from functionalCosmetic faces need a different toolpath and often a different operator.

The first trap is a drawing that changes after the material is cut. A revised STEP file on day two means new stock, a new setup and a new first-article report. Freeze the model before the PO. If you must change it, send the change as a new revision and expect the clock to restart.

The second trap is a finish callout nobody priced. Ra 0.2–0.8 μm across a whole part can double the cycle time compared with Ra 1.6–3.2 μm. Mark the surfaces that need it and let the rest run as machined.

The third trap is packaging. Precision CNC parts with a 0.005 mm tolerance can be ruined by a loose box and a rough courier. Specify how the parts are separated, wrapped and labeled, and include the drawing number on the label. It costs almost nothing and saves an argument at goods-in.

The fourth trap is a single-source schedule with no buffer. If your assembly date is fixed, order the prototype early enough to absorb one rework cycle. Rework is normal in machining. Planning for zero rework is not.

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  Freeze the revisionOne drawing number, one PO, no verbal changes.
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  Price the finish where it is neededSelective callouts, not blanket ones.
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  Write the packaging specSeparators, wrap, label with part number and revision.