United States CNC processing: a working checklist for engineers
This page is for engineers and purchasing managers who need machined parts and are comparing suppliers. We cover the five things worth checking before you send a PO, plus the questions that expose a weak quote early.
In this article
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Key takeaways
What to compare before you award a PO
Five columns an engineer can verify without a site visit.
| Check | Weak answer | Strong answer | How to verify |
|---|---|---|---|
| Tolerance | ±0.01 mm, no size limit | ±0.005 mm with a stated travel | Ask for the machine list and travel |
| Quote turnaround | 3-5 business days | Within 12 hours with DFM notes | Send a test RFQ with one tricky feature |
| First article | Verbal confirmation only | Inspection report on request | Request the report with the first shipment |
| MOQ | 500 pieces minimum | One piece up to 10,000+ | Ask for a one-off prototype price |
| Finishing | Outsourced, no dates | In-house anodizing and plating | Ask who controls the finish schedule |
| Certifications | ISO 9001 only | IATF 16949, ISO 13485, ISO 27001 | Request certificate scope and expiry |
| Late delivery | No data | Historical rate below 2% | Ask how the rate is measured |
Tolerance claims need a size limit attached
Every shop advertises tight tolerance. Few say on what. A ±0.005 mm callout on a 20 mm aluminum bracket is routine for a good 3-axis mill. The same callout on a 1,200 mm titanium frame is a different problem, because thermal drift, tool deflection, and fixture rigidity all scale with part size.
So the first question is not "what tolerance do you hold" but "on what envelope." GreatLight runs 16 simultaneous 5-axis centers, 12 four-axis mills, 27 three-axis machines, and 16 mill-turn centers. Larger travels cover 750 × 1,150 × 550 mm, and the biggest frame reaches 4,000 mm. The tight number applies where the machine and fixture support it.
Material matters just as much. Aluminum 6061 and 7075 cut cleanly and hold size. Inconel and Ti-6Al-4V move under heat and spring back after the cut. If a supplier quotes the same tolerance and the same price for both, they have not thought about your part.
Finish callouts belong in the same conversation. Ra 0.2–0.8 μm needs a separate finishing pass, not just a slower feed. If your drawing asks for Ra 0.4 μm on a deep pocket, expect a small tool, a long cycle, and a higher price.
- 1Bring the envelopeState part size with the tolerance so the shop can pick the machine.
- 2Flag hard alloys earlyTitanium and Inconel change speeds, tooling, and cost.
- 3Separate cosmetic from functionalNot every surface needs Ra 0.8 μm.
Two clocks: quoting speed and machining speed
A fast quote tells you the shop reads drawings quickly. It says nothing about when chips fly. Ask both questions and write both answers into the PO. Otherwise you learn in week three that the quote was fast and the queue was long.
GreatLight returns a quotation and a free DFM analysis within 12 hours and can start production within 24 hours. Standard parts ship in 3–5 days. Those numbers assume a released drawing and available material, so treat them as the shop's side of the clock, not the whole schedule.
The hidden delay is usually outside the spindle. Heat treat, anodizing, plating, and powder coating are separate processes with their own queues. A shop that owns those steps controls the schedule. A shop that brokers them controls nothing.
Ask what happens when a feature is out of tolerance on the first article. A rework plan with named steps is worth more than a promise. Vague answers here are the clearest warning sign in the whole evaluation.
- 1Get two datesOne for the quote, one for the first shippable part.
- 2Map the outside stepsList every process that leaves the building.
- 3Ask about first-article reworkWho pays, who decides, how long.
MOQ and the prototype-to-production gap
Many suppliers quote prototypes at one price and production at another, with a gap in between that nobody wants to cross. The gap is real: setup, fixture design, and first-article inspection cost the same whether you order 5 parts or 500.
A supplier with no minimum order quantity removes that negotiation. GreatLight runs from a single prototype to 10,000+ part runs on the same floor, so the fixture built for the prototype carries into production. That continuity is where the cost actually drops.
Watch for the reverse trap. A shop that quotes a very low prototype price may be buying the job to learn your part, then raising the production quote once you have no second source. Ask for both prices up front, in writing.
Volume also changes process choice. Below a few hundred parts, machining usually beats casting on total cost. Above several thousand, die casting or vacuum casting starts to win, but only if the geometry allows draft and the surface finish is not critical.
- 1Ask for prototype and production pricing togetherOne RFQ, two columns.
- 2Check that fixtures carry overSetup should not be paid twice.
- 3Revisit the process at volumeMachining is not always the cheapest at 5,000 pieces.
Certifications, inspection, and paperwork
Certificates are only useful when the scope matches your part. ISO 9001:2015 covers a general quality system. IATF 16949:2016 is what automotive programs expect. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which matters when your drawings are the asset.
Inspection is the other half. A supplier that inspects 100% before shipment, with raw material checks, in-process monitoring, and a final inspection, can hand you a report on request. A supplier that samples a batch cannot tell you which part is which.
Confidentiality belongs in the same conversation. Uploads should be secure, and an NDA should be available before you send the first STEP file. If a supplier hesitates on that, stop.
None of this replaces a first article. Paperwork proves the system exists. The measured part proves it worked on your geometry.
- 1Match certificate to industryAutomotive and medical have different expectations.
- 2Ask for the report, not the claimA dimensional report on request is a real deliverable.
- 3Sign the NDA firstBefore any drawing leaves your network.
Material range tells you who the shop really serves
A supplier's material list is a map of its customer base. Shops that only cut aluminum are set up for prototypes and enclosures. Shops that cut stainless, tool steel, and superalloys carry different tooling, coolant, and inspection habits.
GreatLight machines aluminum grades 6061, 7075, and 6082; stainless 303, 304, 316L, 17-4PH, and 440C; steels 1018, 1045, 4130, 4140, and 4340; copper and brass including C36000; titanium TA2 and TC4; plus Inconel, magnesium, and engineering plastics such as PEEK, POM, and PC.
That breadth matters when your assembly mixes materials. A single supplier that can hold the aluminum housing and the 17-4PH shaft avoids two sets of tolerances, two inspection reports, and two shipping schedules.
Ask one specific question: what do you use for a 17-4PH part that must hold ±0.005 mm after heat treat. The answer reveals whether the shop has actually run your material class or is reading from a list.
- 1Test with one hard partSend the most difficult material in your BOM.
- 2Consolidate vendorsFewer suppliers means fewer tolerance stacks.
- 3Ask about heat-treat distortionIt changes the finishing sequence.
Step by step: qualify a supplier in one week
Send the same RFQ package to two or three shops and compare the answers, not the adjectives.
- 1Build one RFQ packageInclude a STEP file, a 2D drawing with GD&T, material spec, finish callout, quantity, and target date. Keep it identical for every shop.
- 2Add one deliberately hard featureA deep pocket, a thin wall under 1 mm, or a true position callout at Ø0.05 mm. See who flags it in the DFM notes.
- 3Time the quoteNote the hours until a written response. A supplier that analyzes DFM within 12 hours is set up to read drawings, not to guess.
- 4Request the inspection planAsk what is measured, on what equipment, and whether a report ships with the parts.
- 5Ask about the outside processesHeat treat, anodizing, and plating. Get the names and the added days.
- 6Order one first articleBefore any volume PO, buy a single part and measure it yourself. This is the only test that cannot be talked around.
- 7Compare the production quotePut prototype and 1,000-piece pricing side by side. A large jump between them is a red flag.
Questions buyers ask us
What tolerance can we realistically expect on a large part?
It depends on size and material. On a compact envelope, ±0.005 mm is achievable. On a 4,000 mm frame, thermal and fixture effects dominate, so the practical band widens.
Send the drawing with the envelope and we will tell you which tolerance is realistic on which machine, instead of quoting one number for everything.
Do we need to order a minimum quantity?
No. There is no minimum order quantity. A single prototype and a 10,000+ part run go through the same shop.
That matters because the fixture and inspection plan built for the prototype carry into production, so you are not paying setup twice.
Which certifications should we ask for?
ISO 9001:2015 is the baseline. Add IATF 16949:2016 for automotive, ISO 13485:2016 for medical devices, and ISO 27001:2022 when drawing confidentiality is part of your risk review.
Ask for the certificate scope and expiry date, not just the logo.
How fast can parts ship?
A quotation and free DFM analysis come back within 12 hours, and production can start within 24 hours. Standard parts ship in 3–5 days.
Those clocks start when the drawing is released and the material is available. Finishing steps run on their own schedule, so confirm them separately.
Can you work from a 3D file only?
A STEP file is enough to start a DFM review. For toleranced features, a 2D drawing with GD&T removes ambiguity about what must be measured.
If you only have a model, we will tell you which features need a callout before we quote.
Are our drawings kept confidential?
Uploads are secure and confidential. An NDA is available on request, before any file is shared.
Information security is covered under ISO 27001:2022, which is the standard to ask about when your drawings are the asset.
Send one part and judge the result
Upload a STEP file and a drawing and get a quotation with free DFM analysis within 12 hours.
12-hour quote100% inspectionNo MOQNDA on request
More from the shop floor
We publish setup notes, tooling trials and inspection data from the factory floor.
