How to Vet CNC Processing Supplier Solutions

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  Five checks before you commit
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  Which CNC processing supplier solutions fit which job
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  Tolerance: what CNC processing supplier solutions actually hold
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  Setup count and machine travel decide your unit price
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  Certifications, confidentiality and MOQ in supplier solutions
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  Lead time, quote scope and the questions that expose weak quotes
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  How to run the supplier check in six steps

Every shop prints a tolerance on its website. Very few print the conditions. A ±0.005 mm callout means something specific: a temperature-controlled room, a machine with low thermal drift, and a gauge that can read to a tenth of that. Ask which machine will run your part and what the inspection method is. If the answer is vague, the tolerance is a marketing line.

At GreatLight we hold ±0.005 mm (±0.0002 in) on critical features. That is not a blanket promise across every surface on every part. It applies where the drawing calls it out and the geometry allows it. A 300 mm long aluminum extrusion will move with heat, so we fixture it, let it stabilize and cut in passes. A 20 mm stainless boss can be held tighter because the material is stiffer and the cut is shorter.

Surface finish follows the same logic. Ra 0.2–0.8 μm needs a fine finishing pass and often a different tool than roughing. Ra 1.6–3.2 μm is as-machined and suits most brackets. If your drawing asks for Ra 0.4 μm on a deep pocket, expect a longer cycle and a higher price. Ask the shop whether they can reach it before you assume they can.

One more thing: tolerance and inspection are different budgets. A shop can machine to ±0.005 mm and still ship a part you reject if nobody measured the feature. Ask for the inspection report with the first article. That report is your proof, not the website.

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  Ask which machine, not which toleranceMachine model, spindle, thermal compensation.
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  Separate critical features from cosmeticTight tolerance only where the drawing needs it.
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  Request first-article inspectionCMM or gauge report with the shipment.

A three-axis mill cuts from one direction. A part with features on five faces needs four or five setups, and every setup adds fixture time, re-datum time and the chance of stack-up error. Five-axis machining rotates the part or the spindle so most features come off in one setup. That is where the cost difference lives, not in spindle speed.

Look at your part and count the faces that need cutting. Two faces, three-axis is fine and cheaper. Four or more faces with compound angles, five-axis wins even at a higher hourly rate because you pay for one setup instead of five. We run 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers, so the quote can match the process to the geometry instead of forcing one machine.

Travel limits matter just as much. A part that is 800 mm long will not fit a 500 mm table. We quote against 4,000 × 400 × 150 mm on the large machines, 750 × 1,150 × 550 mm and 600 × 600 × 600 mm on the medium ones, and smaller envelopes on the compact cells. Send the bounding box with the RFQ. It saves a round of questions.

Mill-turn centers are worth knowing about. A shaft with a turned diameter and milled flats normally travels between a lathe and a mill, which means two setups and a re-chuck. A mill-turn center does both in one program. For hydraulic spools, motor shafts and connector bodies, that removes a whole operation from the routing.

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  Count the cutting directionsMore than three faces usually favors 5-axis.
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  Send the bounding boxLength, width and height with the RFQ.
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  Ask about mill-turnTurned and milled features in one setup.

Certifications are a shortcut to knowing which industries a shop already serves. ISO 9001:2015 covers general quality management. IATF 16949:2016 is the automotive standard and comes with control plans and PPAP expectations. ISO 13485:2016 is for medical devices and brings process validation and device history records. ISO 27001:2022 covers information security, which matters when your drawings are the product.

If your part is a medical housing, a supplier without ISO 13485 is asking you to carry the validation work yourself. If your part is a powertrain bracket, IATF 16949 tells you the shop already runs SPC and understands change control. GreatLight holds all four certificates, and they were audited, not printed. Ask for the certificate scope and the expiry date, not just the logo.

MOQ is the third filter. A shop with a 500-piece minimum cannot economically run your prototype, so it will either decline or price it high to discourage you. We run no minimum order quantity, from one prototype to 10,000+ part runs, because the setup is quoted separately and the scheduling absorbs small jobs. That is a deliberate choice, not a discount.

Confidentiality sits next to all of this. Uploads are secure and confidential, and an NDA is available on request. If your assembly is pre-release, get the NDA signed before you send STEP files, not after. It costs a day and removes a category of risk.

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  Match the certificate to the industryIATF 16949 for auto, ISO 13485 for medical.
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  Check scope and expiryA logo without scope proves little.
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  No MOQ suits prototypesSetup quoted separately from piece price.
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  Sign the NDA firstBefore STEP files leave your network.

Lead time is easy to promise and hard to prove. Ask what happens between PO and shipment, and ask for the weak point. At GreatLight we return a quotation and free DFM analysis within 12 hours, production can start within 24 hours, and parts ship in 3–5 days. Our historical late-delivery probability is below 2%. Those are our numbers on our process, and you should ask every supplier for theirs in the same terms.

Quote scope is where budgets break. A low number that excludes material, anodizing, inspection reports and packing is not a low number. Read the exclusions line by line. Does the price include the raw material grade you specified, or a substitute? Does it include the finish, or is finishing a second supplier with its own lead time? Does it include a first-article report?

Packing deserves a mention. A machined aluminum face will arrive scratched if it is loose in a cardboard box. Ask how parts are separated and protected. For cosmetic parts, this is the difference between a usable shipment and a rework claim.

Finally, ask who owns the process window. A good supplier will tell you that a thin wall will deflect, that a deep pocket needs a smaller tool and more time, or that a hard material will wear tooling faster. A weak supplier says yes to everything. The first answer costs you a conversation. The second costs you a shipment.

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  Ask for the late-delivery rateA percentage, not a promise.
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  Read the exclusionsMaterial, finishing, inspection, packing.
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  Ask about packingCosmetic faces need separation.