CNC Machine Services: 6 Checks Before You Order Parts

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  Key takeaways
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  What to check against the part in front of you
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  What CNC machine services actually cover
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  Tolerance and finish: the two numbers that set the price
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  Size envelope, setup count and fixture cost
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  Lead time, MOQ and certifications
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  How to compare quotes without getting fooled
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  Step by step: from RFQ to approved parts

CNC machine services is a loose term. On one quote it means turning a 20 mm aluminum spacer. On another it means a 5-axis titanium housing with a positional tolerance of ±0.005 mm and a sealing face that cannot leak. The machine, the fixturing and the inspection plan are different in each case. So the first job when you compare suppliers is to work out which category your part falls into.

Subtractive work splits into milling, turning and mill-turn. Milling covers pockets, slots, faces and profiles. Turning covers round parts with a single axis of symmetry. Mill-turn centers do both in one setup, which matters when a part has a turned bore and a milled flange that must stay concentric. GreatLight runs 16 mill-turn centers alongside 27 three-axis machines, 12 four-axis mills and 16 simultaneous 5-axis machining centers.

The rest of the shop supports the cut. Surface finishing, heat treatment coordination and light assembly decide whether parts arrive ready to use or need another vendor. Every extra handoff adds a week and a new place for a dimension to drift. When you scope CNC machine services, count the handoffs as carefully as you count the machines.

One more distinction matters. Prototype work and production work use the same spindles but different process discipline. A prototype can be re-cut if a fixture slips. A 10,000-piece run cannot, so the shop locks the process, the tool list and the inspection routine before the first batch ships.

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  MillingPockets, slots, faces, profiles; 3-axis for flat work, 4 and 5-axis for angled features.
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  TurningRound parts; mill-turn when a turned bore and a milled feature share a datum.
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  FinishingAnodizing, plating, powder coating, bead blasting, polishing, laser marking.
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  InspectionRaw material check, in-process monitoring, final inspection before shipment.

Tolerance drives cost more than any other line on the drawing. Going from ±0.05 mm to ±0.005 mm changes the machine, the fixture, the tool path and the number of inspection steps. On a small aluminum bracket that jump may add a few dollars per part. On a 400 mm steel housing it can double the cycle time, because the shop has to rough, stress-relieve, then finish in a second setup.

Finish works the same way. Ra 1.6–3.2 μm is as-machined and comes off a normal finishing pass. Ra 0.8–1.6 μm needs a lighter stepover and a sharp tool. Ra 0.2–0.8 μm usually means a separate polishing or lapping operation, and it applies to specific faces rather than the whole part. Mark those faces on the drawing instead of calling out a blanket finish.

Here is the trap. Tight tolerance on a feature that does not need it is the most common waste in a prototype build. If a mounting hole only locates a cover, ±0.1 mm is fine. If it carries a bearing, it is not. Group your dimensions into functional and non-functional before you send the RFQ, and say which is which. Shops price what you write.

Material changes the answer too. Aluminum 6061 and 7075 cut cleanly and hold tight tolerances with less effort. Stainless 316L and 17-4PH work-harden and move more under heat. Titanium TC4 and Inconel need slower speeds and more rigid setups. The same ±0.005 mm callout costs more in Inconel than in 6061, and a supplier who quotes them the same has not read the drawing.

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  As-machinedRa 1.6–3.2 μm; default for non-sealing, non-sliding faces.
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  FineRa 0.8–1.6 μm; mating faces, O-ring grooves, sliding contact.
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  PolishedRa 0.2–0.8 μm; optical, sealing and wear surfaces only.
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  Callout tipMark tight faces individually. A blanket tolerance raises every price.

Machine travel is the hard limit. A part that fits inside 500 × 500 × 450 mm can run on a compact vertical mill. Past that you need a larger machine, and past 4,000 mm you need a shop that has one. GreatLight's largest travel is 4,000 × 400 × 150 mm, with medium platforms at 750 × 1,150 × 550 mm and 600 × 600 × 600 mm. Send the bounding box with the drawing, because a diagonal feature can exceed travel even when the box looks small.

Setup count follows geometry. Every face you machine from a new direction is another setup, another fixture and another chance to lose datum. A part with features on five sides either goes on a 5-axis machine in one setup or moves through three or four 3-axis setups. The first option costs more per hour. The second costs more in scrap, handling and calendar days.

Fixtures are the hidden line. Soft jaws, vacuum plates and custom clamps can run from modest to significant, and they are often quoted separately. Ask whether the fixture is included, reusable and stored for repeat orders. If the shop scraps the fixture after the first run, your second order pays for it again.

Thin walls and long slender parts behave differently. A 1 mm wall on a 300 mm aluminum extrusion will deflect under normal clamping pressure. The shop has to reduce depth of cut, add support or machine in stages. None of that shows up on the drawing, but it shows up in the quote.

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  Compact500 × 500 × 450 mm and 500 × 310 × 200 mm platforms.
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  Medium750 × 1,150 × 550 mm and 600 × 600 × 600 mm.
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  Large4,000 × 400 × 150 mm travel for long, narrow parts.
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  RotaryØ400 mm rotary table for angled features in one setup.

Lead time has three parts: quoting, production start and shipping. A supplier that quotes in 12 hours with free DFM analysis and starts production within 24 hours is usually a shop with open capacity and a defined process. One that takes five days to answer a drawing is either overloaded or guessing at the price. Both are worth knowing before you commit.

MOQ matters most in the prototype phase. No minimum order quantity means you can order one part, test it, revise the model and order three more without a tooling charge. That is the normal pattern for hardware teams. Once the design freezes, per-piece cost drops with volume because setup and programming spread across more parts. The mistake is ordering 500 parts to get a lower unit price before the design is stable.

Certifications tell you which industries a shop can serve. ISO 9001:2015 covers general quality management. IATF 16949:2016 is the automotive standard and signals process control, traceability and change management that general shops do not maintain. ISO 13485:2016 covers medical devices. ISO 27001:2022 covers information security, which matters when your CAD files are the product.

Ask for the certificate scope, not just the logo. A shop can hold ISO 9001 for one plant and run a second plant without it. Check which site will actually cut your parts. If the certificate list and the manufacturing address do not match, ask why.

Confidentiality sits in the same category. Uploads should be secure, and an NDA should be available on request before you send files. For medical, aerospace and defense-adjacent parts, that is not a formality. It is part of the selection.

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  Quote12 hours including DFM analysis, before any commitment.
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  StartProduction can start within 24 hours of a released order.
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  ShipParts ship in 3–5 days for standard volumes.
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  VolumeNo MOQ; one prototype through 10,000+ part runs.

Quotes are only comparable when they describe the same scope. Two suppliers can both write 'CNC machined aluminum part' and mean different things: one includes material certification and full inspection, the other assumes you supply the stock and accept a visual check. Put the scope on one page before you compare numbers.

Require a line for material grade and condition. 6061-T6 is not the same as generic 6061, and 17-4PH in the H900 condition behaves differently from the annealed state. If the quote says 'aluminum' or 'stainless', the price is not real yet.

Require a line for inspection. For critical dimensions, ask for a first article report and note the sampling plan for the production run. A shop that does 100% inspection before shipment will say so. One that samples will also say so, and that is fine as long as you agreed to it.

Require a line for finishing and freight. Anodizing, plating and laser marking are separate operations with their own lead time. Freight and duties change the landed cost. Compare the landed number, not the ex-works number. If a supplier will not break the quote into these lines, treat the total as a placeholder.

Finally, look at the questions the supplier asks. A shop that queries a datum, a thread depth or a corner radius before quoting has read the file. A shop that only asks for quantity has not.

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  MaterialGrade, temper and condition, plus mill cert availability.
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  InspectionFirst article report, sampling plan, final report on request.
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  FinishingProcess, color, masking and minimum character height for marking.
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  Landed costUnit price plus finishing, freight, duties and packaging.