CNC Milling: How to Choose a Shop Without Guesswork

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  Key takeaways
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  What to check against what your part actually needs
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  Match the tolerance to the feature, not the drawing title block
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  Material choice drives the number of setups
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  Lead time promises are only as good as the queue behind them
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  Inspection, certifications and the paperwork trail
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  What a useful quote should contain
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  Step by step: from RFQ to parts on the bench

Most RFQs arrive with one blanket tolerance in the title block and a note saying tight. That number rarely survives contact with the geometry. A 50 mm aluminum bracket held to ±0.005 mm is routine on a 3-axis mill with a temperature-stable shop. The same callout on a 4,000 mm frame is a different conversation, because thermal expansion alone moves the part more than the tolerance band.

Before you send the file, mark which dimensions actually mate with something else. Those are the ones worth tightening. Cosmetic faces, clearance holes and non-critical pockets can sit at ±0.1 mm and cost far less. When a shop quotes the whole part at the tightest callout, you are paying for inspection time on features that will never be measured by your assembly.

Deep pockets, thin walls and tall ribs are the three features that quietly break a quote. A wall under 1 mm on aluminum will deflect under cutting force, so the shop has to take lighter passes and the cycle time climbs. If the design allows 1.5 mm or 2 mm, say so in the RFQ. That one change often removes the need for a 5-axis setup.

Datum strategy matters as much as the tolerance value. If your drawing dimensions everything from a corner that gets machined away in op one, the shop has to invent a datum and you may not get what you modeled. Define datums on surfaces that exist in the final part and are reachable in every setup.

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  Tighten only mating features±0.005 mm where it seats, ±0.1 mm everywhere else.
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  Keep walls at 1.5 mm or moreBelow that, expect slower passes and higher cost.
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  Put datums on surviving facesAvoid corners that are cut in the first operation.

Aluminum 6061-T6 and 7075 machine fast and hold tolerance well. Stainless 304 and 316 work fine but move more under heat, so the shop may rough, cool and finish in two passes. Titanium TC4 (Ti-6Al-4V) and Inconel cut slowly and wear tools, which shows up in the price long before it shows up in the tolerance.

Plastics behave differently again. POM and ABS are stable, but PEEK and PC need sharp tooling and light chiploads or they chip at the edges. HDPE is soft enough that workholding pressure can distort the part, so vacuum fixturing or soft jaws are common. None of this is a reason to avoid a material. It is a reason to tell the shop which material you have specified and why.

Count the setups on a part before you assume the price is high. A block with features on four sides needs four orientations unless the shop has a 5-axis center that can reach them in one. GreatLight runs 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers, with a Ø400 mm rotary table for round work. That mix decides whether your part is one setup or four.

Maximum processing size is 4,000 mm, with travels including 4,000 × 400 × 150 mm for long parts and 750 × 1,150 × 550 mm for large plates. If your part sits near those limits, ask how the shop plans to support it. Long thin parts sag and chatter, and no tolerance callout fixes that after the fact.

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  Aluminum: fast and stable6061-T6, 7075, 6082 cover most brackets and housings.
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  Titanium and Inconel: slowTool wear and heat control raise both price and lead time.
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  Plastics need sharp toolingPEEK and PC chip at edges; HDPE distorts under clamping.
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  Setup count is a cost driverFour-sided features mean four orientations without 5-axis.

Every shop says fast. The useful question is what happens between purchase order and shipment. At GreatLight, quotation and a free DFM analysis come back within 12 hours, production can start within 24 hours, and parts ship in 3–5 days. Those are the numbers to hold a supplier to, not a vague promise of quick turnaround.

Historical late-delivery probability here is below 2%. That figure matters more than a headline lead time, because a shop that hits 5 days on 80% of orders will hurt you more than one that quotes 7 days and hits it every time. Ask for the number, not the slogan.

Quantity changes the process, not just the price. A single prototype may be machined from billet with generous stock. A 10,000-piece run usually moves to fixtures and possibly to die casting or vacuum casting for the housing, with machining only on the critical interfaces. GreatLight has no minimum order quantity, so the same shop can quote one piece and the production volume and tell you where the crossover point is.

If your annual volume is high and the geometry allows it, ask whether milling is even the right process. Sometimes a cast or molded blank with milled critical faces beats milling the whole part from solid. That conversation is worth having before the drawing is frozen, not after.

Ask what is inspected and how often. At GreatLight, every order gets raw material check, in-process monitoring and final inspection, with 100% inspection before shipment and reports available on request. The qualification rate is 99.99%. If a supplier cannot describe their inspection flow in three sentences, that is your answer.

Certifications should match your industry, not just your purchasing form. ISO 9001:2015 covers general quality management. Automotive and EV work usually needs IATF 16949:2016. Medical devices need ISO 13485:2016. For programs where drawings and CAD data are sensitive, ISO 27001:2022 covers information security. GreatLight holds all four.

Finish specifications belong in the RFQ too, because they change handling and sometimes dimensions. Anodizing, plating and powder coating add thickness. Hardcoat anodize can add 25 to 50 μm per surface, which matters on a thread or a press-fit bore. Bead blasting, tumbling, brushing and polishing change surface texture without changing the nominal size, so they are safer on tight fits.

Laser marking and engraving need at least 1.5 mm character height to stay legible. If your part number or UDI string is smaller than that, it will not read reliably after finishing. Say so early and the shop can adjust the marking layout before the first cut.

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  100% inspection before shipmentReports on request, covering the features you name.
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  Match the certificate to the industryIATF for automotive, ISO 13485 for medical.
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  Finishes move dimensionsHardcoat anodize adds 25–50 μm per surface.
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  Marking needs 1.5 mm heightSmaller characters lose legibility after coating.

A price alone tells you almost nothing. A usable quote names the material grade, the stock size, the machine class, the number of setups, the finish and the inspection scope. If two shops are 40% apart, the difference is usually in one of those lines, not in the hourly rate.

Watch for quotes that exclude finishing, deburring, or inspection reports. Those items get added later and the comparison you made is void. Ask for the line items up front, even on a one-piece order.

Confidentiality is part of the commercial package. Uploads should be secure and confidential, and an NDA should be available on request before you send production drawings. GreatLight offers an NDA and keeps files restricted to the quoting and programming team.

Finally, ask who reads your file. A quote prepared by sales staff and a quote prepared by a programmer who has opened the model and flagged the thin walls are different products. The second one is the one you want, and it usually arrives with a DFM note attached.