CNC Alloy Machining: How to Pick the Right Shop

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  Key takeaways
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  What to check, and what a weak answer looks like
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  Start with the alloy, not the machine
• 4
  What ±0.005 mm actually requires
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  Machine mix tells you what the shop can finish in-house
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  Surface finish and finishing steps change the price
• 7
  MOQ, certifications and data handling
• 8
  Warning signs in a quote or a first reply
• 9
  Seven steps to screen a supplier before you commit

Every CNC alloy machining quote starts with the material. Aluminum 6061-T6 is the default for housings, brackets and fixtures. It cuts at high speed, holds a good finish and takes anodizing well. 7075 gives you roughly twice the yield strength, but it is less corrosion resistant and it moves more after roughing. If your part has thin walls under 2 mm, plan a rough and a finish pass with a stress-relief pause.

Stainless is where quotes spread out. 303 is free-machining and cheap to run. 304 and 316L work-harden, so light cuts and constant feed are required. 316L also galls, which means tool wear climbs and surface finish drifts. 17-4PH (SUS630) machines at H900 or annealed condition, and the heat-treat step changes final dimensions, so leave stock and say so on the drawing.

Titanium and Inconel sit at the other end. Ti-6Al-4V (TC4) has low thermal conductivity, so heat goes into the tool, not the chip. Cutting speeds drop to 30–60 m/min and coolant delivery matters more than spindle speed. Inconel is worse. If a shop quotes titanium at the same rate as aluminum, they either have not run it or they are buying the risk back later.

Magnesium AZ31B and AZ91D are light and machine fast, but chips are a fire risk. Ask how they handle chip evacuation and what coolant they use. Copper and brass families (C101, C110, C36000) cut cleanly and are common for busbars and RF housings. Beryllium copper needs its own controls. Tell the shop what the part does, and the alloy choice usually becomes obvious.

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  6061-T6Default for structural and enclosure parts; anodizes well.
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  7075High strength, weaker corrosion resistance; plan for movement.
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  304 / 316LWork-hardening and galling; expect slower feeds and more tool changes.
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  Ti-6Al-4VHeat stays in the tool; coolant strategy decides the result.

A tolerance callout is a claim about the whole process, not one machine. To hold ±0.005 mm (±0.0002 in) on a production part you need a machine in good geometry, a fixture that does not deflect under cut load, and a shop that controls temperature. A part that measures well at 8 a.m. can drift by mid-afternoon if the spindle warms up and the coolant temperature is not managed.

Ask where the tight tolerance sits on the part. A single bore or a bearing seat is different from a 300 mm bolt circle. Datum choice drives the answer. If the drawing calls A-B-C datums but the shop fixtures from a vise jaw, the CMM report will look fine while the assembly does not fit.

For most parts, ±0.05 mm is enough and costs far less. Reserve ±0.005 mm for mating features, bearing fits and sealing surfaces. When you send an RFQ, mark the critical dimensions. A shop that reads the drawing and comes back with a short DFM note on two or three features is doing the work. One that quotes everything at the same tolerance is not.

GreatLight inspects 100% of parts before shipment and can supply reports on request. Raw material check, in-process monitoring and final inspection are separate steps, and each one catches a different failure mode. In-process checks catch tool wear. Final checks catch handling damage. Ask which steps apply to your order.

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  Tight where it mattersBearings, seals and mating faces only.
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  Loose everywhere elseGeneral ±0.05 mm keeps cost down.
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  Datums on the drawingMust match how the part is fixtured.

A shop with only 3-axis mills will re-fixture your part three or four times to reach the same features a 5-axis center does in two setups. Each setup adds error stack-up and queue time. If your part has angled faces, deep pockets or ports on multiple sides, ask directly how many setups the quote assumes.

Part size sets the floor. A 4,000 mm envelope covers long rails, frames and beams. A 750 × 1,150 × 550 mm envelope covers most automotive and industrial housings. Compact 500 × 500 × 450 mm machines handle small, high-mix work and usually give better surface finish because the structure is stiffer. Match the envelope to your part, not to the biggest number in the brochure.

Mill-turn centers matter for parts that are turned and milled. If a shop has to move your part from a lathe to a mill, concentricity suffers and lead time grows. A Ø400 mm rotary table covers most round work with cross features. For a shaft with milled flats and drilled holes, mill-turn is the cleaner route.

Ask how many of each machine type the shop owns. A single 5-axis center is a bottleneck. Sixteen simultaneous 5-axis centers across 127 high-precision machines means the schedule has slack. That is the difference between a quote that holds and one that slips when a hot job lands.

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  5-axisFewer setups, better position accuracy on complex faces.
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  Mill-turnKeeps concentricity on turned parts with milled features.
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  EnvelopeMatch to your largest part, not the shop's largest machine.

As-machined finish sits around Ra 1.6–3.2 μm on most alloys. That is fine for brackets, plates and internal parts. Ra 0.8–1.6 μm usually needs a finer step-over, a sharper tool and a slower finish pass. Ra 0.2–0.8 μm adds polishing or a controlled finishing operation plus inspection time. Each step roughly doubles the labor on that surface.

Functional finishes are separate from cosmetic ones. Hardcoat anodizing builds a wear layer on aluminum and changes dimensions by a few microns per side, so mask or compensate the tight features. Electroless nickel gives uniform coverage on complex shapes, which is why it shows up on valve bodies. Black oxide and zinc plating protect steel at low cost. Powder coating hides tool marks but adds thickness.

Cosmetic parts are the hardest to quote. Bead blasting, tumbling, brushing and polishing each leave a different texture, and the same process on 6061 and 7075 looks different because the alloy grain shows through. Send a sample or a photo of the target finish. Words like “smooth” or “matte” mean different things to different operators.

Laser marking is the last step and it can scrap an otherwise good part. Minimum character height is 1.5 mm for a clean mark. Below that, the mark gets inconsistent across alloys and finishes. Put the marking note on the drawing with the exact text and location, not in an email that gets lost.

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  As-machinedRa 1.6–3.2 μm; no extra operation.
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  Fine finishRa 0.8–1.6 μm; slower pass, sharper tool.
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  Polish gradeRa 0.2–0.8 μm; adds a separate operation.

Some shops will not quote a single prototype; others will not quote a 10,000-unit run. Both limits are about how the shop schedules and how it documents work. A supplier that runs from one prototype to 10,000+ part runs uses the same process sheet in both cases. That matters because your pilot build and your production build should not be two different processes.

Certifications are a filter, not a score. ISO 9001:2015 covers general quality management. IATF 16949:2016 is what automotive programs expect. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which matters if your drawings are sensitive. Check the scope of the certificate, not just the logo. A certificate for a different site or a different process does not cover your order.

Confidentiality is a real issue with contract machining. Ask how uploads are handled and whether an NDA is available before you send CAD. A shop that wants the drawing first and the NDA later has the order backwards. NDA on request is normal; refusing to sign one is a signal.

The quote itself should be readable. Material with cert, setup, machining time, finishing, inspection and shipping as separate lines. A single lump-sum number tells you nothing about where the cost sits, so you cannot negotiate a design change or compare two shops fairly.

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  No MOQOne prototype or 10,000+ parts on the same process sheet.
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  Sector certsMatch the certificate to your industry and site.
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  NDA before CADSign first, upload second.

The fastest red flag is a round number. Alloy machining cost tracks material removal volume, tool life and setup count. A quote that lands on a clean figure without a breakdown usually means the estimator guessed and will make it up later with a change order.

The second is silence on the alloy. If the shop does not ask about temper, heat treatment or the service environment, they are quoting a generic block of metal. Magnesium chips burn, 316L galls, and Inconel eats tools. A shop that has run these alloys will ask about them in the first reply.

The third is a tolerance promise with no method. “We can hold anything” is not an answer. Ask which machine, which fixture and which inspection method. A supplier that names the machine and the CMM is telling you they have done this before.

Watch for finishing that is always outsourced. If anodizing, plating and powder coating all leave the building, your lead time is now two schedules instead of one. That is not automatically bad, but you should know it before you promise a date to your own customer.

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  No breakdownOne lump sum hides where the cost and risk sit.
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  No alloy questionsA generic quote for a specific alloy is a guess.
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  No methodA tolerance claim without a machine and fixture is marketing.