Surface CNC Alloy Processing Service: How to Pick the Right Shop
This guide is for engineers and buyers sourcing a surface CNC alloy processing service for titanium, stainless, aluminum and other hard alloys. It covers the seven checks that decide whether a shop can hold your tolerance, finish and delivery window, and when a cheaper process is the better call.
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What matters most before you send a PO
Which alloy surface process fits your part
Match the requirement to the process before comparing quotes.
| Part requirement | Process to specify | Typical finish | When it is the wrong choice |
|---|---|---|---|
| Tight tolerance on complex geometry | 5-axis CNC alloy machining | Ra 0.8–1.6 μm | Simple flat plates with one face to mill |
| High wear surface on steel | Hardcoat anodizing or black oxide | Ra 1.6–3.2 μm | Parts that flex under load |
| Cosmetic aluminum housing | Anodizing plus bead blasting | Ra 0.8–1.6 μm | Deep pockets that trap blast media |
| Corrosion resistance in salt air | Electroless nickel plating | Ra 0.8–1.6 μm | Parts needing tight thread fits |
| Medical or food-contact surface | Polishing to Ra 0.2–0.8 μm | Ra 0.2–0.8 μm | Textured grip surfaces |
| Prototype in one-off quantity | 3-axis milling plus hand finish | Ra 1.6–3.2 μm | Features only reachable from five sides |
The short version
Pick the shop that can name the machine, the tolerance and the inspection method for your critical feature. If those three answers are specific, the price is comparable. If they are vague, the price is a guess.
What a surface CNC alloy processing service actually covers
Alloy parts are machined for two reasons: geometry that other processes cannot form, and a surface that has to survive contact, heat or corrosion. A surface CNC alloy processing service combines both. The cutting operations create the shape, then finishing operations set the surface condition the part needs in service.
The work usually starts from bar, plate or a near-net forging. Titanium and stainless move slowly and work-harden at the cut, so feeds and speeds are conservative. Aluminum 6061 and 7075 run much faster but are prone to built-up edge and burrs on thin walls. The shop has to pick the strategy per alloy, not per drawing.
Finishing comes after dimensional checks. Anodizing, plating, powder coating, bead blasting and polishing all change the surface by a few micrometers. If the shop finishes before the final inspection, the coating thickness becomes a tolerance error. Ask where finishing sits in their routing.
- 1Cutting sets the geometry3-axis, 4-axis, 5-axis or mill-turn depending on how many faces the part needs.
- 2Finishing sets the surfaceAnodizing, plating, blasting, tumbling, brushing or polishing.
- 3Inspection closes the loop100% inspection before shipment, with reports on request.
Tolerance and finish: the numbers to ask for
Ask for the tolerance the shop will hold on the features that matter, not the tolerance in their brochure. A general shop may quote ±0.05 mm and still pass, while an alloy part with a bearing bore needs ±0.005 mm. The difference is which machine runs the feature and how often it is measured.
On finish, Ra 1.6–3.2 μm is a normal as-machined surface. Ra 0.8–1.6 μm needs a deliberate finishing pass with a sharp tool, light depth of cut and enough coolant. Ra 0.2–0.8 μm usually means polishing after machining, which adds handling, a separate operation and a risk of rounding edges.
Two traps show up often. First, a drawing that calls for a fine finish over the whole part when only a seal face needs it. That drives cost with no functional gain. Second, a finish callout on a deep pocket, where the tool cannot reach without chatter. Mark the functional surfaces and let the shop finish the rest to as-machined.
- 1±0.005 mmReasonable for bores, bearing seats and mating faces on alloy parts.
- 2Ra 0.8–1.6 μmStandard for most sealing and sliding surfaces.
- 3Ra 0.2–0.8 μmReserve it for optics, seals and medical contact faces.
Alloy families and how they behave on the machine
Aluminum is the easy case until it is not. 6061 and 6082 cut cleanly and hold tolerance well. 7075 is stronger but more brittle at the edge, so it chips if the tool path is too aggressive. Thin walls on any aluminum alloy deflect, and a shop that does not rough in stages will chase the dimension all day.
Stainless 303 and 304 are common, 316L for chemical exposure, 17-4PH where strength and corrosion resistance both matter. Stainless work-hardens, so a light finishing pass on a work-hardened surface cuts poorly. The shop should remove material in one steady pass rather than rub the surface.
Titanium TC4 (Ti-6Al-4V) and Inconel are the hard cases. Heat stays in the cut, tool life is short and the part moves as material is removed. These alloys need lower cutting speeds, high coolant pressure and more frequent inspection. Magnesium AZ31B and AZ91D machine fast but need chip control because fine magnesium chips are a fire risk.
- 1Aluminum 6061 / 7075Fast to machine; watch thin walls and burrs.
- 2Stainless 304 / 316L / 17-4PHWork-hardening; use steady passes and sharp tools.
- 3Titanium TC4, InconelSlow speeds, high coolant, more in-process checks.
- 4Magnesium AZ31B / AZ91DVery fast cutting; strict chip control for fire safety.
Lead time, MOQ and quote quality
Lead time is the easiest claim to make and the hardest to verify. Ask three questions: how fast is the quote, when does production start, and when do parts ship. A shop that returns a quote and a DFM analysis within 12 hours, starts production within 24 hours and ships in 3–5 days has already removed most of the waiting from a prototype cycle.
MOQ is the second filter. Alloy bar and plate have a minimum purchase length at the mill, but that should not become a minimum order for you. A shop with no minimum order quantity can run one prototype and the same part at 10,000+ pieces without a tooling change.
Quote quality tells you how the shop thinks. A one-line price with no assumptions is a warning. A useful quote lists the alloy grade, the machine, the tolerance on critical features, the finish callout, the inspection method and what is excluded. If the quote does not say how the part will be measured, the tolerance is a guess.
- 1Quote in 12 hoursWith a free DFM analysis that flags features that will not machine cleanly.
- 2Production in 24 hoursMaterial on the shelf and machine time already reserved.
- 3No MOQOne prototype to 10,000+ parts on the same process.
Certifications, inspection and documents
Certifications do not make a shop better at cutting alloy, but they decide who is allowed to bid. ISO 9001:2015 covers general quality systems. IATF 16949:2016 is required for automotive and EV work. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which matters when your drawings are confidential.
Inspection is where the tolerance claim becomes real. Ask what is checked, when, and by whom. Raw material check on arrival, in-process monitoring on critical features and a final inspection before shipment is a workable structure. Reports should be available on request, not promised only when there is a problem.
For regulated work, ask how the shop handles traceability. Material certificates, revision control on drawings and a record of which machine ran which operation are the documents an auditor will ask for. Shops that do this daily produce them quickly. Shops that do not will delay your release.
- 1ISO 9001:2015Baseline quality system for general industrial parts.
- 2IATF 16949:2016Automotive and EV programs.
- 3ISO 13485:2016Medical device components.
- 4ISO 27001:2022Confidential drawings and secure uploads.
When a surface CNC alloy processing service is the wrong choice
Not every alloy part should be machined from solid. Die casting makes sense for high-volume aluminum housings with wall thickness above 2 mm and no tight tolerance on the as-cast surface. Machining a 10,000-piece enclosure from billet wastes material and time. The right answer is a casting with machined interfaces.
Sheet metal fabrication wins for enclosures, brackets and panels under about 3 mm thick. Bending and laser cutting are faster and cheaper than milling a flat plate, and they leave less material on the floor. Choose machining when the part has thick bosses, tight bores or features that must be created in one setup.
3D printing is useful before the alloy is fixed. If you are still testing form and fit, a plastic prototype in a few days is cheaper than a titanium one. Move to alloy machining when the geometry is locked and the part has to survive real load, heat or corrosion.
- 1Choose die castingHigh volume, simple geometry, no tight as-cast tolerance.
- 2Choose sheet metalThin enclosures, brackets and panels; laser cut plus bend.
- 3Choose 3D printingEarly form and fit checks before committing to alloy.
- 4Choose alloy machiningTight tolerance, thick sections, complex geometry, real service load.
How to qualify a shop in 6 steps
Run these in order; each step filters out shops that will fail later.
- 1Send the 3D model and the 2D drawing togetherThe model gives geometry, the drawing gives tolerance and finish. A model with no drawing forces the shop to guess, and guesses become change orders.
- 2Ask for a DFM note with the quoteLook for flagged thin walls, deep pockets, sharp internal corners and features that need a 5-axis setup. Free DFM analysis within 12 hours is a reasonable benchmark.
- 3Confirm the machine for each critical featureAsk which machine holds the bearing bore or the seal face. If the answer is vague, the ±0.005 mm claim is not backed by a process.
- 4Check the finishing routingConfirm whether finishing happens before or after final inspection. Coating thickness of a few micrometers can push a part out of tolerance if the order is wrong.
- 5Verify the certification you actually needMatch the certificate to your industry: IATF 16949 for auto, ISO 13485 for medical, ISO 9001 for general. Ask for the certificate number and scope.
- 6Run one part before committing to the batchInspect the first article against the drawing. If the shop ships in 3–5 days, the first article arrives before you have to release the full quantity.
Questions buyers ask before a PO
What tolerance can a surface CNC alloy processing service hold on titanium?
On features that are machined in one setup on a 5-axis center, ±0.005 mm is realistic for bores and mating faces. Features that move between setups accumulate error, so the shop should either keep them in one setup or tell you the relaxed tolerance.
Titanium also moves as material is removed. On thin walls, expect to hold tolerance only after a stress-relief or a staged roughing pass, and plan an extra inspection before the finishing cut.
Do I need a fine surface finish over the whole part?
No. Finishing the whole part to Ra 0.2–0.8 μm adds polishing time and can round edges you need sharp. Mark the functional surfaces, such as seal faces and sliding contacts, and leave the rest at Ra 1.6–3.2 μm as-machined.
If the finish is cosmetic, bead blasting or brushing usually gives a more even look than polishing, at lower cost.
How do I compare quotes from two alloy machine shops?
Line up the same alloy grade, the same tolerance on critical features, the same finish callout and the same inspection method. A lower price usually means one of those four has been relaxed, not that the shop is more efficient.
Ask each shop to state what is excluded. Freight, material certificates, first-article reports and finishing are common exclusions that change the real cost.
Is there a minimum order quantity for alloy parts?
Not at every shop. Material minimums exist at the mill, but a shop can run one prototype and later the same part at 10,000+ pieces without a tooling change. Ask before you assume the prototype is too small to quote.
For low quantities, expect the per-part price to be driven by setup time, not by material. That is normal and not a sign of an inflated quote.
What documents should come with an alloy part shipment?
At minimum, a material certificate for the alloy grade and an inspection report for the features you specified. For regulated industries, add traceability from the raw material lot to the finished part and revision control on the drawing.
If your drawings are confidential, confirm that uploads are secure and that an NDA is available on request before you send files.
When should I choose machining over casting or sheet metal?
Choose machining when the part has tight bores, thick bosses, complex 3D geometry or features that must be created in one setup. Choose casting for high-volume simple geometry and sheet metal for thin enclosures and brackets.
A common approach is a cast or fabricated body with machined interfaces. That keeps material cost down and puts the tolerance only where it is needed.
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