CNC Alloy Processing Service: How to Pick One That Holds Tolerance

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  What matters most when you compare suppliers
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  Which alloy group fits which process route
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  Tolerance claims: what a CNC alloy processing service can actually hold
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  Alloy coverage: does the shop really cut your material
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  Machine capacity and part size: matching the route to the geometry
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  Lead time, MOQ and quotation format
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  Quality system and certifications that fit your industry
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  Seven steps to qualify a CNC alloy processing service

Every supplier page says high precision. The number that matters is the tolerance they will put in writing for your specific alloy and geometry. ±0.005 mm is realistic on aluminium, brass and free-machining stainless when the part is rigid and the features are accessible. On a 4,000 mm long alloy beam, that number is not the same conversation.

Alloy behaviour sets the floor. Aluminium 6061-T6 cuts clean and holds size well. Titanium TC4 and Inconel 718 generate heat at the cutting edge, and that heat moves the part between roughing and finishing. A shop has to leave finishing stock and take it in a separate operation after the part has cooled. If the quote does not mention roughing and finishing as separate steps on those alloys, ask why.

Surface finish is the second half of the tolerance story. An as-machined Ra 1.6–3.2 μm is normal for a milling operation. Ra 0.8–1.6 μm usually needs a finishing pass with a sharper tool and lower feed. Ra 0.2–0.8 μm on an alloy part means a separate finishing operation or a secondary process, and it should show up as a line item.

Ask for the inspection method, not just the tolerance. A ±0.005 mm callout checked with calipers is a different claim than the same callout checked on a CMM with a report. When the part is a mating component, the report matters more than the number on the drawing.

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  Get the tolerance in writing per alloyA blanket ±0.005 mm claim across titanium and aluminium usually means the shop has not cut much titanium.
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  Separate roughing and finishingOn Inconel and TC4, this is how size is held after heat settles.
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  Match finish to functionA sealing face and a bracket face do not need the same Ra.

A machine list tells you what the shop owns. An alloy list tells you what they have actually learned. The two are not the same. A 5-axis center can physically hold a titanium blank, but feeds, speeds, coolant pressure and tool grades for TC4 are a separate body of knowledge from aluminium.

Look for a supplier that names specific grades rather than families. Aluminium 6061, 2024, 5052, 5083, 6063, 6082, 7075 and ADC12 behave differently. Stainless 303, 304, 316L, 420, 440C and 17-4PH cover a wide range from free-machining to abrasion-resistant. Steel 1018, 1045, 4130, 4140, 4340 and A36 sit at different hardness levels. A named list means the shop has process data for each one.

Titanium and nickel alloys are the filter. TA1, TA2, TC4, Inconel and magnesium AZ31B or AZ91D require high-pressure coolant, rigid tooling and slower feeds. If a supplier lists these grades, they have already solved the hard part of the setup. If the page only shows aluminium and mild steel, your titanium part is a first-time experiment on their floor.

Material sourcing is the quiet risk. Alloy stock varies by mill and heat lot. A supplier that buys globally and verifies incoming material against the certificate is doing work you would otherwise do yourself. Ask whether raw material is checked on arrival and whether that record travels with the job.

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  Grades beat families6061 and 7075 are not interchangeable, and the quote should reflect that.
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  Treat titanium as a testA named TC4 or Inconel grade in the material list is a good sign.
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  Ask about incoming material checksHeat lot and certificate verification prevents surprises at final inspection.

Three-axis machines move the tool along X, Y and Z. That works well for prismatic parts with features on one or two faces. When a part has undercuts, angled holes or features on five faces, a three-axis route turns into a sequence of re-fixturings, and every re-fixturing adds position error.

Five-axis machining adds two rotary axes, so the tool can approach the part from an angle in a single setup. That is the main benefit for alloy work: fewer setups means fewer chances to lose datum. It also lets the shop use a shorter, stiffer tool, which reduces chatter in hard alloys. Chatter is what kills surface finish and tool life on titanium.

Size range matters as much as axis count. A 4,000 mm maximum processing size with a 4,000 × 400 × 150 mm travel envelope covers long alloy extrusions and frames. Medium travels of 750 × 1,150 × 550 mm and 600 × 600 × 600 mm handle most housings. Compact travels of 500 × 500 × 450 mm and 500 × 310 × 200 mm suit small precision parts. A Ø400 mm rotary table covers round and indexable work.

Ask which machine your part will run on, and ask what happens if that machine is busy. A supplier with 127 high-precision CNC machines, including 16 simultaneous 5-axis centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers, has room to move a job. A shop with three machines does not.

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  Count the setupsEach extra setup adds stack-up error, especially on alloy parts with tight bores.
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  Match travel to part envelopeA part that fits the table but not the travel will be split into operations.
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  Ask about machine redundancyCapacity spread across 127 machines reduces the risk of a stalled job.

Lead time claims are only useful when you know what starts the clock. A quotation and DFM analysis within 12 hours is a response-time promise. Production start within 24 hours is a scheduling promise. Parts shipping in 3–5 days is a throughput promise. Ask which of these applies to your alloy, because titanium and Inconel take longer per operation than aluminium.

Minimum order quantity is the other lever. No minimum order quantity means the same supplier can run one prototype and a 10,000+ part production run. That matters when you are validating a design and do not want to change suppliers between the prototype and the production order. Every supplier change resets the process knowledge.

Read the quote structure, not just the total. A useful quote separates material, machining time, finishing, inspection and any tooling cost. If finishing is bundled into a single line, you cannot tell whether anodizing or bead blasting is included. Surface finishing options include anodizing in clear, colour, hardcoat and conductive types, electroless nickel, zinc, silver and gold plating, powder coating, black oxide, bead blasting, tumbling, brushing, polishing, and laser marking or engraving with a minimum character height of 1.5 mm.

Delivery risk is worth asking about directly. A supplier with a historical late-delivery probability below 2% is tracking the metric. One that cannot answer the question is not.

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  Separate the three clocksQuote time, production start and shipping are different promises.
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  Confirm finishing is line-itemedAnodizing type and mask areas change cost and lead time.
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  Ask for the late-delivery numberA tracked figure is more useful than a vague on-time claim.

Certifications are a filter, not a score. ISO 9001:2015 covers general quality management. IATF 16949:2016 is the automotive and EV standard and brings tighter change control and traceability. ISO 13485:2016 applies to medical devices and adds validation and documentation requirements. ISO 27001:2022 covers information security, which matters when you are sending confidential CAD files across borders.

The certification has to match the part. A medical implant housing does not benefit from an automotive certificate. An EV busbar does. Ask which certificate applies to your program and whether the audit scope covers the plant that will run your job. A certificate held by a different site is not the same thing.

Inspection depth is the practical test. 100% inspection before shipment, with raw material checks, in-process monitoring and a final inspection, is a process, not a slogan. Reports on request means you can ask for the dimensional data and get it. For alloy parts with tight bores or mating faces, that report is your incoming inspection shortcut.

Confidentiality belongs here too. Uploads should be secure and confidential, and a non-disclosure agreement should be available on request. If the supplier hesitates on an NDA, that tells you something about how they handle customer drawings.

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  Match certificate to industryIATF 16949 for automotive, ISO 13485 for medical, ISO 27001 for data.
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  Confirm the audited siteAsk which plant the certificate covers.
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  Request the inspection reportReports on request turn a claim into a document.