CNC Alloy Complete Service: 7 Checks Before You Order
A CNC alloy complete service covers machining, finishing and inspection on aluminum, titanium, steel, brass and magnesium parts. This guide is for engineers and buyers comparing suppliers. Read it and you can judge a quote, spot a thin scope, and know which questions to ask before tooling starts.
Key takeaways
Which Alloy Work Fits Which Shop Setup
Match the part to the shop before you match the price.
| Part type | Best-fit setup | Watch out for |
|---|---|---|
| Aluminum 6061 / 7075 housings | 3-axis or 4-axis mill, high spindle speed | Thin walls deflect; plan support ribs |
| Ti-6Al-4V and Inconel brackets | 5-axis with through-spindle coolant | Heat at the cut edge; slow feeds, sharp tools |
| Magnesium AZ31B / AZ91D covers | Dedicated mill, chip control first | Fine chips ignite; no mixed-metal swarf |
| Brass and copper fittings | Mill-turn centers, Ø400 mm rotary table | Galling on threads; check tool coating |
| Large 4,000 mm frames | Gantry-class travel, 4,000 × 400 × 150 mm | Fixture sag over long spans |
| Prototype to 10,000+ runs | Same shop, one setup sheet | Re-qualifying a new vendor mid-run |
The Short Version
Judge an alloy supplier on scope, pre-finish dimension control and certification match. Price is the last column, not the first.
Start With Scope, Not With Price
A complete service should cover raw material, cutting, finishing, inspection and packing under one roof. When a quote only lists machining hours, the finishing step is probably subcontracted. That adds a second setup, a second inspection and a second chance for a scratch. Ask for the process list in writing before you compare numbers.
Price gaps between suppliers usually come from scope, not from hourly rates. One shop quotes a bead-blasted part with a final dimensional check. Another quotes as-machined parts with a visual look. The two quotes look comparable on a spreadsheet and are not. Put the scope line next to the price line and the real difference shows up.
For an alloy part, the cheapest quote often carries the highest hidden cost. A re-machined bore or a stripped anodize layer costs more than the original saving. We would rather quote the full route once than patch a partial route twice.
- 1Ask for the route sheetMaterial, machine, finish, inspection, packing. One page is enough.
- 2Name the finish in the RFQAnodize type, plating thickness, blast media. Vague finishes get vague quotes.
- 3Set the inspection levelFull dimensional report or key dimensions only. This changes the price.
Tolerance and Alloy Choice Drive the Real Cost
The tolerance on the drawing matters more than the alloy name. A ±0.005 mm callout on a 20 mm aluminum bore is routine on a 5-axis center with a temperature-stable shop. The same callout on a 900 mm titanium frame needs a different plan: rough, stress-relieve, semi-finish, then finish. That is four setups instead of one.
Alloy behavior decides the cutting plan. Aluminum 6061 and 7075 run at high spindle speeds with generous coolant. Ti-6Al-4V and Inconel generate heat at the cutting edge, so we slow the surface speed and change inserts more often. Magnesium AZ31B and AZ91D cut easily, but fine chips need dedicated collection. Never let magnesium swarf mix with steel or aluminum chips.
Wall thickness is the quiet cost driver. A 1.5 mm aluminum wall will move after clamping and again after anodizing. Adding a temporary web or a soft-jaw fixture costs a little machining time and saves a batch. If your design can accept a 2.5 mm wall, say so in the RFQ.
Surface finish works the same way. As-machined at Ra 1.6–3.2 μm is the default. Ra 0.8–1.6 μm needs a finish pass and a sharper tool. Ra 0.2–0.8 μm on an alloy part usually means polishing or lapping after machining, and it should be quoted as a separate line.
- 1Send the GD&T, not a PDF sketchDatum structure changes the fixture plan and the price.
- 2Flag thin walls earlyAnything under 2 mm on aluminum gets a support discussion.
- 3Separate cosmetic from functional surfacesNot every face needs the fine finish.
Finishing, Certification and Confidentiality
Anodizing, electroless nickel, zinc plating and powder coating all shift dimensions. Clear anodize on aluminum grows the surface by roughly half the oxide thickness per side. Hardcoat grows more. If a bore is already at the top of its tolerance band, the plated part may not fit. The machinist should machine to a pre-finish size, and the finishing vendor should confirm it.
Certification should match the industry, not the marketing page. ISO 9001:2015 fits general industrial work. IATF 16949:2016 is expected for automotive and EV programs. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which matters when you upload CAD files. A shop with all four can serve mixed programs without moving your part to a different quality system.
Confidentiality is a practical question, not a legal one. Ask where the files live, who can open them, and whether an NDA is available before you upload. Uploads should be treated as confidential by default. If the answer is vague, that tells you something about the shop's documentation habits.
Lead time claims need a start date attached. A quote within 12 hours and production starting within 24 hours are useful numbers only when the clock starts at drawing release. Ask what happens if the material is not in stock. A shop that stocks common alloys can hold the schedule. A shop that buys per order cannot.
- 1Confirm pre-finish dimensionsPlating and anodize add material. Machine to the pre-finish size.
- 2Match certificate to programISO 9001, IATF 16949, ISO 13485, ISO 27001. Ask which applies.
- 3Get the NDA before uploadStandard practice for new programs with proprietary geometry.
MOQ, Quote Format and Delivery Risk
No minimum order quantity sounds simple, and for alloy work it usually is. One prototype and a 10,000+ part run can share the same setup sheet if the fixture is designed for it. The risk sits in the middle: a 200-part run with a hand-loaded fixture often costs more per part than a 20-part run with a soft jaw. Ask how the shop prices the transition.
Read the quote format carefully. A good alloy quote lists material grade, machine type, finish, inspection level, packing and lead time as separate lines. A single lump sum hides where the money goes and makes it hard to compare two shops. If a supplier will not break out the lines, you cannot tell whether the saving comes from machining or from skipping the finish.
Delivery risk is worth a direct question. Historical late-delivery probability below 2% is a number a shop should be able to state. Ask what causes the late deliveries that do happen: material, finishing, or inspection. The answer tells you which part of the chain to watch on your own program.
Finally, check the sample route. A shop that runs prototypes and production on the same floor keeps the process knowledge in one place. A shop that prototypes in one building and produces in another loses the setup notes. For alloy parts with tight tolerances, that loss shows up as a first-article surprise.
- 1Compare line by lineMaterial, machine, finish, inspection, packing, lead time.
- 2Ask about the awkward quantityThe 100 to 500 part band is where fixture choices matter most.
- 3Request the first-article planSay what gets measured, on which machine, and when.
Step by Step: Qualifying a CNC Alloy Supplier
Seven steps, in order. Skipping step 4 is the most common mistake.
- 1Send STEP plus a 2D drawingNative CAD is optional. The 2D drawing carries the GD&T and the finish callouts that set the price.
- 2State alloy and temperWrite 6061-T6 or Ti-6Al-4V, not aluminum or titanium. Temper changes feeds and the quote.
- 3List the finish as a separate lineAnodize type and color, plating thickness in μm, blast media and grit. Keep it separate from machining.
- 4Ask for the pre-finish dimension planFor any bore or shaft with a tight band, confirm the size the machinist will hit before plating or anodizing.
- 5Set the inspection level in writingFull dimensional report, key dimensions, or visual only. Attach the report template you expect.
- 6Confirm certification and NDA coverageName the standard your program needs and confirm the shop holds it. Request the NDA before uploading files.
- 7Fix the schedule start pointAgree that the clock starts at drawing release and material confirmation, then hold the shop to that date.
Questions Buyers Ask Next
Can one shop handle aluminum, titanium and magnesium parts?
Usually yes, but not on the same machine at the same time. Magnesium needs dedicated chip collection and a different housekeeping routine because fine magnesium swarf is a fire risk. Titanium and Inconel wear tools faster and need rigid setups. A shop with separate cells for each family is easier to audit than one that swaps materials on a single mill.
Ask how they segregate swarf and whether coolant is changed between families. That answer matters more than the machine list.
How tight a tolerance should I actually call out?
Call out only what the function needs. A ±0.005 mm band on a bearing bore is justified. The same band on a mounting hole adds cost with no benefit. Every tight callout adds an inspection step, and inspection time is real time.
If you are unsure, mark the critical dimensions and leave the rest at a general tolerance block. The shop will quote the critical ones properly instead of averaging the whole drawing.
Does anodizing change my part dimensions?
Yes. The oxide layer grows outward and slightly inward from the original surface. Clear anodize adds less than hardcoat. For a bore at the top of its band, that growth can push the part out of tolerance.
The fix is simple: tell the machinist the finish and the final dimension. They machine to a pre-finish size, and the finishing line confirms the result on a sample before the batch runs.
What does no minimum order quantity really mean?
It means the shop will quote one part. It does not mean the per-part price is the same at one part and at 10,000 parts. Setup, fixturing and programming are spread over the quantity.
For prototype quantities, expect to pay for setup. For production quantities, ask what fixture the shop will build and whether that cost is one-time.
How do I compare two quotes that look different?
Put them in the same format first. Force both quotes into material, machine, finish, inspection, packing and lead time lines. Then compare line by line.
Most gaps sit in finishing and inspection. One shop includes a full dimensional report, the other does not. Once the lines match, the cheaper quote often stops being cheaper.
What documents should come with the shipment?
Ask for the material certificate, the dimensional report you agreed on, and the finish or plating certificate if a coating was applied. For automotive or medical programs, add the traceability record that the standard requires.
Inspection reports are available on request. Confirm the format before the run starts so nobody is surprised at shipment.
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