CNC Aluminum Parts Factory Guide
Picking a supplier for machined aluminum is mostly a paperwork exercise plus a few hard questions about the shop floor. This CNC aluminum parts factory guide gives design engineers and buyers six checks to run before releasing a purchase order: alloy stock, tolerance capability, finishing control, lead time, minimum order quantity, and certification scope.
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Key takeaways
Six checks and what each one tells you
Use this table to sort suppliers before you send a drawing. Each row is one question and the evidence that answers it.
| Check | What to ask | Good answer looks like |
|---|---|---|
| Alloy stock | Which tempers do you hold? | 6061-T6, 7075, 2024, 6082 in stock |
| Tolerance | Which features hold ±0.005 mm? | Named features, not a blanket claim |
| Finishing | Is anodizing done in-house? | Clear, hardcoat, conductive plus plating |
| Lead time | When does the clock start? | Production starts within 24 hours |
| MOQ | One-off and 10,000+ runs? | No minimum order quantity |
| Certification | Which scope is audited? | ISO 9001, IATF 16949, ISO 13485, ISO 27001 |
The short version
Pick the supplier who answers the six questions with documents, not adjectives. Alloy and stock form first, per-feature tolerance second, finishing route third, then schedule, MOQ and certification scope.
Start with the alloy, not the machine
Most aluminum parts that reach a CNC shop fall into a handful of grades. 6061-T6 is the default for brackets, plates, housings and manifolds because it machines cleanly, welds, and takes anodizing evenly. 2024 cuts faster and holds a better fatigue life, but it corrodes without a coating. 7075 gives you roughly double the yield strength of 6061, which matters on aircraft fittings and lightweight arms, and it costs more per kilogram.
The grade you pick changes the tolerance you can hold. Thin walls in 6061 will deflect under a light finishing pass, so a 0.8 mm wall on a 60 mm part is a chatter risk no matter how good the machine is. In 7075 the same wall is stiffer but more prone to stress cracking during heavy roughing. Tell the shop which dimensions are functional and which are cosmetic.
Stock form matters too. Plate, bar and extrusion each have a grain direction and a residual stress pattern. A part cut from the middle of a thick plate can move 0.05 mm after the first face is machined away. Shops that rough, stress-relieve, then finish in a second setup handle this. Shops that cut to final size in one pass often cannot.
- 16061-T6General parts, enclosures, fixtures. Easy anodizing.
- 27075-T6High-strength, low-weight parts. Higher cost, harder to weld.
- 32024-T4Fatigue-critical parts. Needs coating for corrosion.
- 4ADC12Die-cast housings where machining is a secondary step.
Tolerance is a per-feature promise
A datasheet that says ±0.005 mm is not a contract. It is a capability statement for the best case: a rigid setup, a stable alloy, and a feature the tool can reach. Bores, milled pockets and face dimensions behave differently. A deep bore in 6061 with a length-to-diameter ratio above 4 needs a boring bar or a reamer, and the achievable band widens.
Ask the supplier to mark up your drawing with the tolerance they will guarantee on each critical dimension. This is the single most useful document you can get back. It turns a vague claim into a list you can inspect against. Our own shop works to ±0.005 mm on qualified features and quotes a wider band where geometry makes that unrealistic.
Surface finish follows the same logic. Ra 1.6–3.2 μm is standard as-machined. Ra 0.8–1.6 μm needs a finishing pass with sharp tooling and controlled feed. Ra 0.2–0.8 μm usually means a secondary operation, and on large thin parts it may not be reachable at all without distortion.
- 1Name the featuresBore, slot, face, thread: each has its own band.
- 2Watch the ratioDeep bores and tall thin walls widen any tolerance.
- 3Finish costs moneyRa 0.2–0.8 μm is a second operation, not a setting.
Match part size to machine travel
Aluminum parts get rejected at the quoting stage for a simple reason: they do not fit. A supplier with 500 × 500 × 450 mm travel cannot run a 1,200 mm extrusion without a second setup, and a second setup adds a datum shift. If your part is long and thin, ask for the largest travel on the floor before you send the file.
Five-axis capability is often oversold. A simultaneous five-axis center earns its cost on contoured surfaces, angled holes, and parts that would need four fixtures otherwise. For a flat plate with holes on one face, a three-axis machine with a good vise is faster and cheaper. Match the machine to the geometry, not to the brochure.
Count the spindles, not the brand names. A shop running 127 high-precision machines across three plants has real scheduling room. A shop with four machines has less. When your program slips, capacity is what absorbs the shock.
- 1Large travel4,000 × 400 × 150 mm for long extrusions and rails.
- 2Medium travel750 × 1,150 × 550 mm and 600 × 600 × 600 mm for most housings.
- 3Compact travel500 × 500 × 450 mm and 500 × 310 × 200 mm for small precision parts.
- 4Rotary tableØ400 mm for four-axis work on cylindrical features.
Finishing decides how the part looks in a year
Machining gets the geometry right. Finishing decides whether the part still looks and performs correctly after a season outdoors or a few thousand cycles. Anodizing is the usual choice for aluminum: clear for a natural look, color for branding, hardcoat for wear surfaces, conductive for grounding paths. Each type has a different thickness window and a different effect on tight tolerances.
Hardcoat anodizing builds roughly 25–50 μm per surface, and it grows both inward and outward. A bore that measures 10.000 mm before coating will not measure 10.000 mm after. If a mating diameter is critical, mask it or machine it undersize on purpose. Suppliers who do anodizing in-house will raise this before you ask; brokers will not.
The same warning applies to plating, powder coating and bead blasting. Bead blasting changes surface texture and can round a sharp edge. Laser marking needs at least 1.5 mm character height to stay legible. None of these are hard problems, but all of them need to be on the drawing before the first cut.
- 1AnodizingClear, color, hardcoat, conductive. Grows the surface.
- 2PlatingElectroless nickel, zinc, silver, gold.
- 3TextureBead blasting, tumbling, brushing, polishing.
- 4MarkingLaser engraving, minimum character height 1.5 mm.
Lead time is a chain, not a number
A quoted lead time hides four stages: quotation, DFM review, material release, and machining plus finishing. A supplier who answers in 12 hours with a free DFM analysis is telling you something about the front of that chain. One who takes a week to acknowledge the file is telling you something too.
The number that matters is when the spindle starts. Production starting within 24 hours of a released order is a sign of real scheduling room. Parts shipping in 3–5 days is realistic for aluminum when material is in stock. It is not realistic when the alloy has to be ordered, so ask which grades are held on site.
Push back on delivery promises that come without a start date. Ask for the date the material is released and the date the first article is inspected. Two dates beat one vague window, and they give you something to track.
- 1Quote stageQuotation and free DFM analysis within 12 hours.
- 2Release stageProduction can start within 24 hours of order release.
- 3Ship stageParts ship in 3–5 days once machining begins.
- 4Track recordHistorical late-delivery probability below 2%.
MOQ, confidentiality and the paperwork
Minimum order quantity is where prototype work dies. A shop that wants 500 pieces will not run your one-off bracket, and a shop that runs one-offs may not want your 10,000-piece program. Suppliers who quote from one prototype to 10,000+ runs on the same process remove that decision from your plate entirely.
Confidentiality is the other half of the paperwork. Your drawing carries the geometry of a product that may not be public yet. Uploads should be secure, and a non-disclosure agreement should be available before you send anything sensitive. If a supplier hesitates on an NDA, that is your answer.
Certifications belong here too, but read the scope. ISO 9001:2015 covers quality management. IATF 16949:2016 is the automotive standard and applies to production part approval. ISO 13485:2016 is for medical devices. ISO 27001:2022 covers information security, which is what protects your files.
- 1No MOQOne prototype or 10,000+ parts, same process.
- 2NDA on requestSecure, confidential uploads for sensitive drawings.
- 3ISO 9001:2015Quality management baseline.
- 4IATF 16949:2016Automotive production scope.
Running the six checks in order
Work through these before you release a purchase order. Each step takes one email or one call.
- 1Send the 3D file and a marked-up 2D drawingMark functional dimensions, datums and finish callouts. Unmarked drawings get blanket tolerances, and blanket tolerances are where disputes start.
- 2Request a DFM review with the quoteLook for comments on wall thickness, deep bores, tool reach and datum choice. A quote with no DFM notes usually means nobody opened the model.
- 3Confirm alloy and stock formAsk which grade and which temper will be cut, and whether it is plate, bar or extrusion. Plate versus extrusion changes residual stress and post-machining movement.
- 4Get a per-feature tolerance listAsk the shop to return the drawing with guaranteed bands marked per critical feature. Compare those bands against your assembly stack-up before you approve.
- 5Pin down the finishing routeConfirm anodizing type, target thickness and which surfaces are masked. Allow for growth on hardcoat, roughly 25–50 μm per surface.
- 6Lock the schedule with two datesGet the material release date and the first-article inspection date. Ignore any delivery window that has neither.
- 7Settle inspection and paperworkAgree what is measured, at what sampling rate, and which reports ship with the parts. 100% inspection before shipment plus raw material, in-process and final checks is a workable baseline.
- 8Sign the NDA before releasing sensitive filesDo this before the first upload if the part is unreleased. It costs nothing and closes the obvious gap.
Questions buyers ask before ordering
Can a factory hold ±0.005 mm on every dimension?
No, and any supplier who says yes is guessing. ±0.005 mm is a realistic band on rigid features with good tool access in stable alloys like 6061-T6 and 7075-T6.
Deep bores, tall thin walls and long unsupported sections will need a wider band. Ask for the number per feature instead of a single figure for the whole part.
Is there a minimum order quantity for aluminum parts?
It depends on the supplier. Some shops set a floor because setup time is fixed. Others quote from one prototype up to 10,000+ part runs on the same process.
If you are at the prototype stage, ask directly. A supplier with no minimum order quantity will not add tooling charges to a single-piece order.
How does anodizing affect my tolerances?
Anodizing adds material to the surface. Standard clear anodizing is thin enough that most tolerances absorb it. Hardcoat is thicker, roughly 25–50 μm per surface, and it grows both inward and outward.
On a critical bore or mating diameter, mask the surface or machine it undersize. Discuss this with the shop before the first cut, not after coating.
What lead time should I expect for a small aluminum run?
For a released order with material in stock, production can start within 24 hours, and parts typically ship in 3–5 days. Quotation and DFM analysis can come back within 12 hours.
The variable is material availability. Alloys held on site move fast. Grades ordered from a mill add days, and no shop can compress that.
Which certifications should I check for a machined aluminum supplier?
ISO 9001:2015 is the baseline quality system. IATF 16949:2016 matters for automotive production parts. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security.
Read the scope statement, not just the certificate number. A certificate tells you which process was audited, not that every cell in the plant works to that standard.
Do I need an NDA before sending drawings?
If the part is unreleased or the geometry is sensitive, yes. Uploads should be secure and confidential, and a non-disclosure agreement should be available on request.
Sign it before the first file transfer. It is a short step and it removes the one risk you cannot undo later.
Send a drawing, get a real answer
Upload your file and we will return a quotation with DFM notes, a per-feature tolerance list and a production start date.
12-hour quoteNo MOQ100% inspection
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