Custom CNC Machining Aluminum Parts: What to Check Before You Order

• 1
  Key takeaways
• 2
  Aluminum Alloy Selection for Custom CNC Machining Aluminum Parts
• 3
  Why Alloy Choice Changes the Whole Quote
• 4
  Where ±0.005 mm Belongs on Your Drawing
• 5
  Wall Thickness, Corners and Features That Fight Back
• 6
  Finish Calls, Inspection Evidence and Quote Comparison
• 7
  6 Steps to Send a Clean Aluminum RFQ

Aluminum is not one material. The alloy you name sets the cutting speed, the tool life, the achievable finish and often the price. A 6061-T6 bracket cuts clean at high spindle speed and holds a good finish without extra work. The same part in 7075-T6 needs more care, because the higher zinc content makes it stronger but also more brittle at the edge.

Stock form matters too. Plate, extrusion and bar behave differently. A part cut from 6061 plate can move after the skin is removed, especially on thin plates. If flatness matters, say so on the drawing and expect a stress-relief or a double-pass operation.

For most industrial parts, 6061-T6 is the default answer. It machines well, takes anodizing evenly and is widely available in the sizes we run. Reach for 7075 only when the load path is tight or the section is thin. Reach for 2024 when fatigue life is the design driver.

We keep 6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075 and ADC12 in the shop. If your print names an alloy we do not stock, tell us which property matters and we will suggest the closest one.

A blanket tolerance note on a drawing is a common and expensive habit. If the title block says ±0.005 mm and every dimension inherits it, the shop has to inspect every feature to that band. That slows the job and raises the price for no functional gain.

Put the tight tolerance on the features that mate, locate or seal. A bearing bore, a dowel hole, a mating face. Leave the rest at a general tolerance. Most aluminum parts we run hold ±0.05 mm comfortably on general dimensions, and ±0.005 mm where it is called out.

Datums matter as much as numbers. If the print does not define A, B and C, the inspector has to guess the setup and the report will not match your intent. Define the datum scheme before you send the RFQ.

One more thing. Aluminum moves with temperature. A part measured at 20 °C and a part measured at 28 °C on the same CMM can differ by more than the tolerance band on a long dimension. Note the measuring temperature if the part is large.

Aluminum cuts easily, but it is soft. Thin walls deflect under the cutter and the finished part springs back. As a working rule, keep unsupported walls above 0.8 mm, and above 1.5 mm if the wall is tall and the surface finish matters.

Deep pockets need clearance for the tool. A pocket 40 mm deep with a 4 mm corner radius forces a long, slender cutter that will chatter. Open the corner radius or reduce the depth, and the finish improves immediately.

Threads in aluminum strip easily. For small threads, use a forming tap rather than a cutting tap, and keep thread depth at 1.5 × diameter or more. Helicoil inserts are a good answer when the joint will be opened repeatedly.

Sharp internal corners are the other classic trap. A cutter has a radius, so a true sharp corner needs EDM or a broach. Add a radius and save the operation.

Finish is a functional call as often as a cosmetic one. As-machined aluminum lands around Ra 1.6–3.2 μm. A high-quality cosmetic finish sits at Ra 0.8–1.6 μm. Fine finishes down to Ra 0.2–0.8 μm are possible but need slower passes and more inspection time.

Anodizing changes dimensions. Type II clear anodize builds roughly 5–10 μm per surface, hardcoat more. If a bore is tolerance-critical and will be anodized after machining, machine it undersize on purpose and tell the finisher.

Ask what inspection comes with the parts. We inspect 100% before shipment and run raw material checks, in-process monitoring and final inspection. Reports are available on request. A supplier who cannot describe the inspection plan will not be able to explain a failure later.

When you compare quotes, compare the same scope. Cheaper quotes often exclude material certification, first-article inspection or the finish you assumed was included. Ask each supplier to state what is in the price.