China Aluminum CNC Parts: Alloy, Tolerance and Finish Decisions

Most aluminum CNC parts are cut from 6061-T6, and for good reason. It machines cleanly, holds a sharp edge, welds, anodizes predictably, and is stocked everywhere. If your part carries moderate load, needs a decent surface, and will not run above roughly 150 °C in service, 6061-T6 is usually the right starting point.

2024-T4 machines to a better finish and is stronger in fatigue, which is why it shows up in aerospace brackets and stressed skins. The trade-off is corrosion. Bare 2024 will pit, so it needs anodizing or a primer, and its copper content makes some anodize baths tricky. 7075-T6 is the high-strength option for thin walls and highly loaded fittings, but it is less weldable, more expensive, and tends to move when you remove material from one side.

For enclosures, covers, and anything bent or formed, look at 5052 and 5083. They are not strong, but they bend without cracking and resist salt water. 6063 and 6082 sit between the extrusion and machining grades, and ADC12 is the die-casting alloy, not a machining grade, so do not specify it for a billet part. The material list we cut most often is 6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075 and ADC12.

• 1
  6061-T6General machining, anodizing, moderate loads. Default choice.
• 2
  2024-T4Fatigue strength and finish. Must be protected from corrosion.
• 3
  7075-T6High strength, thin walls. Harder to weld, more distortion risk.
• 4
  5052 / 5083Forming and marine service. Lower strength, excellent bendability.

Aluminum cuts fast, which is exactly why it is easy to get wrong. Spindle speeds run high, feed rates are aggressive, and the metal moves under clamping pressure before the tool ever touches it. A fixture that works on steel will often distort a thin aluminum rib. On our floor this is handled with soft jaws, vacuum plates, and sacrificial tabs, plus a roughing pass that leaves stock so the part can relax before finishing.

Five-axis work matters most when a part has features on several faces. Every extra setup means another datum, another clamp mark, and another chance for stack-up error. With 16 simultaneous five-axis machining centers, we can often reach five sides in one setup, which keeps the position of bores and bolt patterns tied to a single origin. For parts that fit on a Ø400 mm rotary table, a four-axis or mill-turn setup does the same job for less money.

Our machines cover a wide size range. The largest travel is 4,000 × 400 × 150 mm, which suits long rails, beams, and extrusion profiles. Mid-size platforms run 750 × 1,150 × 550 mm and 600 × 600 × 600 mm for housings and plates. Compact cells at 500 × 500 × 450 mm and 500 × 310 × 200 mm handle small brackets and connectors where cycle time matters. Maximum processing size overall is 4,000 mm.

Tool choice is where cycle time is won or lost. Aluminum likes polished, high-helix carbide with plenty of flute volume for chip clearance. A three-flute cutter in 6061 will usually beat a four-flute on pocketing because the chips evacuate faster. Deep pockets are the exception. Past about four times the cutter diameter, a longer reach forces lower feed, and chatter becomes the limiting factor rather than spindle power.

A blanket tolerance note on a drawing is the fastest way to pay for capability you do not need. Aluminum expands about 23 μm per meter per degree Celsius, so a 300 mm part measured at 20 °C and again at 28 °C moves by roughly 55 μm before any machine error is counted. General dimensions on aluminum parts are usually held to ±0.05 mm, and that covers most brackets, covers, and housings without extra cost.

When a feature truly needs it, we hold ±0.005 mm (±0.0002 in). Bores, bearing seats, and mating faces are the usual candidates. Tight tolerances on an isolated feature are affordable. Tight tolerances across a long span, or between features machined in different setups, are not, because the shop has to control temperature, tool wear, and fixture repeatability at the same time. If you can tolerance only the functional interfaces and leave the rest open, the price drops noticeably.

Surface finish follows the same logic. As-machined aluminum comes off the tool at Ra 1.6–3.2 μm. Fine stepovers and sharp tooling get you to Ra 0.8–1.6 μm, which is a common requirement for mating faces and seal grooves. Ra 0.2–0.8 μm needs slower passes, better tooling, or a secondary operation, and it is rarely needed unless the surface slides or seals. Bead blasting hides tool marks and is cheap; it also changes the surface texture, so do not ask for blasting and a measured Ra on the same face.

One more thing about finishes. Anodizing builds up and can add 5–25 μm depending on the process, and hardcoat more. That growth lands on the surface, not in the tolerance zone, so call out pre-anodize dimensions on any bore that has to fit a shaft. Laser marking is fine for logos and part numbers, but the minimum character height we can hold is 1.5 mm.

Material cost is the easy part to see and rarely the largest line. Setup and programming dominate on low volumes. A part that needs three setups takes three times the fixturing labor of a part that needs one, even if the cutting time is identical. This is why a small design change, such as moving a mounting hole onto a face that is already being machined, can cut the unit price without touching the cycle time.

Wall thickness is the second lever. Aluminum deflects under clamping and cutting force. Below about 1 mm on a 100 mm part, the shop has to slow down, add supports, and inspect more. That cost shows up as time, not as a line item. If a wall is not carrying load, thicken it. Tolerance and finish come third, and they multiply. Asking for Ra 0.4 μm on a face that also needs ±0.01 mm is far more expensive than asking for either one alone.

Volume changes the answer. There is no minimum order quantity here, and we run from a single prototype to 10,000+ part runs. At one or five pieces, a five-axis setup usually wins because it removes operations. At several thousand pieces, a dedicated fixture and a three-axis cell with a pallet changer often beat five-axis on unit cost. Neither is universally cheaper, so it is worth asking which route the shop intends to take.

Post-processing is the last add-on. Anodizing, electroless nickel, zinc, silver and gold plating, powder coating, black oxide, bead blasting, tumbling, brushing, polishing, and laser marking are all handled as secondary steps. Each one adds a queue, a handling step, and a small risk of damage, so stack them deliberately rather than as a default checklist.

Ask what the shop measures and with what. A CMM report on the first article tells you more than a certificate on the wall. Our inspection flow is a raw material check, in-process monitoring, and a final inspection, with 100% inspection before shipment and reports available on request. If a supplier cannot describe their in-process checks, assume the final gate is the only one.

Certifications are a filter, not a guarantee, but they do tell you which systems the shop maintains. We hold ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022. The first covers general quality management, the second matters for automotive programs, the third for medical device work, and the fourth covers information security, which is relevant if your drawings are confidential.

Confidentiality is worth settling early. Uploads are secure and confidential, and an NDA is available on request. If your part is under a development program, put the agreement in place before the first drawing goes out, not after the first quote.

Timing is the last check. A quotation and free DFM analysis come back within 12 hours, and production can start within 24 hours once the drawing and material are confirmed. Parts typically ship in 3–5 days. GreatLight has been machining since 2011, runs 3 wholly-owned plants across 7,600 m² with 150 technicians and 127 high-precision CNC machines, and operates from Dongguan, China plus a Singapore factory at No.3 Joo Koon Circle, Singapore 629032. Those numbers matter less than the DFM feedback, which is where most cost is removed.