Advanced Chinese CNC Precision Processing

A three-axis mill moves the tool in X, Y and Z while the part stays still. That covers a large share of real work: plates, housings, brackets, pockets, and any feature you can reach from one direction. It is fast to set up and cheap to run. It stops being the right answer when a part has features on five faces, deep side walls, or a bore that must stay coaxial with a face machined in a different orientation.

The step up is a four-axis mill with a rotary table, or a mill-turn center that turns and mills in one program. Mill-turn matters most for parts that are round with cross features: a shaft with a milled flat, or a fitting with radial ports. Cutting both in one setup removes the second-fixture error that usually drives a stack-up problem.

Simultaneous five-axis is the top of that ladder. On a five-axis center the tool tip stays normal to the surface while the rotary axes move with the linear ones. That is what makes contoured impellers, turbine blades, deep cavities with drafted walls, and undercut features machinable without a custom fixture for every angle. At GreatLight the floor runs 16 simultaneous 5-axis machining centers alongside 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers.

Not every part belongs there. Five-axis time is expensive, and a simple bracket that fits in one three-axis setup will cost more if you route it through a five-axis program. The honest answer is that the drawing decides, not the machine list.

A tolerance on a drawing is a promise the process has to keep. ±0.005 mm is achievable at GreatLight, but it is not a default applied to every dimension on every part. It holds on specific features, in stable materials, on a machine that has been thermally settled.

Three things drive the number. First, thermal growth: aluminium moves roughly 23 μm per meter per degree Celsius, so a 300 mm part that warms 5 °C during a long cut drifts about 0.035 mm before any tool wear. Second, fixturing: a part held too rigidly will spring back when the vise opens. Third, tool deflection, which grows sharply with long reach and small cutter diameter.

The practical consequence is that tight tolerances should be applied only where the function needs them. Marking every dimension ±0.005 mm raises cost and inspection time without improving the assembly. Put the tight callout on the mating bore, the bearing seat, the sealing face. Leave the rest at a general tolerance block.

Surface finish follows the same logic. Ra 0.2–0.8 μm is available for sealing and sliding surfaces. Ra 0.8–1.6 μm covers most bearing fits and mating faces. Ra 1.6–3.2 μm is normal as-machined condition and is fine for brackets and covers. Asking for a fine finish on a non-functional face is money spent on nothing.

Aluminium is the default for prototypes and low-volume parts. Grades 6061 and 6061-T6 machine cleanly and hold tolerance well. 7075 is stronger but more prone to distortion when a lot of material comes off one side. 2024 behaves similarly and needs stress relief on thin walls. For castings converted to machined parts, ADC12 is common.

Stainless grades 303 and 304 cover most fittings and brackets. 316 and 316L are for corrosive or medical service, and they work-harden, so the toolpath has to keep the cutter engaged rather than rubbing. 17-4PH (SUS630) is the choice when you need strength plus corrosion resistance in the same part. 440C is for wear surfaces.

Steel grades 1018 and 1045 are straightforward. 4130, 4140 and 4340 are the alloy steels used for stressed components, and they machine better in the normalized or pre-hardened condition. Titanium TC4 (Ti-6Al-4V) and Inconel are slow, generate heat at the cutting edge, and require rigid setups and conservative feeds. They are not impossible, but they change the cycle time estimate.

Plastics are their own problem. POM and ABS are stable. PEEK and carbon fibre are abrasive and expensive, and carbon fibre dust needs extraction. On thin plastic walls, clamping pressure alone can deform the part beyond tolerance before the tool touches it.

Any shop can state a tolerance. What separates suppliers is whether they can show you the measurement. At GreatLight, every order gets a raw material check on arrival, in-process monitoring during cutting, and a final inspection before shipment. Inspection reports are available on request.

The qualification rate on shipped parts is 99.99%. That number only means something if the inspection plan matches the drawing, so the first review step is a DFM analysis that flags dimensions which cannot be measured reliably. A true position callout on a hidden internal feature is a real problem, because it cannot be verified after assembly.

Certification matters for regulated buyers. GreatLight holds ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022. The last one covers information security, which is relevant when your drawings and CAD files leave your network. Uploads are handled as confidential, and an NDA is available on request.

For first articles, the sample center is the place to validate the process before a production run. It is cheaper to find a fixturing problem on part one than on part five hundred.

Ask which machine the part will run on, not how many machines the shop owns. A 127-machine floor with 16 five-axis centers is a capability statement, but the answer you need is whether your specific geometry goes on a three-axis or a five-axis. That single answer explains most of the price difference between quotes.

Ask how many setups the part needs. Each additional setup adds a locating error and a queue. A design that can be reached in two setups instead of four will usually be cheaper and more accurate, and a DFM review should tell you that before cutting starts.

Ask what happens when a dimension is marginal. A shop that stops and calls you is worth more than one that ships and hopes. Late-delivery probability at GreatLight is historically below 2%, and quotation with free DFM analysis comes back within 12 hours. Production can start within 24 hours, and parts ship in 3–5 days.

There is no minimum order quantity. A single prototype and a 10,000-part run go through the same first-article check. That matters when you are still iterating on a design and do not want to commit to volume before the geometry is frozen.