Ask three shops to quote the same aluminum bracket and you will get three different machine assignments - one says 4-axis, one says 3+2, one insists on full simultaneous 5-axis. The prices will differ just as widely. None of the shops is lying. They are each solving your part with the axis configuration they happen to run best. Here is how to read the difference, with photos from our own floor in Shenzhen.
What each axis setup actually is
A true 4-axis machine adds a rotary table under the part: three linear axes plus rotation around one axis. Everything machinable on the outside of a cylinder - flange bolt patterns, radial ports, cross-holes at set angles - happens in one clamping.
3+2 locks the part at two programmed angles, then cuts with three linear axes. It cannot sweep a continuous curve like a simultaneous machine, but for indexed-face work - housings, brackets, manifold faces - it reaches the same geometry at a fraction of the machine rate.
Simultaneous 5-axis moves all five axes at once. That is not "more axes for marketing" - it is the only way to cut impellers, blisks, and thin-wall geometry without re-clamping distortion.
Where aluminum changes the math
Aluminum cuts 5-10 times faster than steel, so machine-hour differences between 4-axis and 5-axis shrink in absolute terms - but setup and fixturing differences grow, because aluminum parts are usually lighter-tolerance and cost-sensitive. A buyer who specs simultaneous 5-axis for a part that is really rotary-machined pays for capability the part never uses.
The dividing line we use when quoting: if every feature can be reached by rotating the part around one axis, it is a 4-axis part. If two faces must be cut relative to each other in one setup, 3+2. If a continuous curved surface defines the part, simultaneous. Our floor runs 100 five-axis centers - 30 of them three-plus-two, the rest full simultaneous - so the assignment comes from the part, not from which machine is idle.
The question nobody asks: how will this part be inspected
Axis configuration changes the datum structure of the part, and the datum structure decides what inspection proves. A 4-axis part measured on a CMM against the rotary datum is a different conversation than a 5-axis part with simultaneous-surface profiles. Before you place an aluminum order, ask which report ships with the parts - dimensional per print, or full GD&T with profile callouts - and whether first-article inspection is included or priced separately. We run CMM reports with every batch, first article in 3-7 days, ±0.005 mm on complex geometry.
A fast self-check before you send the RFQ
- Mark every face that must be machined relative to another face. One axis of rotation covers it? 4-axis.
- Count faces needing two-angle indexing. 3+2 usually beats simultaneous on price here.
- Curved, blended, or thin-wall geometry? Stop optimizing, send it to simultaneous 5-axis.
- State the alloy (6061-T6 vs 7075-T6) and which faces are cosmetic - anodizing quote depends on both.
- Incoterm: we ship EXW Shenzhen only and will happily recommend freight forwarders.
Full breakdowns live on our site: 3+2 Axis CNC Machining, 5 Axis CNC Machining China, and the material side at Small Batch Aluminum Housing CNC.
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