CNC Milling

Most milled parts are 3-axis work. The cutter comes straight down, and the part is flipped between setups to reach the other faces. That covers plates, brackets and most housings.

A 4th axis adds a rotary table so the part can turn under the spindle. It pays off when features wrap around the part, like holes or flats spaced around a shaft, or when four sides of a block need work and every flip would cost another setup. If every feature sits on one or two faces, 4-axis adds cost and nothing else.

A STEP file, plus a PDF drawing if any feature has a tighter tolerance than the general one, a thread, or a surface finish callout. The 3D file gives the geometry. The drawing tells the shop which dimensions actually matter.

State the alloy and temper (6061-T6, not just "aluminum"), the quantity and the finish. For stainless, say whether it needs passivation. When a quote comes back with questions instead of a price, it is almost always one of these that was missing.

Deep pockets with small inside corners, more than anything else. A cutter can't make a sharp inside corner, and a small radius at the bottom of a deep pocket forces a long, thin tool that has to run slowly. Where you can, allow a corner radius of at least a third of the pocket depth.

Thin walls come next. Under about 1 mm in aluminum they chatter and need light passes.

After that it's tolerances and finish callouts applied to every surface instead of only the ones that function.