CNC milling (3-axis): how it works, costs and design rules

A rotating cutter removes material from a clamped block while the machine moves in X, Y and Z. It is the workhorse of prototype and small-batch metal parts: brackets, plates, housings, fixtures and mould inserts.

Typical tolerance

±0.1 mm general, ±0.02 mm on selected features

Quantities

1 to about 1,000

Lead time

3–15 working days

Materials

Aluminium, steel, stainless, brass, plastics, titanium

How it works

The part is cut from a block or plate held in a vice or fixture. The machine steps the tool down layer by layer for roughing, then finishes walls and floors with lighter cuts. Everything the cutter can reach from above is made in one setup; features on other faces need the part to be flipped and clamped again.

Benefits

• No tooling cost, so a single part is affordable.
• Excellent accuracy and surface finish straight off the machine.
• Works with almost every engineering metal and plastic.
• Design changes cost nothing but a new program.

Limitations

• Inside corners always keep the radius of the cutter.
• Undercuts and hidden features need special tools or more setups.
• Unit price drops slowly with quantity compared with moulding or casting.
• Much of the material bought ends up as chips.

What drives the cost

• Machining time, mainly the volume of material removed and the smallest tool used.
• Number of setups (each side that needs work).
• Tight tolerances and fine finishes that need extra passes and inspection.
• Stock size: a part just over a standard plate thickness needs the next size up.

Design rules

• Give inside vertical corners a radius of at least 1/3 of the pocket depth.
• Keep pockets less than about 4× the cutter diameter deep.
• Keep metal walls at least 0.8–1 mm thick, plastic walls at least 1.5 mm.
• Use standard drill sizes and limit threads to 1.5–2× the diameter.
• Put as many features as possible on one face.

Preparing your STEP file

• Export one solid body per file in millimetres.
• Model inside radii as they will be cut; do not leave sharp corners and hope for the best.
• Model threaded holes at tap drill size and call out the thread on the drawing.
• Name the file with part number and revision, for example MP-1001_A.step.

See also the full STEP export checklist and the comparison of all processes.