Beginner's CNC Processing: How a Cutting Program Becomes a Part

CNC processing means the cutting tool moves under computer control instead of a hand wheel. You start with a CAD model, a CAM programmer picks tools and cutting passes, and the software posts a text file of coordinates and feed rates. That file is G-code. The machine controller reads it line by line and drives the spindle and axes to match.

The tool never follows the finished surface. It follows an offset path that leaves material for roughing, then removes the rest in semi-finish and finish passes. Stock size matters more than most beginners expect. A block that is 1 mm oversize on all faces costs less to machine than one that is 6 mm oversize, because the machine spends less time in air and less time cutting waste.

G-code is machine-specific. A program posted for one controller may run on another with edits. This is why shops keep a post-processor library and why minor drawing changes do not always mean a full reprogram. For a first order, send STEP or IGES plus a PDF drawing with tolerances and critical dimensions marked.

One more point. CNC processing is subtractive. If your part is a thin shell or has internal channels that a cutter cannot reach, machining may not be the right first process. Ask before you design around it.

A 3-axis machine moves X, Y and Z. The tool approaches from one direction, so every feature must be reachable from that direction or from a second setup where the part is flipped. Simple plates, housings, brackets and covers are usually 3-axis work. Two setups are common and cheap.

A 4-axis machine adds rotation about one axis, often a Ø400 mm rotary table. This lets the part turn while the tool cuts, so you can machine around a cylinder or hit several faces without re-fixturing. Shafts with flats, cams, and parts with radial holes fit here.

A 5-axis machine adds two rotary axes that move at the same time as the linear axes. The tool can stay normal to a curved surface, and undercuts become reachable. Impellers, turbine blades, medical implants and complex aerospace brackets are typical. Simultaneous 5-axis also shortens setups, which improves position accuracy between features.

The trade-off is cost per hour and programming time. If a part runs fine on 3 axes, moving it to 5 axes does not make it better. It makes it more expensive. We look at feature direction, undercuts, and how many setups a 3-axis route would need before recommending a machine.

Aluminium 6061-T6 is the default for beginners. It cuts fast, holds a good finish, takes anodizing well, and is cheap enough to prototype with. Use 7075 when you need higher strength, and 2024 when fatigue matters. 5052 and 5083 are better for sheet and welded assemblies than for heavy machining.

Stainless 303 machines more freely than 304 or 316, which matters on parts with deep pockets and small tools. 316L and 17-4PH come up in medical and marine work. They cut slower and work-harden if the feed is too light, so the programmer has to keep the tool engaged.

Steels like 1018 and 1045 are common for shafts and fixtures. 4140 and 4340 appear in higher-load parts. Tool steel is usually reserved for molds and dies. Titanium Ti-6Al-4V and Inconel are machinable but slow, and they wear tools quickly, so expect a higher price per part.

Plastics behave differently. POM holds tight tolerances and machines cleanly. PEEK takes heat but costs more. ABS and PC are fine for enclosures and covers. Carbon fibre is abrasive and needs carbide or diamond tooling, and the dust needs control.

A general tolerance block on a drawing means the shop can use a default range for dimensions not called out. Called-out tolerances are tighter and cost more. A ±0.005 mm callout on a 100 mm aluminium part is achievable, but it is not free. It may require temperature control, a finish pass, and inspection time.

Surface finish is written as Ra, the average roughness in micrometres. As-machined surfaces sit around Ra 1.6–3.2 μm. A normal fine finish is Ra 0.8–1.6 μm. Ra 0.2–0.8 μm needs slower passes, sharper tools, and often a polishing step. Do not call out Ra 0.2 μm on a face that only needs to look clean.

Only tolerance the dimensions that matter. Mark datum faces. If a hole must align with another hole across the part, give a true position callout and let the shop plan the setup. Adding tight tolerances everywhere raises the price without improving function.

Inspection follows the same logic. We check raw material on arrival, monitor during the run, and inspect 100% before shipment. Reports are available on request. If you need first article inspection, say so at quote time so it is built into the plan.