How does CNC milling work and what shapes can it produce for metal parts?

The short answer

The four steps from model to finished face

Milling is subtractive, so the part starts as a block or a plate and the machine removes everything that is not the part. The work begins in software: CAM takes the CAD model, chooses the tool for each region, and generates toolpaths that tell the machine where to cut, how fast to move and how deep to go. On the machine, the workpiece is fixed to a table and a datum is established so that every feature is measured from the same reference. The spindle then drives the cutting tool, usually an end mill, a ball mill, a drill or a tap, at speeds that depend on the material and the tool, from roughly 1,500 revolutions per minute in steel to 12,000 and beyond in aluminium, and the tool removes material in layers until the surface matches the model.

Two numbers describe what the machine can do and they move together. Positioning accuracy is how close the machine brings the tool to the commanded point, and repeatability is how consistently it returns to the same place. On a well-maintained three-axis vertical machining centre those are typically 0.0127 to 0.0254 mm and about 0.005 mm respectively. The part in the fixture usually lands wider than that, because tool deflection, thermal growth, workholding stiffness and material batch all add to the machine's own error.

What each axis configuration unlocks

Axis count is the single design decision that most changes cost and tolerance, and the four common configurations are not a ladder of quality but a set of different capabilities. A three-axis machine moves the tool in X, Y and Z and handles prismatic parts, brackets, housings and faces at the lowest cost and the fastest setup. A four-axis machine adds a rotary table, so features on four faces or around a cylinder can be cut without turning the part by hand, and a high-precision fourth axis indexes to roughly 5 to 15 arc-seconds. A 3 plus 2 machine indexes two rotary axes and then locks them, which is the economical way to drill angled holes and machine tilted faces. A five-axis simultaneous machine tilts and rotates while cutting, so it holds one datum across every face and can use shorter, stiffer tools.

The practical consequence is fewer setups, and setups are where cumulative error lives. A part that a three-axis machine needs three fixtures to finish carries the tolerance of all three; the same part in a single five-axis setting carries one. That is why five-axis work often holds a tighter feature-to-feature tolerance than the individual machine accuracy would suggest, and why it is the normal route for free-form surfaces, impellers and turbine blades.

The shape families milling produces, and their limits

Most milled parts are made of a small number of shape families, and each family has a configuration that produces it cheapest and a physical limit that decides whether it is practical at all.

The pattern is that every shape family has a narrower, cheaper configuration than the one that can technically make it. A pocket with a generous internal radius is a three-axis job; the same pocket with a sharp corner becomes a two-operation job because a round cutter physically cannot leave a square internal corner, and clearing it needs a smaller tool or a wire EDM pass. A part with six faces of features is a five-axis job only if the volume justifies it; at low volume, three setups on a three-axis machine can be cheaper even though they are slower.

Design limits worth writing into the model

• Minimum wall thickness. Around 0.5 mm is achievable in metal, but 0.8 mm and up is the reliable zone, because thin walls vibrate and deflect under the cutting force and stainless deforms more than aluminium.
• Internal corner radius. Keep the radius at a third of the pocket depth or larger, and never smaller than the tool radius. Corners below about 0.25 mm need a specialist tool or a second process.
• Pocket depth to width. An end mill is stable to roughly 4 to 1 in aluminium, 3 to 1 in mild steel and 2 to 1 in stainless; past that the tool deflects, the finish suffers and the tolerance drifts.
• Hole depth to diameter. A standard twist drill is comfortable to about 10 times diameter, after which peck cycles or a gun drill take over.
• Floor radius. A flat-bottomed end mill leaves a small nub at its centre, so allow a floor radius of about 0.25 mm or call out that the floor needs a secondary clean-up.
• Undercuts are a five-axis or special-tool feature. If no straight-down view of the model shows every feature, the part contains an undercut and needs a configuration that can reach it.

Where milling stops being the right process

Milling removes material from a solid block, so the geometry has to be reachable, the material has to be cuttable and the volume has to justify the cycle time. Deep, narrow pockets need long, slender tools that deflect, which is why a design with a 10 to 1 pocket is often cheaper as a cast or a two-piece assembly. Sharp internal corners are impossible with a round cutter and belong in wire EDM. Hard materials such as titanium and high-temperature alloys cut slowly and wear tools quickly, so a part with a simple profile may be cheaper as a forging or a casting with a light milling pass. And any geometry that is still changing is expensive to mill at volume, because every revision is new programming and sometimes a new fixture. The honest comparison is always between a milled part, a formed part and a cast part at your real quantity, not between milling and nothing.

How to send a part for milling

Send the STEP model and a toleranced drawing, the material and stock form, the annual quantity, the finish per surface, and a note on which features and datums are functional. Those five items let an engineer choose the axis configuration, the number of setups, the tool sizes and the fixture before a price is set. See CNC machining for the process and its limits, metal bending where a formed shape beats a cut one, and surface finishing for the anodising, coating or polishing that follows the cut.