Basic Knowledge Tutorial on CNC Metal Milling

Milling is subtractive. A rotating cutter moves through a solid block and leaves the shape you asked for; nothing is formed or poured. That matters when you read a drawing, because every pocket, slot and fillet has to be reachable by a tool that is at least as long as the feature is deep.

The cycle starts with a 3D model from CAD. The CAM programmer imports it, picks the stock size, sets the work coordinate system and chooses the toolpath strategy. Roughing removes most of the volume with large stepovers. Semi-finishing evens out the walls. Finishing runs the final pass at the feed and speed the surface finish calls for.

The controller turns that plan into G-code, then into electrical signals that drive servo motors on each axis. A tool changer swaps cutters between operations, so a part with eight features may run through eight tools without an operator touching it. Setup is where the skill sits: fixture rigidity decides whether the numbers hold.

Tolerance is a process result, not a wish. On a rigid setup in aluminium we hold ±0.005 mm on critical features, with surface finish between Ra 0.2 and 0.8 μm when the shop runs a dedicated finishing pass. Tighten the callout without a reason and the price climbs faster than the accuracy improves.

A 3-axis mill moves the table in X, Y and Z while the cutter spins. It handles plates, brackets, housings and any part where the features sit on a handful of faces. If each face can be reached by turning the part over and re-clamping, 3-axis is usually the cheaper route.

A 4-axis machine adds rotation, normally around X. This suits shafts, connectors and cylindrical parts with flats, holes or slots spaced around the circumference. One setup replaces three or four, and angular position stays consistent because the part never leaves the fixture.

A 5-axis center tilts the tool or the table on two extra axes at once. Undercuts, deep cavities, impeller blades and angled holes get cut in a single setup. The cutter can stay short and rigid, which helps on hard alloys where chatter kills the finish. Our shop runs 16 simultaneous 5-axis machining centers alongside 27 three-axis and 12 four-axis mills.

Five axes is not automatically better. For a flat plate with simple holes, the extra kinematics add programming time and nothing else. The honest rule: choose the machine that reaches every feature in the fewest setups without a long, thin cutter.

Aluminium 6061 machines fast and takes an excellent finish, which is why it dominates prototypes and enclosures. 7075 is stronger but gummier, so it needs sharper tools and lighter cuts. 2024 behaves well on structural brackets. None of them need coolant on every pass, but chip evacuation matters more than most people expect.

Stainless 304 and 316 work-harden the moment a tool rubs instead of cuts. Keep the feed up, never dwell, and the surface stays clean. 17-4PH adds strength after heat treatment and is common in medical and aerospace hardware. Titanium TC4 (Ti-6Al-4V) is the hardest of the everyday group on tool life; heat leaves with the chip, so coolant strategy is a real decision, not a default.

Steel grades behave along a spectrum. 1018 and 1045 are straightforward. 4140 and 4340 need more attention to depth of cut. Tool steel cuts well only when annealed; hardened stock goes to grinding or EDM instead.

Copper and brass machine freely but move under clamping pressure, so thin walls need light fixtures. Plastics such as POM, PEEK and PC cut easily yet deform from heat. For those, sharp tools and air blast beat flood coolant.

Pocket depth drives tool choice. A pocket four times deeper than its width needs a long, thin cutter that deflects, so the shop slows down and the price rises. Keep depth under three times the width where you can, and add a corner radius that matches a standard end mill.

Wall thickness sets how hard the part is to hold. Anything below 1 mm in aluminium starts to sing during finishing. If a thin wall is unavoidable, tell the machinist which face is the datum; clamping pressure can be planned around it.

Tolerances should sit on the features that matter. Call out a flatness or bore diameter where it functions, and leave the rest general. A drawing with ±0.005 mm on every dimension costs more than one with three tight callouts, and it does not make the part better.

Threads and small holes follow the same logic. A Ø2 mm hole in stainless is a drill-breaking risk; in aluminium it is routine. Standard thread sizes keep tooling on the shelf. Laser marking needs characters at least 1.5 mm tall to stay legible after finishing.

Send the STEP file and a drawing with the critical callouts. We return a quotation and a free DFM analysis within 12 hours. The DFM note flags features that will raise cost or risk: deep pockets, thin walls, tolerances with no functional reason.

Once the design is settled, production can start within 24 hours. We check raw material certificates before cutting, monitor dimensions in process, and inspect 100% of parts before shipment. Reports are available on request.

Parts ship in 3–5 days for most jobs. There is no minimum order quantity, so a single prototype and a 10,000+ part run go through the same process. Uploads are secure and confidential, and an NDA is available on request.

If you are still at the concept stage, our sample center shows the range of geometries and finishes we cut, which is often faster than reading a tolerance table.