CNC Milling Explained: Process, Parameters and What They Mean

A clear explanation of CNC milling: the principle behind the process, the parameters that matter and how they change accuracy, surface finish and cycle time.

The basic principle

CNC Milling is material removal with a rotating multi-point cutter moving over a workpiece held on the table. Everything else follows from that principle, including what the process can hold and where it struggles.

In practice the shop sees tolerances of plus or minus 0.05 mm and surface finishes around Ra 1.6 micrometres on aluminium, which is the result that most customers are actually buying.

The parameters that matter

Cutting speed, feed per tooth, depth of cut and stepover decide metal removal rate, tool life and the finish left on the part, and they interact rather than work in isolation.

Changing one without revisiting the others is the most common reason a stable job suddenly starts producing scrap.

Tooling and setup

Tool geometry, coating, holder balance and stick-out all change the result, and a rigid setup will out-perform a clever toolpath every time.

Record what worked, including holder and stick-out, so the job can be repeated by someone else six months later.

Typical results

A well-planned operation delivers tolerances of plus or minus 0.05 mm and surface finishes around Ra 1.6 micrometres on aluminium, and those figures should be written into the process sheet rather than assumed.

Where a drawing asks for more, the usual answer is a second finishing operation rather than pushing the roughing cut harder.

Mistakes worth avoiding

The most expensive errors are a setup that moves under load, a tool that is too long for the job, and a programme that was never simulated on the real machine.

Each of these is cheap to prevent and expensive to discover on a finished batch.