The automatic tool changer and full CNC control. A manual or basic CNC mill requires an operator to change tools between operations, so a complex part means constant intervention. A machining centre stores tools in a magazine and swaps them under program control, which lets one program run a complete operation unattended. Pallet changers, probing and through-spindle coolant extend that principle further.

Vertical for general work, prototyping, short runs and anything where capability per unit of cost dominates. Horizontal where volume justifies it, particularly in cast iron and steel where chip evacuation matters and where a pallet changer keeps the spindle cutting while the operator loads. Five-axis where parts need multiple faces machined in one setup, or where tool orientation genuinely requires it. Most shops start vertical and add the others for specific reasons.

Taper, speed and power together, matched to the work. Aluminium at high speed with small tools needs spindle speed above all; steel and cast iron with large cutters need torque at low speed, which is a different machine. The taper determines tooling availability and rigidity. Check the power and torque curves rather than the peak figures, since what matters is what is available at the speeds you will actually run.

They set the tolerances a machine can hold in practice. Rigidity determines how much metal can be removed before chatter, and therefore cycle times as well as finish. Thermal growth moves the spindle relative to the table as the machine warms, which shows up as parts drifting out of size through a shift. Machines intended for close tolerance work use thermal compensation, and warm-up routines are part of normal practice.

Automatic setting and measurement. A spindle probe locates the workpiece and sets the datum without an operator dialling it in, checks features mid-cycle, and can measure the finished part in the machine. A tool setter measures tool length and diameter and can detect breakage. Together they remove a large share of setup time and of the errors that come from manual setting, which is why they are near-standard on production machines.

Because chips left in the work zone recut, damage finish, wear tooling, carry heat back into the part and eventually jam fixtures. A vertical machine drops chips into the fixture, so through-spindle coolant, wash-down nozzles, sloped enclosures and a conveyor matter. Horizontal machines largely solve the problem by geometry. On any machine running unattended, chip handling determines whether it can actually be left alone.

Enough to hold the tools for a complete job plus the standard tools you never want to remove, since a magazine that has to be reloaded between jobs erodes the advantage of the tool changer. Long unattended runs, family-of-parts machining and five-axis work all push the number up. Consider also the maximum tool diameter and weight the magazine accepts, which frequently constrains a job before the count does.