The right choice of a spindle or a milling head is very important. As long as that goes for fun in plywood or soft plastics, a"toy" brush spindle in a plastic housing may be enough. But for the professional use (i.e. to make money) it is totally unsuitable. With hard work it can be finished in a few hours and at economical work it may be sufficient up to six months.
Professional spindles are brushless inductive inverter-controlled spindles on ceramic bearings with a tight engine compartment and absolutely with a metal housing. We have a choice of spindle revolutions and power.
The spindle speed is the nominal value of the identification plate (12.000, 15.000, 18.000, 24.000, 40.000 rpm), which does not mean that the spindle is always spinning with these revolutions. Revolutions can be controlled by the inverter but you have to remember that the spindle power is the value of torque multiplied by rotational speed, so while preserving torque and reducing revolutions by half - about half the power also falls. And here we have to make a compromise ...
There is not a spindle for everything. The higher spindle power the larger are the diameters of bearings and these bearings are the cause of the spindle speed limitation. The larger bearing diameter the higher is the centrifugal force with which the balls are pressing on the running tracks of the outer ring of the bearing, and thus more heat is released (one of the methods to reduce this effect is the use of much lighter ceramic balls). Therefore, 40.000 rpm spindles are usually produced only in low power ranges.
High speed is best for materials such as aluminum, wood, composite materials, laminates, etc. When machining steel, especially stainless steel, thermoplastics or when drilling with HSS drill bits, high speed is rather not applied.
As for the power, the primary criterion is the maximum diameter of milling cutters which will be used and the materials processed with these cutters. For machining aluminum, plastic, wood or laminate with cutters up to 5mm the spindle (0.8kW 24,000 rpm) is sufficient, and accordingly cutters up to 8mm - 1kW spindle, cutters up to 12mm - 3.3kW spindle, cutters up to 16mm - 5.6kW spindle. For steel, we should primarily choose a lower rpm spindle (15,000 to 18,000 rpm) and accordingly cutters up to 10mm - 3.3kW spindle, cutters up to 12mm - 5.6kW spindle, cutters up to 16mm - 7kW spindle, cutters up to 20mm - 10kW spindle.
If you are planning to drill in steel, you always have to choose a more powerful spindle because, e.g., if you want to drill in steel with a 6mm drill at 2000rpm with the 5.6kW 18,000 rpm spindle, we have only 0.6kW at our disposal . You cannot compare the powers of spindles with the powers of conventional milling machines because they have gears and there is no need to have such a torque reserve. And one more thing. The sustainability of such spindles is up to 10 years ...