The choice of spindle or milling head is very important. While a “toy” brush spindle in a plastic housing is enough to play in plywood or soft materials, it is completely unsuitable for professional use (i.e. for earning money). With intensive work, they can be finished in a few hours, and when we save them, it will be enough for half a year.
Professional spindles are brushless, induction, inverter-controlled spindles on ceramic bearings with a sealed motor compartment and necessarily have a metal housing. We can choose the speed and power of the spindle.
The spindle revolutions are the nominal value from the rating plate (12.000, 15.000, 18.000, 24.000, 40.000 rpm) which does not mean that the spindle must always rotate with these revolutions. The revs can be adjusted with an inverter, but remember that the spindle power is torque times the rotational speed, so while maintaining the torque, reducing the revolutions by half – the power also halves. And here you have to compromise …
There is no spindle for everything. The higher the spindle power, the larger the diameter of the bearings, and these bearings are the reason for limiting the spindle speed. The larger the diameter of the bearing, the more centrifugally the balls press against the raceways of the bearing outer ring and thus more heat is released (one way to reduce this effect is to use much lighter ceramic balls). Therefore, 40,000 rpm spindles are usually produced only in small power ranges.
High speed is liked by: aluminum, wood, composite materials, laminates, etc. High speed is unlikely to be used when processing steel, in particular stainless steel, thermoplastics and drilling with HSS drill bits.
As for power, the basic criterion is the maximum diameter of the cutters that we will use and the materials to be processed with them. For processing aluminum, plastics, wood, laminates with milling cutters up to 5mm, a spindle 0.8kW 24,000 rpm is enough, cutters up to 8mm 1kW, up to 12mm 3.3kW, up to 16mm 5.6kW. For steel, first of all, we should choose a spindle with a lower speed of 15,000 to 18,000 rpm and powers of 3.3kW to 10mm, 5.6kW to 12mm, 7kW to 16mm, 10kw to 20mm.
If we plan to drill in steel, we always have to choose a more powerful spindle because, for example, if we want to drill in steel with a 6mm drill at 2000 rpm with a 5.6kW 18,000 rpm spindle, we only have 0.6kW. The power of the spindles cannot be compared with the power of conventional milling machines because they have gears and there does not have to be such a torque reserve.
And one more thing. The service life of such spindles is up to 10 years ..