Achei essa resposta do minilathe no post do muca .

http://www.guiacnc.com.br/motores-em-geral/spindle-de-baixa-rotacao/

Porem achei também essa no guia gringo kkkk CNC ZONE :

I've just received my new 2.2KW water cooled spindle.
Before purchasing I had exactly the same dilemma, as many times when machining steel, or even aluminum with large diameter HSS tools you need rather low RPM.
Now I have this spindle, and a Hitachi X200 inverter.
The very first tests that I did was seeing at what lowest RPM I can run it.
To my amazement, it was no problem at all running even it as low as 200RPM.
At first it had very little torque at this speed. After playing with some parameters of the inverter, I managed to throw the full 10Amps while turning 200RPM.
Of coarse it will start to heat up rather quickly, but this is the advantage of water cooling. If you have sufficient pump and radiator, there is no problem.
I have about 1 liter water reservoir, and a small radiator with a fan.
I can run it at 200RPM, 10Amps for more than half an hour. It becomes pretty warm to touch, and the reservoir temperature climbs to about 45 centigrade, but that's the hottest it gets. I can continue to run it, and it won't get any hotter. I think that putting a bigger radiator would reduce the temperature even further.

I tried to mill steel (low carbon) with 3/8 carbide endmill, 0.1mm DOC at 800RPM. It ate it without any difficulty.
Aluminum is even better.