When I upgraded my BF30 with a belt drive I ran some tests on bearing temperature as a function of time and speed. This was done at the factory bearing preload setting which I haven't touched. I settled on 4000 RPM max with the following graph:
55+ degrees feels very hot to the touch. I would not feel comfortable running hotter than this for any length of time - take a tool holder out and you can't touch the shank for more than a second or so. I could also measure significant spindle length growth based on the increased temperature. I think I looked at an NSK taper roller bearing catalogue and for bearings of this size I seem to remember the continuous RPM rating was around the 5000 RPM mark. No doubt the stock bearings are not up to NSK quality either so should probably de-rate a bit from that. So if I was in your position I would get AC bearings all the way. The BF30 bearings are pretty big. If you could get to 10,000 that would be a very useful speed. You could fly through aluminium with that, with small 2-3mm cutters it would make a huge improvement. With my set up I am pretty happy because I can cut steel and aluminium with various size cutters, and still not have to change the belt ratio (just the VFD speed), but there is no doubt that it would be even better if there was more speed available.
LongRat
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