I have a benchtop prolight 2000 mill with a 10k RPM spindle that I want to do some work on to make it more suitable for general purpose milling (i.e. larger endmills than 1/4" and in steel). I'm going to change the belt gearing to reduce the max speed to 5.5k or 6k to get more torque out of the motor so the bearings will no longer have to run at 10k.
The stock bearings are 6206 radial ball bearings, one on each end of the spindle which are slightly preloaded to remove the axial play. I'm sure they performed fine for wax and aluminum milling at high speed, but in my non-expert opinion these bearings are a pretty lame choice for general milling.
From reading other threads here it sounds like the ideal bearing arrangement would be a pair of 15 deg angular contact bearings at the spindle nose with a light-fitting radial ball bearing or cylindrical roller bearing at the top. To do this I would have to bore a deeper bearing seat in the mill head, it's a solid block of aluminum so that's a possibility. The inner bearing would only have something like 1/3 of the inner ring sitting on the bearing seat on the spindle, which I suppose is a bad idea.
Possibility 2 is to replace the two 6206 bearings with an opposing pair of 7206 AC bearings and give it some preload. I understand the problem with this approach is thermal expansion of the spindle or bearings which could lighten or increase the preload respectively. If I used 15 deg AC bearings from VXB, for instance, I'd end up spending around $180 to $230 for ABEC-5 or 7. The $40 option is a pair of 7206B which I believe have a 40 contact angle.
Possibility 3 is to replace the two 6206 bearings with a pair of 30206 tapered roller bearings, and run them in oil. I can get a pair of brand name ABEC-5 tapered roller bearings for $90. From what I've read so far this would give me more spindle rigidity and load/shock resistance but people seem down on using tapered roller bearings this way. 6k rpm should be within spec for the bearings when oil lubricated. This seems like an attractive option to me.
Is this a dumb idea, or does anyone have suggestions?