PM833 Spindle Upgrade - AC Servo vs Inverter Duty Motor

For more information on the actual spindle bearing replacement, check out this post:

Spindle Bearing Replacement - CNC Mill Conversions and Builds - Dr. D-Flo (

With respect to this the spindle upgrade, I ran the new spindle motor with the new bearings installed and milled a bunch of aluminum. All went great! As the bearings are not fully broken in, I ran at 4500 rpm for one hour. The spindle temp never exceeded around 84F! I verified little to no runout even when intentionally forcing the spindle laterally.

I am currently running with a 47 x 72 x 10 seal. My previous tests suggested that the tighter seals could add around 35 degrees to the spindle temp. I am going to experiment with the seals a bit more. When running a high helix aluminum specific 1/2" end mill, it was literally throwing chips vertically right at the seal. Ideally, I want as tight a seal as possible to prevent contaminants from entering the bearings. The seal is the only thing safeguarding against this. I 3d printed a jig to allow me to install the seal in such a way that the seal is not driven all the way in, but 3.5mm short. This will guarantee that the seal is not rubbing on the bearing race. My goal is to use the tightest seal possible and keep the temps in range.

Prior to the above test run, I ran a spindle break-in procedure. 500 rpm for 30 min, increasing by 500 rpm every 30 minutes until I reached 5000 rpms. As noted above, I then milled aluminum at 4500 rpm for about an hour. I then removed the oil seal to inspect. The image below shows a fine layer of grease coating the rollers. From what I have read about lubrication, the goal is to lightly coat the rollers with grease but to minimize any excess. Excess grease between the rollers causes heat. I believe that this is the key to keeping these bearing running on the cooler side. I used a syringe to inject Kluber Isoflex grease into the bearings. I then rotated the inner race with respect to the outer race of the bearing a number of times by hand. This expelled a bunch of excess grease out the other side of the bearing. I removed this excess grease with a small dental tool.

I am really happy with the AC servo spindle upgrade and would highly recommend it. Next stop… Rigid Tapping!

Best… Richard