Clunking / Knocking / Vibration Problem '88 Series I, Variable Speed

Thanks derf, that makes sense to me. Looks like a belt change to me. How do you determine the speeds with your conversion? I also have a VFD on my mill but the display shows the electrical Hz, not motor speed. Did you add a digital tach to the head?

Yes, I did indeed add a digital tach that I bought off ebay for 16.00. It will be a little tougher to mount one with the variable speed pulleys and such taking up all that space. Once I got all the vari-speed stuff off, I made poly v-belt pulleys from aluminum. The spindle pulley is 3". I mounted a 4" lexan disc (1/2" thick) to the bottom of the pulley, and used a flat head machine screw for the sensor flag.
Since all the speed control is now out of the way, I had plenty of room to mount the pick up sensor.

Derf is that a custom or kit pulley/belt mod to a Series I Variable speed head? -Details?

I had to make all that from scratch. You might have noticed the motor pulley has fan blades for spokes. The reason for that is the motor fan will run slower below 60 hz, so an auxiliary fan is suggested for cooling. I added a 6" muffin fan on the top cover, but it is hard to push air through a 7" pulley that is only 1/2" away from the fan. By fashioning the spokes in the pulley like a fan blade, it allows the air to be suctioned through instead of being deflected.

derf, was the work on the pulley (fan blades) done on a CNC? Nice install overall.

My noise issue might not be due to the belt hitting the pulley shaft. I put it into low range and ran it up to max speed and quiet as can be. I then lowered the speed back down but it didn't seem as loud as before. When I put it back into high gear the noise was back but only loud at the lowest speed. It seems to make a difference whether it is in low gear or high gear. I am back to thinking it might be the plastic sleeve in the upper pulley assembly above the spindle. I still plan on replacing the belt first in case that is the issue. Since I have a VFD to run the 3 ph motor another option I guess would be to do a conversion similar to yours. Is your VFD a standard one or different to enable your setup to work?

I machined the pulley on that very machine before I gutted it, yes it is CNC.

I believe I would install a new belt and the plastic bushings in both pulleys and see if that cures it. If that doesn't, it would tell me that the spindle pulley hub and the splined gear hub need replaced(that's about $500).
The VFD is a AB 523, and is probably no different than any other vfd. The forward/reverse/stop control is wired into the cnc control. I use remote pot and built a dial around it for ease of operation. I also incorporated 2 adjustable limit stops for the pot. This makes it easy to run 2 different tools at different speeds just by flipping the dial one way or another.