Just thought I would post this for others to learn from.
My 4 year old SL-10 suddenly started slipping the cog belt on the spindle. When I first heard it I thought a tool broke. If I tried to run the spindle from 0-500 RPM's it would hesitate, rock back and forth then jump to 500 RPM's and the belt would slip. After I got it past 500 rpm's I could increase it to 4000 with no slippage. When you hit spindle stop it would jump teeth on the belt again. The belt was tight, no gear box, and the spindle turns freely. My local Haas Techs were scratching their heads as I was too. I figured it was either the spindle motor, encoder, or Haas's Vector drive. I didn't want to just start spending money without knowing what was wrong so I decided to call Haas in California. That Tech thought the belt was too loose, after I insisted that it wasn't the belt and that it just started all at once he suggested changing the accel/decel parameters and had no other advice. Just out of couriocity I lowered the parameters (one from 2mil down to 30k). I finally got the belt to not slip, but it was still not right. When I tried to run the machine I noticed that when I tried to make a cut in the low rpms the machine labored and the spindle load went way up. So I called my local tech back a couple more times until they thought about checking the "Delta-Y contactor" for the spindle motor. Come to find out that one of the lugs on a jumper on one of the contactors was never tightened at the factory, and had been just barely touching. So it ended up being a cheap fix but with a day of production lost. This contactor is the one you hear "Click" when you increase your spindle speed from low to high. Hope this saves someone else some time.