I have a Delta DP350 variable speed drill press less than 6 months
> old that has started having vibration problems at slower speeds (maybe
> 1100 rpm and less). It seems fine at higher speeds. I removed the
> cover and looked at the belt and pulleys and noticed that the rear
> drive pulley (above the motor) is controlled by pressure from a spring
> that forces the pulley plate downward to force the belt outward to
> increase the speed of the chuck. When I reduce the speed, at about
> 1100, the spring starts dancing around on the pulley plate and the
> vibration gets worse as I further cut speed. The spring seems to have
> roughened the plate around the hole.
>
> Anybody had this happen?
>
> Agkistrodon
Yep, had the same problem myself. Below is what I had posted last year, should solve your problem. I had called Delta and they sent me a new pully, but I did this fix in the meantime and it's still good.
For those of you with one of these who may have a problem with the bushing separating from the upper rear belt pulley, I found it's easy enough to fix. First move speed indicator to fastest speed, this loossens(relaxes) belt, then it's not too difficult to release the spring. After you get the upper pulley off, drill and tap a 10-32 hole and install a set screw to hold bushing in place. To re install spring I just compressed it in a vise and tied it with plastic tie straps to hold it while I got the keeper ring back on, then cut the tie straps and pulled em out. Hope this helps someone.