So the drive pulley on my Sherline started coming loose again, and it was allowing the pulley to spin while the spindle sat still. The set screw was tight, against something. Finally had enough and tore it apart. Turns out the first time it came loose it rolled some metal into the thread and was preventing the set screw from getting to the spindle. It's a 10-32, so I chased it and the screw went through freely. Put it all together and things are all locked together nicely. That's a relief.