The most common error I've found relates to the quill advance mechanism and depth indicator. The index mark should be straight up, in line with the setscrew which holds the spring housing in place. I have found many where the setscrew wasn't tightened enough and the spring housing slipped out of position. This limits how far the spring will stretch and retract. You can spot a factory error on this easily when the putty is still in place over the setscrew. On used machines that owners' have messed with, you often find advance mechanisms reassembled wrong, particularly the spherical washer inside and the various external washers.

Recently I found two examples of a trickier error. Evidently some of the fan sheaves on the motor were mis-manufactured. If set up on the motor shaft correctly, the "fingers" of the sheave halves don't mesh. (See photos) The floating sheave physically hits the fan sheave and won't allow for complete speed control. What the factory did was to install the spacer on the shaft, then put the fan sheave on, twisting it slightly and tightening the setscrew onto the shaft of the motor. Then the key was installed butted up to the fan sheave and the floating sheave slipped on over the key. I only found this error on these 2 machines because the fan sheaves started rubbing on the motor housing and I had to take it all apart to fix it. There is no way to save the fan sheave, you just have to replace it. An owner or inexperienced repair person would have no idea why it wasn't going back together correctly since it was meshing fine before dissassembly.

Problems arising from this: setscrew can damage motor shaft, key is loose, fan sheave is not locked in by key, spacer can wear prematurely.