I am working on an electric motor for my Atlas 10" lathe. One day when I was disassembling the lathe to put a new headstock on there, when I re-assembled it, the motor didn't want to start. It just hums. I can start it by hand and it works fine. I took the motor apart and checked the resistance of both the starter and the running windings and they are both less than 3 ohms, so there are no opens. The contacts of the centrifugal switch were pretty gunked up (There is a lot of gunk inside) so I cleaned them up with emery cloth and then sprayed contact cleaner on it to get rid of the grit from the cloth. Still doesn't start on it's own. The centrifugal switch moves freely and is working properly. At this point I don't know what else to do. The motor does not have a starting capacitor so I don't know what else there is to check. One bizzare thing I found inside the motor, there was an old screw that seemed to be free-floating, as well as a spring. See attached picture. This is approximately where I found it. The armature looks like the screw has been rubbing on it. I have no clue what this is for and why it is in there. Someone clearly made a repair or something, but it just seems very odd. Any thoughts?