When Set Screws Go Bad

I am reinstalling the clock drive on Big Bertha.  While disassembling it for shipping,  I went one step more then I needed,  and removed a spur gear from the motor.   Like most such schemes,  the spur gear is held by a set screw to a flat part of the motor shaft.   The problem is that the gap between bottom of gear and flat part of shaft is very small.  The set screw will not fully engage in the threads in the gear while still allowing the gear to slide on to the shaft.  This was not enough flat shaft.

My immediate plan is to put the motor in the freezer at -8 and the gear at 200 so the contraction and expansion give a bit more play to slide the gear on the shaft while the set screw is still engaged, then tighten it down. 

Cold and heat did not do the job.  I put the motor in the mill and applied almost no pressure to hold it in place and milled of a few thousandths of an inch on the flat.  The set screw was now able to engage threads while still sliding into the shaft,  then I could tighten it down hard enough.

Now I see why I removed the set screw; to remove a motor mounting plate.   I took the gear off again and overtightened the set screw which is steel stripping the threads in the brass gear.   Now I need to find a replacement gear.  Maybe a brass set screw as well.  No, I tried again and it is tight.