
Okay. Problem solved and I feel really dumb.
Middle of the day I went back down to the mill and got my tools and turned it on one last time. Concept was to put one hand on the spindle end and the other on the motor housing and see if I could detect which one vibrated first as an alternative to disassembling everything.
Kind of surprised when nothing unusual happened. The spindle spun very nicely with no clunks whatsoever. So, I stopped everything and put the end mill back in and tried again. Flawless operation.
All thoughts of totally disassembling the spindle head and motor housing now went on hold while I went and got a cup of coffee and pondered the discrepant behavior.

Something was different between yesterday and today and the last time the mill ran perfectly.
The last time was also night time just like last night. It ran fine then, but not last night. Today it runs fine again. Only difference I could think of was last night I.. Oh! I wonder if? Last night was cold so I plugged the 50K BTU torpedo heater into the same wall outlet to heat the basement up while I milled. Today the basement is already warm, so the heater is not sharing the wall socket.
I started the mill and plugged the heater into the wall socket. The heater started and a few seconds later the mill went CLUNK and the fault light came on and the mill stopped. Then a few seconds later the fault light went out and the spindle began spinning again. About 4 seconds later, CLUNK and the mill stopped again and the fault light came on.
I unplugged the kerosene heater and the mill ran just fine. The outlet is a 20 amp GFI box.
I was thinking it was about current, but when I plugged in my watt/amp meter I see the SX2 actually uses way less than 500 watts. Varying the speed varies the current load but it never passed 3 amps draw even at maximum 2500 speed. Likewise the kerosene heater uses only about 1.4 amps. Nevertheless the two devices somehow conflict when on the same line.
Resolution? I plugged the kerosene heater into a different wall outlet.
I will leave it to brainier folks than me to figure out why the two devices are incompatible when on the same outlet box. All I care about is the mill works and I have no need to take it apart.
