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Assume that the SCR, the 2-100ohm resistor and the 2-50uF caps aren't there. Since the SCR doesn't conduct when first start up. Then the only path to your electric drill (if it is a drill conected to the AC socket) is thru the POT.
See if you follow me, out of the rectifier into the POT, out of the pot into the AC socket and back to the rectifier. Lets say whatever you connect to the AC socket (like the drill) pulls 5 amps. So you have pretty much a 10K resistor limiting the current of the drill. At 120Vdc and the drill trying to pull 5A......and the POT limiting this current , converting it into heat......
120V @ 1A is like a 120W bulb to relate it to something.
So I believe this is why your POT keeps on burning.