I've been trying to dismantle the apron on my old South Bend 9A lathe in order to clean it all up. The gears are jam packed with crud and oil channels are all plugged up. I got it all apart except for the clutch. The clutch has a left hand threaded nut holding a star shaped knob. This nut is notorious apparently because people turn it the wrong way and break it off. So it seems to me that I am turning it the right way for a left hand thread (clockwise) and still I have been unable to loosen it. One complicating factor is that the screw has only a shallow slot so a screwdriver pops out very easily. I've tried heating the knob and hammering on the screw to loosen it without success. Any thoughts on how this stubborn problem can be conquered?