I have a 4×21 craftsman sander as well. There should be a black knob on the top toward the front of the belt. This is a tracking knob. Turn the sander on and turn the knob slowly. If the belt continues to come off in that direction turn the knob slowly in the other direction. As you turn the knob in the correct direction the belt will come back to center. If you over turn the knob the belt will move to inside and rub. Remember turn the knob slowly and watch the belt move.