If left over an extended period of time, the threads on the grinding wheel shaft that thread into the right angle may become cemented with tooth debris, thus making removal difficult. Obtain graphite (i.e. Liquid Wrench) from a local hardware store and lubricate underneath the wheel, enough to penetrate the threads. Let sit for 10 minutes and then re-attempt to loosen wheel by turning in a counter clockwise direction using a pair of vise-grip pliers. Repeat procedure until successful. Apply grease to the threads on the new grinding wheel before replacement.