I got this grinder out of the scrap at work. The air motor would turn, but not the spindle. Taking it apart revealed the bevel gears were worn to the point they no longer meshed. It looked like it hadn’t been oiled in a very long time. I decided to try machining a little off the spacer(for lack of a better word) that spaced the gears apart. Turns out it worked. I question the longevity of this repair, but it works for now.