I'd chalk up spinning the cutters on the shaft to your learning curve. I think we've all done it early on. The trick I have found is to always make sure the spacers and the head nut have enough clearance to really be tight. But, if you get a piece of wood flying and it wedges in the fence or otherwise allows the cutter to get "hammered", it can do the same thing.
Good news is it can probably be pressed back out at a machine shop if you can't manage it with your own tools. I wouldn't suggest using heat expansion techniques on the precision parts. I've always managed to get mine free again after using enough?expletives.
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Brett Wissel Saint Louis Restoration 1831 S Kingshighway Blvd (at Shaw Blvd) St Louis, MO 63110