Heard about not using a hammer on a hammer myself, but confess to doing it fairly often. When removing a propeller from a boat there's not much room; using a puller doesn't always work and usually requires removing the rudder as well. Now, swinging a lump hammer at a prop will rarely arrive where it's needed on the boss and will most likely cause serious damage. My way is to rest a ballpein hammer against the boss and give it a hefty swipe with a large lump hammer. Works most of the time. Never had any shards flying about, but I agree glasses or safety glasses are recommended. Just why two hammers is regarded as a risk but not a hammer on an anvil (which is pretty hard) puzzles me.