Grinders are made on CNC (Computer Numeric Controlled) Milling machines. Place the proper sized billet in the unit, feed in the coordinates and let her rip.
i took metal shop in school, you are exactly right sir. CNC work sucked, it took so long to program coordinates for whatever you were cuttin. i made a real nice pipe on one of the lathes...sadly, it got taken away from me by my teacher though...
the dude that owns the local smoke shop gets a local machinist to make his grinders and then sells them for a 200% profit. i know this because we had the same machinist make some shit for us and when i went to pick it up i saw a shit load of grinders sittin there and i was askin him about them.
but more than likely thats what most of the smaller companies and sellers do, as for the larger companies...they proly make their own....fuck im stoned...