Usually called a buck..
Sometimes though you may build an actual forming die , for say stretching into..
Good example, you make a circular saucer shaped. Drepression in a piece of wood, with a few grooves across. Then using multiple screws, you screw the annealed blank to the wood form, the screws being outside the actual finished part. You then hammer the metal into the form, smooth it as much as possible. Then remove the part, trim around the outside..and presto..you have an ashtray.
But that only works on parts or assemblies. That can pull out of the form. Like for example a sidecover. The rear fender could done that way,but involves left and right sides..and the deeper the cavity, the harder it gets to make the die and do the work..... So they use bucks instead..
Have a buddy that has done a few bodies and tanks for his bikes..he learned fast..
