I know you'll ask what it's for because you are certain I'll put an eye out, kill the neighbor's poodle, build a nuke or am up to no good anway, so here goes. I'm building a press that presses a dimple into thin leather. For that purpose a 3/8 ball bearing is by means of a C-clamp (not shown in drawing) pressed down through layered aluminum sheets . A wet leather strip is sandwiched between the aluminum plates. The plates are held together with M5 machine bolts and wingnuts. The ball will press dimple into the leather. I need to cut a spherical depression into a 3/8" aluminum cylinder. How do I best go about it? I was thinking of using maybe a 1/4" drill bit in my drill press to get a little pilot dimple going. Then switch over to a 3/8" ball burr and remove material down to a depth of 3/16." Looks like ball burrs come in two flavors" carbide or diamond coated. I'm not a machinist, so I am asking which burr is more suitable for my task? I guess the carbide cutter? As always, enjoy the drawing.