Is 4140 the best to use....

[f350ca]

I like machining 4140, gives a beautiful finish, but have heard its hard to work with small machines. (could be hear say) As for heat treating I've never needed to yet, for a slotting cutter in a home shop environment your great grand kids wouldn't wear it out. I think you want the bright surface on the taper to get the best grip, any irregularities from treating will reduce contact and cause problems. Your into a project I need to follow some day, have a chinese R8 one that has way to much run out.