Had a couple of 20" real hard cutter head, scalloped disk blades that I broke off my House HD disk set, long story, helping a neighbor cut fire breaks at a rapid pace on a burn. No good deed shall go unpunished.
Tried to drill the disc blades with drill bits, and a flat end mill bit, no go. So I locked the blades in my Bridgeport with the vice, a 2x4 juniper block and a bar clamp. Then got out a ball end four flute solid carbide bit, a squeeze bottle of tap magic and cut four 3/8 holes 1/6" at a lick, for carriage bolts and heavy chain to hang them at 250 meters on my range. Should make a good cast bullet target set.
If you need to cut or drill spring steel, the ball end mill bit in a mill will do it, using plenty of oil and taking your time to cool the bit.