...let's here yer tricks.
I've tapped thousands of holes in my life, and never, ever, once, have I broken a tap....until Friday...in the bowl when I was cutting 5/16-18 threads for the ride plate saddle. It broke about 1/4" below the surface. I got it out tonight.
I tried a couple different things, but the last one worked.
I tried some carbide tipped glass drill bits...no dice
I tried a 3/8 plug cutter bit (for wood...but aluminum is almost as soft) to try and cut around it...That made it about 1/8" in the hole and then gave up the ghost.
I ended stacking tack welds on top of each other on the tap the weld had reached the surface, then stuck a 7/16 nut on there, and welded the nut to my pile of tack welds. I went through a couple of nuts before I got it right....apply light force back and forth in both directions and it started to move a bit....kept doing this for about 5 minutes and it came right out. I ended up drilling out the hole to 5/8", turned up an aluminum plug (which I drilled and threaded on the lathe) and tig'd it in. A bit of work with a flap wheel and some paint and it looks like new.
any better ways (besides EDM)?
When using tap sockets instead of the tap handle, use a stubby ratchet. My 12" snap-on ratchet offered more than twice the leverage of the tap handle and I didn't really think about that when I was running the tap in...took very little pressure to snap the tap.