I tried sharpening a broken tip engraver bit .125 variety and used it to bore out a remnant of a buried 1/4-20 tap, approximately 1/4" deep in a steel plate. It worked suprisingly well.......

I do this in the bridgeport so I can feel the spindle pressure but I usually crunch the tool at some point in the process so I tried one of the dull engravers with a bit of creative sharpening and it worked, and the price is right on those used bits.