What Kind of Carbide Drill Bit to Drill Out a Tap?

Pneumatic 1/8" die grinder and a diamond burr does it for me. I've done 4-40s in the past, takes a steady hand and good lighting. You cut the center out of the tap from the side(s) and pick the pieces out. I've never, ever, had good luck with any sort of tap extractor, the taps are usually siezed in the material and those itty-bitty fingers have absolutely no chance to back the thing out without snapping across. Since I've gone to using a tapping jig with bushings, I don't break taps, large or small, anymore. I can't understand breaking taps in cast iron, though, that stuff is usually one of most free-cutting materials there is. You might have hit a chilled spot or aren't cleaning the tap out enough. One way to do blind holes is to fill them with cutting wax, I use Castrol brand, then tap them. The extruding wax pushes the cuttings out of the hole. The wax can be cleaned out with a blast of brake cleaner, if needed.

Stan

First of all, thanks to everyone for all of their suggestions. A few people had questions and some wanted to know how it turned out:

A lot of people wanted to know "How did you break off 2 taps in cast iron?" Well, I am first to admit when Ido something stupid - the second one was me being stupid, but the first one snapped off easily and without warning. I know when I've gotten lazy and pushed something harder than I should. You know, "One more half-turn and then I'll clear out the chips". But the first one must have been hard spot in piece or defective tap. I ended up getting it out with a tap extractor - further evidence that it was not going in too tight when it broke.

The second one, well, I, um, kind of tried to learn how to use a Tapmatic I bought at an auction by starting with a 5-40 tap on a blind hole...probably not my brightest move. I wasn't precise when I drilled the holes and one was not as deep as the others. The tap bottomed and snapped in a heartbeat. Yes, I could've had the clutch on the Tapmatic set lighter, but I did not. This bad boy was in there

- I couldn't get the fingers of the extractor in. I heated this one up with the torch. I didn't really expect it to help much, but it softened it considerably. I was able to drill it out with a combination of a 3/32" carbide end mill and a regular old drill bit.

Now, how to avoid breaking off teeny-tiny taps in the future? Well, I took an evening and some pieces from the scrap bin and made a small tapper! Check it out at:

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Nuthin' fancy, but it works fine. The large brass knob gives me more than enough power to tap small holes but keeps the "feel" of the cut.

Thanks again, Wallace