We ran into a very similar problem trying to bore a hole to a larger size in a planer blade. Those things are harder than woodpecker lips!

Carbide-tipped masonry bits work real well for this type of job, but it was a weekend, we don't have a decent hardware store in town...and we didn't want to drive to Sioux City just to buy a cheap drill bit.

We took a broken 3/8" twist-drill bit and chucked it, upside down, in the drill press chuck & clamped the planer blade in the drill press vise. Then we centered it where we wanted to drill the 5/16" hole. We then started the press on a medium speed and held the flat end of the bit (with no lubricant) against the blade until it started turning red, stopped the press, but continued to hold the bit in contact with the blade. As the bit cooled, we started the press again, and again and again, in the same manner....that transferred the heat to the area we wished to drill in a controlled manner....and in a very small portion of the hardened blade. After a while the hole itself started turning red, also. That was the time to bury the blade in a small pile of wood ash, which insulated the annealed portion and prevented it from cooling too quickly. Result: Retained hardness in the rest of the blade, but a small, soft spot that we could easily drill with even a cheap Chinese bit off the bargain table at Harbor Freight. It helps to have an old retired machinist around town that has run into almost every metal-related problem at one time or another to ask about such things.......like we did.

EDIT:

ing auto-correct
