[DJ Morrow]

I thought this might be worth a new thread so it didn't get buried in the old one.

Previously I had asked how to remove the hardness from some steel rails so I could drill and tap them. A couple of really good replies resulted. In my continuing quest, I found this on the web:

"Spot-Annealing Hack Saw Blades and Other Hardened Steel
© John A. Swensen 1996. All rights reserved.

The hardened steels used to make hack saw blades and other edge tools are very difficult to drill using ordinary drill bits. However, it is easy to soften the steel in the area of the hole by spot-annealing the metal. Note that this technique probably will not work with high-speed steels, which are designed to retain their hardness even at red-hot temperatures.

This technique was described by Alexander G. Weygers in his The Making of Tools , Van Nostrand Reinhold Company, 1973, ISBN 0 442 29360-7. This wonderful book is an introduction to blacksmithing and toolmaking in general, and is highly recommended.

Another of his books, equally good, is The Complete Modern Blacksmith , Ten Speed Press, 1997, ISBN 0 898 15896-6.

Spot-annealing requires a drill motor and a nail or other steel rod about the diameter of the desired hole. Cut off the nail head and smooth the end, then chuck the nail into the drill motor. At high speed, attempt to drill a hole in the hardened steel. The attempt will fail, but the resulting friction will heat the metal until the point of contact turns straw, then bronze, then peacock, and, finally, blue.

Stop the heating process and allow the steel to cool while you replace the nail with a drill bit of the desired size. At low speed, using a cooling/lubricating fluid, drill the hole through the softened steel. "