[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. "

[IndHobby]

Have you tried a square carbide drill bit? You can get them at KBC tools.

Just drill and counterbore then drop the screws through, or tap them with a carbide tap.

I would guess that spot annealing would only work for somthing thin. A rail would take the heat away pretty quickly.

[HuFlungDung]

I highly doubt that method of spot annealing would work on a Thomson rail. It would be a good way to make a spot hard, though The surrounding cold metal would "quench" the spot, if you could get it red hot.

BTW, professionals bend some pretty large bars by spot heating, so it is not ideal to do this to a single spot your straight rails.

[TommyB]

I actually had a similar problem this past weekend while working building my diy cnc. I bought thomson rod and needed to drill and tap it for support rails. Well, let me tell you, my first attempt frustrated me for hours. I created flats on the shaft with a grinding stone on a mill. I went in about .05 inches thinking it was enogh. Went to drill the holes and nothing, still hard, ruined one end mill and 2 drill. The next morning a professor saw what I was doing(I'm a student at Rutgers University and have access to the machine shop) and told me he once had the same problem and that he went to Home Depot and bought a carbide masonary bit to drill through hardend shafting like thomson rod. He said that the carbide bit would build up heat high enogh to locally soften the material and cut it and not effect the bit. I was skeptical but went out and bought a 5/16 bit for $5 and WOW, it worked like a charm!!!!!!! The chips flew off dark blue and at times glowed red hot, but the area being drilled showed no signs of heat. I used it to drill through the hardened case up to the soft steel, about .12" in 1" rod. From that point, a #7 drill and 1/4-20 tapped. The drill lasted me for 18 holes, with plenty of life left.