Dan, thanks for the offer. I have seen the bit and it is clearly tool steel. That being the case, I am not going to try to weld it and chance creating brittleness. I am going to try silver brazing with some 55% silver braze that I have.
I've been watching this thread, and I just don't see the appeal of welding a bit like this. I would be afraid it would just come apart again, and the next time it would be wedged between a fill stone and a piece of rebar so it wouldn't come out of the hole.
That being said, my Makita SDS hammer drill is 18 or 19 years old, and I have never broken a bit in it like you picture/describe. Its been used to drill plenty of stuff. Some of which should have been done with a core drill, but I had to run what I brung. I have broken the carbide tip of one or two bits, and just plane worn out a few tips, but I have to be honest. I don't ever recall twisting one of these bits off. Some of my larger ones are as old as the rotary hammer, and I still use them.
(I also have a heavier Milwakee spline drive that is only a year or two younger than the Makita.)
I'm sorry. I am doing one of those things I hate. Not answering the question you asked, and I am sorry for that, but I would throw the bit away and buy another one.
I think the appeal is that it broke with only 10 inches more to drill. So anything that would get 10 more inches drilled is a winner. The second appeal is that it is for Habitat for Humanity. Not an organization with unlimited funds.
If that does not work let me know if you want to try some 312 stainless. You would have to grind off all the silver braze, but for welding you would probably have to do a lot of grinding to get ready for welding anyway.
Can you go to the other side of the block, and measure from the completed holes very carefully to figure out very accurately where this last hole is supposed to come out, and then start drilling from the other side with a shorter drill bit that you do have?
If you have no access to the other side of the block then you still have a problem.
Hammerdrill bits and chisels are made from "S" steels. They are extremely resistant to shock. They are air-hardening steels so to weld it you would have to bevel the edges of both sides of the break, preheat both ends to above 700 degF, and weld using a high strength filler. Stainless would be a bad choice here for fracture toughness.
You could stick weld it with 8018 or 9018 rod, or bust the flux off and use either of those as TIG filler.
Keep it hot during the weld process and let it cool slow, buried in powdered lime, ashes, vermiculite or at very least, clean dry sand. You could also roll it up in a piece of Kaowool. You want it to cool down over a few hours if possible.
But there is probably someone volunteering at HFH that has the skills and k nowledge.
My choice would be to weld it. You can start drilling with it and when you get to where the weld is close to the start of the hole, switch to the new bit. That would save some wear and tear on the new drill bit. And you wo uld have it for a proven ready replacement if anything happened to the new drill bit.
Meanwhile why don't you post the URL of the site with the info on masonry d rilling. I don't have any projects in mind that require masonry drilling, but at always open to learning how to do things correctly. Yes it is off topic for a welding group, but not very far off topic.
Some years back I found a pretty big hunk of granite in my yard. And follo wing some things I learned on this newsgroup, I used a weed burner on it, w hich split it into chunks that I could lift. If anyone tries this at home, wear goggles.
Some years back I found a pretty big hunk of granite in my yard. And following some things I learned on this newsgroup, I used a weed burner on it, which split it into chunks that I could lift. If anyone tries this at home, wear goggles.