Long read but it's late and I'm wide awake and I may be having a little drink. I really can't say.
Anyway, a guy getting a new AR15 barrel for Modern Military Matches wanted the windage adjustable front sight mod.. I took on the job and started two days ago. It involves four drilled and tapped holes for set screws rather than through pins and four milled flats on the barrel corresponding with the screws.
Of course you can't drill the sight housing on the barrel like you would if you were pinning because you don't want holes through the barrel. The method I use is to drill the sight housing on a 3/4” mild steel mandrel. This accomplishes a coupe of things; drilling straight through the sacrificial mandrel provides continuous, full support of the drill bit, and it ensures the holes on both sides are in alignment in case somebody wants the sight pinned in the future.
So here's the weird part. First hole, all the way through no problem. Second hole, through the pad on one side of the sight housing and the mandrel, no trouble. Easy going. But then the drill bit snapped off. Dam. I thought, well maybe I pushed it too much. I pressed the mandrel out of the sight housing, shearing the drill bit, not damaging the sight or the mandrel.
New drill bit, take it easy this time cowboy. The bit does not want to cut at all. I pushed a bit more. No go. Took the bit out and looked at it with magnification. Cutting edges smooshed over forward into the flutes. It was the last one I had. I sharpened it (ever try to sharpen a #38 drill bit?). Tried again. No bueno. The bit make a snapping sound as I applied force. Better stop! Looked at it, smooshed cutting edges again.
So I ordered new bits. They came today. I went back to work on it. New bits won't cut. I don't understand it. I thought try drilling this final hole from the other side. So I set up and located the hole. When I used a solid carbide spot drill it was obvious the the material was hard. After I got the hole spotted I switched to a new twist drill and it instantly died. That one pad was harder than the proverbial woodpecker lips. Even though the other three were of normal hardness and drilled easily. Even if I did manage to eventually wear a hole through this thing, there was no way I could tap something that hard.
And I didn't have a spare. So, out came the oxy-acetelene torch with a small tip and I heated that one pad to orange heat. After it cooled, it drilled and tapped just like normal. Maybe even weirder still after the job was done, cleaned up and lightly oiled you can't tell it had been heated.
I don't know how these sight housings are manufactured. They're forged as far as I know. Out of what material? Some kind of steel but how that one area could be so hard and the rest of it not is beyond me. And I never would have expected it to take that heat and not show discoloration but...I guess all's well that ends well.
Anyway, a guy getting a new AR15 barrel for Modern Military Matches wanted the windage adjustable front sight mod.. I took on the job and started two days ago. It involves four drilled and tapped holes for set screws rather than through pins and four milled flats on the barrel corresponding with the screws.
Of course you can't drill the sight housing on the barrel like you would if you were pinning because you don't want holes through the barrel. The method I use is to drill the sight housing on a 3/4” mild steel mandrel. This accomplishes a coupe of things; drilling straight through the sacrificial mandrel provides continuous, full support of the drill bit, and it ensures the holes on both sides are in alignment in case somebody wants the sight pinned in the future.
So here's the weird part. First hole, all the way through no problem. Second hole, through the pad on one side of the sight housing and the mandrel, no trouble. Easy going. But then the drill bit snapped off. Dam. I thought, well maybe I pushed it too much. I pressed the mandrel out of the sight housing, shearing the drill bit, not damaging the sight or the mandrel.
New drill bit, take it easy this time cowboy. The bit does not want to cut at all. I pushed a bit more. No go. Took the bit out and looked at it with magnification. Cutting edges smooshed over forward into the flutes. It was the last one I had. I sharpened it (ever try to sharpen a #38 drill bit?). Tried again. No bueno. The bit make a snapping sound as I applied force. Better stop! Looked at it, smooshed cutting edges again.
So I ordered new bits. They came today. I went back to work on it. New bits won't cut. I don't understand it. I thought try drilling this final hole from the other side. So I set up and located the hole. When I used a solid carbide spot drill it was obvious the the material was hard. After I got the hole spotted I switched to a new twist drill and it instantly died. That one pad was harder than the proverbial woodpecker lips. Even though the other three were of normal hardness and drilled easily. Even if I did manage to eventually wear a hole through this thing, there was no way I could tap something that hard.
And I didn't have a spare. So, out came the oxy-acetelene torch with a small tip and I heated that one pad to orange heat. After it cooled, it drilled and tapped just like normal. Maybe even weirder still after the job was done, cleaned up and lightly oiled you can't tell it had been heated.
I don't know how these sight housings are manufactured. They're forged as far as I know. Out of what material? Some kind of steel but how that one area could be so hard and the rest of it not is beyond me. And I never would have expected it to take that heat and not show discoloration but...I guess all's well that ends well.
