my friends and i have been discussing the possibility of making the ar-15 upper from a bolt. in essence, making a threaded tube from an already appropriately threaded bolt.
there are two problems, as i see it:
1.)by its very nature, a 1 1/4 inch fine threaded bolt is gonna be on hard, tough, son of a bitch to drill through. they come in two varieties, "5-star" and "8-star" bolts, and these by their very nature are very hard, and very tough.
2.)now, "deep hole" drilling is very tough even on a lathe, where most of it is done, let alone deep drilling a bolt on a drill press as i want to do it. (this is just in keeping with the "keep it simple, stupid" theme of this whole exercise: i want to show that firearms can be made by simple means without access to complicated shop tools.) for one thing, how do you hold a bolt straight and true while being drilled on a drill press, with the simple vises available for drill presses. how do you hold a bolt still, to be drilled?
of these concerns, the second presents the most problems. drilling a tough and hard object is handled by setting the proper speed for the cutting tool, and exercising patience on the feed rate. but, how do you put a bolt straight and true in an ordinary vise, and how do you set up the drill bit to cut dead center through a bolt on a drill press.
i think i have a potential solution.--
it is easy enough to state clearly, and it presents a way to hold a bolt straight and true while being drilled. it should be a relatively straight forward proposition to drill a true 1 1/4" hole through a piece of medium hardness steel about 8" thick, say 9" or so square. one would want to surface grind this bit of steel on both sides, so that it has parallel surfaces, and so that it can be clamped to the drill press surfaces.
this hole would be threaded in a standard fine thread, and cut to the pitch of the 1 1/4" bolt on the face of the standard ar-15 upper receiver.
on one side of the piece of steel described above, a recess could be cut for the head of the bolt to be drilled. then, the bolt could be tightly threaded into the piece of steel, and snugged down to 50 or 60 pounds of tension. the end of the bolt to be drilled would be snugged flush with the surface of the block, and it's end flushed and trued.
this whole mess could be clamped to the drill press deck, and a true center on the end of the bolt found. a succession of pilot holes could be drilled into the bolt, to a sufficient depth where a 1" drill would not be subject to forces causing the drill bit to drift or wander. drill press chucks are not the precision instruments that a lath chuck is, and are not supported like a bit on a good bench lathe. nor are they supported and held as rigidly and precise as on a vertical mill.
but, i believe that were a bolt head and bolt shaft held as securely as described above, a bolt could be through drilled on its shaft axis, sufficiently true to be ground or honed to a final dimension more than true enough to accommodate the tolerances necessary for the proper function of a bolt carrier group in an ar-15 rifle. (there is quite a bit of "slop" between the walls of an ar-15 upper receiver and the bolt carrier group, believe me. lots of "wiggle room" involved in that relationship, from a machining and mechanical point of view.)
and, once made, a steel block 8"x8"x9" with a 1 1/4" hole drilled through it would be "durable" enough to be used several times, for the fabrication of more than one or two ar-15 upper receivers. from standard sized bolts, available from any supply house. since bolts are not gun parts, and subject to any registration requirement, and since upper receivers are not gun parts and likewise not subject to registration, a reasonably proficient hobbyist could make as many ar-15 uppers as his little heart desired with the gd gubment having the slightest notion of what he or she were up to.
john jay @ 09.14.2015
p.s. did i mention that a 8"x8"x9" chunk of steel is not gonna be moving around on a drill press table? clamped down, such a piece of steel & press table would be rock solid, and rock stable. in other words, as my oldest brother used to say, it ain't gonna go anywheres!!
Seems to me, tempering the bolt would save you a lot of grief. You could do it yourself with a small home made forge (better option) or even with a torch in a pinch. Torch, tho, will yield less uniform results. Or, find a machine shop with a tempering oven. Then, when you've completed drilling, re-harden the bolt. Anyhow, that's how I'd approach it. Good luck.
Posted by: Wynne | September 16, 2015 at 12:50 PM
wynne:
agreed on all points.
i have a friend who is a knife maker, and something of an expert w/ laminated steels.
the last time i visited his shop, he'd made an oven out of a mail box, and a big tank of propane.
the mail box wasn't big enough to line w/ fire brick, so he just used some sort of foam stuff, sprayed it on, and away he went.
it was still in use, the last i knew. it was more than sufficient to heat a fair chunk of steel really really red hot.
btw, thinking of how to make a bolt holder out of something a little lighter than steel. my friend the engineer reminded me that a hunk the size i speculated upon was about 168 lbs, or so.
laughing.
oh, well, i'll figure something out.
john jay
p.s. grandpa jay was a blacksmith/welder, grandpa beckius was a blacksmith/welder, and my dad was a blacksmith/welder/mechanic. i became a lawyer. too bad, so sad, that i didn't learn something more or less useful.
(so i do know about tempering & hardening, and that sort of stuff, along w/ the various hardening states of steel. but, i cannot weld a lick. *sigh*)
Posted by: john jay | September 17, 2015 at 09:58 AM
This is a great idea john. You got me thinking. I might try a fabricated lower/upper assembly in 4130 Chromemoly myself. (One in Titanium alloy would only be a bit more complex. Ti would be considerably less weight than Aluminum Alloy. Though nobody has bolts in Ti the size required for the barrel nut. Ti is difficult to drill, and some milling operations it is a bear to machine. Have to cut that thread on my lathe. I have the experience in both metals, so its a toss up between the two.)
A piece of tubing, 4130 chromemoly would be the top choice, but any seamless steel tubing will do for the receiver section. Take your bolt, cut off the appropriate length of threaded end of it, heat that thread section to 1100 degF, (dark red in a darkened room), sink it in a pail of wood ashes, let it cool to ambient. Better yet, if the bolt isn't shouldered, i.e. threads right up to the hex cap or socket head if it is an allen socket screw, leave the head on. Still cut the required length of thread, anneal as above, now you have a carbon steel bolt, likely 1045 carbon steel or similar, if it aint chinese junk, which you can drill easier. The cap is good, because you are going to need to jump from the thread OD to the tube OD, (or ID, for a press fit), you can chuck the bolt by the threads, drill, or bore the ID, and turn down the cap, and shorten the offset down to the minimum thickness. This way it is easier to line up your thread piece to your bolt carrier section. Because these two parts can be welded, or brazed together. If you have the bolt/carrier assembly, barrel with extension, and the barrel nut, you have a dandy perfect jig to line everything together for tack welding. Or, you can turn the OD of the cap of your bolt to the ID of the tubing, and use small cap screws drilled and tapped thru the tube OD, into the OD of your bolt head, while it is inserted into the tube stock, and your dandy jig lines it all up. No weld method option there.
Be a stout assembly for sure.
4130 is a good choice. It is precision drawn, comes annealed, good mill finish, has around 100,000psi tensile, and astronomical yield strength. Easy to weld or braze in all commercially common metal joining processes. Machines, drills, taps and turns like a dream using regular high speed steel tooling.
No need to stress relieve it after welding either for this use.
Great stuff.
Decent selection of 4130 tube:
http://www.aircraftspruce.com has a
Cap screws here:
http://www.mcmaster.com/#socket-head-cap-screws/=yzgalp
I could fabricate an upper in my basic shop in say about 8 hours on the first one. If I ganged up on each process, saving set up time by producing lots, maybe some simple filtering and jigs, cut the time to under an hour per piece.
Posted by: Doug | September 17, 2015 at 06:49 PM