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Old January 20, 2014, 05:28 PM   #1
hhunter318
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Bolt Action Machining Process

Out of shear curiosity, how are the angles cut on the lug abutments in a bolt action? I know some people get out of this by chamfering the edge of the locking lugs on the bolt itself. But this just baffles me. Any experienced machinists throw your knowledge at me.
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Old January 20, 2014, 07:55 PM   #2
James K
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A broach.

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Old January 20, 2014, 08:29 PM   #3
johnwilliamson062
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A broach is like a lathe in that it is very simple, but if you use your imagination you can do incredible things.
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Old January 21, 2014, 03:34 AM   #4
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I guess you are talking about the cams in the receiver itself. Now days I would say live tooling. The old days, probably a shaper. Some military models look as if it is forged in. The angle on the bolt lugs you are talking about would be like on a Carcano bolt? I know what you mean about curiosity. I had been given a small .25 auto made in Spain in the late 20's, I believe. There was a slot in an area of the slide I never could figure out how it was done. Look through some books on old ordnance plants and see what they were using back then. It is amazing what they turned out with what they were using.
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Old January 21, 2014, 10:40 AM   #5
hhunter318
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Yes, I was referring to the cam angles on the lug abutments within the receiver. I knew the raceways were broached or done by wire edm. I just wasn't sure how this one specific part was done with a shaper or broach. I'm picturing the action being rotated as the broach or shaper moves in and out..?
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Old January 21, 2014, 11:06 AM   #6
Unclenick
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It took a lot of ingenuity. The Garand bolt and receiver, for example, have helical lug mating surfaces with a 0.2 inch pitch at a 1.16 inch pitch diameter that has to be radially square within half a thousandth. That's easy to put on the bolt lugs with a lathe and toolpost grinder. But the receiver is a tougher nut to crack. When bidders first looked at Garand's receiver drawings a number of them said there was no way to make it. Garand then went about proving them wrong by designing the jigs and fixtures that made it all possible to do.

I think you'll find every design had somewhat unique methods of approaching the problem based on when and where the work was being done.
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Old January 21, 2014, 12:22 PM   #7
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Could have been. Many machines are built for one specific job. You have to think past standard looking machinery. Unclenick is right, you can do a lot with fixtures. I made a brass rotor for a 99 Savage once with a lathe and bridgeport. (I did have a surface grinder to make my tooling) That was probably the toughest part I ever made for a gun.
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Old January 26, 2014, 01:38 PM   #8
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I ran a big American Broach machine for awhile.Amazing machines.Mostly a big hydraulic up/down ram with a chuck on it to accept the broach and draw it through the part.
Generally the point was to start the broach through a round hole,draw it though,and the hole would become splined or hex or whatever shape was ground into the broach.
Generally speaking ,the broach is along tool with a series of cutting teeth on it designed to change the hole to the desired shape.

There is a very important limitation to a broach.

As each tooth pulls a chip,the chip curls up into a gullet.There is really no place else for that chip to go.The length of cut a broach can perform is limited by the chip clearance in the gullet.Too long of a cut = too much chip and the broach will lock up and break.Drills,taps,reamers,etc have the same problem,but you can back them out and clear the chips.

A broach goes through in one pull.

I can't say exactly how a Mauser or Springfield was done,but I suspect a fair part of the raceway for the locking lugs was done via forging or some preliminary cuts with something like a Woodruff cutter to just get most of the steel out of the way.

Getting around this problem shows up in the rear lug Rem 788,the fat bolt Weatherby,and the AR barrel extension is a perfect part for a broach.

I had occasion to work on a Remington Hepburn kit...just castings.

I made a fixture out of a cheap cast angle plate to hold the receiver via the barrel thread and a slot in the face of the angle plate.I then bolted on blocks to precisely hold and guide common keyway broaches to cut the ways for the breech block.It worked sweet!
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Old January 26, 2014, 11:15 PM   #9
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On those old receivers none of the internal cuts were done by forging or casting them in. They were all machined into rough forgings. The locking lug seats were cut in different ways. In the M1903, they were machined by an end cutter, since the barrel was not installed and the OD of the lug seats was the same as the ID of the barrel hole. The cam angles were then shaved using a special tool.

In the Mauser 98, which uses an internal collar, the cuts were made by a using a rotary cutter that was offset in the hole in the receiver. (The same technique was used to cut the locking lug seats in the 1911 pistol slide, which is why a barrel bushing had to be used to fill the oversize hole in the front of the slide that was needed to admit the cutting tool.)

I apologize for my early error in saying a broach was used; for some reason I was thinking of the bolt lug raceways, not the lug seats.

Jim
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Old January 27, 2014, 10:09 PM   #10
Dixie Gunsmithing
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The bolt lugs on some bolts were actually milled on a horizontal mill, using a cutter with the partly round profile of the bolt, then it stopped off to leave the lugs. The bolt face was turned to the outside diameter of the lugs, and the profile cutter milled the top and bottom, leaving the two lugs.

The receiver is generally broached in a hydraulic broach, where it pulls a long toothed carbide cutter of the profile through the receiver, out the front, and cuts the two recesses that match the lugs. Each tooth cuts about .005+" each, which steadily get larger, and a long row on each side of the broach cuts both recesses in one pass. This is the same way keyseats are cut in gears and sprockets.

Another way is to use a shaper, with a rotary indexer holding the frame. The broach is much faster by far.

The inside face of the frames lugs can be milled several ways, including a boring bar in a mill.

I forgot to add this on tapered cam surfaces. Cincinnatti, and a few makers of large vertical mills, had a setup that had a rotary table sitting on the longitudinal table. It's rotary motion was connected to a gearbox, and then onto the table feed. They used this to cut some forms of worms, and also tapered cam surfaces on ID's and OD's of round objects and faces too.

Last edited by Dixie Gunsmithing; January 27, 2014 at 10:48 PM.
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