notebook: cutting raceways
TRX
August 17, 2013
Older Mausers with forged receivers had the bolt lug raceways broached. This
was done with spear-length broaches pulled by machines the size of cars. For
production work, it was fast and inexpensive.
Most of you have probably seen the PDF from the guy who modified some standard broaches to cut Mauser-style raceways in a bar stock. One copy is here:
Benwood did it at home, but he had to grind standard (and expensive) broaches down to make them work for 98 Mauser raceways.
Back in the old days, any machinist would have looked at the raceways and said "this is a job for a shaper!" But shapers aren't common nowadays, and you'd need one with enough stroke to traverse the length of the receiver. It might also be possible with a slotter, which is basically a shaper stood on its nose, normally used for making keyways and splines. But those are even more rare than shapers now.
Nowadays, the usual method of doing the raceways is with EDM. The going rate seems to be in the two hundred dollar range, which isn't a bad price considering how much metal has to be removed. But unless you have a big enough EDM machine, it's something you'd have to farm out.
Recently I've become seriously interested in rifling cutters, and just now realized that that's another process that might be used. The Voices proposed something like one of those DIY power hacksaws shuttling the rifling head, and further noted that a 5/8 to 3/4 inch rifling box would be a whole lot easier to build than something in .30 caliber. And since the receiver is short compared to a rifle barrel, you could take a righteous chip out with each stroke.
Yes, it would be a whole lot of work, and not particularly fast; I present it simply as "yet another way." Benwood had to grind standard broaches down to match Mauser specs, but if you designed your receiver from scratch to use standard-broach-sized raceways, you could skip all that.
When Hiram Berdan designed his No.2 rifle, which later became the Mosin- Nagant, he turned the raceways 90 degrees and simply milled a slot all the way from the tang to the receiver ring, leaving only a 3/4" or so section at the top and a tiny section at the bottom that had to be broached or shaped. James Paris Lee also used vertical raceways on his designs (SMLE and earlier), and moved the lugs to the back so no broached sections were needed at all, though he did have to do some A-axis milling to cut lug pockets in the sidewalls.
Some modern rifles use a "fat bolt" system where the bolt body is the same diameter as the locking lugs. This means the raceway is just a round hole. A fat bolt design generally has some kind of guide track and lug arrangement to guide the bolt handle when the bolt is opened, so it doesn't just flop around. I can't think of an example of a fat bolt gun by name at the moment, but it's a popular layout for benchrest rifles.
I'm getting really bored with remodeling the house, and I'm in need of some serious shop time...
TRX
August 27, 2013
I was just reading Otteson's "Actions and Triggers" pamphlet. It's from
1980, and about benchrest stuff. One of the actions he described had the
raceways done with a rifling head on a specially-built machine.
He said they got an excellent surface finish, too.
A benchrest maker that does a handful of receivers a year is probably about the limit of where this method would be economical, though. After that, having proper broaches made would be a better tradeoff in investment vs. time.
update: That was Stolle who used the rifling setup to do raceways.
TRX
September 06, 2013
Further information from Otteson: The early Hart benchrest actions had the
raceways formed with a shaper, but the cuts were rough-looking because of the
way the cutter dragged on the return stroke. So they went to using broaches.
Also, he says the Remington 788 (rear locking, multiple lugs) had its lugs hammer-forged into the receiver over a mandrel, just like many companies make barrels.
TRX
September 06, 2013
I've been keeping an eye out for a small shaper for years. I haven't found
anything much under $500 by the time shipping or gas get involved. But I'm
still watching...
The "fat bolt" designs are usually multiple lug, like the later Weatherbys or some of the exotics, in order to get enough bearing area to avoid lug setback. Unless the bolt is *really* fat, the lugs wind up being rather shallow.
The fat bolt eliminates a lot of machining hassles, but it gives you a new one - the receiver walls are farther away from the cartridge, so they're of less use to guide it into the chamber, and the bottom of the bolt is now in the area where the magazine lips once were, before they got moved down to clear the bolt. Neither is an insurmountable problem, but apparently it's not easy to get right.
Feeding is actually a lot simpler on an autoloader. The bolt snaps back, then snaps right back forward with a cartridge. You don't have to worry about guide ribs or Mauser-style "controlled feed"; the breeching happens faster than the eye can see, and there's no time for a loose cartridge to go anywhere. The bolt rifle guys make a big thing about turning their rifles upside down, working the bolt slowly, short stroking, etc., to *try* to cause a misfeed, then use that to "prove" that the rifle at hand is either the be- all or junk.
TRX
September 16, 2013
How did Mauser cut the left raceway on the model 98, since one end is blocked
by the inner collar?
In Colvin and Viall, page 58, the 1903 Springfield used a rifling head to cut the raceways even though it didn't have the inner collar. There are detailed drawings of the rifling head. They refer to the process as "shaving."
"Cut Data - Strokes of machine, 30 per min.; feed, about .003 in. per stroke; hand feed. Coolant - cutting oil, 1/8-in. stream. Average Life of Tool Between Grindings - 250 pieces."
So Mauser probably used a rifling setup.
TRX
September 16, 2013
That's basically how the Maadi-Griffin receiver is done. The plans show one
and two piece variants; the two piece one has the locking lugs and barrel
threads, so it's basically a "barrel extension."
Note that besides some machine guns and AR-15 and AR-10, many of the newer Browning rifles also use separate barrel extensions to hold the locking lugs. With a barrel extension, the receiver is just a bolt guide and bracket, like the AR's aluminum upper.
If you don't want a round action, you could mill the raceways in left and right halves, then bolt, braze, or weld them together.
The Stolle Panda benchrest actions were aluminum, raceways and all, with a barrel extension holding the locking lugs. If you anchored a "chassis" stock to the barrel instead of the receiver, the receiver could be extruded aluminum, plastic, carbon fiber, or even wood, if you were trying to make a point. If the recoil load isn't going through the receiver it doesn't need much strength.
TRX
September 17, 2013
The problem with a shaper is that you're trying to cut a 3/16" deep, 3/8"
wide slot, with the bottom being about nine inches deep in a .70" hole. The
cutter support bar can't be much more than 1/2", which means there's a lot of
flex down in the hole.
Otteson's "Benchrest Actions and Triggers" says a couple of the benchrest action makers started off slotting, but the cuts were rough and they eventually went to broaching. And those were short actions, and lacked the long rear tang of the Mauser shape.
So, I don't think Mauser cut the left raceway with a slotter or shaper. Granted I don't have any experience running one, but lathe tools sure don't like that kind of stick-out, and they're only making a tiny cut instead of trying to pull a 3/8" chip.
TRX
September 17, 2013
Put the bar and cutter between centers and mount the receiver to the
carriage? Hmm, I don't see any problem with that. And it could be done on
any lathe as long as it had a bit more than 2x the receiver length between
centers.
If you got too much spring in the bar, you could run a narrow cutter and just rotate the receiver a bit to widen the channel.
TRX
September 18, 2013
Quote from: Boltlug
The raceways weren't machined out in their entirety; The receiver was rough
forged, and then the raceways were probably just cleaned up.
I don't know about the Mauser, but the Springfield forging just had a rough center hole and the entire raceway was "shaved" as they called it.
The Parker-Hale "Mauser" raw receivers on Gunbroker a few years ago were castings, and the Ruger 77s are castings. Ruger runs a broach through for clean-up.
TRX
September 18, 2013
Quote from: viper dude
The raceways on my FN '98 Mauser receiver are broached full length through
on the left side. The right side raceways are broached or shaped to an
undercut at the barrel thread area.
The broach through is so the non-rotating extractor can reach the extractor groove on the cartridge, which is on the barrel side of the inner collar. From what I've read, FN has made 98 actions with the traditional C-shape collar, a "()" shape where they broached both sides, and no collar at all.
I'm guessing the speed and cost savings of the broach were the deciding factor.
TRX
September 18, 2013
Quote from: Hllrsr
Aren't the Ruger 77's a rear lug action? I pretty sure the 77/357 I was
looking at recently was.
The bolt body is factory assembled in 2 parts, and only the rear section rotated for locking.
I bet you're thinking of the Remington 788, which I think was discontinued a few years ago. That was a rear locking design with multiple lugs.