barrel making 101
I think I just met someone today that makes barrels in their back yard.... Got me thinking about how tough it really is?? I am a CNC shop owner but never been around gun drilling or rifling. Other than the tight tolerance that should be held, what is the procedure for this?
Shouldn't be anything for you. Buy the steel, cut it to length. Cut centers in each end, deep hole drill, lap, ream, cut the grooves, contour, and stress relieve. Ready to ship.
Are most shops using a specific rifling machine that is basically a rotary with a broach press? I am not real interested in making barrels, I am just curious of the process. I am kinda wondering how people could really get any accuracy without some pretty specialized machines.
Asked one guy how they mic the barrels he said nada, just test for accuracy when shooting. I was surprised. Looks like good barrels might be air gaged to =+/- 0001" but that still does not account for straightness. I could imagine the broach process could get things a little iffy.
Barrel steel is special as it cant have any real imperfections or it can become a grenade. Depending on the type of production used it can be hammer forged like most AK type barrels. Where lengths of barrel grade steel are cut to length and bored, have a rod with the rifling on it inserted into the barrel blank and the barrel is run through a swage with I believe 4 hammers pounding the short bar out lengthening it and compressing it onto the rifled rod imparting the rifling into the bore. After that the barrel blank gets profiled for whatever series or rifle it is intended for.
Barrels are also made using barrel quality steel bar stock using the button method. Stock is cut to length heat treated,bored and a hard button in the shape of the rifling is pulled through the bore until the rifling is complete. Then its chambered,profiled and heat treated.
Another method is basically the same but uses a cutter to cut the rifling one pass up and down the barrel at a time very slowly cutting each land and groove. I believe this is the most time consuming.
Like you, I too have wondered just how they do it. I know in the old days, it was a "Drawn on Mandrel" operation. They actually had a mandrel that had the rifelings on it and they would heat the barrel and beat it on the mandrel. To make things even more interesting........some barrels have progressive type rifelings that change helix angles as it gets nearer the open end of the barrel (opposite the blind end or bolt end). In a case like that....how would they retrieve the mandrell? HUMMMMM?
There is a pretty good 101 on the Border Barrels site. They've re-instated it after years of absence.
and a word of warning - avoid buying Harold Hoffman's books, there are links posted in old threads here to download scans of Vickery and of Howe, which are both excellent, and they appear to be where Hoffman plagiarised his information, he also appears to have copied a piece about barrel steels that appears in one of Ackley's handbooks (which are of middling quality IMO), again without citing his sources. Hoffman's contribution to the books in his name appears to consist of poor spelling and grammar, which you could forgive, if he'd only cited his sources, for you to check against.
Steel selection depends on the pressure / velocity of the round.
For black powder, and cartridges with black powder type pressure levels and performance, for example .22lr, free cutting mild steel is apparently adequate.
For higher pressure levels, jacketed bullets and higher velocities, or for lighter weight -where you want to use reduced wall thickness, then the pre hard and stress relieved 4140 seems to provide the combination of strength, wear resistance, good surface finish and price needed, along with taking hot bluing well. I've read widely differing opinions of 1350 (stressproof), apparently it is even freer cutting, but wall thickness may need to be increased to compensate for lower yield strength.
For stainless barrels, 416R (R for stress relieved) provides the strength and free machining. When I was organising a gunsmith to get a one off barrel made by Border Barrels, the smith reckoned that Border preferred to use 416 over 4140, due to the 416 being easier on their tools. Disadvantages with stainless are greater density, and poorer heat conduction. 4 series (martensitic = hardenable) stainless also suffers a brittle transition around about zero degrees Centigrade. In practice, the embrittlement doesn't seem to occur until you are into big negative Fahrenheit figures; something to bear in mind for winter shooting in very frosty weather.
Design assumptions for barrels are that the stress is pseudo static, and that the max allowable hoop stress on the inside of the bore is the yeild stress of the steel! in practice you'd put a factor of safety of 1.5 or so on that.
For air rifles, I've read about amateur builders using free machining brass, as high tensile strength and resistance to erosion aren't needed.
Industry standard would be a commercial Gun Drill, although before Pratt & Whitney developed what we would recognise as a gun drill, people were drilling with D bits of various descriptions, used through hollow centres. There is an article in an air gun magazine describing home barrel making experiments peck drilling with a D bit in a 7" swing Myford lathe. Guy Lautard also sells a DVD showing a home built drilling and rifling machine.
Viktor Astakhov's site has excellent info on self piloting drills and drilling, and he occasionally contributes on PM.
In the past, barrels for the trapdoor Springfields were hot rolled over a series of mandrels, and in the more distant past, barrels were also forge welded from twisted strip, in a spiral.
The Border Barrels article shows a modern multi flute reamer, unless things have changed, I understand that these use Machinery's handbook figures for radial spacing of flutes to avoid chatter.
Traditional methods of reaming used "spill boring" with a square sectioned "armourer's bit" backed up with a whittled piece of bamboo ("spill"), and adjusted by inserting pieces of fine paper between the steel bit and the wooden "spill".
Supposedly, in skilled hands, spill boring could hold tolerances of a tenth of a thou.
Pictures of the style of tool are shown in older books, for example WW Greener's "The Gun", where it is shown in connection with choke boring shotgun barrels. Lubricant was apparently peanut oil. I don't know whether this was for some specific property of the oil, compared to other oils, or whether it was just a tradition that had never been questioned.
single point Cut rifling cutter boxes are well shown in Vickery and in Howe. A search of patents will turn up some later refinements, along with patents by people like Gerlich and Lovely (yes, that is a surname!) for cutter boxes for rifling squeeze bores, and the rifling machines to go allong with them.
Positive and negative rake
There appears to be a lot of repetition of the mantra of "negative rake" for rifling cutters. I can't remember whether it is Vickery of Howe who clarifies this. Apparently both were used. Negative rake was termed "scrape rifling" and removed hundredths of a thou a pass or less, but supposedly gave a better finish than the positive rake "cut rifling", although there doesn't seem to be a problem with cut rifling, there may have been a difference with hand powered operation.
getting a twist
constant pitch rifling is equivalent to the print you'd get by rolling a cylinder over an ink soaked thread layed at an angle, and this was the principle used to lay out the helix for hand cutting a wooden rifling guide, fore runner of the more modern rifling machine in the Border Barrels article.
The helix feed can also be generated by a rack (this is the principle shown in the Lautard DVD).
Both of those are fixed pitch
Faster or slower pitches can be achieved using a "sine bar", which is equivalent to altering the angle of the ink soaked cotton thread, except you can alter it as often as you like.
The sine bar is usually straight, but, if you want a "gain" twist, you can rig up a curved guide. A Lindsay compilation reprint "Ordnance 1900" from a few years back, has an article about improvising gain twist rifling on a large scale, for a shop made artillery piece during the siege of Kimberley, in the Boer War.
Normal practice for single point cut rifling is to index to the next groove after every pass of the cutter, and to advance the cutter after every full rotation of the barrel, that way, each groove should end up identical shape and depth.
Multi point cutters and broaches were attempts to speed up production of cut barrels, well done, they were probably adequate, but as cut rifling is now for the premium market, the cost of the broaches and of maintaining them would be prohibitive, and single point cutting is used.
A micro stepping stepper motor could probably be used to rotate the barrel for generating the helix, and for indexing it too, but old fashioned mechanical means seem to work very nicely, and the poor torsional stiffness of the rod carrying the rifling cutter box would likely limit any accuracy gains for either mechanical or numerical rotation.
Two approaches were developed during the Second World War: hammering over a mandrel was tried in Britain, but it seems not taken to full development, and the rifles with "dollied" barrels were re barreled. In Germany, the system was fully developed to hammer forging.
US developments culminated in Merle H ("Mike") Walker's patent for button rifling, where an over sized hard metal "button" with the impression of the rifling and its twist is pushed or pulled through the the lubricated bore.
Hammer forging, in its fullest form, allows the chamber and throat to be formed in the same operation as the bore.
Either operation will allow the forming of exotic shapes of land and groove, and the formation of many, many grooves or lands - if you want them to.
Both swaging operations induce severe residual stresses in the barrel, this can be used to advantage, for example turning down most of the barrel but leaving a section at the muzzle at full diameter produces a slight choke effect, used in some target rifles. the work hardening may also have benefits in wear resistance, and the residual stresses can be used to counter tensile hoop[ stresses during firing, allowing for a slight reduction on the weight for a given strength (a sort of one piece equivalent to the artillery gun practice of shrinking sleeves on)
For turning to say sporter profile, a swaged barrel would need to be stress relieved first, or the bore would become slacker towards the muzzle.
Last edited by Alpacca Fortyfive; 04-29-2011 at 10:00 AM.
SAAMI specs for the area of bore for each calibre are given in their pressure testing documents for rifle and handgun calibres. There are old copies scanned on "Steve's Pages".
Normal practice appears to be "Enfield" type, straight sided grooves and narrow lands with square corners.
Rounded rifling forms are used to allow an even thickness of hard chrome plating to be carried out, and allowance is made for the thickness of the plating.
It would probably be naive to expect hard chroming to build up perfectly evenly throughout the bore, without some lumps and some thin areas appearing. Chroming appears to be a nice touch in making guns soldier proof, and is a pleasure to clean, but would be out of place in a bench or varmint barrel.
The border barrels article has a very good discussion of lapping. It's value appears to be in conditioning the bore allowing it to be "worn in" without the throat area becoming "worn out" first.
Chambering, throating and crowning
These appear to be where problems creep in. I posted a link a few months back to an interesting discussion on Mike Belm's site, about the problems with current production practices in cutting chambers, throats and crowns and ending up with them asymmetric to the bore.
Great responses.....are the barrel makers making any money???
Or just barrels?
Of course. Go check the prices for a blank. The prices are held low due to the liberal application of the appropriate black magic for the individual barrel maker's process.
Originally Posted by S_W_Bausch
Well its actually simpler than you think. Most deep hole drills like the Pratt D and smaller will turn the barrel blanks at about 4000 RPM which is sort of a requirement to get proper cutting and chip flow. But it is possible to convert an old lathe. You need something with about 60 inch centers and you have to mount a chuck on the off side of the headstock as well as the standard chuck. You have to gear up the machine to rotate as fast as possible without the bearings flying apart. 4000 RPM is probably minimum but higher is probably better. You also have to regear the apron so that the feed can be reduced to less than .0005 inch revolution. The drill is fed from the apron. You also need a high pressure oil pump that will give you 400 to 700 psi flow to clear chips. The oil used is a high sulfur cutting oil. The rifling machine can be made as well. Buttoning is the best and it really takes very little pressure to draw the buttons. A hydraulic affair capable of pulling a couple ton or so is sufficient. The button must be twisted as its drawn through and thatís quiet easy to do with a geared sine or if your really cheep you can do it with a cable and drum sine. I have seen some rifling machines that use thrust bearings and let the button head turn itself but I can see that causing problems if something hangs up. Besides the sine is easy to make. Buttons drills and reamers are available from Danjon and Iím sure JGS or Pacific Tool would also supply you with drills and reamers. Is there any money in it? Of course not thatís why no one does it. Even at $425.00 for a shipped match grade blank you are just working for wages here in Canada. And there are a million guys selling cheap buzzard grade barrels already. One of the big problems is steel. Good double stress relived ordnance steel must be special ordered and they probably wont talk to you unless you are willing to drop $30,000.00. Sure you can make barrels out of a truck axle if your so inclined but if there is no consistency in your accuracy you beet yourself before you started. The trick to making them shoot is simple. Stress reliving after cutting to length, smooth holes, button rifled, double stress relieved steel of the highest grade and light hand lapping. I wouldnít do it on a bet. No money in it at all.
They probably have a better way now, but straightness used to be checked by eyeballing down the barrel. It's actually a very accurate method. They used to eyeball, then straighten with a fly press device, back and forth until straight.
Alpacca, it may be different in your area, but in the US Stressproof refers to 1144 that has been strain hardened and stress relieved. I believe the process was developed by Carpenter Steel many years back. I have read references to stressproof not being suitable for anything other than blackpowder barrels if at all.
Thanks for pointing that out.
Originally Posted by ulav8r
After you posted, I remembered a post about "stressproof" here.
I can't remember how it got there, but it was lodged in my brain that "stressproof" was a trade label used for 1350.
Another manifestation of the dangers of
"It's not what we don't know, it's what we think we know that ain't so!"
FWIW, Crucible steel manuf. (or use to) 2 hardenable stainless steels for rifle barrels, 416R and174SXR the later being very stable in heat-treatment, and highly corrosion resistant, both can be hardend to a min of 35 Rc.
There is a show in July in Kempton, Pennsylvania. These guys are only after the original methods of making muzzleloaders and strick judging of finished rifles. I saw a video of a guy who rifles his own barrels by putting a piece of paper under the rifling cutter for each pull until he gets the proper depth. He indexes and starts on the next rifling groove. Could not tell you how many cycles he goes through to cut all the grooves.
Dixon's Muzzleloading Shop - Home of the Gunmakers Fair
There is also electrolytic rifling. Similar to EDM. It makes things like gain twist simple, as all grooves are cut in a single pass. Its more expensive than button rifling, but doesn't leave any stresses in the barrel.
Is any one actually doing this? Last I heard it was a pie in the sky idea that extrude hone owned.
Originally Posted by gbent
I have heard of it used for rifling like features in "mud pumps" but not in actual barrels.
I recently got the deluxe in your face 3 1/2 hour tour by a well known button rifled barrel maker. They use mostly 416 stainless. He has a mix of newer and older deep hole drills and can do from a couple mm up to 20mm barrels for the military. The way he explained drilling them straight is the cutter follows the path of least resistance. 99% of the time that is right down the center as it is moving the slowest in inches per minute at the cutter than getting further off center. Then the drilled blank gets pull button rifled. That takes all of a minute and a half. He said one of the benefits of button rifling is it leaves the interior work hardened making them much more wear resistant. Then they go to the oven for stress relief. They hang vertically in a kiln at 1000 degrees per inch of thickness per hour. (2" barrel soaks at 1000 degrees for 2 hours) then is brought back to room temp very slowly. The stress relief is the choke point in the whole process. Then they get lapped and air gauged. From there they get profiled in a high speed cnc lathe at 4000-5000 rpm. Anything with detectable run out gets scrapped anywhere along the process. I saw more than a few barrels go to scrap at each station while I was there. He gave me a couple handfuls of culls to take home and machine on and dissect. The losses are all figured in. Every barrel is custom made to order. If it gets tossed they go on the back order list and is first up in the morning. The last step is fluting. Then they get shipped. That's it. They said all that crap about working on a button barrel after stress relieving and lapping is total crap. Every barrel is profiled and cut on after final lapping. All further machining is done cold so no stress is introduced. They are very stable and I did hear the words ,Aberdeen, Army, Marines and military a lot and they routinely test the barrels with multiple 20 shot groups to check for point of impact changes from heat and there is none. With almost no advertizing they are making barrels by the thousands and they go all over the world.
I went there initially believing the cut rifled barrels had an edge over button rifled barrels. I took a list and asked every internet objection I could remember for 3 1/2 hours straight and he patiently answered all of them and it came down to people repeating things that are not true. They air gauge them before and after and the internal bores just don't change when metal is cut off of them and they were pumping out a lot of pretty skinny tubes. He said no problem with fluting or reprofiling any of his tubes after it leaves the plant. If you send one back to them they will reprofile any of their own barrels to something slimmer. After scoping and slugging the culls I found they were nicer finished inside and straighter than some of the barrels I was paying for. When I left I cancelled my cut rifled barrel orders and ordered some custom profiles from them.