Barrel lapping

[uscra112]

Yes, I can see the problems with the button process would be far worse than those of a thread mill or your sinebar machine or the geared process. I'd believe an order of magnitude.

Since I now earn my crusts and sour milk by engineering turnkey projects for the world's premier builder of CMMs, it would be fun to hear how the twist measurement is instrumented. (Not that I don't know how to record rotation as a function of displacement.) A slightly undersize (and rather short) button on a rod, passed thru the bore with an encoder on the rod, maybe?

Since I've got you on the hook, and you obviously have the professonal creds - is it current practice to let the lap actually extend thru the muzzle end as a convenience, knowing that the fixtures will be cut off? The conventional wisdom of ages being that the lap was to be held short of the muzzle, I've always followed that dictum. But then I'm always lapping an existing bore or a rebore, not making brandy new ones. I even go so far as to do my charging at the breech end. Which means that about 3 inches of the lap never gets charged directly.

Now, about this rifling machine - you are still USING it? Send me some pics! (Lots of people collect guns, not many of us collect machine tools, but in a small way I am one of those. Makes it awfully hard to relocate, let me tell you. ) I'd have thought even a rifling machine would be CNC by now . . . .

[mhb]

Yes, I'm on the hook, but I'm gonna wriggle just a bit.
You have the idea ref. measuring the pitch of rifling: a contact point on a long rod rotates a digital encoder, which records angular rotation per linear unit of travel, which data can then be used to calculate incident pitch at any point along the bore.
In lapping new barrel blanks with the fixtures still in place, I charge the lap at both ends, and the lap does protrude some inches for the purpose, and at the end of each stroke, but the charging of the lap is only done once per lapping cycle, and the charging of both ends merely insures full abrasive coating of the entire length of the lap - I do remove the lap from the breech end and rotate it one groove (per number of grooves) and repeat the cycle until each orientation has been lapped with the same grit and the same lap.
As to the rifling machine photos - here is where I'm gonna wiggle a bit -
If you visit Benchrest.com, and look-up the extensive discussion listed as 'A Pratt and Whitney Sine Rifling Machine', started 2-19-06 in the Centerfire BR forum, you will find some very nice photos and a VERY extensive discussion of the machines and processes, much of it a note comparison between myself and the barrel maker at Border Barrels in Scotland.
mhb - Mike

[uscra112]

Thanks for the info about that thread, Mike. Made me all nostalgic for the machine tool business all over again. I'd heard of them, but hadn't ever seen any pictures.

I heartily agree that a "CNC" rifling machine would be easy enough to do. No 100K rotary head involved. (They don't cost that much anyway - I've had to price them for a project not 3 months ago.) A decent heavy 12" swing lathe carcass with a nice long bed would be a good start. I seem to remember some American Pacemakers that fit that bill. And you wouldn't even need a full-blown CNC controller - every PLC maker I buy from today, which is to say Fanuc, Allen-Bradley and Siemens, makes a low-cost PLC with servo management capability. The program is just two G01 blocks, repeated over and over, unless you want gain twist.

I once converted to CNC a Gould & Eberhard rack hobbing machine that had been built in 1898, and I'd consider it a doddle to convert one of those P&Ws, but I also wouldn't do it. Looks pretty wonderful just the way it is. I doubt I could actually improve it much. It might make setup quicker, but that's all.

Something I've observed over the years - in the period from Civil War to about WW1, the real smart guys were as often as not employed in the precision mechanics field, whereas today they're recombining DNA or figuring out how to mash another million junctions onto a computer chip. No wonder we got so much brilliant mechanical design in those days.

PW

[mhb]

The P&W sine bar machines have been the backbone of cut-rifled barrel making since , well, a long time ago. There are CNC rifling machines in existence, and working, but they DO NOT MAKE BETTER BARRELS. The machine in our shop originally went into service at Springfield Armory in 1917, and has been civilianized since just after WW2. It's true that these machines are now very difficult to find (at least, any that the owners are willing to part with), but, with care and proper maintenance, they can hardly be said to wear out. As I said in that other discussion, if I wanted a new machine, I'd try to have the Chinese build an exact copy of the P&W, which I believe they could do at reasonable cost and of good quality. FWIW, when I toured the Mauser museum in Oberndorf (years ago), they had a nearly identical machine on display, but built by Ludwig Loewe.
I'd advise against gain twist, for several reasons - but that is for another discussion.
If you find yourself in this corner of SE Arizona, stop by - I'll arrange a 25-cent tour of the barrel mine for you.
mhb - Mike

[uscra112]

The old machines are better sometimes. I once sold off an old Pratt & Whitney engine lathe (the kind with the stepped flat-belt-pulley headstock), and bought a Monarch 10EE long-bed. I had the guys in the grinding shop regrind the bed for me, and I fitted it all up myself, but could I get the same surface finishes off it that the old Pratt could do? Not on yer life. My current home-shop barrel lathe is a plain-bearing big-hole South Bend that I paid a pretty penny for, because I was damned if I was gonna let it get away.

'nother story - I went to see a guy in North Carolinas once who had the damnedest aerospace shop I've ever seen. Drive up to it and you'd have expected to find cattle inside. Most of the floor was DIRT. He was making rings for Pratt & Whitney Aircraft, and his shop was a rank of ww1-vintage Bullard Spiral Drives, machines with the scraped Babbit pit bearing for the table. He swore by 'em - said nothing else could give him the surface finishes he was getting. Wanted us to rebuild some for him. My boss almost fell over laughing when I turned in my report. We didn't even quote the job, but later on there was a competitor in Connecticut that was actually converting them the CNC and making a profit.

There's whole a lot to be said for the dampening effects of plain bearings and all that cast iron. And with that long skinny rod holding the tool, you need dampening!

As a pro, what do you have to say about the old Pope groove form?

[mhb]

Harry Pope credited his lead-bullet rifling form to George Schalck (who almost certainly did not invent it, either). It would be difficult, if not absolutely impossible, to dream-up a rifling form or geometry which has not already been tried at least once. Pope himself used the nearly flat-bottomed groove of original bore diameter at the groove center (and, of course, deeper at the corners), very narrow lands and gain pitch rifling ONLY for his lead bullet guns. Barrels which Pope made for jacketed bullets and high velocities were rifled with conventional, concentric form rifling. I personally feel that the gain twist for a muzzle loading rifle (as Pope's best were - breech-muzzle loaders) is wrong in principle, requiring the bullet to be forced from a faster to a slower pitch, and then fired the other way - consider that the forces on the engraved lead bullet's surface are shearing ones due to the pitch change once engraved, and that doing it again under high acceleration in the other direction cannot be good for the bullet's integrity - I think the excellent accuracy he got with this method is due to the forgiving nature of the soft lead alloys used and the relatively low velocities involved.
Now, no one admires or honors Pope more than I do, but my professional opinion is that the unexcelled performance of his barrels and rifles (at the time) was due more to his painstaking craftsmanship than to the design of rifling he used, or the gain twist. His conventional pattern rifled barrels shot as well as any made at the time, too. My belief is that his barrels and rifles would have been as good had he used conventional and fixed-pitch rifling.
Gain pitch rifling has always had advocates, but, in rifle barrels (as distinct from certain artillery caliber applications) offers no demonstrable advantage in accuracy, and introduces several disadvantages in manufacture (the gain twist rifling cannot be lapped in the conventional manner, and, worse, the width of the grooves and lands changes with the pitch because the aspect of the cutting tool changes with the angle of the cut, while attempts to produce buttoned gain-twist barrels fare even worse because it is necessary to try to drive the button at pitch rates other than the basic one built into the button - and given the already unsteady nature of the button's travel in many cases, you can imagine the variations possible in a gain twist buttoned barrel).
At bottom, I feel that there is no magic form or geometry of rifling - and that none is better than the conventional concentric pattern in terms of accuracy - the quality of the barrel depends, as always, on the experience of, and qualiity control exercised by, the maker. Then, too, the barrel is only one component (a very important one, true) of the system made up of all the components of the rifle and its ammunition, and it is quite possible to make a very poor rifle with a very good barrel, or get poor performance from a basically good rifle due to bad ammunition or shooting skills.
Just my opinion, as stated, and FWIW.
mhb - Mike

[felix]

Mike, I am in total agreement with your synopsis. ... felix

[uscra112]

I was thinking more on the groove form, not so much about the gain twist. Aside from all the reasons that have been written about, I've been pondering for a long time, wondering if it didn't perhaps give him one more way to see that all the grooves were uniform depth. If one were to blue up the bore (with, say, prussian blue, not layout ink), then scrape until the cutter just removed the band in the middle. You see where I'm going? He was after all doing it all by hand.

[mhb]

IIRC, was made (by Pope) from a lathe - I don't recall that it was hand-operated, and think it unlikely - but it should have made no difference whether the cutter was power driven or pulled by hand. If, as I suspect, he took one or more cuts at every groove at each height setting of the rifling cutter, until it stopped cutting (which is the way it is properly done, whether using hand or mechanically powered rifling machines), and inspected/measured the groove depth as work progressed, he would have had no great problem in achieving and maintaining the desired depth of cut. It is much better practice to rifle the barrel in this fashion (which requires only that the barrel blank or rifling rod be indexed for each groove between passes - exactly what any rifling machine is made to do) than to attempt to rifle separate grooves to the same depth, finishing one before beginning on the next. This is best practice, and I don't believe that Pope would have done it any other way.
mhb - Mike

[Bass Ackward]

Every year we have a pioneer festival and there used to be an elderly gentleman that cut rifle barrels by hand power. Now that I think of this, I haven't seen him for awhile.

Interestingly enough his lubricant was vintage for the day as well. He used bear grease. Made muzzle loaders just as accurate as can be purchased today. Especially, considering all the room for error.

I would say that I agree with pretty much everything that you have said. Great class .... so far. It's what I am interested on what you might yet say that prompts this. You don't mention of inducing stress into steel by various rifling methods. This stress can affect your chambering dimensions and trueness with the bore when the rifle is made.

After the gun is assembled, stress can alter chamber dimensions and angles and momentum paths for lead, bore dimensions and twist rates, plus even elongate and warp the straightness of bore. And thus how temperature affects this stress / change. And then there is memory to consider. None of this hand lapping can anticipate.

All this change can and will change vibration patterns, especially as steel moves and stress is relieved under the heat of firing. More movement in stress areas will alter twist rate angles and expose areas of the bore that previously may not have been exposed to bullet travel depending on how well the initial barrel was made. Any change will remain .... in an unlapped state that the bullet will correct (wear away)following it's new path.

Internal dimensions and finish are keys to lead, but only two pieces of the accuracy trifecta. And in my opinion, for a barrel to be fully (lapped) broken in for lead, a barrel must go through an entire climate cycle of your area to fire lap under all conditions that the barrel will be exposed to, before .... you know it's true condition / potential. Because this lapping, in the cold, will then again slightly alter your warm weather conditions (dimensions) for lead again as a walking barrel returns to a warm temperature state and previous changes were worn away.

I believe cut rifling results in the most temperature stable situation as it induces the least stress of all methods. So temperature affects it less, if the blank was made uniform and stress relieved before you started. This rifling method results in internal dimensions that remain the most stable and true to what was origionally cut. Once fire lapped, (worn away) cut rifling results in the best conditions for lead usage and accuracy year round.

[mhb]

I think you are pointing to the importance of stress (or lack therof) in the finished barrel - and I agree that it is important both in the manufacture of, and performance from, a rifle barrel.
Any manufacturing process affects the steel's stress level - whether in the steel mill or in the barrel making process. Most barrel makers order barrel-quality steels of specified physical characteristics - and most are dependent on the steel supplier for the accuracy of the specification - really careful ones have the means to test the physicals in-plant, and any batch which does not meet the requirement is returned to the supplier.
Practically all modern barrel makers stress relieve the bar stock before machining it at all - some, like the button-rifled barrel makers, must also stress relieve the barrel after rifling and before contouring (rifling must be done in the full-diameter state, for reasons of induced stress previously discussed, while machining the rifled blank in the stressed state can result in undesired variations in the bore and groove dimensions). Some stress-relieve the finished blank yet again.
Hammer-forging is a different animal - the external dimensions of the finished barrel can be forged at the same time as the bore, so no further heavy external machining is required - and the stresses induced in the finished barrel, while severe, are, theoretically, evenly distributed - hard experience makes me recommend against external machining of hammer forged barrels after they leave the maker.
That leaves traditional cut-rifling and broaching, which are merely different approaches to achieving the same end - formation of the grooves by removal of metal with cutting tools. In both cases, the stresses induced by the small amount of stock removed are quite small, and, assuming the blank was not already heavily stressed, can be ignored.
In our shop, the blank is stress relieved before any machining, the barrel blank is fixtured and drilled, then contoured to near final external dimensions, reamed, and rifled. All external machining operations are done under coolant flood, temperature rise during drilling and reaming is controlled by cooling the flood lubricant and rifling is done so slowly under continuous lubrication that little additional stress is induced. Finally, the exterior dimensions and finish are established by grinding between centers and under coolant flood, never becoming heated at all (and, of course, removing little material). These procedures result in a barrel which is straight, of very uniform internal and external dimensions, with good surface finish inside and out, and with little residual and less non-symmetrical stress - such barrels pass the ultimate test, which is shooting accurately under all conditions.
As to external factors affecting the straightness of the barrel in use - well, maybe. Uneven heating from exposure to the sun can induce point-of-impact change, but usually will not affect group size.
Good practice on the part of the riflesmith in fitting and chambering will pretty much preclude any sources of induced stress which might negatively impact the accuracy or stability of the assembly - with poor practice, all bets are off.
I do believe that cut-rifled and broached barrels are less likely to be stressed in manufacture (with proper care) than are buttoned or hammer-forged barrels.
The old-timers were very conscious of varying performance of given loads between warm and cooler weather - they attributed this (and I believe rightly) to the lubricants used - the entire range of ambient temperatures encountered in shooting would hardly have any measureable effect on the bore and groove diameters, and certainly not enough to noticeably affect the grouping ability of the barrel, given loads and lubricants proper for the season.
mhb - Mike