Truing a barrel.
I’ve been reading up on this. The general tactic seems to be to establish how far out of true the bore is to the OD and then use a four-jaw chuck to accomodate the mis-alignment. Which is fine for the job at hand, but one is still left with a barrel that has it’s bore out of line once the barrel is bushed and mounted in the frame.
So I propose to make a .25 cal push-fit brass insert for the chuck-end and use a live centre at the other end, so that the barrel revolves true to it’s bore, and then turn the OD the full length of the barrel ( a bit will be missed, due to the constraints of the cutter, but I’ll lop that bit off).
In such a case, by how much might I expect the centre of the barrel to flex, given a starting OD of 15.5mm? I have a follow-rest, but I don’t like using the gouging POS. If I take very light cuts (like 0.10mm) will there be any meaningful flex at all? Overall barrel length is 23.5″ which I can fit onto my lathe with about an inch to spare.
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We both knew it was only a matter of time Jerry !!
Really looking forward to this one. BSA has a very good rep’ with its barrels.
Lang, bless him, is gonna do the work on it for me, including new bushings. If its anywhere near the usual standard of his work, its gonna be good… 8)
Hope your boys are still slaving over a hot mold as I’m hoping to be ordering some more slugs soon…. 😉
Aw heck Paul, you know you want a new .25 with proper rifleing anyway!
I’m really enjoying this thread and some of the others in this section. Makes me think I might make something out of metal instead of wood some day and avoid at least one mistake…
As long as he doesn’t test the limits on MY barrel…. 😆
hahahah, that’s the spirit lad. More power to you, and good luck. Nothing wrong with trying to make something work. I’ve chased many many such ideas in my time. Not all pan out, but a few do, enough to keep you coming back for more. Just remember; old timers get to be old timers for a reason. Some are really good. Some are just accumulations of algorhythmes from other, wiser men. But none of them get to keep doing things that waste time and metal. Turning on centers is time tested, proven without question, and something I would tend not to second guess very often. But, we all learn from our failures as well as our triumphs. If you never test an item to failure, how will you ever know where the limits are?
walt
I’m thinking if the pilot-bars are tight enough in the barrel’s bore, plus I squeeze a bit with the wheel on the tailstock. there’ll be enough grip to take very light cuts along the barrel.
Yes, my lathe did come with a drive-plate. The dogs look good, but it looks like using that method would prevent a considerable amount of the barrel from being machinable. I suppose a person would flip the barrel in that case, but then he’d have to match cuts to get the exact same OD on the full length.
Looks like I’ll be bucking the trend again though, unintentionally on this occasion. I’ve already ordered the tailstock-chuck, so I’m commited to the idea now. What’s the worse that can happen, the barrel will spin on the pilots and lock still when I try to cut it. And bend like a pretzel, knowing my luck. 😆
?? You don’t need a live center, it’s just convenient. A dead center will also do fine. You can just make one up, or buy them since MT2 tapers are cheap.
What are you using 2 chucks for? a pilot in each end of the bbl will get you a nice indicating setup, but you can’t cut on it. You sound like you are trying to approach the method known as ‘turning on centers.’ This method uses a live or dead center in the tailstock, and a dead center on a drive plate instead of the chuck. You attach a lathe dog to the bbl at the drive plate end, which is an L shaped piece of metal that has a hole though it at one end. Each dead/live center goes into the bbl hole to absolutely center it; the lathe dog is used to tighten on and provide the turning motion to the bbl; it dogs into, or locks, into any of the slots provided on the drive plate. This ensures that shafts are turned absolutely true to center. Works great for bbls. You bought your lathe new, yes? It should have come with a drive plate, it’s one of the most basic forms of the lathe setup and frequently in old times, was more commonly found than use of a chuck due to the accuracy of setup it allows (chucks are always off by a bit and, I’m sure you’ve found, can’t even be depended upon to regrip something the same way each time). This was prior to collets. Collets now allow very uniform gripping and rechucking, but turning on centers is still unparalleled for barrels.
walt
I tend to try and work around difficult stuff by finding an easier way. The 4-jaw is a perfect example, and I’ll be avoiding that bugger for as long as humanly possible. 😆 I’ve found an alternative to a live-centre for the tailstock too, it’s a small 3-jaw chuck on a 2MT arbour. So I’ll have a piloted-bush in each end of the barrel, squeeze it all up and spin. I span my .22 BSA up today and it was very clearly out. If the .25s are the same, or even half as much, they’ll be getting turned.
The 4 jaw is another world entirely. Difficult to learn and REALLY taxing on one’s patience. But, you may as well learn it, you’ll need to sooner or later. There are lots of projects out there that will require you to learn how to use it eventually. Also, the 4 jaw hole down the center is frequently larger than your 3 jaw, so sometimes you may have to use it.
As to perfect, well, the 3 jaw method is not technically perfect, but it’s damn close. Close enough that you wouldn’t be able to find any difference with a micrometer. If perfection bothers you, then just run 2 cycles on it. The first time you do it, it gets very very very close to perfect. Run it (the entire process) 1 more time and each time, you gain an order of magnitude in centering accuracy. I’ve never found it necessary to do it again, my chucking accuracies suffered more introduced error than I was able to gain from any increase in accuracy from re-doing the process again. With the new 6 jaw, that may change as it is considerably more accurate than my old 3 jaw; but time will tell.
walt
Thanks Walt. So that’s how it’s done with the 4-jaw. I never would have thought there’d be enough movement in the pilot-bar. I can only imagine how long it would take to fiddle with the 4-jaw before I’d take a large hammer to something. 😆
The 3-jaw method sounds a lot easier but, as you say, not perfect in terms of accuracy. But what are we talking about here, thousands of an inch? I guess I’m kind of expecting to see the OD wobbling around the bore’s axis like a pissed snake when I spin up the barrel, but if the Lothar’s aren’t that bad the BSA’s will probably be insignificant (also stated by Voltar).
Either way, I’ll be taking extremely fine cuts. Apparently the bore’s dimensions can be screwed up if too much heat goes into the barrel while turning, depending on the method that was used to cut the rifling. IIRC a button-rifled bore will expand, a cold-forged will reduce and the third method (forget it’s name) it usually remains unchanged. Plenty of pitfalls to catch the unwary with this malarkey.
Sort of ties in with your other post, Lang. If you ask different gunsmiths you’ll get a different answer as it relates to alignment of chamber when cutting for high power. Since airguns have no chamber, the point is moot but still has relevance for alignment issues.
My order of LW barrels has not yet come in. I’ve ordered them before, but only as onesies or twosies, and they were always OK; not perfect, but reasonably close to centered. Keep your cuts small, you don’t want to push the barrel away from the cutter. I don’t usually run a follow rest unless the barrel is a super lightweight. Cut only what you have to in order to true it and no more.
The way to align by bore is as you surmised; with a 4 jaw. You make a little jig, a bar that holds a pilot bushing (the ones that go onto the ends of chamber reamers… they come in very small increments, fractions of a thousandth) and insert the pilot into the bore. The bar with pilot is held in your drill chuck on the tailstock. The barrel is chucked in your 4 jaw. You indicate on the bar, which is stationary, and turn the 4 jaw. Zero out the bar movement.
For a quick and dirty without the pilot and using only a 3 jaw scroll, insert the barrel all the way leaving only a small piece sticking out. Face off. Now you have a face that is reasonably true (albeit 90 degrees) to bore axis. Debur at the bore. Rechuck, taking barrel out almost all the way so you are holding only an inch or less. Insert live center at other end and if there is no swarf under the live center, you should be close to centered on the bore; BUT ONLY AT THE LIVE CENTER, not at the chuck. Take a light truing cut on the live center end just to true; now you have outer diameter true to bore at live center end. Take out, flip barrel over and repeat. FAcing cut, then true diameter. You can only chuck it as deep as your facing cut because that is the only true surface. But now you can cut the entire length of the barrel and it should be reasonably true…. but you need to be careful. Reasonably true is a relative term. Bores are not always straight. This truing method will get you a bbl with OD centered on the bore at each end, but if the bore wanders mid barrel (and yes, they all do, just a matter of how much) this centering is only applicable at each end, not the middle.
walt
never worried about it and never had any problems. Mind you I buy mostly HW barrels and they all seem to be perfect 🙂
even the lowly Crosman barrels are central.
Walter….
Another thought….. if it truly bothered you then mount the bushings oversize and turn them down with the bore running true.
Thanks Walter, nice tips. 🙂
I’ll run the barrels on a dial and see how far they’re out, if at all. Funny thing is, if they’re way out they’ll really need to be done, and if they’re out only a bit they’ll be easy to do simply for the sake of anal perfectionism. 😆
I want them perfect for the sake of a good crown (90-degree or otherwise), and I really can’t be arsed with all that four-jaw chuck malarkey. Don’t even understand how that’s done, to be honest, short of making a bushing that snugs into the spindle-bore and also has a probe that snugs into the barrel-bore prior to adjusting the chuck’s independant jaws. Seems far simpler, and better, to true the barrel by turning it.
Turns out I don’t have the live-centre I remember ordering, I must have sent it back for a refund (seem to remember it was too big for what I needed it for at the time and it ran rough as well). I’ll get another ordered.
I’m quite looking forward to it, it’ll be something new to master. 🙂
Not something I would bother doing. The barrels are close enough that there is little reason to do it. If there was a problem set it in the gun so the bore is located in the vertical plane with the bore pointing up at 12 oclock. Then use your scope adjustment ot fine tune it.
never worried about it and never had any problems. Mind you I buy mostly HW barrels and they all seem to be perfect 🙂
even the lowly Crosman barrels are central.
Walter….
Another thought….. if it truly bothered you then mount the bushings oversize and turn them down with the bore running true.
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Hi Paul,
Since Lang is working on the perfect live center or bore pin or whatever…, ask him to slug the bore for you as well. I’m certain Chris has not yet seen an order for a .25 BSA barrel and we NEED to get the sizing perfect the first time!