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.
All Replies
ive always just turned the barrel btwen centers.w/ a live cnter in the tail stock and a dead one in the chuck that you turn tru. every time you put it in and a drive dog on the barrl to keep it spining..
I said that to Lang’ about the expensive barrels having the final trueing down by hand and literally being bent by some old boys. He wouldn’t have it.
Somethings only the human eye and brain can accomplish to near perfection.
Ahhh, That makes sense!
😉
Mike
That’s a tough call. Anyone with a lathe can see where the outsides of a barrel are not straight, you can see it wobbling when you chuck it up. The LW barrels I just got were strange that way, there appeared to be a mild depression in several of them, running about 1/3 of the way down. Just a few thousandths, sort of like a crease running lenghwise, but enough that I had to take heavier cuts to get to clean metal. The trick is to be able to see if the bore is straight, and that is much much harder. If you hold it up to the light, though, you might be able to determine if the barrel is bent or if they straightened it according to the bore and the outsides are running out. That, by the way, is the proper way to do it; by sighting down the bore and bending the barrel until the bore runs true… they do this for high end guns, but I have serious doubts that they would have done this for an inexpensive airgun barrel. I could be wrong. The commercial rigs look like an old ship’s wheel turned sideways and mounted overhead. The wheel is mounted to a fine pitch screw base which bears upon the barrel. You sight down the bore and figure out which way the bore is turning and then gently massage the barrel using the wheel to precisely control how far you push on the barrel; but surprisingly, the best barrels come form old timers using nothing but sight alignment, no measuring tools other than their old eyes.
Lang, the out of true measurement is not as important as the differential between the out of true measurements from one end to the other end. If they are both out of true in the same axis and by the same amount, the pellet will still go straight out the extrusion. When one end is more off than the other, then the difference between the two measurements is how far the pellet will be off axis when it exits the bbl. Then it must travel the distance to the end of the shroud and as it goes farther forward it deviates more and more since it was launched off axis. This is why you should true barrels on centers to zero this out.
walt
I was going to use the rest and take 1/50th of a mm cut on each pass, so I think I might have got away with it. It turns out the barrels are slightly bent as the dial reads next to no error at each end. One is approx 0.5mm out, the other is about 0.2mm out (I took better readings). I can see the wobble on the worst with the naked eye, the other is imperceptible.
Given that the bore on each is almost perfectly concentric I see no point in turning them. Following tips given elsewhere I’ll make sure the bends are aligned vertically in the frames. I’m going to have to turn the bushes while on the barrels though, so that’ll be a chance to bannana them for you. 😆
You’ve neshed it you puff, I wanted to see you make the first banana barrel cos’ I’m sure it would have bent if you turned the OD down.
I’ve tested both barrels, one comes up to 12/100ths mm out at the breech end, the other to 30/100ths. Both about 3/100ths at the muzzle. I see no need to turn either of them. 😀
In Black powder terms a ‘swamped barrel’
For sure 🙂
Walter….
I couldn’t machine shiny turd but for some reason I sense a double taper barrel on the horizon…
just the lightest cut while geting a full cut all the way around.
I’m sure barrels are stressed relieved.
walt
As long as you didn’t sell any of it to BSA…
Funny, I had the same experience with a batch of stainless tubing I bought. I usually specify mill finish, which is sort of flat/sanded. But this batch, they sent me the wrong stuff and it was covered in paper to protect the bright, polished finish. I figured, ok, just prettier but the same 301 stainless. Nope. EVERY time I finished a piece from it, it always turned out just a tad off. I sat and puttered with it for a couple days and finally sighed and thought, shit, I’m getting old and must have just measured wrong or cut something crooked or or or… so I redid the project and at the end, same result; part was off by just enough so I had to scrap it. Funny notion started creeping into my head, so I made a 3rd part, this time from old mill finished 301 and bingo, no problems whatsoever. The damn new stuff was stressed in an odd way, so that it would change dimensions ever so slightly after machining. I still have yards of it in the garage, useless shit. But pretty and shiny.
walt
By the way Sean, how deep a cut were you taking on the piece that bent?
So what you guys are saying is to use the 4-jaw even though the job doesn’t strictly require it, for the sake of learning it. I take your points, but my brain doesn’t work like that. I learned DW and PS because I had to (needed to for a job), but when I tried to add Premier to it, just because I fancied it, it wouldn’t gel. And I don’t even fancy messing with the 4-jaw. 😆
Thanks for that info Sean, I think in light of it I’ll be doing a dummy-run with a bit of old stock first.
- You must be logged in to reply to this topic.
Your are right Pab! That is exactly how it is done! 😀
Mike