Setting up a barrel in a lathe

[oldred]

In another thread the method for setting up a barrel in a lathe was *set up to Indicate both ends of barrel with a snug fitting pin into zero in a 4 jaw chuck and spider on the back* and I am NOT disagreeing with this, in fact the reason I didn't bring this up in the other thread was that I didn't want to take anything away from that excellent reply. However it does bring up something I have been thinking about for a long time, while that method is the accepted norm and the way I too have been doing it something didn't seem quite right to me when doing this. I made my barrel centering plugs from 360 alloy brass so I could get a snug slip fit for three inches or so into the barrel without fear of scratches but perfectly aligned with the bore, I then turned another three or so inches to a larger diameter to indicate from. What I was finding was that when I indicated center at any point along this protruding shaft the center would be slightly off at different points along it's length, there seemed to be a corresponding difference on each end of the barrel. Furthermore the differing indications seemed to show the shaft, while centered at least at any one point, to not be perfectly aligned parallel with the center axis of the lathe since the error would reverse on either side of the point that indicated true center, that is if true when checked near the middle of the shaft I would get opposite readings near the tip vs near the end of the barrel.

I was sure these center indicator plugs were perfectly straight and snugly fitted to the bore and then it occurred to me that few if any barrels are TRULY straight, usually extremely close but not perfect. Sure enough when I trued the bore by indicating near the middle of the shaft on the chuck end of the setup and then making adjustments on the spider end I could easily get an even center indication along the entire length of my gauging shaft of the plug. Again I am not disagreeing in any way with any method and readily admit my reasoning could be flawed, mostly because I am talking about extremely small errors anyway but it sure seems to me that for truly perfect chambering, etc, it would be better to assure that the part of the bore being machined is aligned rather than just the very end being centered.


I think what I am saying might be confusing so please bear with me, I tried my best but find it hard to put into words so I apologize if what I am saying is confusing. I am sure the normal method is just fine and I am not suggesting anyone do it differently, I am simply talking about a principle and likely a totally insignificant error if indeed there even is an error. To make an extreme example let's envision a barrel with a visible curve near the center but with perfectly straight and centered indicating plugs in each end, even such a bent barrel as that could indicate center at some point along the length of the indicating pin but would change if the dial gauge was moved to another point on the pin. The spider end of the setup could be adjusted, albeit off-center, to align the chuck end of the bore to then be parallel with the center axis of the lathe.

[KCSO]

Old Red
Yep this was what I was told to do on the old South Bend. It still works.

[Ballistics in Scotland]

I'm strictly an amateur, with no need to work on several barrels of the same contour, from a single maker. But it looks to me like Shilen, at least, turn the contour true to the bore, and probably give all of the same contour the same taper. If this is the case there may be a case for drilling and boring that contour in a soft emergency collet, in a size like 5C.

I know some of the quality makers don't straighten barrels, but reject and bore larger any that aren't very close to straight. on the basis that a very slight curvature isn't as bad as one that might walk back to the way it used to be. In such a case I wouldn't worry too much about runout on the left of the headstock by a barrel that was trued up on the right. A nylon or rubber bushing there might minimize vibration. 

[HollowPoint]

When you say "Protruding Shaft" or "Barrel Centering Plugs" are you referring to a type of home made "Range-Rod?" If so, and it is as you say that the runout you're detecting is very minor, it could have little to do with your barrel or bore being less than true and everything to do with your home made brass "Range-Rods" being less than concentric along their entire lengths.

In my mind I can kind of picture what you are saying and I know that even some of the most expensive barrels are not absolutely perfect. When you ad an imperfect home made "Barrel Centering Plug" to an even slightly imperfectly turned or bored barrel it tends to have a cumulative effect in the runout measurements; doesn't it?

The store-bought Range-Rods are Hardened and Ground to very tight tolerances. At one point, I too made a couple of brass Range-Rods to center the muzzle of my barrels to cut threads for my suppressor. Because my suppressor was a 30 caliber suppressor that was to be used on a .223 cal rifle, I felt that a home made Range Rod would be good enough. I still believe it would have worked well enough for this application but, in the end I used a store bought Range Rod and I was able to true up my .0005" dial indicators to near zero runout using them. I say, "Near Zero" because even with these highly accurate professionally manufactured Range Rods my indicators would still pickup microscopic imperfections on the surface of these indicating rods. I don't think I could have achieved that level of accuracy with my home made brass Range Rod.

HollowPoint

[oldred]

The rods are ground concentric and to a snug fit for each particular barrel with my tool post grinder and are essentially zero run-out so the rod that extends from the barrel is parallel to the bore and concentric along it's entire length.

[Geezer in NH]

I always used a three jaw and a steady rest nor the head of the lathe with a spider. Indicate to the bore at the breech in the steady rest I cut threads and chamber this way.

A few dollars invested in some of the old gun smithing books and machine shop manuals is better spent than anything else.

Also adult courses at night at the local trade college or school will pay off handsomely.

[B R Shooter]

Oldred, you are correct, no rifle bore is perfectly straight. The tools you made are in effect the range rods mentioned before. They intentionally extend out beyond the end of the barrel so you can indicate the barrel "straight" at that end. Through the headstock is relatively easy with a spider on the left end. You have to grip the barrel in a way to allow the barrel to move without bending. You don't want to grab the barrel with the full depth of the chuck jaws and then try to move the muzzle. A simple piece of copper wire around the barrel so the jaws grip on the wire, will allow the movement. Go slow and with very light jaw pressure to get the barrel where you want it, then start tightening things down.

Now, this method you describe is used more in the very long range shooters, where they indicate the barrel and let the muzzle run out, and thread the barrel and fit the action. THEN, you see where the "up" side is on the muzzle end, and cut the barrel shoulder so the action tightens up with the muzzle pointing "up". The idea is to get the elevation advantage for long range.

Most will indicate the barrel zero on both ends, the important area is the barrel where the throat will be. This has to be indicated to zero, drill and bore the hole for the chamber to match the barrel where the throat will be, then chamber. You will have a chamber perfectly in line and the throat will be perfectly centered in the bore. This makes an accurate job.

Thankfully, over the past decade, barrel makers have made great improvements on deep hole drilling. It is more common now with premium barres to have a very straight bore, than one the looks like a cork screw. This kindof makes all this mute since they are good to begin with.

[oldred]

BR, that's along the lines of what I was thinking and while dialing in the centers on both ends it occurred to me that getting the bore where it was to be chambered aligned parallel with the lathe axis and centered was more important than getting both ends centered. After centering both ends at a middle point on the rod if I moved the indicator to the extreme end of the rod or back toward the barrel it would start to indicate a slight out of center condition, however by adjusting from the spider end I could get the rod on the chuck side to indicate centered along it's entire length although this meant the spider end of the barrel would then indicate slightly off-center.

BTW, I do use that wire trick. I learned that one some time back in another discussion.

[country gent]

When we had to drill deep holes both ends were indicated in to keep the hole as close to center and true as possible. WHen chambering I set up a 4 jaw and lightly snug fitting pins roughly 4" long in both the muzzle and breech ends. (One pin may not fit both lightly snug due to bore taper). I use a 4 jaw on the spindle and a spider at the back end ANd indicate spindle then spider and back and forth until I get zero readings or close to it. The pins are insert so that only 1/2"-3/4" protrude from the ends this is more than enough to indicate on and gives more stability to the rods. A light coat of grease on the pins also helps sometimes. You should be dead true or as close as possible at both ends. Another thing is to make sure the tailstock is straight and true not high or low to center of spindle. It is common practice to turn long tapers with the tailstock offset and they do sometimes move also due to wear and other things they may not set as straight or true when moving back and forth on the ways. If the tail stock isnt true then the cut wont be either. Setting up and getting everything dead true is most important.

[EDG]

If it helps you to sleep better indicate your next barrel in and pull your plugs out.

You can shift the spindle out of gear so it is free spinning and look through the bore while rotating it by hand, You can also turn on the spindle at 15 to 50 RPM if your lathe will run that slow. Look through the bore with light at the other end.

The first time I did this my engineering manager wanted the paper patch chamber neck of a .40 -2.5 Sharps Straight chamber in an original Remington Hepburn bored out.

That original Remington barrel flopped around like a jump rope. It must have been straightened because it had 2 different curves. Even so it must have had at least .040 of bend. The barrel must have had .080 run out that I could see.

[oldred]

You should be dead true or as close as possible at both ends.


This is where the subject starts and again I am not disagreeing but rather simply discussing a principle. I can see where that would be desirable as long as a barrel bore is dead-on straight but that's rarely going to be the case and if both ends are centered the bore is unlikely to be turning perfectly along the lathe axis except for where it has been trued at the ends. Take an archer's bow for example, an extreme example for sure but the principle is the same, if the bow were made from a hollow tube like a barrel and each end centered in a lathe using the same methods as the barrel then the ends would indicate center BUT while the string would be centered would the the bore really be centered? With the ends centered while spinning in a lathe a curved tube, such as a less than straight barrel, will have the bore revolving around the center axis rather than parallel with it except at the ends and while normal straightness flaws in a barrel may not be readily visible they are usually there no matter how minor.

My point from the start is the question of would it be better to perform machining operations on a barrel with the longitudinal axis intersecting a less than perfectly straight bore only at the ends or run the outer end slightly off-center enough to make the machining operation aligned straight with the bore at that point? Going back to hollow tube bow example again, if the ends are centered using the short pins then while spinning any machining operations are going to be based on the turning axis, or the string of the archer's bow, rather than the curved path of the tube's bore. The bottom line is that up until now I have concentrated on getting both ends dead-on center as much as possible but it has become obvious to me that while we may be getting the ends of the bore centered we would likely be shocked to see how much off-center that bore may be somewhere else along the barrel if we had a way to check it! By using a longer pin (range rod?) I am finding that while I may have been centering the ends of the barrel it does not necessarily mean the machining operations have been perfectly aligned straight with the bore, centered at the end yes but not necessarily perfectly parallel.


Once more, please don't think I am trying to argue but rather I am trying to better understand this procedure and rather than disagreeing I am simply thinking out loud as to how I am seeing this, I am not insisting that I am right.

[country gent]

Longer pins with a spreical ball end at inside end and one 6"-8" with a straight round end 5"-6" might give an idea of the amount the bore is "bowed" after it has been indicated in. A snug fitting set of balls and indicate the length of the stem at 0* 90* 180* 270* and again at 360* The added lengths of the pin in the bore and indicating stem along with the balls more limited contact. Another way though not as accurate is look at a light thru the bore can you find a round hole or is it always oval A true straight hole ( even if at an angle but still straight) will appear round and true tho this a bowed hole will appear oval and not round. Some armories and makers used this test.A crosshair or wire in front of the light will help to see the holes ovalness. Another issue will be stresses in the barrel blank itself. These can cause movements with temp changes or heat generated from machining. There are alot of variables involed working with someone else start. Ideally would be to drill ream rifle countour and chamber fit the barrel all in one set up.

[oldred]

Spoke with a gunsmith in Knoxville Tn about this yesterday and he not only confirmed my suspicion but also said what I am looking at is commonly done. When I mentioned that I had been centering each end as closely to center as possible he said that method too is common but why does the outboard center, referring to the spider end of the setup, matter when none of the rest of the bore is usually centered due to normal run-out? If both ends are centered then other points along the bore will not be centered so what makes that outboard end any more important than any other point along the bore? He went on to point out that actually the farther apart the centering points are then the less important they become. We discussed the slight error I had seen when checking the bore center at different points along the range pin I had inserted into the bore and he then, just to make a point, asked ok if you get different indications along the length of the pin then what point along that pin would be correct? I told him that the original question was that if I centered the bore on each end by taking an indication on the pin as close to the barrel as practical I then got the error as I moved outward and indicated again in different spots with the greatest error right at the end of the pin, the longer the pin the greater the error. I was then told that was due to the pin being aligned with the bore as it should be but that with both ends centered the bore is unlikely to be perfectly aligned with that center axis I had dialed in by centering both ends of a less than perfectly straight bore! He then pointed out that whatever error I was getting at any given distance outward along that pin would be the same error in the bore at that same distance from the point I had centered! So in essence if the bore is centered on both ends right at the barrel itself and indicates zero run-out at that point but then exhibits, for example, a .002 run-out on the pin 4 inches away from the point indicating dead center right at the barrel then the bore would be out-of-center by the same .002 if indicated 4 inches inside the bore!

We had a good discussion about this and I see it clearly now, the outboard center is indeed meaningless relative to machining operations on the other end, what IS important is having such machining operations, chambering, crowning, etc, centered at that end and ALIGNED properly with the bore at that end regardless of what the other end indicates. By having a range pin (thanks guys for educating me on the proper name for that pin) extending outward from the bore and adjusting the outboard (or spider) end of the setup so that this pin indicates zero along it's entire length rather than just at any one point we can be assured of being better aligned with the actual bore regardless of center on the other end. The machining operation being performed will then be better aligned with the bore rather than with the imaginary line running through the two centered points on each end of the barrel which will be the center turning axis of the lathe. To visualize this let's use the overblown, but still relative, example of the archer's bow, if both ends of that bow were centered in the lathe then the turning axis would be the string and any machining operations would be along THAT axis rather than the bow itself which represents the obviously much less but still somewhat less than perfectly straight barrel. Now if the outboard center is ignored this error is reduced more and more as the outboard centering point is moved closer to the end where work is being done, by extending the indicating point out past the end of the bow, such as is done with a long range rod, the outboard end can be adjusted so that the work end bore is parallel to the spinning axis of the lathe!


Wow, I think now I'm getting a headache! :sad: