Gunsmithing bore curvature

[Unknown]

OK, I'll show my ignorance...I saw a posting where a company said they will time the curvature of the bore to the 12 o'clock position as part of the accurization. I have never heard of this, and the company that mentioned it has done wonderful work on a number of rifles for 2-3 friends of mine, so they have plenty of credibility.

My first impression is that timing the curvature of the bore means that the bore is bent (has a curve), and it would seem that having a barrel that is straight to begin with would be the way to go. Obviously, I'm missing something, and hope someone can educate me about the statement: "time curvature of the bore to the 12 O'clock position". Does that really mean that barrels start out bent/curved?

 

[elephantrider]

Re: bore curvature

Yes there is a very small amount of curvature in even the highest quality barrels. Some barrel makers will typically be more than others on average, but all will have some small amount of curve. It isn't a problem as long as it is timed to 6 O'clock or 12 O'clock, so there is no zero/windage or barrel bedding issues.

 

[Al_Ski]

Re: bore curvature

Gun drills don't follow a straight path through the centerline of a barrel. The hole won't be a concentric curve, the drill will wander around in different directions.

There are two trains of thought on indexing a barrel. You can index the barrel so it points up at 12:00 when the barrel is tightened to the action. Or you can indicate the bore in at the muzzle and at the neck area of the chamber to chamber the barrel, no indexing necessary.

 

[seanh]

Re: bore curvature

the last part is what I understand that most companies do.

 

[RTO]

Re: bore curvature

Barrels can range from .015 to as much as .300 runout across the length of the barrel. When you look at it as a 26" barrel or whatever the length of the barrel is it is a very slight runout. All barrels have some runout. Your better barrels like Bartlein and Rock seem to be much less than others. The reason to time a barrel at 12:00 is for several reasons. Its as simple as appearance of gap in barrel channel, once action is bedded with one barrel then rebarreled your gap on either side wont change if clocked at 12:00. (assuming it was clocked at 12:00 when rifle was bedded originally and barrel was centered in barrel channel) Its also easier to adjust elevation and for a lot of scopes you have more elevation to adjust than windage. This is a very good practice most smiths use. It takes a little bit of time to do but worth it. I have chambered a quite a few and if you indicate 1" inside and then 2-3" inside and true both to .0001" you can actually see the runout at the muzzle by eye. I know some people dont time them and they still shoot great. Its just a good practice that takes a few mins longer.

 

[Edds]

Re: bore curvature

What % of guys who chamber barrels bend them slightly when setting them up to make them run true then have them spring back when they remove then from the fixture?

 

[RTO]

Re: bore curvature

There is some degree of stress induced in fixtures but this will still not affect the runout across the barrel. Typical fixtures are indicated at two points at 4" centers typically so this will not affect the direction of runout to the muzzle. I dont use a fixture as I dont like how far out from the headstock it puts the chambering. There are many ways to skin the cat and get great results. The amount of stress induced in a fixture is nothing like the runout the OP was referring to.

 

[RTO]

Re: bore curvature

Even for those who use a fixture how much stress are you really inducing across 4" of barrel at its thickest point. Second depending on cartridge chambering only the first 2.5" with the first 2" to 2.5" outside of that 4" centers so that point is stress free. With the large if not all of the chamber outside of those centers that are stress free of the chambering fixture and a piloted reamer. The reamer is following the bore, the bore is trued on these two points to create a zero following the bore down the length and not just at one point. Lets assume for a moment you do have spring back but you are indicating at the chamber aprox .5" to 1" inside and again at 2-2.5" inside or if you use a range rod same thing across just a shorter distance. This is being indicated and zero'd at a non stress induced locations where the cuts are being made. Whether stress is induced or not its no affect on the point of cut. Like I said before I dont use this method primarily just because it puts the work to far out from the headstock for my liking but I do have a chambering fixture and have used it and know many people who do use them with outstanding results.

 

[Unknown]

Re: bore curvature

Thanks very much to all of you ffor giving me straight answers, without any of the often seen "you are a dummy because you don't know what I know" nonsense.

The company which made the post that prompted my inquiry has my utmost respect..so much so that I would love to have them do a build for me. But that will have to wait until I have more spendable cash, and their workload slows to where they will accept more work.

At least now I know more than I did two days ago. The last company that spent so much time educating me about rifle work gave me an hour explaining things to me before I even bought anything. I was so impressed with their willingness to spend that amount of time with someone who had not even bought anything, that I ended up buying a complete custom action from defiance machine. I have never regretted the decision. Even after they had my money, the great service continued.

Mods, if I did something wrong with this post, please just delete the wrong stuff.

 

[Bill Larson]

Re: bore curvature

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Edds</div><div class="ubbcode-body">What % of guys who chamber barrels bend them slightly when setting them up to make them run true then have them spring back when they remove then from the fixture?</div></div>

Not inducing stress into the barrel when chambering is key to being a good gunsmith.There are several ways of doing a chamber w/o bending barrel.That`s what makes a steady rest so appealing.....
bill larson

 

[Gene Poole]

Re: bore curvature

I'm trying to wrap my head around this concept. I've always known that you can't expect the bore axis to be perfectly concentric with the barrel axis, but are you saying that it is not only non-concentric, but may be something besides a straight bore? Like a bore that is more or less an arc through the barrel, or even worse, like a corkscrew through the barrel?

 

[Mgordon]

Re: bore curvature

I have a few videos that should help explain it.

When we dial in barrels, we always release the tension on the chuck/spider that we are working on before we make our adjustments. Furthermore, we watch carefully how much torque we are putting on the locking screws or chuck jaws.

Video

Video 2

Mark

 

[Edds]

Re: bore curvature

I am not a gunsmith and I have never chambered a barrel.

Those videos may show a good way to find the curvature of a barrel that is already chambered and threaded but I don't think that would be a good setup to chamber and thread a barrel.

 

[Al_Ski]

Re: bore curvature

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Like a bore that is more or less an arc through the barrel, or even worse, like a corkscrew through the barrel?</div></div>

Yes...

Scary thought, isn't it!

 

[former naval person]

Re: bore curvature

Aren't you supposed to calculate the Coriolis effect and calibrate the curve of the barrel to compensate?

 

[Sonic Crack]

Re: bore curvature

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Edds</div><div class="ubbcode-body">I am not a gunsmith and I have never chambered a barrel.

Those videos may show a good way to find the curvature of a barrel that is already chambered and threaded but I don't think that would be a good setup to chamber and thread a barrel.</div></div>

How do you think that barrel was dialed in to cut that chamber and threads?

 

[Edds]

Re: bore curvature

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Sonic Crack</div><div class="ubbcode-body">How do you think that barrel was dialed in to cut that chamber and threads?
</div></div>

I don't know how those barrels were dialed in to be chambered and threaded.

If they were dialed in at the neck and muzzle, they were either bowed when that setup was made or they are bowed in the setup in the video.

 

[Mgordon]

Re: bore curvature

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Edds</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Sonic Crack</div><div class="ubbcode-body">How do you think that barrel was dialed in to cut that chamber and threads?
</div></div>

I don't know how those barrels were dialed in to be chambered and threaded.

If they were dialed in at the neck and muzzle, they were either bowed when that setup was made or they are bowed in the setup in the video. </div></div>



It makes more sense to me to have the bore straight as possible for chambering. This is done by having the bore co-axially true and this ensures the chamber will be very straight with the bore. What happens to the bore after that is not as important. The muzzle work and threads are done the same way. When we are crowning a barrel, we do not just stick an indicator in the bore and indicate at one point, that would guarantee angular runout the farther away you go from that point. But by co-axially dialing in the bore in two separate positions, we are making the work we do as concentric and square to the bore.

People may not desire a timed bore on their rifles, but having machine work that is co-axially true to the bore would be a must.

Mark

 

[Edds]

Re: bore curvature

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: M.Gordon</div><div class="ubbcode-body"> But by co-axially dialing in the bore in two separate positions, we are making the work we do as concentric and square to the bore. </div></div>
Why would you not want those two positions to be the muzzle and the neck?

If they aren't bowed in the setup, I don't consider the chamber of those barrels in the videos to be machined true to the bore. Maybe now would be a good time define bore.

 

[Mgordon]

Re: bore curvature

I understand why you would want the neck area dialed in, but how would you be dialing in the just the neck area?

What benefit would it be to have the muzzle dialed in while you are doing work on the chamber?

If you take fixtures like the True Bore Alignment system from Straight Shot Gunsmithing True Bore Alignment System and dial a barrel in, co-axially at two separate points in the chamber area, I guarantee you will find similar results as to what I am showing with the barrel having a curvature to the bore. So with this being said why would you concern your self with the muzzle when you are doing all of your work on the chamber end?

I know its been an older and well accepted practice to place ground pins in the muzzle and on the chamber end, then dial in .001", but that approach does not give me as straight of a chamber as what I'm getting right now.

I would be happy to take one of our long stem indicators, and run it up and down the bore infront of the chamber (after the chamber is cut), to show that the bore is dialed in within .0001"-.0002" then move the indicator to the chamber to show that it has the same or less runout.

I would also be happy to take a barrel (not one that I paid for) and dial it in the same way you are suggesting and see if there is any runout in the chamber after the work is done. By dialing a chamber in the way you are describing will also promote a larger diameter chamber than the way I am dialing.

Different strokes for different folks, I know there have been many accurate rifles built doing the pin in the muzzle method, however I feel its just not as good of a chambering job than a timed curvature co-axially indicated bore.

Mark

 

[Edds]

Re: bore curvature

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: M.Gordon</div><div class="ubbcode-body">I understand why you would want the neck area dialed in, but how would you be dialing in the just the neck area?</div></div>
You would dial it in by positioning the indicator on the neck area.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">What benefit would it be to have the muzzle dialed in while you are doing work on the chamber?</div></div>
It would allow the bore to be aligned with the chamber. I consider the bore to be the whole bore not just a 3 inch long section at one end of the barrel.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">If you take fixtures like the True Bore Alignment system from Straight Shot Gunsmithing True Bore Alignment System and dial a barrel in, co-axially at two separate points in the chamber area, I guarantee you will find similar results as to what I am showing with the barrel having a curvature to the bore.</div></div>
Exactly, that is why I said I didn't think it should be done that way.
<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">So with this being said why would you concern your self with the muzzle when you are doing all of your work on the chamber end?</div></div>
So the muzzle will be aligned with the chamber.
<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">I know its been an older and well accepted practice to place ground pins in the muzzle and on the chamber end, then dial in .001", but that approach does not give me as straight of a chamber as what I'm getting right now.</div></div>
I don't see what this has to do with chamber straightness. Why can't the chamber be straight regardless of bore alignment. Maybe chamber straightness needs to be defined also.
<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body"> By dialing a chamber in the way you are describing will also promote a larger diameter chamber than the way I am dialing.</div></div>
Why would it make a difference in the diameter of the chamber?

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Different strokes for different folks, I know there have been many accurate rifles built doing the pin in the muzzle method, however I feel its just not as good of a chambering job than a timed curvature co-axially indicated bore.</div></div>
Why would it not be as good of a job if it resulted in less runout of the muzzle bore ? Seems like it would be a better job.

 

[Al_Ski]

Re: bore curvature

Are you pre-boring your chamber before running the finish reamer?

 

[Mgordon]

Re: bore curvature

I have a barrel in the lathe getting ready for muzzle work. Iwould like to post a video of the muzzle and the chamber end dialed in within .0002".

I then take the indicator on the muzzle end and check what the bore is doing about 1" in from the muzzle. Again, this is with the end of the muzzle and the end of the chamber dialed in within .0002". When I move the indicator in from the tip of the muzzle to about 1" in, I have over .001" of runout in the bore.

It just does not make any sense to me to thread a barrel for a suppressor, or to put a reamer in with that scenario.

I will post the video when its done uploading.

Mark

 

[JJones75]

Re: bore curvature

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: M.Gordon</div><div class="ubbcode-body">I have a barrel in the lathe getting ready for muzzle work. Iwould like to post a video of the muzzle and the chamber end dialed in within .0002".

I then take the indicator on the muzzle end and check what the bore is doing about 1" in from the muzzle. Again, this is with the end of the muzzle and the end of the chamber dialed in within .0002". When I move the indicator in from the tip of the muzzle to about 1" in, I have over .001" of runout in the bore.

It just does not make any sense to me to thread a barrel for a suppressor, or to put a reamer in with that scenario.

I will post the video when its done uploading.

Mark
</div></div>

Mark , give it up bud , trying to explain things to some people is like trying to push a rope up a wall , especialy when thay dont understand the concept.

 

[turtlesniper]

Re: bore curvature

What he doesn't understand is the bore is not straight it's shaped like a strung long bow. If the bore was straight that would work

 

[turbo54]

Re: bore curvature

Edds:

This is a very exaggerated depiction of your proposed chambering method:

Though exaggerated, I hope it gets the point across. Your method creates an IMAGINARY bore axis your throat and cartridge will be concentric to. As you can see, the bullet will enter the throat concentric to something imaginary.

I'd prefer the bullet enter concentric to the actual bore.

 

[turtlesniper]

Re: bore curvature

Nice pic there turbo.. I was trying to do that same thing in paint shop but it wasn't working for me

 

[Al_Ski]

Re: bore curvature

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: JJones75</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: M.Gordon</div><div class="ubbcode-body">I have a barrel in the lathe getting ready for muzzle work. Iwould like to post a video of the muzzle and the chamber end dialed in within .0002".

I then take the indicator on the muzzle end and check what the bore is doing about 1" in from the muzzle. Again, this is with the end of the muzzle and the end of the chamber dialed in within .0002". When I move the indicator in from the tip of the muzzle to about 1" in, I have over .001" of runout in the bore.

It just does not make any sense to me to thread a barrel for a suppressor, or to put a reamer in with that scenario.

I will post the video when its done uploading.

Mark
</div></div>

Mark , give it up bud , trying to explain things to some people is like trying to push a rope up a wall , especialy when thay dont understand the concept. </div></div>

Plenty of winning short range benchrest barrels are chambered in this manner. Indicate the muzzle and the The area of the bore when the throat will be. Pre-bore chamber .01" undersized and then ream. The chamber/throat will be concentric and will not cut an oversized chamber. Hell you can leave the pilot off the reamer...

The bore is curved. Indicate at the muzzle and then again say 3" deep into the bore. Get those two points dead nuts concentric. What is the barrel bore doing between those two points?

 

[Adam B]

Re: bore curvature

I work in IT and have very little machining experience and even I understand Mark's method

 

[JJones75]

Re: bore curvature

I've discussed chambering at length with Don Geraci while watching him build several guns , he uses a bushing near the muzzle end of the barrle that fits snug in the bore of his lathe he then dials in the chamber end to dead nuts at the point where the throat will be ad at the end , this small 2-3" span is all that matters to be strait , if the bullet doesnt start off strait it cant shoot strait is the line of thought here , when i asked about indicating the barrel so that the muzzle would be clocked to the 12 o'clock position he said that in the BR game it doesnt realy matter if the barrel is crooked or not because its a known range but in long range shooting from 100-1000 yds it would make a hugh differance.

I persoanly do it the way Mark and many others do with the range rod in the chamber end , then after everything is cut the muzzle is indicated to find the high spot so that it can be indicated to TDC.

If the bore is dialed in dead nuts at the chamber end preboring the chamber isin't needed because the reamer is going to follow the existing hole and as long as you are using a floating reamer holder their isint any issue with the chamber being perfectly strait and round

 

[Al_Ski]

Re: bore curvature

I've done Benchrest barrels both ways, generally do my LR rifle barrels by indicating the throat area and the area at the start of the chamber. Both methods get pre-bored. The LR barrels get clocked.

I haven't seen a difference accuracy wise... YMMV

 

[eddief]

Re: bore curvature

Some barrels benefit from clocking 12 o'clock but a lot of good tubes don't need it.

Dial in at the throat and muzzle, then pre-drill, go back in and check throat and muzzle once again and tweak back in if needed. Now pre-bore with carbide boring bar. This should create a pefectly concentric hole that your reamer will follow. This pre-bored hole is lined up co-axially with your throat to the back of your chamber and your muzzle. Now you have created 3 points that are concentric and co-axially lined up with each other (beginning of chamber, throat and muzzle). Note: this method you can't clock the barrel..

Dialing in the muzzle with the chamber is more beneficial for the shortrange BR guys. When they switch out a barrel for another that was chambered with the muzzle dialed in, the first shots are usually within 1MOA of the previous barrel.

If you are going to clock a barrel at 6 or 12 because of runout, then you will have to dial in the muzzle after chamber work like Mark is doing.

Merit to both methods if you ask me, and in the end the result on paper is all that matters.

More food for thought...tennon and threading pressure while cutting. Are you knocking your finely tuned bore out before you chamber by cutting your tennon and threading first? Once those are finished, you can't dial back in your throat and chamber if they are off. If you do, now your tennon and threads aren't concentric with the chamber.

But, if you cut your chamber first, you can definitely tweak it back in before you finish the tennon and threads. If you don't dial check your work when finished (barrel shoulder, threads, throat, neck and rear of chamber) you don't know if your work is done right.

So many ways to skin the cat, and if done right they all work very well.

 

[Mgordon]

Re: bore curvature

EddieF,

I can see how that would a very good way of doing it. My first couple chamber jobs on my own rifles were done with the ground gauge pins in the bore, dialed on both ends with an indicator method. Not the best way to do it in my opinion. This video I'm posting represents that technique and the weakness associated with that style.

If you are dialing in, drilling, then dialing in farther up the throat/neck area, then boring while the muzzle is dialed in, I can see how that would seem acceptable, my brain just likes our method.

Here is the video I promised. Please excuse my phone ringing during the video.

Mark


Video of bore runout when dialing on muzzle/chamber end only.

 

[eddief]

Re: bore curvature

I'm not a fan of gauge pins either, I use only direct reading from the interapid and Mitutoyo indicators. Pins and rangerods are fine to get you close, but then I like to make final adjustments with the indicator direct. Probably splittings hairs, but it makes me feel good..

I've had some high minded discussions on this topic with Gordy, M.Bryant, Alan Warner, Jim Borden etc. and always learn something.

Thanks for sharing...I need to make a video

 

[Mgordon]

Re: bore curvature

EddieF,

I spent some time with Gordy, and I model most of my chamber work after what he is/was doing. Like you said, tons of ways to execute, you have to do what makes you sleep at night. I'm a light sleeper!

Mark

 

[turbo54]

Re: bore curvature

Mark, you are the man.

There is a good reason your rifles shoot the way they do.

Bravo.

 

[eddief]

Re: bore curvature

Gordy is a great guy and personal friend of mine too. He has helped a lot of smiths out there, including me over the years.

Hell, one of the funnest weeks I ever had was back in 2000 at the Varmint Hunter's Jamboree in S.D. I met Gordy out there and we hung out all week (he ran the match). I didn't know anybody and Gordy treated me like family. We ate a lot of steak that week and I made some great friends. Take Care

 

[Al_Ski]

Re: bore curvature

Lots of good information being passed around. Thanks!!!

Biggest thing is to have confidence in your method, and a little of Chad's pixie dust...

 

[Edds]

Re: bore curvature

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: turbo54</div><div class="ubbcode-body">Edds:

This is a very exaggerated depiction of your proposed chambering method:

Though exaggerated, I hope it gets the point across. Your method creates an IMAGINARY bore axis your throat and cartridge will be concentric to. As you can see, the bullet will enter the throat concentric to something imaginary.

I'd prefer the bullet enter concentric to the actual bore. </div></div>

That's good, I like it.

Now use the same level of exaggeration and see what you get when you dial in the chamber only.

This isn't my proposed method of chambering. Someone early in the discussion said this is the way most people do it. I do believe the barrels in the first two videos could have been chambered so there would have been less muzzle runout.

 

[elephantrider]

Re: bore curvature

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Edds</div><div class="ubbcode-body">I am not a gunsmith and I have never chambered a barrel.

</div></div>

I think this is the source of a lot of your confusion. Once you've seen a barrel chucked up for chambering and the runout that you are getting with either end of the barrel it starts to make sense. Trust the explanation that has already been given. The method that Mark describes is a sound way of doing it. His and may other gunsmith's results validate it.

Although a little too exagerrated in my opinion, the drawing above does show the reality of chambering a barrel. The bore is not concentric with the barrel OD. It is actually slightly bannana shaped within the barrel. This is just a reality of how barrels are made, and you wont be able to straighten it out or find an imaginary line in the bore to indicate everything off of. As long as the chamber is concentric with the bore on the breech end, the overall curvature is at 12 O'clock, and any muzzle threads are concentric with the bore at the muzzle end you are good to go. Who cares what it is doing in between those to points?

 

[300sniper]

Re: bore curvature

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Edds</div><div class="ubbcode-body">

That's good, I like it.

Now use the same level of exaggeration and see what you get when you dial in the chamber only.

This isn't my proposed method of chambering. Someone early in the discussion said this is the way most people do it. I do believe the barrels in the first two videos could have been chambered so there would have been less muzzle runout. </div></div>

more exaggerated drawings showing both methods. both are using the exact same radius for the arc. the dots are showing the points indicated to zero tir.

throat and muzzle at zero tir:

same picture, zoomed in on chamber end:

throat and two inches in front of throat at zero tir:

same picture, zoomed in on chamber end:

 

[Edds]

Re: bore curvature

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: elephantrider</div><div class="ubbcode-body"> you wont be able to straighten it out or find an imaginary line in the bore to indicate everything off of. </div></div>
Some people who have chambered barrels don't agree with you.

If those videos are the norm, I'm glad I'm not a Gunsmith. I'd be out of business before I ever produced anything because I'd not let anything that out of whack leave my shop.

 

[Rhys]

Re: bore curvature

By indicating to be square with the first couple inches the bullet gets a start straight down the bore. It will follow that bore wherever it goes. Then Indicating the last couple inches at the muzzle end the same way you can ensure that your crown and any muzzle device will be set up ezactly square to the bore at that end.

The best barrels I've worked with using this methosd deviated from the ideal centerline by .004, and the worst that I recall was about .060. All of these shot better than anything that I chambered indicating in each end, and ignored what was in the middle.

 

[elephantrider]

Re: bore curvature

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Edds</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: elephantrider</div><div class="ubbcode-body"> you wont be able to straighten it out or find an imaginary line in the bore to indicate everything off of. </div></div>
Some people who have chambered barrels don't agree with you.

If those videos are the norm, I'm glad I'm not a Gunsmith. I'd be out of business before I ever produced anything because I'd not let anything that out of whack leave my shop. </div></div>

I know of at least two gunsmiths who do top notch work that chamber barrels the way that Mark has described. One of them even has built gun for top PRS competitors. I think you are having a hard time discerning between "out of whack," and something being done differently than your gut tells you it should be. This is not out of whack, it is simply understanding how the barrel comes from the factory. Even the best barrels exhibit this to some degree. When they are chambered this way they still shoot awesome. Do you still need to argue with the finished product even though it performs flawlessly?

 

[7mmRM]

Re: bore curvature

Interesting discussion.

I have been using the range rods and indicating on two points, using the chuck and spider to get them running true. I've built several rifles this way and they have performed very well. (Keep in mind I am fairly new to this.)
I accepted the range rod method as the way to do it from being taught that way and seeing so many here do it that way.

I had some reservations about the method from the get go, but went with it for the above reasons . My reservations are - tolerance stacking and the big one (in my mind) is how the hell is it possible to align two points in a bore, that are obviously not aligned due to curvature, without bending the curvature straight?

These reservations, especially the last one, came to head the last rifle I built myself. In order to get the ass end of my range rod true, I had to jack the spider end waaaay more than anything I've ever dialed in before. So much infact that I thought this cant be right, so I loosened everything up and started over. Ended up the same way. Fuck it. Its mine . Lets start cutting. Finished it all up, went to the range. Shoots awesome. Think to myself, "self you got lucky." Time to find a better way.

Start reading, a bunch... Come across posts by Chad, Eddie, Dave, and others about indicating the throat, prebore, chamber. No influence from the muzzle end while working on the chamber end. Bingo. This makes sense and is how I will now do them. Ordered my long stem indicator and micro boring bar the other day.

All that rambling leads to my next question, and the crux of the thread in general- curvature timing. Say the next barrel I put in the lathe, dial in the throat, go to the muzzle end and find the obvious high spot. My question is, would that not be the "natural" high spot? And time accordingly? Back up a bit, and say we are dialin in on two points using the spider, is it plausible that jacking the spider to get the ass end of the rod true would give you a false high spot on the muzzle end?

 

[Al_Ski]

Re: bore curvature

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body"> My reservations are - tolerance stacking and the big one (in my mind) is how the hell is it possible to align two points in a bore, that are obviously not aligned due to curvature, without bending the curvature straight?</div></div>

It's fairly simple...

The point where your chuck jaws contact the barrel has to be very narrow for starters. The barrel has to be able to pivot at this point. The same for the jackscrews in the spider. Some people us a single wrap of 12ga copper wire wrapped arount the barrel at the chuck and set screws with brass tips for the spider.

Secondly the force used to tighted the jaws/spider can influence bending the barrel. It isn't a good idea to slip a piece of pipe over the chuck wrench to get more leverage. When you're getting close to having the barrel dialed in release each jaw/screw keeping an eye on how much the barrel moves. If the indicator shows excessive movement your jaws are too tight. It's a "feel" thing that comes with experience.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body"> My question is, would that not be the "natural" high spot? And time accordingly? </div></div>

Stick your range rod into the muzzle of the barrel, place your indicator tip next to the face of the muzzle. When your indicator shows the high spot you are there....

 

[7mmRM]

Re: bore curvature

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Al_Ski</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body"> My reservations are - tolerance stacking and the big one (in my mind) is how the hell is it possible to align two points in a bore, that are obviously not aligned due to curvature, without bending the curvature straight?</div></div>

It's fairly simple...

The point where your chuck jaws contact the barrel has to be very narrow for starters. The barrel has to be able to pivot at this point. The same for the jackscrews in the spider. Some people us a single wrap of 12ga copper wire wrapped arount the barrel at the chuck and set screws with brass tips for the spider.

Secondly the force used to tighted the jaws/spider can influence bending the barrel. It isn't a good idea to slip a piece of pipe over the chuck wrench to get more leverage. When you're getting close to having the barrel dialed in release each jaw/screw keeping an eye on how much the barrel moves. If the indicator shows excessive movement your jaws are too tight. It's a "feel" thing that comes with experience.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body"> My question is, would that not be the "natural" high spot? And time accordingly? </div></div>

Stick your range rod into the muzzle of the barrel, place your indicator tip next to the face of the muzzle. When your indicator shows the high spot you are there.... </div></div>

I do use brass tipped screws and copper wire.. nor do I use a pipe on the chuck wrench.

Im well aware how to find the TDC, what Im asking is would TDC be the in the same place if it were not influenced by the spider.

 

[Al_Ski]

Re: bore curvature

You should be GTG

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Im well aware how to find the TDC, what Im asking is would TDC be the in the same place if it were not influenced by the spider.</div></div>

If the barrel is allowed to pivot freely in the chuck jaws, yes.

 

[jrm850]

Re: bore curvature

Spider stress... I'm seriously thinking about just using the action trueing fixture for this dasher I'm about to chamber. Anyone else do it this way?

 

[eddief]

Re: bore curvature

7mmRM,

Your TDC should be fine. If you ever want to check/verify yourself, mark TDC as you normally would. Pull that TDC marked barrel out and stick it in an action truing jig and dial in your two spots like you normally do by using the action truing jig jack screws, leaving the muzzle end completely alone.

Once dialed in, rotate the barrel as slow as you can in back gear on the lathe while taking a peek down the bore. You will be able to visibly see the curvature in the muzzle end, and should be able to tell if it's in the same TDC marked spot. No spider or jacking screws are near or on the muzzle end, so you know it's not being bent or torqued out of normal.