Gunsmithing Finding TDC on a barrel blank.

[crumpmd]

Before I chuck up another barrel I want to ask, again, what method do you use to find top dead center of a barrel blank. It is given the bore and outer surface of the barrel are not true one to the other and there is always some curve in the barrel no matter how small.
I remember 300sniper and Wm Roscoe mentioning it. More recently Mark at SBC mentioned it. But the practical details elude me. I appreciate any input.
Dave

 

[LRI]

Re: Finding TDC on a barrel blank.

By "TDC" I'm assuming your referencing the highest point of runout on the outer surface of the barrel cylinder when the bore is indicated to the center of the lathe's spindle. (right?)

Assuming that is the case:

Lets say we were to sit down and draw 360 little tick marks along the circumference of the cylinder portion of a barrel blank and duplicate the same at regular intervals along the barrels length. next we chop the barrel up into pieces that coincide with the marks spacing. We'd likely discover that the "TDC" point of the barrel will change (in terms of clocking position) along its length. What I mean is runout between the bore and the OD shouldn't be counted on to be consistent/linear. It's quite likely to vary significantly.
It also rarely matters unless its grossly off. Then things can get weird as the barrel warms up during a long course of fire.


On the other hand:

If what your talking about is "timing" the barrel so that it's natural curve (barrels are rarely perfectly straight) is oriented in a specific clocking position (usually vertically) then that's completely different.

So, exactly what are you after?

-C

Does that make sense?

 

[GasLight]

Re: Finding TDC on a barrel blank.

He is after timing the runout on the bore so that it is at its highest point at 12:00 on the muzzle. That way it will allow the scope to utilize the maximum amount of elevation under its mechanical limitations. This also assists in a minimum of windage correction.

The typical way of doing this is to have the barrel indicated at the chamber end and have the muzzle extending through the headstock out of the rear spider. You can then simply use a test indicator to tell you where the highest point of the bore is at the muzzle with the chamber end indicated to zero run out. Then you would mark this on either the top or bottom side of the barrel at the chamber end, and use this mark to trim the shoulder of the barrel until it matches with the cooresponding (top or bottom) side of the action.

Hope that is clear

 

[crumpmd]

Re: Finding TDC on a barrel blank.

Yes. To both of you.
I want to measure the runout and then time the barrel as mentioned.

I do have a rear spider on the headstock.

I am understanding the measurment on the muzzle end would be done on an indicator rod in the bore rather than the outer surface of the barrel at the muzzle. Correct?

 

[GasLight]

Re: Finding TDC on a barrel blank.

Correct, you are only concerned with the bore not the exterior of the barrel. You can use a rod, or you can simply insert the tip of an indicator into the bore, this will tell you which is the highest point of the bore. You don't need to be concerned about runout at the muzzle at this point, but using a rod would be perfectly acceptable

 

[300sniper]

Re: Finding TDC on a barrel blank.

i place an indicator off the od of the barrel where i plan on cutting it. i figure if the od of the muzzle is running out .020", the bore is going to be facing that way too. true, the bore may not be perfectly in the center, but i highly doubt it will be more than .020" out on a quality barrel.

 

[Wild_Bill]

Re: Finding TDC on a barrel blank.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 300sniper</div><div class="ubbcode-body">i place an indicator off the od of the barrel where i plan on cutting it. i figure if the od of the muzzle is running out .020", the bore is going to be facing that way too. true, the bore may not be perfectly in the center, but i highly doubt it will be more than .020" out on a quality barrel. </div></div>

Hi i have seen a few digerent brands of barrels that the runout inside the bore was as much a 20 thou. i would cut the barrel to length before indication it. also if you have a good eye you can get a pretty good idea by looking down the barrel before setting it up in the lathe and have a ball park of where you will index the barrel. we use a lot of 30-34" barrels and it becomes essential to make sure that any curve faces up because at 1200 yards you can run out of windage quickley if the barrel has been fitted with abend to the sides. The drill wanders on its way down the bore and it gets worse the smaller the calibre. we fit lots of full bore anf F Class barrels and they are usualy 30" in 223 and 30+ in 308 for Standard and some barrel makers dont like making a 224 barrel longer than 28" because it is so hard to drill the hole perfectly straight.

Hope it helps but i would cut it to length then indicate it.

 

[Texar2]

Re: Finding TDC on a barrel blank.

lock the barrel between centers and run with it. All cuts will then be centered around the bore. Or is your lathe table not long enough so you have to feed it through the chuck? For instance. When i chamber a barrel blank i put a dead center in my 4jaw and a live center in the tail stock. With the muzzle in the chuck and the shank in the live center and then tighten the jaws. Then i make passes on the shank until it is round. Then i thread. Then i put the steady rest on the remainder of the shank (whick is now concentric with the bore). After securing the steady rest then i back off the tail stock. At which point i remove the live center and insert chuck to hold my reamer. After chambering and fiting to action then put live center back in tailstock and retighten. Next remove steady rest. Now measure to desired length and make a mark. If adding muzzle brake then it is now time to move your cutting bit to this mark and make this piece of barrel round. Then take barrel out and cut with band saw jus past the spot you rounded. Put barrel back between centers but with shank in chuck and muzzle in live center. Now secure the muzzle in the steady rest on the area you made round and remove from tail stock and face off crown. Now put back in tail stock and thread. Now pick up shavings. Remember this technique only requires that you take out of lahte 1 time. Its stupid proof and dead on concentric. Please do not cut to lenght first! this is the last step! If you mess up the threads or chamber and have to start over you will end up at least an inch short. I also use a chucking reamer to cut the muzzle while it is in the steady rest. lets say a .325 reamer for a .308. Makes a nice crown.

 

[GasLight]

Re: Finding TDC on a barrel blank.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Recoil</div><div class="ubbcode-body">lock the barrel between centers and run with it. All cuts will then be centered around the bore. Or is your lathe table not long enough so you have to feed it through the chuck? For instance. When i chamber a barrel blank i put a dead center in my 4jaw and a live center in the tail stock. With the muzzle in the chuck and the shank in the live center and then tighten the jaws. Then i make passes on the shank until it is round. Then i thread. Then i put the steady rest on the remainder of the shank (whick is not concentric with the bore). After securing the steady rest then i back off the tail stock. At which point i remove the live center and insert chuck to hold my reamer. </div></div>

This is definately one way people have done this, and many have had good results using this method. However, I think the OP and many others prefer the method of dialing out the runout in the chamber end. The reason to do this, is so that at the exact point the bullet will first be engraved, they want the chamber to be perfectly concentric to the bore. This will allow the most perfect engraving of the bullet by the lands, which will determine the way the bullet will ride the rest of the bore, and will determine how the bullet exits. This is all on a very minute scale, but it makes the most sense to me. Using centers to do barrel work has been done by many top smiths and I am sure it will be acceptable for many and their gains. The only problem would occur if the bore wanders quickly in a short space... YMMV

Dave

 

[Texar2]

Re: Finding TDC on a barrel blank.

Sorry, I was not clear. I edited my post. oops please reread.

 

[GasLight]

Re: Finding TDC on a barrel blank.

I think it would be a fun experiment to do it the way you describe and then after you remove the live center, insert a range rod and measure the run out of the bore. I think it would be somewhat eye opening. At least on many barrel blanks

 

[crumpmd]

Re: Finding TDC on a barrel blank.

i appreciate the responses.
I am doing all the work through the headstock and I don't know why I was confounded by the concept of looking at the muzzle when chamber is indicated at zero to find the top of the curve.

Once somebody says it it becomes clear and you wonder why you had such a case of the dumbass about it.

I hot linked 300sniper in the the OP to show I had actually searched it again. But it was not clear to me how the OD of the muzzle would be a precise indicator of the curve, and now I understand he is allowing for a small amount of deviation.

 

[Texar2]

Re: Finding TDC on a barrel blank.

Never tried indicating the barrel while it is in the rest. I am about to barrel an m2008 with a shilen 308 select match. I will try it and post the findings. I hope its not too eye opening!

 

[BlackOps Tech]

Re: Finding TDC on a barrel blank.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: DebosDave</div><div class="ubbcode-body">I think it would be a fun experiment to do it the way you describe and then after you remove the live center, insert a range rod and measure the run out of the bore. I think it would be somewhat eye opening. At least on many barrel blanks </div></div>

+1....
I'm not going to spoil your surprise in the event you want to indicate the barrel while it's in the steady rest, but I think you'll be looking to change methods once you read the indicator....

 

[300sniper]

Re: Finding TDC on a barrel blank.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Wild_Bill</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 300sniper</div><div class="ubbcode-body">i place an indicator off the od of the barrel where i plan on cutting it. i figure if the od of the muzzle is running out .020", the bore is going to be facing that way too. true, the bore may not be perfectly in the center, but i highly doubt it will be more than .020" out on a quality barrel. </div></div>

Hi i have seen a few digerent brands of barrels that the runout inside the bore was as much a 20 thou. i would cut the barrel to length before indication it. also if you have a good eye you can get a pretty good idea by looking down the barrel before setting it up in the lathe and have a ball park of where you will index the barrel. we use a lot of 30-34" barrels and it becomes essential to make sure that any curve faces up because at 1200 yards you can run out of windage quickley if the barrel has been fitted with abend to the sides. The drill wanders on its way down the bore and it gets worse the smaller the calibre. we fit lots of full bore anf F Class barrels and they are usualy 30" in 223 and 30+ in 308 for Standard and some barrel makers dont like making a 224 barrel longer than 28" because it is so hard to drill the hole perfectly straight.

Hope it helps but i would cut it to length then indicate it.

</div></div>

i could maybe see .020" of eccentricity between the bore and the od if you cut the blank in half. i highly doubt you would be anywhere near that within a few inches of the blank end. i must be doing something right, the last couple barrels i have done were within 1.25 moa of the scope's mechanical center at 100 yards.

maybe a non-tapered barrel may be worse. i don't know if they turn/grind them between centers after boring like i assume they do with contoured barrels.

edit: here is my 18.5" 308 barrel. i cut around 6" off of the blank and had .0025" of eccentricity between the bore and od of the blank.

 

[LRI]

Re: Finding TDC on a barrel blank.

Here's some things to consider:

First, lets draw a picture. Get your pencil sharp and a sheet of paper.

Draw something that resembles a gun barrel. Now draw the bore and be sure to draw it like a banana since we know they aren't straight anyway.

Now look at what you have. You've got a circle. If we extend this arc out far enough eventually it's going to come right back to us.

So now what? Well we have some choices/decisions to make.

Indicating the bore concentric isn't good enough because the bore's center changes along its length because it's wrapped into a circle. The hole size doesn't change (much if any) but the center point in relation to the spindle of the machine certainly does.

I like to pick my battles. I personally feel it best to indicate where the bullet is going to engage the rifling. I've invested considerable time, effort, and money building specialized tooling/set up devices to do this very thing. I can say with 99.9% certainty that my chambers are concentric to within .0001" where the bullet bites the lands and within .00015" back at the case web/breech area. It's been checked, double checked, triple checked, by myself and a half dozen other people that make good livings at holding tolerances to the far right of the decimal point.

This way I at least know that the bullet is truly getting "bit" evenly around its circumference. From there I'll bore the back side so that the reamer runs concentric all the way up to that point. I don't use a drill as it'll just follow the existing hole. I single point it with a solid carbide boring bar made by Sandvik. (CXS series of inserts/holders)

Now, here is the part that'll be sure to get the fires, pitch forks, and ropes out of the closet and a lynch mob at my front door.

I said we have decisions/choices. If given a choice do we want to have our bullet TANGENT to the bore (which we've already decided is shaped in a big circle) or do we want it off axis? (meaning pointing straight down the spindle bore of the machine?) You can't have both because the barrel has a curve to it and the moment you pull it from the machine it's going right back to that arc. So for me I decided I had to make a choice.

IF I want it tangent then it means I have to (dare) defy one of gunsmithing's 10 commandments. I must allow the crown end of the bore to flop around inside the spindle with no support what so ever. BECAUSE if I indicate it in what have I done? I've pushed it to the center point even though we are all smart enough to figure out it's not going to stay there once I take the barrel out of the spindle.

I know, I know, it's blasphemy and I'm destined to Hades for all eternity for even suggesting it.

-But I'm here to tell you gents that IT WORKS and it works pretty darn well.

Fire the ovens!

 

[crumpmd]

Re: Finding TDC on a barrel blank.

I don't think anyone is going to flame you over that.

I chamber with the barrel through the headstock for the very reason you pointed out.

I use a rear spider to hold the muzzle when I indicate the chamber end of the bore. I indicate only the chamber but use both ends and adjust them until my test indicator has no movement on the indicator rod. At that point I am reasonably sure the bore is concentric with the lathe spindle centerline and I cut my chamber.

While the barrel is set up like that I can put an indicator rod in the muzzle end of bore and find the high spot and mark that spot and index my receiver to that mark.

300sniper is indicating the outside of the barrel and doing the same thing with the expectation that it will be no more than .0025inch off at any point close to either end.

And so my original question was answered by him and DebosDave, but if you do it differently at LRinc or if Mark at SAC would chime in it would be helpful. Maybe Roscoe would add his opinion.

 

[GasLight]

Re: Finding TDC on a barrel blank.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: C. Dixon</div><div class="ubbcode-body">
Now, here is the part that'll be sure to get the fires, pitch forks, and ropes out of the closet and a lynch mob at my front door.

I said we have decisions/choices. If given a choice do we want to have our bullet TANGENT to the bore (which we've already decided is shaped in a big circle) or do we want it off axis? (meaning pointing straight down the spindle bore of the machine?) You can't have both because the barrel has a curve to it and the moment you pull it from the machine it's going right back to that arc. So for me I decided I had to make a choice.

IF I want it tangent then it means I have to (dare) defy one of gunsmithing's 10 commandments. I must allow the crown end of the bore to flop around inside the spindle with no support what so ever. BECAUSE if I indicate it in what have I done? I've pushed it to the center point even though we are all smart enough to figure out it's not going to stay there once I take the barrel out of the spindle.

</div></div>

Chad,

I think you are doing the same thing as described above, only the typical way discussed by myself is using the rear spider to support the barrel at the same point where you would 'allow it to flop around unsupported.' The rear spider isn't used to indicate at the muzzle end, it is used to indicate the runout. I hope that makes sense. If you have ever watched Gordy Gridder's video through Grizzly, he is doing basically just what you are saying, but he is indicating the entire length of the chamber area, and supporting the muzzle end (NOT indicating the muzzle end).

 

[LRI]

Re: Finding TDC on a barrel blank.

Very well.

The first time I made any confession about doing it to this way I was literally running for the hills.

I've never seen Mr. Gritter's video. I came up with it after a long discussion with a really sharp M/E friend of mine from Seattle.

Perhaps a good thing. If two people can independently arrive to the same conclusions/processes it may very well help to validate the procedure.

A former employee of mine now resides in the DC area. There's a gunplumber out there named "Dan" (can't remember his last name) He's been chambering guns this way for about 20 years. I've been told his NRA service rifle (308 winny) is known to shoot half inch groups at 300 yards. Never seen it but its what I've been told.

-Assuming its the case (and I have no reason to believe otherwise) it sure as hell isn't hurting anything!

Enjoy the long weekend folks.

-C

 

[300sniper]

Re: Finding TDC on a barrel blank.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: crumpmd</div><div class="ubbcode-body">
300sniper is indicating the outside of the barrel and doing the same thing with the expectation that it will be no more than .0025inch off at any point close to either end.
</div></div>

not exactly. i can see the bore being WELL over .0025" eccentric from the od of the barrel. even if it is .010" out where i am going to finish it, if the od is running out .025" at that spot, the bore high point is going to be pretty damn close to the od high point.

the picture i showed was what pretty close to center considering i cut about 6" off of the blank. not close enough that i would do any work on it setup like that though. i just checked it to satisfy my own curiosity.