Off Center throat question...

[Eddie Harren]

You can sure tell who has successfully chambered barrels by reading some of the answers. Be careful Eddie, I think you should be able to separate the wheat from the chaff.

+1 Butch. It doesn't take long for the "experts" to expose themselves.

 

[Eddie Harren]

So are you experts going to help this man with some advise? Funny how gunsmiths seem to think their trade is rocket science. Sorry, its just another blue collar trade, not too much brains needed. Kind of like being a mechanic.

You are a gunsmith. Help the man out.

 

[hogpatrol]

Just a WAG but maybe the drill that was used initially was ground/sharpened off center? It would cut the hole large but my memory banks aren't working well enough to remember if it would just follow the bore or move the hole axis off center with the existing bore.

 

[Steelnecktie]

I don't think it would be a high land in a 6 groove barrel. I would say your pre drill method isn't right, or you didn't use proper size bushing. Just my guess.

 

[Tozguy]

Don't know how bad the lack of concentricity is that you mentioned eww but from what little we have to go on, and assuming that you indicated well, you have mixed two different approaches to reamer control. If you prebore the chamber to fit the body of the reamer then you should not use a tight fitting bushing on the reamer. When the chamber is properly prebored you don't need a bushing at all. The reamer body is guided by the machining you did.

If you want to use a tight fitting bushing to follow the rifling, don't prebore or drill in any way as Dusty said in post 7. Let the pilot guide the reamer from beginning to end.

This is not about which approach is better but more about not using two different approaches at the same time. Don't blame the barrel for that.

Eww, in my opinion the reamer pilot no matter how tight will not override a crooked prebore so in your case my guess is that the bore job was off.

 

[butchlambert]

Don't know how bad the lack of concentricity is that you mentioned eww but from what little we have to go on, and assuming that you indicated well, you have mixed two different approaches to reamer control. If you prebore the chamber to fit the body of the reamer then you should not use a tight fitting bushing on the reamer. When the chamber is properly prebored you don't need a bushing at all. The reamer body is guided by the machining you did.

If you want to use a tight fitting bushing to follow the rifling, don't prebore or drill in any way. Let the pilot guide the reamer from beginning to end.

This is not about which approach is better but more about not using two different approaches at the same time. Don't blame the barrel for that.

I agree. I indicate a Deltronic pin in the bore first. I drill large enough to use my Mitutoyo 514-503 tenth indicator to reach in and double check the groove and bore. If you have a difference in the land height to bore, split the difference. Now it is time to taper bore and that allows your reamer to follow your taper bored hole with a loose fitting bushing.

Is that what you are looking for zmalibu? Ain't no secrets, just good solid machining techniques as Jackie has posted.

 

[paperpuncher]

If you wan[/QUOTE]

I agree. I indicate a Deltronic pin in the bore first. I drill large enough to use my Mitutoyo 514-503 tenth indicator to reach in and double check the groove and bore. If you have a difference in the land height to bore, split the difference. Now it is time to taper bore and that allows your reamer to follow your taper bored hole with a loose fitting bushing.

Is that what you are looking for zmalibu? Ain't no secrets, just good solid machining techniques as Jackie has posted.

This method sounds very similar to that of the original poster. He does not state if he uses a taper bored or just a straight bored hole and he does not state if he confirmed the land to groove geometry. He does state that he used a snug fitting reamer bushing but if the lands and grooves are concentric and if the prebored hole is concentric to the grooves Im not visualizing how he can have this issue. Butch I have used the method you described with good results, I only do barrels for myself so my sample size is relatively small.

 

[Tozguy]

He does not state if he uses a taper bored or just a straight bored hole and he does not state if he confirmed the land to groove geometry. He does state that he used a snug fitting reamer bushing but if the lands and grooves are concentric and if the prebored hole is concentric to the grooves Im not visualizing how he can have this issue.

But he does have the issue!
It seems like the OP relied on the tight fitting pilot to keep the reamer concentric. However the pilot no matter how tight will not over ride a crooked pre bore. If a pre bore is used it has to be perfectly tapered and concentric. If results are off then the pre bore was off. How mysterious can basic machining become?

 

[Tozguy]

Seems we are not the only ones 'whittling away the days of winter' by talking chambering.
http://benchrest.com/showthread.php...el-with-a-long-headstock-spindle-length/page2

 

[Roeder]

I don't think it would be a high land in a 6 groove barrel. I would say your pre drill method isn't right, or you didn't use proper size bushing. Just my guess.

Button Rifled barrels can and often do have some out of concentricity between the bore (Lands) and the grooves, 3, 4, or 6 grooves doesn't matter.

If you truly want to get to the bottom of this issue, set the barrel back up in your lathe and indicate the lands just past the throat, then slowly rotate the barrel and note how deep the indicator shows each groove. It doesn't take a lot to produce a visibly off center lead as viewed through a bore scope. The geometry of the typical 1.5 degree throat dictates that a .001 difference in the groove depth would move the forward edge of the lead about .038. This is very visible through the bore scope and looks worse than it really is.

The method Butch describes is probably the best way to mitigate this. Attempting to correct for this would require cutting enough barrel tenon to allow doing the pre bore over. I would test it, it may still shoot.

 

[Tozguy]

This is a case of a picture being worth a thousand words. In this case it should already be obvious through a bore scope if one land is taller or shorter than the others. Or whether the length of the leads phases from long to short and back.

Thanks for doing the math Roeder. Is a .001'' difference in groove depth in the realm of possibility for a custom barrel?

 

[Roeder]

This is a case of a picture being worth a thousand words. In this case it should already be obvious through a bore scope if one land is taller or shorter than the others. Or whether the length of the leads phases from long to short and back.

Thanks for doing the math Roeder. Is a .001'' difference in groove depth in the realm of possibility for a custom barrel?

Absolutely. More likely, is a total runout difference of .001 which would be .0005 on each side. This would reflect the approximately .038 difference in the length of the visible throat from one side to the opposite. This is really a small amount and does not keep a barrel from being a shooter. Really good cut rifled barrels are rifled by a machine that cuts each groove sequentially in steps of .0002 or so, allowing better control of concentricity, at least that has been my experience.

 

[eww1350]

I agree. I indicate a Deltronic pin in the bore first. I drill large enough to use my Mitutoyo 514-503 tenth indicator to reach in and double check the groove and bore. If you have a difference in the land height to bore, split the difference. Now it is time to taper bore and that allows your reamer to follow your taper bored hole with a loose fitting bushing.

Is that what you are looking for zmalibu? Ain't no secrets, just good solid machining techniques as Jackie has posted.

Butch, why is taper boring necessary when I am trying to create a hole for the reamer to follow that is true to the indicated throat..will straight boring not be concentric and straight with what I have indicated..??

 

[butchlambert]

Butch, why is taper boring necessary when I am trying to create a hole for the reamer to follow that is true to the indicated throat..will straight boring not be concentric and straight with what I have indicated..??

Eddie, If you indicated the bore- groove properly you would not have the problem. You want your reamer to follow your prebored hole and not the bore with a tight bushing. Normally with a tight bushing and a bore that wanders in front of the throat it will make the base of your chamber slightly oversize. Kinda what Scott indicated that happened in the throat. I want my reamer to follow my prebored hole that is indicated to the throat.
The prebored hole will be concentric to what you indicated, but a tight bushing could alter your result as it can move the back of the reamer to cut heavier. I hope you understand what I am saying. Now, is your perbored hole larger than the diameter of your reamer at the shoulder. If not it will follow the bushing and not your prebored hole.

 

[eww1350]

I usually pre-bore about .010"-.015" under the shoulder diameter then start the reaming process..

 

[butchlambert]

Eddie, it needs to be about .005 larger than the shoulder diameter or it will not guide the reamer. An example is my PPC is approx .430 at the shoulder and .440 at the web. If you do not want to go to the trouble of taper boring, just bore a short step(.250)at .005 over shoulder diameter. That will allow your bored hole to center your reamer.
The way you are doing it just removes material.

 

[DaveTooley]

Just a question. What's the distance between the bushing and the throat? I have a reamer for a 2.500" magnum made on a long blank. The bushing is a considerable distance in front of the throat. I have to use an undersize bushing and allow the reamer to self center or the throat is not concentric.
Dave

 

[eww1350]

It was a standard length reamer that was a rental..I have never used a long neck reamer, but have seen them..

 

[butchlambert]

Just a question. What's the distance between the bushing and the throat? I have a reamer for a 2.500" magnum made on a long blank. The bushing is a considerable distance in front of the throat. I have to use an undersize bushing and allow the reamer to self center or the throat is not concentric.
Dave

You are correct.

 

[tightneck]

I'll add my 2 cents. I use a Interapid long reach indicator, indicate the bore 2.75" in. Then I use a .0001" indicator and indicate the far breach end. I go back and forth untill I get .0001"-.0002" Which is as good as my bearings will allow. I then will run my reamer in untill I have cut .1" of body. Then I will drill/bore .2" short and finish reaming the chamber. The Initial reaming, gives the reamer a place to start after you drill, and I found letting the reamer do all the cutting for the last .2" gave the tightest chambers. I inside mic all my chambers and the base measures what the print reads. As Butch stated, if your indicated right the reamer wont even need a bushing. I also go in after and check concentricity between the bore and throat after the work is done. I am very confident in this method.

I use a hybrid method similar to this. I first drill to .2" of the finished chamber, then indicate the throat and bore the first .5" to just over shoulder diameter. I then run the reamer in with a tight fitting bushing to establish the start of the chamber. Bore out the rest of the chamber to just under shoulder diameter, and finish ream.