Chamber Runout?

[Bob3700]

OK, I am in need of some chambering guru's once again.

Here is what I have and can't figure out that happened.

I have a 6mm bbl set up in my four jaw chuck. I get the bbl centered with my indicator on the lands of the bbl and the runout is .0002",two tenths). I cut a register on the outside over the chamber area and runout is only .0001". I am good to go.

Start chambering at 40 rpm, using a piloted reamer and Omega oil. The reamer is in a floating reamer holder. Chambering is going great. When I am finished, the chamber has .0006" runout! WTF!!!!!!!

I check the register on the outside of the bbl. Still .0001" I take the reamer and insert it in the bbl and just tap it in with my finger until there is no play. Put the indicator on the reamer shank, there is .0001" runout?????????????

Now how can that be. The OD of the bbl has the same true register as when I started. With the reamer in the chamber, the shank is as true as when I started. The chamber itself has .0006" runout.

I am baffled on this one. Please help if you can.

Bob

 

[Bob3700]

OK,

Some further info. I just inserted my range rod with the pilot that I used on the reamer. The range rod, for those who are not familiar, is a piloted tapered rod. The pilot slides into the bore and the tapered rod then centers up on the lands of the bore.

Put the indicator on the range rod and guess what? That throat indicates between .0001"-.0002" of runout.

Could it be that somehow, the bbl steel has an inconsistent hardness that the reamer ran into? The reamer never chattered, cut smooth all the way to the finish. I took very shallow cuts, approx. .030" at a time. The chamber finished smooth with no imperfections.

Man, I am stumped as to why the big end of this chamber has .0006" runout.

Bob

PS, I still think it will shoot OK as I have seen some chamber runouts larger than this from rifles that hammered at 600 yds.

 

[JDJones]

Did you pre drill the barrel or prebore it ? or just run the reamer in their?

 

[Bob3700]

I used a piloted roughing reamer.

Bob

 

[Rustystud]

Most lathes have runout associated with their bearings even the best lathes. If you reverse your lathe during the process. You will almost always encounter runout. You are removing the load from the bearings. I always run my lathe in reverse for about 5 minutes and the then run it in forward for about 5 minutes at 600 RPMs before starting to set up. This gets the bearing warmed up and lubricated. Most lathes run about 99 percent of their life in one direction forward. After I set up, I run my lathe at 24 RPMS watching my .0001 indicator. I don't start turning, threading or chambering until my .0001 indicator needle sits idle with out any movement. If you limit your feed to .010 you should not put enough presure to cause yourself headaches of getting your stock/barrel out of the desired indication. Ideally a floating reamer holder should have both coaxial and concentric float. Some do some don't. If your floating reamer holder has both coaxial and concentric float and your bushing is properly fitted your barrel should not have any runout that is readable while in the original set up. Once you leave the original setup you will never get a "perfect" reading again.
Rustystud


Rustystud

 

[sjk]

Right on Rusty. Many are unaware of this condition merely because they have not been taught the capabilities of machining. You can make a machine,including mills) do a lot more, but for some of the chinese junks out there, there's no hope. I'm not saying all but comparing them to something of the Monark, Hardinge, Sheldon etc They're not on the same ball field

 

[Bob3700]

Rusty,

Thanks for the machining knowledge/tip. Never thought of running the machine in reverse.

I have chambered up a few bbls recently, but this was the first one that didn't meet my expectations for low chamber runout.

What puzzles me though is how can my register read .0002" runout, the range rod at the throat reads .0002" runout but the chamber reads .0006" runout? That is what is confounding me. If something wasn't right, all those readings would be large. Yes/no?

Bob

 

[Rustystud]

Bob3700:

First, measuring of the range rod is measuring off the rifling and bore,one plane). Measuring the chamber can or can not be in the same plane as the bore. Most likely your pilot was not properly fitted to the bore and it allowed the reamer to yaw and or wallow as it cut the chamber. This could also be a situation where the barrel had a loose spot in it. Floating reamer holders can be the best thing since sliced bread or be the biggest curse there has ever been. A yawing reamer will give an round but oversized chamber. A wallowing reamer will give a egg shaped chamber. Measure the diameter of the web area of the chamber and see if it is round or egg shapped. If it is egg shapped then the throat area will also be egg shapped. The reamer because it is solid cuts in one plane. Because you have two axis one concentric and the other co-axial. If you shoot a piece of brass the in this chamber the fire formed brass will be contorted in shape. A fireformed piece of brass will be visably out of round or have a bulge. No puns intended.
Rustystud

 

[Bob3700]

Rusty,

I appreciate you taking the time to share you knowledge with me. Chambering bbls is relatively new to me and most of my acquired knowledge is 70% reading and 30% doing right now.

If you will. What do you mean by the "Barrel had a loose spot". I have experienced bbls having an inconsistent heat treat. The turning tool will be cutting perfectly and suddenly it starts to chatter or is no longer cutting the same depth chip. Then it goes right back to cutting just fine. That seems to me to be a metal hardness factor.

As far as the pilot not fitting correctly, I guess that is possible. I have two bbls from the same manufacturer in the same bore diameter. Chambered both with the same reamer and holder. Duplicated my steps as exactly as possible. First bbl cut and measured like a dream. Chamber runout was .0002" Great. Second bbl is the one that I have written about.

I could see if the bbl had a "Loose or Hard" spot and the reamer cut inconsistently because of that.

The .0006" runout is disappointing but my gut feeling is that it will shoot just fine. I have pulled several custom bbls off rifles that the owner said was a hammer at 600 yds before it wore out. Put the bbl in the lathe to check the accuracy of the work and the chamber had .001" runout from the bore. So there has to be a bit of lattitude in what really "Works".

Bob

 

[Rustystud]

Bob3700:

Barrels are deep hole drilled. As we all know and agree drills don't drill round holes. Reamers don't always make drilled holes round. We also know barrels will have hard and soft spots. Barrels sometimes diring stress releiving will expand or contract from what the original bore was before stress relieving. You don't see the low end barrels offering that they have been air gauged to check for consistency. You can take a tight lubricatred patch in a jag and check for tight and loose spots in a barrel.

Rustystud

 

[Bob3700]

Rusty,

Thanks for your thoughts and help. Working with metal is as much art as science. Just when you think you have found an absolute, a piece of metal will make a fool out of you.

As I continue to increase my bbl fitting and chambering experience, the realization is that you have to guard against picking fly shxx out of the pepper.

Bob

 

[Clark]

http://www.bryantcustom.com/articles/rebarrel.htm

he roughs out his chamber with a twist drill bit. This is not a piloted drill, bit but just an ordinary twist drill bit. He explained that he then trues up the hole with a small boring bar to give a true hole to start the chamber reamer into the barrel. This also allowed for less wear and tear on his finish chamber reamer.

The boring bar will also produce a hole that is more concentric than a piloted reamer will. So as much metal as possible should be removed with the drill and boring bar.

I don't bother with this in 17M2, 22LR, .223, etc.

But I do with 6mmBR, 300WM, etc.

A hunting rifle that checks out at .0010" before it is taken out of the lathe can be an extremely good shooter.

 

[Rustystud]

Clark:

I don't wish to start an arguement but I have a question:

A reamer bushing has .0005 -.001 ID clearance for .625 inch bearing surface. If used properly the bushing shood have .0002 OD clearance or less with the bore. This gives very little wiggle room.

A boring bar is a single point cutting tool with a .002-.020 cutting surface dependent on setup. Most lathes acceptable for doing gun work have spindle bearings that have between .0005 and .005 run out. The run out is mostly consumed when the bearings have a load on them.

It has been my understanding that there were a number of reasons reason for pre drilling and then boring. I have heard that a properly set up piloted reamer was less accurate in cutting a hole than a boring bar. However I don't believe it and I have been told by one of the best reamer makers it is not true. Both reamers and boring bars can cut out of round holes believe me I have done it before.

I will agree if the barrel is not drilled straight and the reamer follows the bore it may cut a chamber with runout. Barrels are set up off the bore. If the barrel is not dead nuts on the boring bar will also cut an out of round hole. The two main reasons for pre drilling and boring were to speed up the process and save wear and tear on both the reamer and the gunsmith. The second and more important reason was to allow the reamer to freely beging cutting the chamber closer to the point where the bullet will enter the barrel. In other words align the chamber with the bore so the bullet starts off with as close of alignment with the bore as possible. The coaxial and concentric alignment of the bore may or may not be the same at the tenon face as compared to the throat dept of the chamber. Reamers don't flex much, so they must cut out of round holes if the bore moves in relationship to the starting cutting point of a chamber to the throat of a chamber.

Most gunsmiths don't have insturments to read runout below .0001.

Is this not correct?

Rustystud

 

[butchlambert]

Nat,
How can a boring bar cut a cam shaped hole?
Butch

 

[Rustystud]

Butch:

If the barrel is not centered right to begin with. Butch a boring bar cuts a cylinder shape hole. If the muzzle and the tenon are not both centered in alignment the bored hole may not be both coaxially and concentrically aligned. This is one of the things that concerned me about Gordy's methology in his chambering video. I think Jackie may have had the same concern. I think he said soamething about tollerence stacking.

Nat

 

[butchlambert]

Nat,
What I am saying is the bored hole itself will not runout. It will not be coaxial to the bore if it isn't indicated properly, but even at that the bored hole will show virtually no runout if the bearings are good in the headstock.
Butch

 

[Rustystud]

Butch you and I are saying the same thing. If the barrel is indicated off the tenon end and drilled and bored. Then the reamer starts off in the bore begining at the depth of the amount drilled and bored. If the barrel is at a different location concentrically and coaxially in relationship to the original tenon end. Do you not reindicate after the barrel is drilled and bored? We will agree the reamer is going to remove more material and should locate it's own center based on the bore and pilot. My contention is the reamer is going to be influenced more on one side than another. The natural tendency is for the reamer to want to yawl in the oposite direction of the presure influence. When a reamer yawls the run out is increased exponentially by the distance from the point it is not in alignment. If a floating reamer holder is used that yawling of the reamer will be reflected with an out of round chamber. In exagerated cases the neck will also be coaxially out of alignment with the bore. This can also be seen in chambers cut with the reamer held tight and the barrel set up in a bind. You yourself and mike Briant introduced me to using a 4 gauge copper wire in my set up to prevent the barrel from flexing during chambering. We both agree the goal in our chambering is to get the chamber cut at close to perfect coaxial and concentric alignement with the bore. A bullet started off straight has less correction to end up straight.

It is great that you exercise my tiny brain, I have always found four eyes see things differently and you can learn from seeing through the other mans eyes. It would be interesting to have seen this same post on BRC. I am confident Jackie, Mickey, and William would have added their thoughts. I would love to take a couple of barrels chambered by Gordy and a couple of barrels chambered the conventional method set up on a test rail gun and shot with the same loads in the same conditions off the same receiver and see the results. This might be a fun project and get Gene Beggs to shoot them in his tunnel.

Nat

 

[tightneck]

Rustystud, I don't know how everyone else is doing their drilling and boring, but re-indicating AFTER boring as you mentioned in your last post would be detrimental IMHO. I adopted my method after watching and reading others posts on the subject, and a bit of trial and error. The chamber and crown ends are indicated off the grooves and then the appropriate drill is used to rough out the chamber about .015"-.030" short of shoulder diameter, depending on the drill. Then both ends are re-indicated and the grooves must show almost zero run-out AT THE FUTURE THROAT. From there, all cuts are made, from boring the hole to just under chamber dimensions, finish reaming the chamber, turning the tenon, threading etc. The reamer will obviously follow the bored hole, so that hole MUST be concentric with the bore. I've had my best results indicating the throat as have many of the best BR smiths. I'm not saying its the only way to do it, just that it works very well.

 

[Clark]

Rustystud,
My brain is at least as tiny.

I never thought to compare the headstock bearing runout with the reamer pilot to bore misfit.

Now that you got me started, I can see the the reamer tilt and reamer offset from pilot misfit is either/or/partial each.
With 100% tilt, the pilot is fitting tight.
With 100% offset, the tilt is zero.

Now that you got me started, headstock runout would be out of round, but would still be average centered. This might have no effect on accuracy.

I did figure out on my own that the barrel would just flex between 4 jaw and spider, but I picked up the Copper wire gimbal solution later on the internet.

I did not figure out on my own that the barrel is bent, and the barrel only needs to be locally and longitudinally concentric as dialed in over the length of a spud.

I did not figure out on my own the mathematical relationship between ammo concentricity and accuracy that I read in the NRA and US Army in the A A Abbatiello 1960 article "Gauging bullet tilt" and the 1969 patent 3440923 issued to Clarence Purdie was for the sliding sleeve seater die. Concentricity came to me as gun culture accuracy folklore in my 2000 quest for a 1" group.

 

[rbertalotto]

The chamber and crown ends are indicated off the grooves and then the appropriate drill is used to rough out the chamber about .015"-.030" short of shoulder diameter, depending on the drill. Then both ends are re-indicated and the grooves must show almost zero run-out AT THE FUTURE THROAT. From there, all cuts are made, from boring the hole to just under chamber dimensions, finish reaming the chamber, turning the tenon, threading etc. The reamer will obviously follow the bored hole, so that hole MUST be concentric with the bore. I've had my best results indicating the throat as have many of the best BR smiths. I'm not saying its the only way to do it, just that it works very well.

This topic comes up time and time again.......I just don't understand how you can indicate the concentricity of the bore and the chamber any better than this method. I've tried them all and IMO this method obtains the best results.....