lobed chamber from chatter - further analysis

[lrgoodger]

After taking further measurements on the fired brass with the concentricity gauge, I'm convinced that the posters who said it was caused by reamer chatter are correct. The gauge will measure roundness instead of runout if you put the gauge tip directly over the case support balls. I measured roundness in various places all the way from the tip of the neck to the head. The 5 lobe pattern was deepest on the neck. It got less and less as I moved toward the head until it was gone at the head. That tells me the reamer was perfectly centered in the lathe and that the tip of the reamer was bouncing off something at 72 degree intervals at the pilot end. It can't be slop between the pilot and reamer because the reamer has six flutes (and I can detect no slop at all in it). My guess is that the pilot size was too small for the bore. My pilot measures .276 as near as I can tell with my $10 Chinese caliper. The bartlein barrel headstamp read .277/.284. That one thousandth difference between the bore and the pilot would allow exactly the half thou, 5 lobed scalloping in am seeing. I would think one would want to use a pilot with about two ten-thousandths clearance for best results.

The question now is; should I be concerned about it? (I suspect this condition exists on a lot of chambers and just went un-noticed because the owners did not know to look for it or how to find it.)

My original post on this topic had a response that 5R barrels had an issue with this and that there was a solution, but the poster did not say what that solution was. Now the post has been removed (derogatory to 5R barrel makers?) and I can't remember who posted that. I would certainly be interested in the solution so that I can make sure my gunsmith knows about it. I have a new Barlein on the way from Grizzly and I want to make sure this one is done right.

 

[lrgoodger]

I deleted it as I grew tired of "experts" giving off the wall bad advice. I find it irritating when guys that know little, can't spell, can't type but by God they're gonna' get their two cents in anyway. Facebook and Twitter are the place to do that. When that happens I go in and delete all the posts I can still reach. Sorry.

The five "flutes" come from the five lands in the barrel. It can also be a multiple of five - as I said mine had ten high-low "flutes" in a 5R barrel. My 'smith followed Gordy's advice when he recut the chamber and it worked fine.

Here's the response from Gordy Gritters courtesy of Benchrest Central to someone having a similar problem:

MIT, I seriously doubt the reamer is the problem here. Measure the OD on the freebore section on the reamer, and if that measures OK the reamer won't be the problem. It may be something else, but you very well might be experiencing something I frequently see to one degree or another, especially in canted land barrels. Go in with your long-reach indicator and see if the free-bore section of the throat is smooth. My guess is it's not, which means you have experienced reamer chatter.

It's amazing how much reamer flex can happen in these chambers, and it's especially prone to do this in canted land barrels (it can do it in conventional land barrels also, but not nearly as readily). It will do it no matter how true you have the bore dialed in, how well-fitted the reamer bushing is in the bore, how carefully you feed the reamer in, or how big and expensive your lathe is. Carbide reamers are much stiffer and won't do it as bad, but most reamers are HSS and they will definitely do it!

I started seeing this unexplained reamer chatter phenomenon as soon as I started using canted land barrels quite a few years ago. I called the barrel maker on it a number of times, but he told me I was the only guy in the country he knew of who had seen this. I told him maybe I was the only guy who consistently measures for this all the way through the chambering process, but I see it to this day on about every canted land barrel I chamber. I still use and like canted land barrels, but I just know to be extra careful when chambering them.

I often demonstrate this in my classes (and will be showing this in-depth in an upcoming "Advanced Chambering" DVD) - I'll tell the students exactly when the chatter will start. I will be chambering along and the indicator shows absolutely no chatter whatsoever anywhere in the chamber. When the reamer is far enough in to start cutting the throat, we will then instantly be able to measure chatter through the whole length of the chamber. In every single case, when I can feel the reamer start to cut the throat, I'll tell them we'll now put the indicator in and see what happens - sure enough, we can now measure chatter beginning, not only in the throat area, but in the entire chamber. Usually its very minor and if you didn’t measure for it you’d never know anything was going on, but it’s there and can get worse if you don’t catch it in time.

Then I show them the method I now use to prevent this as much as possible - simply prebore the lands away clear to the end of the neck while you are pre-drilling and boring the body of the chamber. This allows the reamer to be completely seated (and now fully supported) before it finally gets to the throat area as it comes to full depth. When the reamer is fully supported and cutting completely before it finally picks up the throat way at the end, this really minimizes chatter problems.

Now if you have to cut a longer throat with a separate throating reamer after you've done using the main chambering reamer, you really have to be careful the chatter doesn't get away from you, since the body of the throater is completely unsupported. Both the throater and the chamber reamer will benefit by using the wax-paper wrap trick since this dampens the flex somewhat and will help keep you out of trouble.

I've tried quite a few things over the years trying to figure out what causes this and how to prevent it. In my opinion, what happens is the conventional land barrels have straight vertical sides on the lands, so when the straight vertical flutes on the reamer cuts into them, they meet square and there is no side pressure on the reamer flutes so they cut very nicely. But in canted land barrels, the lands are sloped at a pretty good angle, so the flutes of the reamer want to ride up the lands instead of cutting straight into them. Most of the canted land barrels are an odd number like 5 land/groove, so each time a reamer flute tries to ride up a land, it pushes away from that land making the opposing flutes dig into the grooves between the opposite lands (you can usually see this in a borescope if you know what to look for). Measure it and you'll see that the freebore is now 5-sided and not round!

This can happen severely enough that the effective diameter of the freebore section in the throat (the tops of the chatter) is smaller than the reamer itself. The reamer just flexes up and down in these grooves and flexes sideways just enough to form "lands" which is what you're measuring with your indicator. I've often seen it (and measured it) where the reamer chatter in the throat made the effective freebore smaller than the bullet diameter like you've seen here.

Check it out and see if maybe this is what you're experiencing. Hope this helps!

Outstanding, Dave! I agree with you on the off the wall, maybe it was this, maybe it was that by folk who really don't have a clue, but I know there are some knowledgeable readers like yourself out there who can help me. It's a little frustrating to have to weed out the good from the bad, but usually someone with something valuable will chip in. Your post stood out like a gem among the others. That's how I was able to remember it. Let me encourage you to overcome the frustration and leave those gems out there for the benefit of those smart enough to understand them.

I had a few mechanical engineering college classes while completing my electrical engineering degree (aced all of them, too), so I know enough to put together the clues from measurements I took. I don't have any practical machining experience though (I spent all my time building circuits), so I look to those who do when I run into the limits of my knowledge.

Your outstanding post by Gordy Gritters will be forwarded to my gunsmith. It's very probable that he was not aware of the information it contains.

Best regards,

Ron Goodger

 

[Kings X]

I wondered what happened to this thread.
The first time I had a reamer chatter - I call Clymer for advise. He told me to wrap the reamer in cotton square cut from an old shirt. That worked in the 70's and works today. We also has 4 flute roughing reamers that you could kiss the chattered chamber with a cut it back round.
Today I still use a "filler" some of the time. Now I don't "see" the relections until the lather is slowing down during chamber polishing.
I may need to change my method of chambering the 5R barrels. As I am in the process of ripping out my worn out manual belt drive and installing a electronic controled speed unit.
Dave - sorry for the mis-spelling. I am just a dumb machinist - not a gun writer.

 

[amlevin]

Out of curiosity, would a reamer with a live pilot have the same issue with the 5-R barrels? It would seem that the non moving outer surface would be immune to the riding up on the canted rifling, or would it?

 

[lrgoodger]

Out of curiosity, would a reamer with a live pilot have the same issue with the 5-R barrels? It would seem that the non moving outer surface would be immune to the riding up on the canted rifling, or would it?

If you read the post again carefully, you will see that Gritters said, "It will do it no matter how true you have the bore dialed in, how well-fitted the reamer bushing is in the bore, how carefully you feed the reamer in, or how big and expensive your lathe is."

Also, my original post says, "It can't be slop between the pilot and reamer....", which indicates my reamer has a live pilot.

The answer to your question was already there. In Gordy's statement, if its not a live pilot there is no bushing. In mine, if it is not a live pilot there is no clearance between the pilot and reamer because it is one piece.

I sometimes have to read longer and more complex posts several times to make sure my question isn't already answered (it can be easy to miss something if you don't), but it keeps me from sticking my foot in my mouth (most of the time, anyway).