Both Bros BAT were in the shop today. I had both of them look at the big scratch. One of them said a bad spot in the barrel was as good of an explanation as any, but then showed me a tiny chip weld on the reamer. He's a very diplomatic guy.
He also showed me how to remove any chip weld. He said lots of methods work but he likes a flat diamond file the best. So I am bringing mine from home. Since I now know what to look for, every reamer will now get inspected before each use and dressed if needed.
I asked how it was possible that a new reamer, run with a flush at 80 PSI (I have been turning it up lately), with a speed of 140 RPM could get a chip weld. He said that's the machining life. He said it is a combination of the particular reamer, the particular barrel, and the particular chambering method.
We then talked through how I chamber. 140-180 RPM, in .050" and back out to ensure all chips are flushed. He said in-and-out works fine and that is how he did it before he added his flush system.
He thinks 140 RPM is too high. I pointed out that JGS says 200-300, and he disagrees. He likes to push the reamer in very slowly at 70 RPM and make one pass. He doesn't have a high pressure flush system. It barely dribbles out.
He then asked the money question. Was I stopping the lathe before withdrawing the reamer. I said no. He recommends backing off the pressure at the end of the cut and then stopping the lathe by letting it wind down.
He, like
@butchlambert and others here, pointed out that withdrawing the reamer with the lathe turning creates space that can catch a chip.
Thinking through this, that could have happened. The 260 Rem is a very tapered case with a small neck. Meaning there is a lot of room for a stray chip. I don't think I have cut a chamber with a more tapered reamer. I did a bunch of .308s a couple months ago, but they have a bigger neck, and they were better quality barrels.
So I am thinking that a combination of factors caused this. Gummy barrel steel with a heavily tapered small neck case, a reamer that was perhaps a little on the too sharp side, and a sub-optimum chambering method by me. The flush may have actually worked against me in this instance by pushing a chip into the wrong spot.
Going forward immediately:
1. Reamer is inspected and dressed if needed before each use
2. Stop lathe before withdrawing reamer
3. Slow down RPM even more
Next steps:
1. Figure out how to drill and prebore
2. Modify all the reamer screws
3. Consider making longer passes
4. Build pusher and use (parts are on order)
Like I have been whining about, being at the sophomore level sucks. However, these are lessons that I need to learn. I am very thankful to have a pair of master machinists close by willing to teach me in person.
On the positive side, the chamber I cut on Tuesday is beautiful (6.5 PRC), and an experienced customer (a gun shop owner) complimented me on how nice my muzzle threads looked. I have 15 more barrels of his to thread. I apologized for being so slow. He that's okay, that it was worth the wait. He said he can find fast and sloppy everywhere.
I currently have a 338 Lapua IMP chamber to cut. Things have been so busy that all I got done yesterday was turning the tenon, and today I got it threaded. The other BAT Brother said my threads looked good.
The new lathe operating is progressing nicely and soon it won't all be on me.
