Chamber polishing

Al,
Without seeing the OP's chamber, I can't pass judgement. I put the groove in my chamber. It was just back of the shoulder and hard to see as I didn't have a borescope at that time. It was not a tool mark. It was swarf caught between the reamer flute and the chamber. Since I shut my spindle down now before retracting the reamer, no more troubles.
Butch
 
I misspoke re 'tool mark,' obviously even if the lump which produced the groove WERE ground into the tool it wouldn't manifest as a groove. I would though still use 'tooling mark' assuming that everyone reading would know that it's the result of chips or swarf, not the grind of the tooling.


Communication is hard :)

al
 
OK boys and girls read the following test results--

Notice that when the steel finish gets to about 32rms or slicker what happens (about 600 grit)
Also notice the effect of a lubed chamber

http://varmintal.com/afric.htm
 
Not to take anything away from the tests he did but I would like to see test results that used live loads and actual pressures. That would involve an expanded case with very high pressure within a chamber. I don't know how such a test would be done. Once pressures reach a level where the brass will stretch to fit the chamber, bolt thrust will be increased regardless of the chamber polish. It only makes sense that a 'rougher' chamber will grip the case more than a 'smoother' chamber but once the higher pressure 'takes over' it would be difficult to design a test to distinguish the differences.

One of the first things I noticed in the test results at that link - Estimated coefficients of friction used...
 
Those varmint al tests are LAME!!! Lissen to leeroy, the Blish effect is real. Establishing a "coefficient" by dragging blocks acros't he table is beyond lame, it's flawed beyond belief!

al
 
I wouldn't say Varmint AL's test is flawed, but in no way does it duplicate what happens with 60,000 psi applied. What
appears as a smooth unblemished finish in a polished chamber actually has lots of little hills and valleys. They are closer
together and not as deep, but they are there. That 60,000 is 30 tons per square inch and for that moment might as well
be welded together.
 
The original Tommy gun design actually put this 'stiction' feature to use, using a lighter breechblock than would be required for a straight blowback design. Of course the flaw is/was that if you leave oil in your chamber the effect is largely neutralized.

That said, when using a dry chamber the case DOES NOT SLIDE with normal loads. This is easy to prove via simple testing. Varmint Al again used a computer model to predict an effect. Problem is, his model simply doesn't match reality because of a flawed initial premise. John Blish was an engineer too....difference was, he did real-world testing not modeling.

al
 
I'm not sure that Leeroy's claim is absolutely correct.

Parker Ackley ran some experiments way back with a Winchester 94 & a 30/30 AI chambering. He was able to demonstrate that the case held the full pressure of the load he was using by firing the unit without the breech locking blocks in place & with up to .1" of clearance behind the case head (headspace or whatever you are happy calling it). The rounds didn't back out, though the primers did, which says to me that the case he used alone is capable of holding that sort of pressure - just like straight cases do in revolvers. When he oiled/lubed (I lost the books in the january floods, I think) the cases or chamber, they stretched back to the breech block face, but didn't hassle the inertia of the assembly.

I think that it is therefore reasonable to conclude that there is a certain amount of pressure being restrained by the brass case alone & it's just the residue that comes back onto the bolt face.

John
 
Ben Dover said:
cartridge head gap vs. headspace ???
Click to expand...

I am assuming this is a question...

My thoughts on it...

This is about terminology... and it isn't just to be anal about it. If the incorrect terminology is used often enough, people will start to believe it and knowledge is lost. This has already happened with some of the various tools on the market that are claimed to measure headspace when in fact they are measuring cartridge head gap.

Headspace is set and not adjustable when the gunsmith installs and chambers a barrel. A standard chamber is correctly set using a steel go gauge. Nothing you do with a reload will alter the headspace.

Cartridge head gap (I use the word gap rather than space to make the point it is different than chamber headspace) is adjustable by the re-loader. You can push the shoulder back and have a lot of gap to the point of being dangerous, but the rifle's headspace is correct and considered safe.

Any measurement using a cartridge case is measuring the cartridge head gap. Any measurement of the chamber using a steel gauge made to an exact measurement is measuring headspace.
 
Some of these post's make me wonder! An earlier post was truing action's, lapping scope ring's, faultless bedding, then seeing nothing wrong with a 1/4 lb. bolt handle. Now we're going from dialing in a barrel to zero to putting a piece of emery cloth on a stick to polish a chamber
 
Not just any ol' stick. One that has been surgically split to the proper length, and the emery cloth cut to exacting lengths.
 
I kind of wondered the same thing that B. Johnson mentioned.
We make every thing so precise and yet we use sand paper on a stick inside the chamber to remove metal.
I would think that it is quite possible to change the perfection of concentricity that we strive for by possibly removing metal in order to polish the chamber.
If we remove .001 to polish then couldnt it be .001 out of round somewhere? Or even l.0005?
 
Butch! I was thinking precision. As for another question! How do you cut emery cloth to an exact dimention, and how do you figure that dimention?
 
Spinning the lathe and moving the abrasive in across hatch motion lightly for a few seconds isn't going to make anything out of round. Spinning the abrasive on a Black and Decker for a minute or two with the barrel in a vise will.
 
Obviously the posters asking the questions have never been in a precision machine shop environment........ :) never actually fitted parts.

So, B.Johnson, you think truing on a lathe is an "accurate" procedure VS polishing? A lathe is a rough-in machine VS polishing....if the person doing the work has a clue.

And somehow a "1/4 lb bolt handle" is bad for ????? "precision?" Accuracy? What?

al
 
You are absolutely correct Al, I have not. That is why I was asking. Hoping to learn something.
Thanks Dennis that clears it up a little.
 
I have been in a precision machine shop! more than one. When a referred to the bolt handle, there is a book by a Gen. Hatcher who was at Springfield Arsenal. They did test's where they cut the bolt handle off. It made a difference. Those people had equipment probably as good as the home gunsmith. A lathe is not considered a finish tool, but the end result can be made worse by the wrong approach. My Machinery Handbook make's no note of emery cloth as a precision finish tool.
 
Well, opinions vary.... this is what makes the world go 'round :)

But you can't argue with results, THIS is what determines who brings home the goods.

:)

al
 
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