Dialing in the bore "test"

[Dennis Sorensen]

Just thought I would share this.

First the barrel is set in the lathe and closely lined up… a 4-jaw chuck in the head stock, a 4-screw spider at the muzzle end.

Picture one shows the muzzle dialed in. That portion of the barrel is true with the bore of the muzzle. It only needs to be within a thou or two this far from the chamber. It requires adjusting as the chamber is being adjusted. One affects the other.

Picture two shows the bore right at the end of the chamber (for ease of this test only) being dialed in using the long rod with a removable pilot in the bore and the other end held lightly by the tail stock chuck and an 8 ounce weight hanging on it to pre-load the tension so any movement up and down can be accurately measured. The barrel was dialed in at both ends until the indicator did not fluctuate one 10 thousands of an inch when the barrel was rotated.

Picture three shows a Deltronic gauge.

Picture four shows the Deltronic gauge firmly seated in the bore. A tight fit. No adjustments were required and there was no movement detected when the barrel was rotated.

Picture five shows the actual bore being indicated. I detected possible movement of one or two ten thou in the lands… but I could not indicate it reliably and repeatably enough that I would make any changes.

Measurements at the chamber end were done using a test dial indicating to one 10 thousands of an inch.

Now I don’t know about everyone else but my experience with measuring one 10 thousands of an inch over bumps and hollows is that it is extremely difficult… The little ball bounces in and out of the lands, sometimes rocking the barrel back and forth gets it to ‘settle’. My point is one or two ten thousands of an inch is difficult to determine inside a barrel with a mechanical gauge.

I know in theory some believe you should only indicate to the lands, not the bore (top of the rifling) as the depth of rifling may vary and the bullet is always going to fill the lands… but in practice quality barrels are better than very good and when using a good reamer with a pilot the pilot always fits the bore, not the grooves.

I have confidence in using the piloted rod method as shown here.

 

[spencer313]

Now, if you run your piloted, weighted rod in and out so that you measure at different points, without touching the chuck or spider, you should be able to measure the curvature of the bore. It would be interesting to hear what you observe.

 

[alinwa]

Dennis, did you mean "did not fluctuate 1/10,000) or did not fluctuate .0001 in your post above?

In other words, zero wobble on a .0001 indicator???

al

 

[Dennis Sorensen]

Thanks Al, I corrected the text... I was referring to one 10 thousands...

I don't think it is possible to measure any curvature in a bore with a mechanical set up as this.

There was no change from where I indicated and 2 inches into the bore.

 

[mike in co]

your saying THIS bbl has no detectable difference in runout at the throat and the base of the chamber ?

i would never claim that all bbls have no curvature...but i would also never cliam one had none detectable over 2 inches or so....

but have you measured with an indicator at the throat...to see how close the setup is ??

and thanks for the setup/pics and info

mike in co

 

[alinwa]

Thanks Al, I corrected the text... I was referring to one 10 thousands...

I don't think it is possible to measure any curvature in a bore with a mechanical set up as this.

There was no change from where I indicated and 2 inches into the bore.

Well that's a conclusive statement, from your one setup!



lol

al

 

[jackie schmidt]

When I indicate the throat portion of a barrel, I can move the indicator 1/2 inch one way or the other in most barrels and get runnout, (after indicating one sopt dead true). If there is one of those squirley spots just where I am trying to indicate, you can get as much as 1/2 thousanth in an inch.

What I do is declare that spot I have decided to indicates as "it", and I don't worry about things I have no controle ove..

For many of you who are not skilled in Machine Shop Practice, what we are really engaged in is the proccess of 'splitting the difference'. More times than not, that is your option when dealing with items that are not as truly straight as you would like.......jackie

 

[Dennis Sorensen]

Jackie, I would trust your indicating over mine any day. 1/2 of a thou (5 - 10 thou) I have no problem indicating, it's when I get down to one ten thou and a 6 groove barrel I wonder how accurate I am. I have had 'squirley' spots and always thought it was me... I guess I am going to have to believe my indicators more...

Mike, I did not measure the bore between the set up at the rear and 2 inches into the bore... after dialing the bore in, I set the pilot in 2 inches and did not indicate any difference at that point. I did not do a 'travelling' indication.

I am going to continue 'testing' on the next few set ups... and I wish everyone made a 4 groove barrel.

 

[kgcoker]

Corrected spelling

I have a question along the same line. I received a barrel today from a well know maker. It is marked 218/224. A .218 range rod will go in about 1/4 inch in either end then get tight. A .216 will go in freely for about 3" or 4" and get tight. I know I can cut the muzzle at the tight spot and that is good. I can chamber the other end and that is good. What about the other area of the bore? I know I can slug the barrel and get a better idea. Is this common? I have never experienced this before.

Thanks,
KG

 

[Dennis Sorensen]

I have lightly pushed a Deltronic gauge (1.5 inches long) from one end of a barrel to the other... the occasional barrel may feel uniform but I have seen barrels with large variations where the gauge will stop firmly somewhere in the middle... the worst I saw was on a hunting rifle brought to me with inaccuracy problems. There was a very tight spot about 2 inches ahead of the throat and then it was a full thou + larger down the spout...

 

[alinwa]

what we are really engaged in is the proccess of 'splitting the difference'. More times than not, that is your option when dealing with items that are not as truly straight as you would like.......jackie

I see this differently....

whether or not one tries to "straighten" the barrel at the chamber or one chambers "between centers" I feel it's best to try and hit the throat deads-nuts-center instead of averaging in any of the runout fore and aft.

"Splitting the difference" is for when you have to give on both ends......

al

 

[frwillia]

Your results say to me that the bore at the breech end that barrel blank is very well aligned with the muzzle of the barrel blank. The fact that it didn't change when you replaced the probe with the pin is the key piece of data for that conclusion. That says if you moved the probe back and forth in the bore it wouldn't change the reading noticably. That's a nice barrel blank for at least that characteristic. Hard to know how the bore is without slugging it, but I could sure like the alignment.

IMO: the probe the way you used it is probably the most accurate way to measure at the throat with out preboring first.

Fitch

 

[sailhertoo]

You sure keep your lathe clean! Good pics

What is a Deltronic gauge?

 

[Dennis Sorensen]

What is a Deltronic gauge?

A very precisely made piece of steel as perfectly round and as true as can be made, about 1.5 inches long, available in various diameters in one ten thousands of an inch increments.