doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Question on Wall Thickness Gage

Last summer I made a horizontal wall thickness gage to take the place of the vertical one I had. For the vertical one, you need a secretary to write the numbers, and since that isn't happening, and the dog can't write as of yet, I decided to try and make a horrizontal one.
I had posted some pictures then and was happy with it, but noticed that the indicator needle was not centered on all of the ball bearings for getting zero. At the time I didn't have acess to a milling machine and used my drill press with a xy compound table. Very hard to drill five holes on a 36" rod in perfect alignment.

On this left side you can see the anti-rotation pin and the ball bearing for setting zero.
On the right side, the calibrating ball was not close to zero, so I put a jack screw in the back and used it to push the rod over to zero, went back and checked the other ball at 24" and the one all the way to the left set at 4 3/4" from the stop (well enough past the chambers and forcing cone.

Now my question, the dial needle is not centered on every ball bearing, they are all reading zero +/-.002. I realize that a sphere is zero anywhere, but what does it do when a barrel is placed on the rod and is resting now on 2 balls that might not be perfectly in-line with each other?

The reason I ask is on a few barrels I was getting some low readings, so I went to the vertical one and the readings were better.
So for those more knowledgeable, is the needle reading the barrel on the balls or is it distorted by not being in-line.

The thing that is nice about the horizontal one is that once the barrel is on the rod, I can rotate it from top rib to bottom rib and get readings, very hard to do vertical.

After I posted this, I started thinking, outloud, and yes it makes quite a bit of difference. The needle is at zero on the sphere, but the barrel is at it's own zero on another part of the sphere and could be as much as .010 difference.

I have absolutely no idea how that tool is supposed to work.

Can you please take a picture with a barrel being measured. I also have no idea how this is supposed to work. Do you remove that bottom aluminum block? That would put the entire weight of the barrel on the bottom rod causing a false reading.

Pete

The mice are holding their breath, Dave.

jack

It is quite simple, the barrel is supported on 2- 3/8" ball bearings on a 5/8" dia. rod. The rod is setting in a V on each pedestal, the barrel is placed on the rod and the breech end is pushed up against the stop. The dial is then moved from the set position to anywhere along the barrel you want to measure.
There are only 2 points along the whole length of the barrel where it is in contact with the ball bearings, so there is only about .001 sag. That is the reason for the other ball bearing all the way to the right, it is there to set zero and to see if there is any sag.
The two points are 4 3/4" and 24" along the barrel's length.

The only problem I see is that you are assuming the bore is perfectly straight.

David:

If you are zeroing your gauge while it is resting on the stands, then putting the rod through the barrel and resting it on its stands, does either end of the barrel rest on the stand? Or is the other barrel resting on the bench thereby raising the height of the rod through the barrel? How do you keep two pressure points on the gauge head if the ball is fixed at the breach and muzzle ends? Any flex or sag in the rod?

I am more familiar with the vertical, free standing BWT gauge from Galazan and others of that ilk, then horizontal bar gauges. Russ

Peter, yes I am assuming that, and would hope that they are. If they are not, then this would be useless. But, I also think that in drawings of good barrel wall thickness, a greater/lesser reading that is close to the one before or after would show this.

RWG, the barrel is resting on only 2 points on the rod, one 4 3/4" from left hand stop and the other at close to 24" from stop on left. This way it is away from the chamber and forcing cone and on the 24" is away from the start of the choke area.
When the rod is placed in the left and right pillars, I zero the left side and then the right side, and will check the other two balls as well. Once the barrel is placed on the rod, the dial needle is already on the left zero point, I then move it all the way to the right and check it again for zero. Sometimes the sag is .001-.0015.

Interesting device but I still can't visualize the whole set-up. Does the indicator slide along that top rail? If so, I bet it sags or deforms several thou at least.

If your balls read at .002 difference, that isn't really acceptable error.

It may be a design flaw that you have to get balls lined up to less than a thou over a 30-inch length. Very hard to do

Of course, I may be simply not understanding the entire device - can you post a picture of the whole thing with a barrel on it? Also we need to see the end-blocks - are they one-piece or are they split?

How repeatable are your measurements? Have you tried to take a piece of tube of known wall thickness and tried to measure it?

I have made several wall thickness gauges in my shop and know that even if they appear simple they are hard to design so they work well.