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

So, do you measure the wall thickness at the chamber? The problem barrels I have found mostly have had the chambers lengthened.

Pete

Gnomon, yes the indicator slides on the 1/2" square tube, I find it hard to believe it would sag.

The end blocks are one piece.

I haven't tried a tube of known thickness and that would be a good idea. I will have to try that.

I will post some pictures later, hard to post close-ups in one picture because of length, maybe show a split and post two.

Pete, the thickness can be measured there, but since the barrel is resting on the ball at bore diameter, the wall thickness at the chamber would not be correct for wall thickness.

I believe you would have to subtract bore diameter from chamber diameter to get wall thickness. This would have to be done the entire length of the chamber, and since they are tapered, kind of hard to do accurately.

A $100 Manson gauge will measure to the thousandth, is convenient, portable, and provides repeatable measurements. I won't be designing any tool in an attempt to improve on it.

Bill, I'm not sure which type of Manson gage you have, but I believe that only the hanging one is accurate, and it has to have the spring on it to push the barrel away from the rod. The troulble even with this one, if you don't hold the barrel parallel to the rod as you are pushing the barrel farther up, you will get variations in the readings, because now the barrel is acting like a see-saw and the needle will reflect that.

I am only trying to come up with something that can read a barrel's wall thickness in more than a 180 degree arc and still be able to watch the dial needle instead of watching the barrel.

David- you have an interesting instrument here but as with any measuring instrument you have to determine such parameters as repeatability and you have to determine the internal error.

If you take a measurement; then take the barrel off and replace it and re-take the measurement is it the same?

Another potential problem with your design is torquing - unless the aluminum ends are on a flat and level surface (in a machine sense) any slight twist will affect measurement - sort of like a twisted lathe bed.

I bet that if you were to put a perfectly flat straight non-sagging piece of metal between the end pieces and ran your indicator along the length of it you wouldn't get the same reading all along the piece.

By the way, did you mill the end pieces on your drill press also?

It looks like the block that holds the indicator has a square hole into which the horizontal bar fits. In order to get as sliding fit you need several thou clearance just here and that will make for a loose fit and decrease repeatability. That horizontal bar needs to be a precision piece and some sort of semi-kinematic design for the sliding part to maintain precision.

David, I'm not being critical but I've struggled with a lot of stuff like this in instrument design.

Gnoman, you bring up some interesting points. If I do take the barrels off and put them back on, I do get readings that are within .001-.002 of the previous ones.

To check out for the torquing, I could mount another indicator on a stand and put the dial needle on the top left side or right.

As for finding a perfectly flat non-sagging tube, not sure where to look for one to try. I would think a piece of 3/4" copper pipe 30" long and checked with a straight edge that is made for checking straightness would be sufficient, just wonder if it is exruded or rolled.

Yes I did mill them in my drill press.

I started out making a block to fit the square tubing by filing it square, too much work and it didn't come out right. What I did was measure the square tubing across the diagonal then drilled a hole that size in the block for the indiactor. It is loose front to back, but the forward weight of the aluminum and the dial indicator's weight hold it down.

Thanks, I appreciate any input and critisim isn't always a bad thing as this is how we all learn.

I see that you are in eastern Pa, also, I live near Cabela's, maybe we could meet and you could look at it.

I may be seeing it wrong, but I think Peter B. is on to something. Assuming no deflection, there can be a lot of variation along that nineteen plus inches between the two support points of your bearings. I believe the gauge might work best if it was zeroed right on one of the support bearings, then make the measurement at that same point. But, then you'd have to move things to get the barrels on the jig.

Again, sorry if I'm seeing the jig incorrectly, I like to see shop solutions and how folks work through problems.

craigd, you have a valid point. The bar with the ball bearings is 36", roughly 3/4" is resting on each pillar, so now you have 34 1/2" and at half or 17 1/4" would be I would think, the most sag. My theory, since the balls are closer to each pillar, the barrels weight is being distributed evenly across these two points and does not effect the center at all.
Most of the weight would be on the left side, naturally where the barrels are joined and where the lug is, which is forward of where the first ball is.

In the scenerio you suggested, it would be the same, once you moved the barrel to read that ball under it, then zeored it and moved the barrels over it again, the readings would be the same as if the barrels weren't moved at all.

In my post before this, I made a mistake, the aluminum block holding the indicator was filed square to fit the square tube, but it came out loose, and as Gnoman stated, it has to have clearance for it to slide. But in re-checking it more than once, it comes back to within that .001-.002 from the original.
The pillar blocks were drilled round and a set screw holds it tight to each pillar.

David- If you use a tube it should be extruded and not rolled. I would suggest a length of milled flat steel bar such as one gets from MSC. 1/4 x 2-inch standing vertically and the dial indicator on the edge. Its sag will be negligible.

1-2 thou wiggle may be too much. If it is additive you may mis-measure thickness by 4 thou. It may not matter if your barrels are 30 thou thick but if they're 21 thou thick then it is potentially serious.

The movable indicator is an Achilles heel to this design - It is very difficult to mount even a light dial indicator in such a way that it doesn't wobble or flex over the distances you are trying. At the very least you would probably need a precision bar and a semi-kinematic design in the block to maintain precision.

A length of extruded brass tube, obtainable from MSC or McMaster-Carr would be ideal for experimenting.