Gary,
I consider you the master of working with close tolerances. So, I wouldn't knowingly disagree with anything you stated on the subject.
I would simply ask the question as to how you would consistently take such a precise measurement as requested by the OP (.0001") if you aren't certain that your case head is square to the body to at least the same degree of precision?
Most digital calipers will give a reading to the nearest .0005". I have no idea how trustworthy that reading is. But as a relative comparison, spinning the case will cause the reading to fluctuate which I suspect is a function of (a) pressure/technique, and (b) case heads that are slightly out of square.
i.e. ye olde banana case theory
-- richard
I did super precision measuring for years, and most ofit is getting into a proper frame of mind. When you start talking one tenth, you need to move into arc seconds, as that's where finite measurments start.
Now with my above disclaimer(<G), here's how I start. Everything you do has to be super consistent. Otherwise you cannot develop a base to start your measurments off of. So if you in this case measure twenty cases with the headspace measurment on the datum line, and you see everything running plus or minus seven tenths. Now you can start trying to tweek a couple tenths here and there. But your never going to gain much beyond the mechanical movement in the press and the flexing of the dies in the thread. A good many folks also think that with the large diameter case bodys so much in vogue today that the die diameter is too small. And honestly a one thousandth window is not all that bad (+/-.0005"). Basicly that .0005" is about ten and one half arc seconds (.000048"), but don't let those zeros scare you. They're just numbers
Now we want to accurately measure something that may vary as much as .0025", but we also want finite accuracey. Picture a centerline with the line going thru the tip of the bullet & primer pocket. We can figure somekind of an imaginary gauge line in the shoulder. I like to split the shoulder O.D. with the neck O.D. You'll want a bushing that has an I.D. of that diameter, but you also want that bushing to have a slight radius that runs very true with the I.D. (.015"R. is about right). Reason for a radius is that it's near impossible to duplicate the same angle as the shoulder. The base of the case must contact something that is flat and smooth (forget shell holders) (good idea to make sure the base of the case is also flat and has no raised areas). Now in theory you have all the pieces to measure that datum line. Right? Wrong!! But there's still enough there to measure it down to about .0005". Make your bushings out of soft steel, or you can simply use a Redding or Wilson die bushing (make sure the two faces are parallel with the bore by the way). Mr. Scott shoots a 6.5x.284, so a good mumber to work with is .385" on the I.D., but even .360" would work. It's your personal number you've setup for the case after all.
It's pretty hard to measure the chamber of a rifle right down to one tenth, but you can get fairly close with some trail and error. I like to very gently size a case down to where I can feel the bolt getting resistence when I close it (last quarter of an inch of travel (just me). When I get a case length that does this, I'll use it for a gauge (actually four or five in a row). Drill the primer pockets out so you won't loose them. I also prefer once fired cases if that matters much.
You can use a digital indicator to measure these case, but you want the bushing to be centered up with the indicator stem (they don't like anykind of a side load). The indicator must be ridgidly mounted and very strait with the centerline; otherwise you will develop error. But you still can simply measure over the bushings with a good caliper. So we still want to be accurate to one tenthousandth of an inch! Now we start to seperate the men from the boys, and gain grey hair in the process. I like the device made for a Bridgeport head, and they are good for one tenth. Just indicate them in to run parallel with the centerline you are measuring. .0001" in six inches is plenty good enough here. Reason why is that those grid lines are a certain width, and if they are angled they actually get wider. Sounds hard, but isn't hard at all to get strait. I made a gauge to do this that used a piece of 3/4" Thompson rod running thru a hard bushing that was honed for a slight slip fit. The bushings were held in place with a small set screw in the end of the rod. The frame was "L" shaped, and simply bolted to a flat piece of steel plate (I used 3/4" O-1 gauge stock). But it'd be fairly easy to setup a digital indicator to the samething. Then all you have todo is slip the case under the ram and measure it (I would kinda jiggle mine around to get a good solid reading.)
Last time I looked you could buy the digital head for a Bridgeport for about $70, and the reast can literally be culled out of a scrap tub. I cut mine out on a band saw, and then finish milled it. The last thing you do is the bore for the bushing, and this has to be very square with the mounting base. Make the bushing about 1.5" long or longer, and shrink fit it to the bore. Then all you need to do is hone it for the slip fit (I used a wheel cylinder hone used to rebuild brakes on cars). Just not that hard to do! You could weld a flat plate on the bottom of the L frame that had three set screws in to also align the head square if that mattered, and that would be easier yet. I did add a spring on the ram to make it always go upwards when not in use, and just used a light spring and drill stop collars. I probably had less that $90 dollars in mine, and the worst part was the band saw work!
I traded mine for some things I wanted kinda bad, and was thinking about doing one that used the Wilson case holders in a v block setup. I think this would have been even easier to build.
gary
