Just received my digital hornady caliper. two gauges diff readings

[HiBC]

This is rather interesting.
The gentleman who suggested starting with a headspace "go" gage is,IMO,the most correct.
If you feel the need to verify the calibration of the hardened and ground steel gage,OK.Not wrong.

But that done,its a repeatable standard to go back to what a minimum SAAMI chamber should be cut to. In other words,it represents a standard to size ammo to that will fit in ANY SAAMI spec chamber..
Now the handloader can set his tool to that baseline,and THEN,from his notes,decide I want plus .003 for THIS rifle.

The idea that any aluminum bushing with less than razor sharp corner clamped to a caliper can more accurately represent a standard than a hardened and ground gage is amusing.
You certainly CAN set your Hornady bushing on the headspace gage,hit "zero" and then measure an approx. + or - from there.

I have made a lot of military,aerospace,and other high precision parts.
There are different levels of precision,but generally calipers and micrometers are .."navigation"tools for the machinist.

Often,they are not acceptable to qualify a part. Holes get accepted/rejected with a calibrated plug gage.Rounds are qualified good/bad with a ring gage.

There is a device called a "comparator" that is a stand with a very flat stage.A very sensitive indicator is attached to the stand,often graduated in 50 millionths of an inch. You set the gage with carbide Johannsen blocks,traceable to the Bureau of Standards.
And,dial bore gauges,etc are calibrated to a Johannsen block set to zero for each job,the indicator showing + or - in to 50 millionths.

Point,the yes or no qualifying standard is a a gage,

Now,with all due respect for a feeler gage,how flat/straight is it?

I have,on my loading bench,a flat of black granite with a post on it and a clamp for a dial indicator. If I take my Wilson case gage and stand it on the granite, I can set my dial indicator zero on the min or max step of the gage,and the little limit flags on the indicator dial face to whatever tolerance I want from there.
I can drop the brass in the Wilson gage and pass it under the indicator to get a read for,example -.003 from max length or actually +.003 to reduce head clearance for a particular rifle.

Its just another way.

Mic setting standards are OK,I always used Jo blocks

A pretty good bargain for a decent set of "standard" blocks to use in a shop isa set of "space blocks". they are about 3/4 in in dia,round,with a threaded hole in the middle.They are steel,hardened and ground.
I think maybe $50or $60 will buy a set.
You get a set that can be stacked..like .101,.102,103,etc.THen .110,120,130 And a .050,a.0625,
A .100,.200,.300,a.500,a 1 in,a 2.00 in,etc. You can stack them on an allen set screw.Use them with lathe and quill stops,sine bars,measuring slots,etc.

 

[F. Guffey]

Hey Jeep, real good question.How do I know my standard is true?

I have said there is something about the datum reloaders do not understand. I have also said there is something about the head space gage a reloaders does not understand. I have also said there is something about gages and micrometers the reloader does not understand. I have said I make datums, and I collet datums and there are times I purchase datums; Bascialy I am told there is no such thing as 2 datums; it would seems I am the only reloaders that understands the plural of datums.

Anyhow, a reloader with a dial caliber that has the shop skills to zero their dial caliper has the ability to measure the length of a head space gage from the datum to the head of the gage if they have the ability to make a datum.

At the end of last month I had the opportunity to purchase 200 pounds of datum makers, instead of the datum makes I purchases a cabinet and 3 Binks pressurized paint cans for $25.00 each. If I go back to purchases the tools I will need the cabinet; it is one of those 22 drawer file cabinets.

F. Guffey

 

[condor bravo]

To try to determine which caliper is closer to being right, do something simple like measuring a bullet diameter. If you have access to Sierra 168 gr .30 caliber Internationals, now called Matchkings, they should measure right on at .308. My recent acquisition from Harbor Freight measures exactly that while a more expensive set I had dropped was a good .001 off. Or try other .30 caliber bullets, whatever you have.

 

[Unclenick]

The plural of datum is data. You hear people say they took a single data point, but they actually mean they recorded a datum.

 

[F. Guffey]

The plural of datum is data. You hear people say they took a single data point, but they actually mean they recorded a datum.

True, I hear that all the time and then say to my self; "if they are not careful they are going to talk it to death". I can not take a single point data because I measure from and to meaning I have to measure from two points.

Datums: Datums make it possible to find one of the points. In the big inning I saw the same thing everyone else saw; the difference? I was not the one that was confused. In the big inning smiths and reloaders found a drawing of a case that was supposed to explain everything. Everyone recognized the drawing of the case but there was a line drawn through the shoulder with an arrow that identified the line as a datum line and an explanation. The explanation gave the rational; "and that is how they do it". Do WHAT! ? From that day on reloaders used up a lot of space on the Internet talking about 'the lline'. You will not believe how difficult it was to convince reloaders the line was a round hole/circle.

So, I made datums, I made circle/round holes, to verify a head space gage I select the correct datum/round hole, If I have two .375: round holes I have two datums for the 30/06 family of cases that will also work on the 300 H&H.

Again, L.E. Wilson had this figured out in the early 50s, when a case is placed into a Wilson case gage the case sits in the datum of the case gage. One more time, the Wilson case gage datum has a radius but when Wilson is grinding the case gage to length; how can they miss?

Because this stuff does not lock me up I used a straight edge and a feeler gage, I have found a flat surface works better; problem, reloader believe I am doing a magic trick like turning nickels into times.

Again, I make datums, I collect datums and when I can sneak up on someone I purchase datums. And then there are tools that are used to make datums. When I mess up and my datums do not work out I turn them into comparators.

Hey Jeep, real good question. How do I know my standard is true?

You need to find someone that can teach you how to zero your gage.

F. Guffey

 

[Unclenick]

I can not take a single point data because I measure from and to meaning I have to measure from two points.

Datums: Datums make it possible to find one of the points.

That's exactly right, of course, because I know you understand it. But I am going to go ahead and spell it out for others interested in learning about it.

The datum makes it possible to find other points because the datum is the first, basic location, aka, the reference location from which measurements to other points are taken. The SAAMI drawings even designate it with the letter "B" for basic. It can be a point or a plane or any other geometric designation from which other measurements are to be located.

Back in the 80's, when engineering work was slow, a partner and I would repair machine tools. I got a great book back then on reconditioning machine tools. It was written in the 1950's, but no principle of geometry has changed since Archimedes, so I used as a reference work for tricky figuring jobs. Except that people have gotten lazy and mostly ship their worn equipment to a place with gigantic CNC precision grinders now, nothing has changed since then in principle. There's just a lot less hand scraping now.

If you wanted to figure a worn Bridgeport mill, for example, to get the table ways squared and perpendicular to the knee ways, you pulled down the table, scraped the knee way wear flat and declare the plane of that surface to be your datum plane. From that plane there were (are, actually) numerous ways to set up indicators and use precision scraped aged iron castings to show you where to scrape the table ways on the knee to get them perpendicular to the knee way datum plane and evenly level on both sides and put true 60 degree angles on the dovetail forms for the gibs to slide against. Same with the table ways. The main thing is to establish that first datum plane to serve as the reference from which all other measurements are taken and surfaces figured.

If you used the datum to find only one other measurement, then used that measurement to find the next measurement and so on, measurement errors would accumulate. That's what establishing the datum prevents: cumulative error. By referencing every measurement back to the datum, the only error possible is one least significant digit of measuring resolution.

For the cartridge case the datum is the plane that slices through that shoulder diameter that is declared, by definition, to be the datum diameter for the cartridge. Where it meets the shoulder diameter is the only dimension that is the same for both the case and chamber alike. That allows measurements of either to reference that same plane and cone intersection.

 

[TruthTellers]

Grab a piece of paper, close your calipers on the paper and slide the paper out from the jaws to clean the jaws.

Then, close your calipers shut. Hold them up to a light source. Do you see light coming in between the closed jaws? If yes, the jaws are not flat, send them back.

If no light is coming through, then check the reading on the calipers. If it says zero, the calipers should be accurate. If no, then hit the zero button on the calipers.

All calipers, even the best of the best, have a margin of error of +/- .001 inches. Which is the reason they're not the most trusted measuring tools in a manufacturing factory.

The cheap reloader's calipers are garbage that no machinist worth his salt would ever buy and probably have an MOE of more than +/-.001, but they are good enough for reloaders trying to measure their slugged bores and case lengths and cartridge OAL.

 

[F. Guffey]

Unclenick, as the man said; "I have two speaches, one is a short speech and the other speech is long speech".

I will start with my short speech; Unclenick, Thank you. And then finish with my long speech: Unclenick, Thank you very much.

Somewhere back there I said I had the opportunity to purchase datum makers. The seller is building a 78 year old car, those projects do not come without problems, he has a problem with the car he has been trying to work around. I understand professional jalousies so I am trying to figure a way to make the suggestion his ideal and if I am correct 'we' will decide how to correct the problem, and while I am there I will ask him about his datum makers.

F. Guffey