You cannot buy or own a datum. They only exist in theory. You can buy a ring gauge or bench centers or a surface plate to represent the theoretical datum,but it is not the datum.
AGAIN: I was at a gun show when I noticed a box of miscellaneous box of stuff, good stuff, stuff that would be useful to me and considered as junk to others. I said to the dealer; "dautms" he responded with " I do not have any datums".
It was a little crowded, the happy shopper ahead of me ask "Where?", then next happy shopper ask "What does a datum look like?" and another said he did not see any datums.
In about 1938 L.E. Wilson designed the case gage, they did not name the Wilson case gage a case head space gage, that was because the case did not have head space. The case did not have head space until after the Internet was invented. After the invention of the Internet all someone had to do to become a reloaders was to sign in to social media and claim they were a reloader.
Before the Internet the Rock Chucker was not a cam over press; before the Internet a reloaders should have been able to determine the difference between a cam over press and a non cam over press. The Vise-Grip plyers have been with us for over 90 years; the Vise-Grip plyers have 'leaver lock'. The Rock Chucker press has 'leaver lock'. The Rock Chucker is not a cam over press, if it was it would be a 'bump press', meaning the ram would bump once on the way up and again; and once' on the way down.
Bump? And now reloaders believe they can bump the shoulder back on the case with a die that has full body support. And I ask; "How is that possible, how do they do that?" all I get back are responses from socially dysfunctional members that claim they are reloaders.
The Wislon case gage is a datum based tool, Wilson built the case gage with a datum that has a redius, Hornady/Sinclair makes a comparator, the Hornady Comparator also has a datum
, problem, the Hornady comparator had a datum with a radius.
The Wilson case gage is accurate to .001" and the Hornady comparator is a comparator because Hornady has never found a way to zero the gage with a datum that has a radius.
I do not believe it is fair to you for tool manufacturers to sell you tools without datums. I made sure my first Wilson case gage had a datum; for me that was not a problem for most reloaders ? They never knew there was a datum in each and ever case gage made.
I know; you believe making a cast would give you all the information you need to check for a datum, there are very few smiths and reloaders that are better at casting than I am, And I taught him.
We have had the Sinclair/Hornady comparator with us for decades without an understanding why it is necessary to verify the accuracy, we all should know the comparator is off, what we do not know is 'by how much'. Well that is not correct, one of us knows, he does not use the Hornady comparator. 'UNLESS I MEASURE BEFORE AND AGAIN AFTER BECAUSE THAT IS NECESSARY WHEN USING A COMPARATOR.
And then? Where you when the Digital Head space gage came onto the scene. It was not a head space gage, it was a dial indicator stand, in my opinion it was a cheap one. And the man claimed he invented the 3 point contact at the shoulder of the case. Because I did not start reloading after the invention of the Internet I gave credit for the 3 point contact to the first person that build the first milking stool and the man that leveled his furniture in the beginning back when they had dirt floors because the 3 legged stool, table and chair always made contact with the floor at the same time.
F. Guffey