‘'”We had no datums”
And I claim I collect datums, I make datums and......I purchase datums. I was at a Gun show in Dallas at Market Hall, I walked up to a table and said “DATUMS!!”. the dealer said he did not have any datums, one shopper ask “WHERE?”, another happy shopper ask “What does a datum look like?” I purchased the +/- 15 pound box of junk for 5.00$.
polyphemus, it all started with a an arrow pointing at a line, the line is identified as ‘DATUM’, and for years that is all smiths and reloaders knew about ‘datum’. I made tools in the 70s, the only tool I use that is flat is a feeler gage, all the rest have three dimensions. The concept that the datum was not a line but a round hole was not something that was acceptable to smiths and reloaders.
Anyhow, to a machinist, datum is ‘measured from’. The box of junk I purchases was a box of holes drilled into metal plates, cylinders and die parts. A few of the pieces were case holders for Wilson case trimmers. To use the holders for datums all I had to do is butt grind the ends to remove the radius.
The datum/round hole for the 30/06 is .375”. It is not difficult to drill a .375” hole perpendicular to the surface of a plate. For me it has never been necessary to use a .375” hole,
http://www.saami.org/PubResources/CC...pringfield.pdf
I can use a round hole that is larger in diameter than .339” and smaller in diameter than .441”, I do not recommend getting close to the edge but when using a round hole the smith/reloader needs to understand they are using a comparator, they are measuring a fired case and comparing the length of the case from the datum/shoulder with a sized case, or they do as I do, I measure the case before firing and again after firing because I want to know the effect the chamber had on the case when fired. As a side benefit, when measuring the case before firing the reloader will. always be able to use a comparator to determine why a sized case will not chamber.
“diameter than .339” and smaller in diameter than .441” for years it has been claimed the datum was half way between the case body shoulder juncture and the shoulder neck juncture, for the same numbers I have ask: “How can that be when the the .375” datum is used for the 8m06, 30/06 280 R, 270W, 7.7 Japanese, etc., etc., 25/06, I can make datums/round holes that are half way between the shoulder junctures because I understand the information I want can be obtained with a comparator.
If I choose to tie all the loose ends together and get the chamber, case length from the datum to the case head and die and case gage and head space gage along with the shell holder to agree I have to start with SAAMI specifications. Problem! SAAMI does not require anyone reaming chambers to be exact, so I do the next best thing, I measure the length of the chamber from the shoulder/datum to the bolt face, this allows me to leave out head space gages that do not fit except for +/-.
I am the fan of transfers and standards and verifying is not far behind.
F. Guffey