Join Date: July 18, 2008
Today, 06:34 AM #20
Join Date: November 13, 2006
Posts: 2,942 Datums
From ASME Y14.5 2009: "Geometric Dimensioning and Tolerancing"
Datum — A theoretically exact point, axis, or plane derived by contact a datum feature; it then becomes the origin from which geometric characteristics are measured.
You cannot buy a datum.Datums are theoretical geometric features,such as planes ,axis,etc..
I make datums, I purchase datums, I collect datums, I thought the reloading forums had moved beyond believing the datum is beyond their ability to understand and or comprehend. When I first started the holy grail was a drawing with an arrow pointing to a line labeled datum line, the total amount of information understood from the drawing? “It is measured from this line and that is how it is done”, no one understood the concept.
If someone choses to use a set-up table I will help them, and, at the same time discourage them for doing such a dumb think. I have a steel set-up/layout table, drilling through the table would be mindless, something like grinding the bottom of a die and or top of the shell holder, it is just not necessary.
The datum is a round hole/circle, anyone with a drill index set of drills and a means of drilling a straight hole is very capable of making datums, I am convinced they (reloaders/smiths) can do it, problem? It seems they are afraid if they take the time to understand the concept they will be required to ‘do it’.
Anyone that can not drill a 3/8” hole is very short on shop skills, the 3/8” hole is the .375” datum for the 30/06, 270 Winchester, 280 Remington, 25/06, 7.7 Japanese on and on etc..
“Datum — A theoretically exact point, axis, or plane derived by contact a datum feature; it then becomes the origin from which geometric characteristics are measured”
Measured from, measured from the datum, the length of the case is measured from the datum/shoulder to the head of the case. The case sets on the top of the hope, the hole for the 30/06 is .375” in diameter, when measuring from the surface the hole is drilled through the reloader is measuring from the datum, measuring the length of the case from the head of the case down to the surface of the plate the hole is drilled through will indicate the length of the case ‘from the datum to the head of the case’.
A plane!, a plane! The plane is the surface the hole is drilled through, the point measured from is located at the .375 intersection on the shoulder, again, the shoulder is tapered, the shoulder is round, the round tapered shoulder forms a cone, a cone has varying diameters, and that is what locks everyone up, the diameter ‘WE’ are looking for on the shoulder with varying diameter shoulder is .375. the only way to find the .375” diameter location on the shoulder is with a straight 3/8” hole drilled through a flat surface called a plane.
The 308 Winchester 243 Winchester etc., datum is .400”, for most that get driven to the curb that is it, not me, because of the tapered shoulder that forms a cone, I can use .375”, .340'” etc..My chamber, my dies, my cases,
my reamers and my gages. It has taken years to get reloaders pass the arrow pointing to a line labeled datum line,
The L. E. Wilson case gage has been around since the 50s. The shoulder of the case when placed in the gage is supported by’ THE DATUM’, the datum at the bottom of the L E. Wilson case gage for the 30/06 is .375”.
Back to HIBC
Datum — A theoretically exact point, axis, or plane derived by contact a datum feature; it then becomes the origin from which geometric characteristics are measured”
My opinion, HIBC you would not recognize a datumn if you were looking at one, again, again, the Wilson case gage has a datum.
If ‘you’ in your post was not referring to me, forgive.