Your dimension of 302 mm is correct, more correct than the nominal 12". However, Steve's assertion that it is inch based is also correct, as written in the specification. Since the International size is claimed to be 300 mm, and 302 mm differs from it by a few multiples of the tolerance, as an engineer, I have to say they were engineered to be different sizes, even if they are "close." Assuming the International size has a similar tolerance, they will not overlap.
--- On Sun, 4/5/09, Jeremiah MacGregor <jeremiahmacgre...@rocketmail.com> wrote: From: Jeremiah MacGregor <jeremiahmacgre...@rocketmail.com> Subject: [USMA:44375] RE: Records To: "U.S. Metric Association" <usma@colostate.edu> Date: Sunday, April 5, 2009, 9:59 AM John, Thanks for verifying what I have said. We now have proof that I am correct and Stephen is wrong. But I highly doubt Stephen will change his position and continue to spout error. The RIAA spec is very interesting as it butts up against an interesting problem in converting millimeter dimensions to fractional inches. I would say that in 1963 when the spec was made (possible from an older spec) that decimal inches were rare and not popular and it was common to express all (or most) inches in fractions. So, how do you convert a rounded metric number to a fractional inch and then come up with a usable fractional size that is in either 16-ths or 32-nds? Anything smaller is not practical. 302 mm converts to 11.89 inches. The nearest fraction is 11.875 mm which is 11-7/8 inches. This however is only 301.625 mm. The reason for the asymmetrical tolerances is to accommodate rounded numbers in both units, the 302 mm in metric and the 11-7/8 in inches. If we add the 1/32 tolerance to 301.625 mm we get 302.42 mm. The average of the two is the 302.02 mm you noted, which for all practical purposes is the 302 mm intended. However, in inches there is no common fraction to equate to 302.02 mm. So the closest common fraction was chosen and the tolerance was made asymmetrical. If the RIAA spec were ever to be updated it could simply drop the 0.02 mm extra you noted and simply make it 302 mm +/- 0.4 mm. Would you agree? I do find it interesting that the RIAA wanted their records to be an extra millimeter in radius bigger then the standard 300 mm. Could be they wanted to have extra leader space at the beginning. As a side note I wonder if the 1963 spec was an update to an earlie spec and how far back the spec really goes? If so, and the inches were in fact not post 1960 inches, then how would pre-1960 inches or even pre-1900 inches affect the outcome of the conversions? At least we now know that the dimensions are what is intended and that the reference to shrinkage is just wishful thinking. It just goes to show that the extremist propaganda that claims everything in the past started out as inch based is wrong and that there are many examples that actually started out as metric based and were later corrupted by the English world in inches and the metric history was forgotten. It is time to revive the truth about products that originally were metric to begin with. Jerry
