Hi Hal, I had always used 25.4001 or .03937 to do my conversions. So, I looked online and found the .039370078 and did the reciprocal. It is, indeed very very close to 25.4. If you google "25.4001 conversion" you can find lots of tables using that as the conversion factor online. I don't know where the error came from or why it's quoted so regularly. But, it appears to be the rounded result of taking the reciprocal of a rounded number. Don't machinists use this number for conversion?
Thanks for the discussion, everyone. bob ----- Original Message ----- > From: Hal Murray <hmur...@megapathdsl.net> > To: Discussion of precise time and frequency measurement <time-nuts@febo.com> > Cc: > Sent: Tuesday, June 25, 2013 10:53 AM > Subject: Re: [time-nuts] OT Prototype Boards > > I think many many years ago, the metric-inch conversion was slightly off from > 25.4 mm/inch, but that was back before PCBs and it was only off a tiny amount. > > Wikipedia's inch article has a history section: > http://en.wikipedia.org/wiki/Inch#Modern_standardisation > The (a?) old conversion was 39.37 inches/meter. In 1959, that was changed to > 25.4 mm/inch. > > For those of you reading the surveying discussion, there is still a US Survey > inch using 39.37. :) > > 25.4 mm/inch is 39.370078 inches/meter. That's under 2 ppm from 39.37. A > 50 > pin connector with 0.1 inch spacing would be off by only 0.001 inch. You > could probably see or measure that if you looked carefully, but I doubt if > there would be any problems inserting a part. > > ---------- > > I've never had any problems with 0.1 inch spacing. > > I have seen problems with surface mount parts that were metric at 0.65 or 0.5 > mm pitch where somebody rounded off too early. That's easy to do if you > look > at the drawing and use the inch numbers without realizing that you should be > using the metric numbers. > > I just looked at a couple of data sheets. They omitted the inch numbers for > the drawings that were really metric. > > > -- _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
