The many wonders of the Metric system

[fno]

To avoid derailing [silly expression deleted] discussion this week I wanted to start a new one to see how many of us can relate interesting observations, factoids, rumors, and even some not so interesting observations. I will start the discussion with an interesting (to me at least) method of determining the correct drill bit for a given Metric tap.

Like U.S. taps and threads, Metric has two series of taps for various purposes but for the sake of simplicity they are fine thread and course thread just like our UNF and UNC threads. The interesting part of this is that you never need a chart like the one shown below.

Fig. 1. Tap Drill Chart

tap_drill_chart.jpg (56.24 KiB) Viewed 2400 times

Most threads are described by the diameter and the pitch of the thread, for example 1/4"-20, 3/8"-16, M10-1.5, M20-2.5. The beauty of the Metric system is that the tap drill diameter is easily determined by subtracting the pitch from the tap diameter. In other words, that M10-1.5 will need a 8.5 mm drill bit for a tap drill and the M20-2.5 tap drill will be 17.5 mm.

[jimh]

Very interesting, and I never heard of that relationship before.

The SAE fastener measurement system has hung on a lot longer than many people thought. The automotive manufacturers have gone to Metric sizing for most of their screw fasteners, but the rest of the USA seems to be hanging on to threads like 8-32 and 10-24 and so on.

Here is a really odd one: I bought a bicycle carrying rack that fits into the trailer hitch on my truck. The manufacturer, SWAGMAN, supplies a threaded shoulder bolt that holds and tightens the carrier into the hitch receiver. This bolt is threaded with some completely odd-ball diameter and pitch, so if you lose the bolt you cannot replace it easily at a hardware store. You have to order it from the bicycle rack manufacturer for about $6. It's some ancient British thread standard. I don't know why they chose that. May it was to make sure their rack was fastened with the proper grade steel bolt, instead of some made-in-China low-strength-steel bolt.

[Jefecinco]

Our Swagman was made in Australia. Perhaps they continue to use the British standards down under. I believe we continue to use British Wire Gauge (BWG) for some products. I seem to recall using the standard for ordering replacement boiler tubing for an old (1943) express boiler.

[fno]

No more interesting, weird, or unusual observations about the metric system or it's history?

[jimh]

The French decided that the distance of one meter would be equal to one ten-millionth of the distance from the North Pole to the Equator, measured along the meridian passing through Paris. In a recently published (2002) book by Free Press, New York, New York, there is an excellent account of the actual process of measuring that distance, which began in 1792, and what went wrong in the measurements.

The Measure of All Things:
The Seven-Year Odyssey and Hidden Error That Transformed the World
by Ken Adler
(Available on Amazon.)

I read this fascinating account several years ago. The author describes how an error made in measuring some segments of the meridian and was covered up. If interested in the history of the (French) metric system, this book is a very good source. The book has received several awards and has been translated and published in 11 other languages.

[Jefecinco]

Put your math and research skills together to determine the correct length in inches of a meter if the French calculations had been correct. The delta should be interesting. I wonder if the length of a corner edge of a cubic liter may have been a good length for a meter.