The intelligence of the Metric System

Guest blog by Randy Bancroft, P.E.

metric Ignorance more frequently begets confidence than does knowledge.
— Charles Darwin

I began working as a consulting engineer in 2007. From the outset, I wanted to work exclusively in metric. It’s been quite an education, and the first lesson has been that U.S. engineers have no idea how to use the metric system. Something like the Dunning-Kruger Effect seems to be operating.

Examples of measurement folly are legion. Given engineering drawings in inches, I generally find awkward dimensions with several decimal places. Converting these to millimeters, I discover simple whole numbers. Designs originally in metric, almost always from other countries, are converted to non-metric, at some expense. Why? This happens even when manufacturing is to be done overseas.

U.S. aerospace companies prefer an inch-based, colloquial unit called a mil, over micrometers and millimeters. One reason: NASA demands it, in spite of official commitments to align its engineering with the space agencies of other countries.

I’ve designed GPS antennas for which the conversion from metric to Ye Olde English measures nearly ended in creating a considerable amount of expensive scrap. The length of the GPS antenna resonator was converted from metric to Olde English, with a numerical transposition. This was caught just before fabrication.

Electronic components are in metric dimensions to conform with international standards, but U.S. designers force them onto incompatible mil (inch-based) grids, necessitating hand-routing of printed circuit boards. Despite international agreements, redundant, inch-based size designations for surface-mount electronic components were created at the demand of U.S. industry, with some designations identical to metric, causing world-wide confusion.

U.S. fastener sizes are a mixture of awkward fractions of inches—and meaningless “gauge” numbers, without rational or logical correlation. In contrast, the metric screw designation M5 instantly tells you it is 5 mm in diameter, and that a five millimeter drill bit will produce a friction fit. Quick! What drill bit will produce a friction fit for a 4-40 screw? I’ve yet to meet a machinist who could answer this question. What is the size? There isn’t one. With the metric system, an average high school student could answer the question. When an Australian Ford plant implemented metric threads in 1972, they cut bolt-sizes stocked by 88% and nuts by 72%.

Australia’s rational mandatory metric implementation produced savings throughout their industries. When I tell fellow engineers that the Australian construction industry uses millimeters only, it elicits guffaws. “That’s ridiculous! The unit is too small to build houses with!” American ignorance begets confidence. In fact, the use of millimeters (centimeters are banned) means that all dimensions on a construction site become whole numbers, from rough concrete forms to kitchen cabinetry. Eliminating decimals eliminates a great deal of scrap from mis-cut materials. This has saved Australia 10-15% in construction costs since the 1970s, compared with the U.S.

Because two sets of measurement units exist in the U.S., OEMs, and individuals, needlessly maintain two sets of tools, Ye Olde English and metric. In the U.S., there is instantly a discussion of the costs of going metric-only, but never how much cost is involved in maintaining a 15th-century set of irrational measurement units for design. We in the U.S. have 14 definitions of gauge: wire gauge, drill bit gauge and sheet metal gauge are but a few. I’ve seen situations where a fixture needed equally thick polycarbonate plastic and metal sheet. If they were both manufactured and called out in millimeters, this would be a trivial exercise.

I’ve seen adhesives fail in the U.S. because they were cured at 100° F instead of 100° C. The non-U.S. manufacturer documentation just said 100°, unambiguous everywhere but here. Which set of tools should be used for production equipment repair is often unknown, as many offerings have both Olde English and metric in the same design. For start-up companies, time is everything. Product development is often confined to the most commonly available parts and materials. In US hardware stores you will not find metric drill bits, or nylon metric screws, or millimeter-only scales or tape measures. To save time, one has the choice of using what is in the hardware store, or purchasing online.

In the U.S., we see ourselves as world class designers, but what is more fundamental to design than a measurement system and its effective use? An examination of how best to use basic measurement units seems to be beneath the attention of U.S. engineers and designers. Because of this, they remain certain that the metric system is something to be addressed in the future. For the other 95% of the world’s population, the future is now.

Randy Bancroft is an engineer and author. He blogs as The Metric Maven at themetricmaven.com.

Related: Editors On Topic: Metric Vs Imperial