# Soft (In The Head) Metric

By The Metric Maven

Not long ago I was in an engineering meeting, and the persons involved were using inches. I attempted to convince them to use metric instead. The design was then quoted using a strangely odd number. With millimeters, the dimensions should now be round integers for the mounting plate and other design options. I protested that the design did not appear metric even though the dimensions were dual. The engineer looked back at me and said, yes it’s metric, we use soft metric here. I’m sure my jaw became slack when I realized what he was saying. I replied “soft metric is no metric at all.” After considerable discussion, I realized that I was yet again going to lose when it came to using actual metric in a US engineering environment.

One of the first times I encountered the idea of soft metric was in the monograph Metric Implementation in U.S. Construction by Andrew J. Holland. This report was written for Holland’s Master’s Degree in 1997. Here is how it defines hard and soft metric:

Optimistically Holland states that only a few products will need to be resized for hard metric:

A handful of products which are considered to be modular products, such as suspended ceiling grids, drywall, plywood and rigid insulation, raised access flooring, brick, and concrete block fall in the category of “hard-metric” and therefore will need their dimensions changed to the new rounded metric numbers.

…and that 95% of products will not need to be altered. While this sounds promising, those affected in the US will ask for “exemptions”  which will become permanent, and then nothing will change. Take NASA for example, the rocket scientists there have been issued metric exemptions for decades. Any drawings they would ever generate in a “metric switch-over” would probably have both metric and Ye Olde English,  would continue to be drawn in Olde English inches, and after a time no one would see why metric is a superior idea, and go back to inches, like the California DOT did. Here is the US escape clause for construction:

The idea is to change all of the important (modular) stuff to metric so the numbers will all be simple ones:

Unfortunately, we must be “flexible,” so there are exemptions for sheet metal thicknesses and such:

The term gauge is meaningless and a fountain of confusion. Gauge values need to be given priority for reform. I have written about this problem in my essay Don’t Get Engaged With Gauge.

This type of metric change might possibly work in the building construction industry, if a mandatory metric-only metrication (no dual units) was implemented and most soft metric eschewed. In practice it has been crushed by the building industry. The idea of soft metric has been exported to other engineering disciplines in the US by those who want to maintain the status quo. That is what I’ve experienced in industry. Soft metric was waved in front of me as a distraction so Ye Olde English could continue to be used as soon as I left the room—and so that no metric changes would despoil our perfect engineering life in the US. The drawings might continue to have metric dimensions on them side-by-side with Ye Olde English, but the metric values would be ignored for the familiarity of the inch versions as Naughtin’s first law predicts.

Soft metric is pretend metric.

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The Metric Maven has published a new book titled The Dimensions of The Cosmos. It examines the basic quantities of the world from yocto to Yotta with a mixture of scientific anecdotes and may be purchased here.

## 2 thoughts on “Soft (In The Head) Metric”

1. Can you say 5 meters?

“LOOMIS, Calif. (KCRA) —Caltrans is preparing to raise nine bridges along the Interstate 80 corridor from Colfax to Loomis in order to meet federal highway standards.

Federal highway guidelines state that bridges must have a minimum clearance of 16 feet, 6 inches.

The overcrossing at Horseshoe Bar Road in Loomis only has a clearance of 15 feet, 1 inch.

The Horseshoe Bar Road bridge along with six others will be jacked up and reconstructed to support the new height.

Caltrans said three other bridges will have their roadways below excavated to create the necessary clearance.

The project starting this month is expected to last through 2015.

During the nights when workers are raising the bridges, I-80 traffic will be diverted off the roadway.

Those closures are only expected to last one night in each direction.

However, foothill residents will want to pay close attention to detour information since lanes across some bridges will remain closed up to 22 days.

The “Raise 80″ project is expected to cost \$36 million.”

2. Soft metric is meant to give a semblance of metric without changing anything. Its intent is to make metric values look clumsy compared to old inch values and discourage engineers and designers from taking the plunge.

But soft conversion doesn’t have to be that way. About 20 years ago, I had the job of redrawing the companies support products from hand drawings to CAD. They were all in inches. But, I redrew them in metric. They were what could be referred to as pseudo-soft. The metric values were rounded to give the impression that is what they were intended to be.

It can be done with no loss of precision nor accuracy and complete interchangeability with parts made in rounded legacy units. It is really quite simple.

One has to consider that a dimension consists of two numbers, a length and a tolerance. In my case, those numbers that were in fractional inches had a tolerance of +/- 1/16-th inch and those in decimals had a tolerance of 0.030 inches, or in millimetres, +/- 1.6 mm and 0.76 mm. The true length of the dimension was the stated length plus the tolerance and minus the tolerance.

A dimension of say 7 inches would be 7 x 25.4 = 177.8 mm +/- either 1.6 mm or 0.76 mm depending on the tolerance. One would think that the soft conversion would result in a dimension 177.8 +/- 1.6 mm or 0.76 mm, but not so. The dimension can be rounded to 178 mm +/- 1 mm or +/- 5 mm and still be withing the acceptable limits of the original dimension.

177.8 -1.6= 176.2 and 177.8+1.6= 179.4. Thus the acceptable range of the dimension is 176.2 to 179.4 mm based on the original dimension. The new metricated dimension of 178 mm +/- 1 mm fits into the original range and thus any part made to the metricated dimension will be completely compatible with previously made inch parts.

The same holds true for the tolerance of 0.76 mm. 177.8-0.76= 177.04 and 177.8+0.76=178.56. A new metricated dimension of 178 mm +/- 0.5 mm fits into the range of the original inch dimension and a compatibly of parts is maintained.

Any machinist or parts maker working from the pseudo-soft metricated drawing will not be aware that the drawing may have been converted from a drawing once done in inches.

If you are forced to soft convert, than fool the Luddites with pseudo-soft metrication.