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Bobs Rule: Popular Woodworking

This article first appeared in Popular Woodworking magazine in June 2005

Out of the Woodwork

Bobs are real, not just a theory. Robert "Bob" Dunstan manufactures and sells tapes and rules that use bobs. We have a set and are giving them a try. Senior Editor Bob Lang is particularly smitten with the system.

Farewell Fractions – Hello Bob


A new unit of measure attempts to trump the troublesome imperial and metric systems.

For as long as I can remember, the woodworking community has been divided between metric people and imperial (feet and inches) people. Each is willing to argue their cause until blue in the face but unable to persuade the other. I know why – I grew up in Britain during the transition from the imperial system to the metric system and used both systems in my workshop to the extent I can categorically claim neither is suited to the type of measurement and calculation woodworkers and other craftspeople use day in and day out.

I propose an elegant solution in the form of a new unit of measure I call the "bob." Twenty-four bobs comprise the inch just as 12" comprise the foot. The bob allows the integration of all that is good in the metric system into our familiar imperial system and should please adherents of both. My argument is as follows:

The metric system excels at smaller measurements while the imperial is better suited to larger ones. The metric systemís real asset is the particular size of the millimeter. This happens to be an ideal "small" unit of measure, smaller than a 1/16" and larger than a 1/32". In other respects the metric system is unsatisfactory. The reason we divide the day into 24 hours not 20 and the circle into 360į not 300į is because the building block of 12 is vastly more flexible than 10.

The imperial system offers a manageable series of larger increments. Feet and inches break up what in the metric system tend to be long, easily corrupted numbers into readily recalled chunks. For example, a metric length of 2,362mm equals 7' 11". While 2,362 might easily become 2,326 in our fallible memories, itís hard to not notice 7' 11" becoming 11' 7". The inspired division of the foot into 12" rather than 10" allows easy division into halves, thirds and quarters.

The great failing of imperial measurement is the multitude of unsatisfactory fractions that make addition, subtraction and division an exercise in mental agility, and provide ample opportunity for error. Nobody can quickly and reliably add 11 7/16" and 4 5/32" then divide the result by two, let alone by three.

The bob sidesteps all these problems. Like the foot, which can be divided many ways, an inch made up of bobs can be divided in two, three, four, six, eight, 12 and 24. The bob is very close in size to the millimeter and so it has the same natural advantage as an ideal small unit.

By giving this new unit a name I hope to allow measurements to be remembered in much the same way as British currency was before decimalization, when a train ticket to Oxford might have cost "One pound, seven and six" (1 pound, 7 shillings and 6 pence). For example, the width of the desk Iím working at would be "four-foot eight and 10 (4', 8" and 10 bob) which is a lot easier to remember and identify than either "four-foot eight and 1 7/32nds" or "1,436mm." Where a greater level of precision is needed the bob can easily be divided in half by eye and further still by working to one side or the other, rather than the middle of the rule markings (the terms "fat bob" and "thin bob" come to mind). Even without the confusing profusion of tiny markings found on the standard ruler the woodworker using bobs can be consistently accurate to about 1/100".

I doubt the bob will have immediate impact against either current system but it may have a promising future in the workshop as by far the best way of measuring, marking, remembering and calculating. It is within the context of that environment that I pin my hopes! PW

Bob Dunstan lives and works in Jackson, Wyo. His daytime work consists of ceaseless promotion of Whitechapel Ltd. Furniture making remains his secondary business and primary hobby.