Thou (length) - History of Usage

History of Usage

The introduction of the thousandth of an inch as a sensible base unit in engineering and machining is generally attributed to Joseph Whitworth who wrote:

...instead of our engineers and machinists thinking in eighths, sixteenths and thirty-seconds of an inch, it is desirable that they should think and speak in tenths, hundredths, and thousandths...

Up until this era, workers such as millwrights, boilermakers, and machinists measured only in traditional fractions of an inch, divided as far as 64ths. Each 64th is about 15 thou. Communication about sizes smaller than a 64th of an inch was subjective and hampered by a degree of ineffability—while phrases such as "scant 64th" were used, their communicative ability was limited by subjectivity. Dimensions and geometry could be controlled to high accuracy, but this was done by comparative methods: comparison against templates or other gages, feeling the degree of drag of calipers, or simply repeatably cutting, relying on the positioning consistency of jigs, fixtures, and machine slides. Such work could only be done in craft fashion: on-site, by feel, rather than at a distance working from drawings and written notes. This in turn limited the kinds of process designs that could work, because they limited the degree of separation of concerns that could occur.

The introduction of thou as a base unit for machining work required the dissemination of vernier calipers and screw micrometers throughout the trade, as the unit is too small to be measured with practical repeatability using rules alone. During the following half century, such measuring instruments went from expensive rarities to widespread, everyday use among machinists. Bringing more metrology into machining increased the separation of concerns to make possible, for example, designing an assembly to the point of an engineering drawing, then having the mating parts made at different firms who did not have any contact with (or even awareness of) each other—yet still knowing with certainty that their products would have the desired fit.