Engineering Problems in Dimensions and Tolerances

The basic unit in most considerations of dimensions in the United States is the inch. The value of the inch is so important that many companies including the Bell System maintain in their measurement laboratories a standard yard bar calibrated against the standard at the National Bureau of Standards. In spite of this it is an interesting and curious fact that though all have been much concerned over the legal value of the dollar there has been little interest even among engineers in the exact legal value of the inch. Actually there is no single answer to so simple a question as "What is an inch?" In fact, we have changed from a British inch and our own legal meter, to our inch and the International meter and now through action of the American Standards Association we are actually using an inch based on conversion from the International meter which is neither our own legal inch or the British legal inch--and the British are using it too. Table I shows this history of the legal inch in the United States. It will be seen that under the present status there exists a difference of two parts in a million between the legal inch and the inch used in the dimensional work of industry. This difference is more theoretical than real in small dimensions and industrial use. The bill before Congress, sponsored by the Bureau of Standards is intended to eliminate this as well as any possible ambiguity in the U. S. inch. DECIMAL DIMENSIONING In subdividing the inch the modern trend in industry is toward the use of decimals instead of the older common fractions although fractions continue to be used, especially for dimensions of certain materials such as iron pipe, lumber, phenol fiber.