PRECISION GAUGES AND COMPARATORS. The World War was responsible for a considerable development and improvement of apparatus intended for the measurement of engineers' gauges and products.
Improvement in ac curacy of measurement necessarily goes hand in hand with im provement in accuracy and perfection of manufacture of the articles to be measured. The original measuring machine of Sir Joseph Whitworth would have been of little value apart from the system of accurate standards (both end gauges and cylindri cal gauges) which he produced for use with it. The next great advance was the introduction by the Swedish firm of Johans son, in 1908, of flat parallel faced slip and block gauges, of such perfection of workmanship that any two of them, when cleaned, would "wring" together. (See METROLOGY.) The gauges were made in series differing by definite small amounts, so chosen that by wringing together a suitable combination of pieces, any desired size, to the nearest 0.00oi in., could be produced. To ensure this accuracy, it is necessary that the individual pieces should each have a guaranteed accuracy of (say) o•00001 in., in order that the cumulated error of a group of four or five should not exceed 0.00005 inch. And in order to assert with confidence that this degree of accuracy is, or is not, in fact at tained in an individual piece it is necessary to be able to measure with an accuracy of the order of o•00000i inch. When the Johansson gauges were first introduced no appliances were available which could be relied on to give measurements of an accuracy of the order of o•00000r inch. Three entirely different methods have since been developed, however, which enable it to be done, and which, with due precautions, give mutually con sistent results. These are : Tilting Level Comparator.—The "tilting level" comparator, due to A. J. C. Brookes, in which the two gauges, or groups of gauges, to be compared are stood side by side on a level surface plate, and the difference in their heights is determined by the reading of a highly sensitive level, which rests by point contact upon their upper surfaces, through two ball feet.
Millionth Comparator.—The "millionth" comparator, due to J. E. Sears, jun., wherein measurements are made between two flat parallel anvils, giving a local surface contact, the one anvil being fixed, and the other connected, through a series of spring suspensions designed to eliminate all frictional effects, with a sensitive tilting mirror which causes a spot of light to move across a scale, giving a magnified image of the displacement of the anvil on a scale of about 30,00o to 1.
Optical Interference.—The method of optical interference developed at the Bureau of Standards, Washington, wherein the whole surfaces of the gauges under comparison may be in spected, being marked out into contours of approximately 0•00001 in. difference in height by the alternate light and dark bands forming the interference pattern.
The method of transference from external to internal measurement constituted a considerable difficulty, and the sizes of such objects as ring gauges, either plain or screwed, were usually estimated by the nature of their fit upon corresponding plug gauges. As the fit depends to a very marked extent upon the amount and nature of the lubricant used, considerable uncertainty existed as to the correct interpretation of the observed results. If well finished and liberally lubricated with thick grease, a cylindrical plug gauge will enter and pass through a ring gauge definitely smaller than itself without damage to either. Two instruments have now been produced for the measurement of internal diameters of either plain or screwed rings, which give results mutually con sistent, if due care is taken, and the work being measured is sufficiently uniform in its dimensions, to the order of o•0000i inch.
In the chord contact machine, due to G. A. Tomlinson, the diameter of the ring is deduced by calculation from the meas ured displacement, in a direction perpendicular to its length, of a double ball-ended distance piece of known length, which is allowed to make contact inside the ring, first on one side, and then on the other side of the diameter.