DELICATE MEASURING MACHINES
The upper illustration shows a Pratt-Whitney Measuring Machine in operation to decide the thickness of a cigarette paper, which is one-thousandth of an inch thick. This machine will measure variations of length or thickness as minute as one hundredth-thousandth of an inch. The lower illustration shows a Whitworth Measuring Machine which is sensitive to variations of one-millionth of an inch.
"He (Sir J. Whitworth) obtained the subdivision of the yard by making three foot pieces as nearly alike as was possible, and working these foot pieces down until each was equal to the others, and placing them end to end in his millionth measuring machine; the total length of the three foot pieces was then compared with a standard end-measure yard. These three foot pieces were ground until they were exactly equal to each other, and the three added together are equal to the standard yard. The subdivision of the foot into inch pieces was made in the same way."[2]
A doubt may have arisen in the reader's mind as to the possibility of determining whether the measuring machine is screwed up to the exact tightness. Would the measuring bars not compress a body a little before it appeared tight? Workmen, when measuring a bar with callipers, often judge by the sense of touch whether the jaws of the callipers pass the bar with the proper amount of resistance; but when one has to deal with millionths of an inch, such a method would not suffice. So Sir Joseph Whitworth introduced a feeling-piece, or gravity-piece. Mr. T. M. Goodeve thus describes it in The Elements of Mechanism: The gravity-piece consists of a small plate of steel with parallel plane sides, and having slender arms, one for its partial support, and the other for resting on the finger of the observer. One arm of the piece rests on a part of the bed of the machine, and the other arm is tilted up by the forefinger of the operator. The plane surfaces are then brought together, one on each side of the feeling-piece, until the pressure of contact is sufficient to hold it supported just as it remained when one end rested on the finger. This degree of tightness is perfectly definite, and depends on the weight of the gravity-piece, but not on the estimation of the observer.
In this way the expansion due to heat when a 36-inch bar has been touched for an instant with the finger-nail may be detected.
One of the most beautiful measuring machines commercially used comes from the factories of the Pratt-Whitney Co., Hartford, Connecticut, the well-known makers of machine tools and gauges of all kinds. It is made in different sizes, the largest admitting an 80-inch bar. Variations of 1 / 100,000 of an inch are readily determined by the use of this machine. It therefore serves for originating gauge sizes, or for duplicating existing standards. The adjusting screw has fifty threads to the inch, and its index-wheel is graduated to 400 divisions, giving an advance of 1 / 20,000 inch for each division: while by estimation this may be further subdivided to indicate one-half or even one-quarter of this small amount. Delicacy of contact between the measuring faces is obtained by the use of auxiliary jaws holding a small cylindrical gauge by the pressure of a light helical spring which operates the sliding spindle to which one of these auxiliary jaws is attached.
On one side of the "head" of the machine is a vertical microscope directed downwards on to a bar on the bed-plate, in which are a number of polished steel plugs graved with very fine central cross lines, each exactly an inch distant from either of its neighbours. A cross wire in the microscope tells when it is accurately abreast of the line below it. Supposing, then, that a standard bar three inches in diameter has to be tested. The "head" is slid along until the microscope is exactly over the "zero" plug line, and the divided index-wheel is turned until the two jaws press each other with the minimum force that will hold up the feeling-piece. Then the head is moved back and centred on the 3-inch line, and the bar to be tested is passed between the jaws. If the feeling-piece drops out it is too large, and the wheel is turned back until the jaws have been opened enough to let the bar through without making the feeling-piece fall. An examination of the index-wheel shows in hundred-thousandths of an inch what the excess diameter is.
On the other hand, if the bar were too small, the jaws would need to be closed a trifle: this amount being similarly reckoned.