We have now got into a region of very "practical politics," namely, the subject of gauges. All large engineering works which turn out machinery with interchangeable parts, e.g. screws and nuts, must keep their dimensions very constant if purchasers are not to be disgusted and disappointed. The small motor machinery so much in evidence to-day demands that errors should be kept within the ten-thousandth of an inch. An engineer therefore possesses a set of standard gauges to test the diameter and pitch of his screw threads and nuts; the size of tubes, wires; the circumference of wheels, etc.
Great inconvenience having been experienced by American railroad-car builders on account of the varying sizes of the screws and bolts which were used on the different tracks—though all were supposed to be of standard dimensions—the masters determined to put things right; and accordingly Professors Roger and Bond and the Pratt-Whitney Co. were engaged to work in collaboration in connection with the manufacture of tools for minute measurements, viz. to 1 / 50,000 inch. "To give an idea of what is implied by this, let it be supposed that a person should take a pair of dividing compasses and lay off 50,000 prick-marks 1 / 8 inch apart in a straight line. To do this the line would require to be over 520 feet, or nearly a tenth of a mile long. Imagine that many prick-marks compressed into the space of an inch, and you have an imperfect idea of the minuteness of the measurements which can now be made by the Pratt and Whitney Co."[3]
The standard taps and dies were supplied to tool-makers and engineers, who could thus determine whether articles supplied to them were of the proper dimensions. Nothing more was then heard of nuts being a "trifle small" or bolts "a leetle large." And so beautifully tempered were the dies made from the standards that one manufacturer claimed to have cut 18,800 cold-pressed nuts without any difference being perceptible in their sizes.
To appreciate what the difference of a thousandth of an inch makes in a true fit, you should handle a set of plug and ring gauges; the ring a true half-inch internally, the plugs half-inch, half an inch less one ten-thousandth of an inch, and half an inch less one-thousandth, in diameter.
The true half-inch plug needs to be forcibly driven into the ring on account of the friction between the surfaces. The next, if oiled, will slide in quite easily, but if left stationary a moment will "seize," and have to be driven out. The third will wobble very perceptibly, and would be at once discarded by a good workman as a bad fit.
For extremely accurate measurements of rods, calliper gauges, shaped somewhat like the letter Y, are used, the horns terminating in polished parallel jaws. Such a gauge will detect a difference of 1 / 20,000 inch quite easily.
So accurately can plug gauges be made by reference to a measuring machine, that a gold leaf 1 / 30,000 inch thick would be three times too thick to insert between the gauge and the jaws of the machine!
You must remember that in high-class workmanship these gauges are constantly being used. As time goes on, the "limit of error" allowed in many classes of machine parts is gradually lessened, which shows the simultaneous improvement of all machinery used in the handling of metal. James Watt was terribly hampered, when developing his steam-engine, by the difficulty of procuring a true cylinder for his pistons to work in with any approach to steam-tightness. His first cylinder was made by a smith of hammered iron soldered together. The next was cast and bored, but stuffing it with paper, cork, putty, pasteboard, and "old hat" proved useless to stem the leakage of steam. No wonder, considering that the finished cylinder was one-eighth of an inch larger in diameter at one end than at the other. Watt was in advance of his time. Neither machinery nor workmanship had progressed sufficiently to meet the requirements of the steam-engine. To-day an engineer would confidently undertake to bore a cylinder five feet in diameter with a variation from truth of not more than one five-hundredth of an inch.
Before passing from the subject of measuring machines, which play so important a part in modern mechanism, we may just glance at the electrical method of Dr. P. E. Shaw. He discovered recently that two clean metal surfaces can, by means of an electric current, feel one another on touching with a delicacy that far transcends that of the purely mechanical machine. The mechanism he employs is thus devised: A finely cut vertical screw having fifty threads to the inch has a disc graduated into 500 parts. The screw can be turned by means of a pulley string from a distance, and it is thus possible to give the top end of the screw a movement of 1 / 25,000 inch, when a movement corresponding to one graduation is made.