Page:Popular Science Monthly Volume 46.djvu/630

612 The reverse would be the case if this lathe-work was printed from the plate as it is engraved by the machine. This reversal (making that sunk on the plate which is left raised by the lathe, and vice versa) is effected by a process that is one of the carefully guarded secrets of the trade, and therefore can not be described here. Its effect, however, is evident.

We may suppose, for instance, that a very careful engraver might possibly cut upon a plate a very fair imitation of the lines forming the figure in our last diagram, but these cut lines, remember, would print black and would give an entirely different effect; therefore the black diamonds between the white lines must be cut and the line left standing; and what hand, be it ever so skillful, could cut these black interspaces and leave the white lines in their purity and regularity? Yet this is just what the engraver must do who would reproduce on steel this figure; and, we repeat, this is far less elaborate than those in actual use on bank notes.

The tool which cuts these delicate lines is made of the finest steel; made very hard and very carefully tempered. It passes through each line of the cutting about twenty times, cutting down about one three-thousandth part of an inch each time, then continues going over the line about fifty times more to clean out the burr and polish the work. The machine must, therefore, work with perfect precision, for a deviation of one tenth part of a hair's breadth would destroy the whole cutting.

On account of this required precision, these lathes are made adjustable in every part—that any loss-motion caused by wear may be taken up. They are so sensitive as to be affected by a sudden change of temperature in the room; and if a partially finished line should be entered by the tool after such sudden change, the result would be a defective piece of work.

We may watch one of these machines for hours and each moment discover some new movement. In the hands of a skillful operator (for, after all, the machine itself, to produce the required effects, must be under the direction of human intelligence) it will produce almost any form desired.

There are only three or four first-class operators of these machines in the world, and they are all Americans. Only one of these is able to make a cutting from a pencil sketch and figure out the required combination of wheels, set the lathe, and know it is properly set and adjusted before turning a wheel. The turn of a screw, the substitution of one wheel for another with the variation of a single cog—the shifting of the axis of an eccentric, will produce an entirely new effect; it may give distortion where perfect regularity is demanded; therefore a perfect and long familiarity with the machine is necessary to the successful handling of it.