Files have to be very carefully hardened and tempered. If heated too strongly, or made too hard, the steel is so brittle that the teeth tear off; if too 'soft, they wear down rapidly, and the file soon becomes useless. Great care is also required in keeping them straight, as the sudden cooling necessary for hardening is very apt to warp the steel.
At first sight, it would appear, from the simplicity and continual repetition of the movements required in file-cutting, and the precision and regularity of the work, that it is an operation specially adapted for machinery. Many attempts have been made to cut files by machinery, but with only partial success; the chief difficulty arises from the necessity of modifying the force of the blow to suit the hardness of the steel. It is practically impossible to supply a large number of blanks all of exactly the same hard ness; and if the machine be adjusted to suit the hardness of one blank, it may strike too heavy or too light a blow for the next; whereas the workman feels at once the hard ness of the steel he is working upon, and adjusts his blows accordingly.
FILING.—To the uninitiated, this may seem a simple operation of rubbing one piece of metal upon another, and requiring only muscular strength and no skill. This is far from being the'case, for a skillful workman will, in a given time, with a given amount of muscular work, cut away a far greater quantity of metal with a file than one who is unskillful, for lie makes every tooth cut into the work, instead of rubbing over it. To do
this, he must adapt the pressure and velocity of motion of the file to the coarseness of its teeth, and the hardness, brittleness, and toughness of the material he is working upon.
To,file flat, that is, to avoid rounding the sharp edges of a narrow piece of work, is very difficult, and some years of continual practice is required before an apprentice can do this well, especially in " smoothing up" or finishing work before polishing, and there are some who never succeed in filing, smoothing, and polishing without rounding the edges of fine work. The power of doing this constitutes the main test of skill among mathematical instrument makers and other metal-workers. The flattest surface can be obtained by laying the work, where its form admits, upon a piece of cork held in the vise, and filing it with one hand, the pressure on the file being communicated by the forefinger.
It is mainly to aid the workman in filing flat that the rounded or bellied form is given to files; this partially compensates the tendency of the hands to move in a curved line with its convexity upwards when! they move forward and apply pressure, as in the act of filing.