A file manufactured to work on iron or steel is not suitable for filing brass. In filing steel it is better to use second-cut files than those with coarser teeth, and sometimes when an extra finish is required, the finer cuts will be found essential.

For ordinary lathe requirements a coarse or milling file is the most suitable. Its performance will be improved if it is kept specially for this work and carefully cleaned after use. When good work is called for it is a mistake to use for lathe work a file that has already been used for vice-work. In the same way a file used for lathe work should not be used for vice work.

A standard bastard file is usually more economical than a wider bastard file for most jobs on which that kind of file is used. With the same weight applied it removes more metal in a given time.

A sharper file is needed for the non-fibrous metals such as brass and cast iron, than for wrought iron or for steel. There are, in fact, files specially cut for work on brass and other non-fibrous alloys. A sharper file is also needed for a broad surface than for a narrow. Use a new file, therefore, first for broad cuts on brass or cast-iron, next on narrow surfaces of these metals, and on edges. A new file should never be used for edges, as the teeth bite too quickly, and may be torn out, ruining the file. Next, wrought iron and steel may be filed, with the same file, and better results are here obtained than if a perfectly new file were used. Old files should be employed to remove sand and scale from castings and forgings. New files would be quickly destroyed by this work, without doing it very much better, because the scale is very hard, and a rough finish only is needed. The edges of a flat file, seldom used for other purposes, will do this work effectively. Although all filing should be done forward, it is not necessary to lift the file during the return stroke, except when using special files as for instance mill saw files with special " float " teeth. The only thing to avoid is the exertion of any pressure.

The best results from the forward stroke will be obtained by giving it a slight motion sideways, alternating to right and left with every few strokes. To get the right grip for correct filing, look for the thick part of the file, rest it on the work, with the first two fingers or the base of the thumb directly over the work. Then grasp the handle with the other hand and begin to file. The hand which rests on the file should not leave it either at the beginning or the end of the stroke.

Large files for which both hands are to be used should be gripped so that the end of the handle fits into and rests against the fleshy part of the palm below the joint of the little finger, with the thumb lying along the top of the handle in the direction of its length and the finger-ends pointing upwards.

For filing work held in a vice the height of the holder from the floor is important. In general, the jaws of the vice should be level with the elbow of the worker. A good average height is 42 inches from the floor, and this will be found to suit most men. If the work to be fitted is small and delicate, so that the user of the file need only move his arms slightly, or may need to move only one hand and arm, the vice should be higher, because this enables him to stand more erect, and also facilitates examination of the work. When the work is heavy and massive, needing considerable exertion, its surface should be below the elbow joint. In this way the weight of the body is brought to bear on the stroke of the file.

A way to economise on files when facilities are available is to use the sand blast for resharpening. They should not have the teeth worn down too much, or else the blast will have little effect and re-cutting alone will serve the purpose. Another method of cleaning and, to a certain extent, resharpening old files is the following which was published some time ago in " The Motor " :-To remove all greasy matter, boil the files in a strong solution of caustic soda for a quarter of an hour, then take out and wash thoroughly in water. The caustic solution should be about Jib. to 3 quarts of water. On no account must it come into contact with the hands, owing to its corrosive action. There may still remain metallic particles between the teeth. To remove these the files should be placed in a mixture of one part of strong nitric acid, half a part of strong hydrochloric acid and three parts of water. To prevent the files rusting after using the above preparation, they should be well rinsed in strong lime water and dried. When perfectly dry, they should be well oiled, and brushed and wrapped in paper, each file separate one from another, as two hardened files rubbing together removes the cutting edge from both of them.

It has been said by experts that it takes three thousand hours of filing for a man to become efficient in the use of the file. The question at once arises : if it takes him that length of time to learn to file efficiently, how many more thousands of hours are required for him to become efficient in choosing and taking care of his files ?