A smooth file, which has finer teeth, is used for finishing work, and a dead smooth file for polishing lathe work. Dead smooth files should be employed wherever a very fine finish is required. No pressure should be used, and these files should never be put to work on rough surfaces. The reason is apparent. This class of file has as many as 120 teeth to the inch in a 4in. file, and 60 to the inch in a 16in. file. Therefore if used on rough surfaces the teeth will in all probability be ripped out and the file irretrievably spoiled.

Turning now to the way in which money can be saved by using a file correctly, a golden rule already mentioned is : always file forward and never backward. In some respects a file is like a circular saw. No one would dream of trying to make a circular saw cut backwards, and in the same way no one should ever file backward. The reason why so many mechanics fail to realise the truth of this comparison, and continually use their files both ways, is probably that there are so many teeth to the inch on a file. Damage done to one of them does not, therefore, seem of very great moment. This is a fallacy, however ; every tooth is of value and importance to the work.

Files should never be left unprotected in such a way that they come into contact with hard surfaces. This is one of the surest ways in which their teeth can be spoilt. It is as foolish to throw a good file into a drawer full of other tools, or to carry them about in a tool roll without any cover on them, as it would be to open a good razor and keep it in a cricket bag with bats and things.

Once again every tooth on a file matters. Every time its sharp edge is forced against other hard substances it is bound almost inevitably to break or lose some degree of efficiency. The next time a file which has been mishandled in this way is put to work there will be a number of places where it does not cut. All this means more time required to finish the job and consequently a loss of money in excess labour.

The teeth of a iile should always be carefully brushed when the file is put away. It is the slack workman who leaves his tool with bits of metal clogging all the teeth, and this same man will find the efficiency of his work impaired when next he uses that particular file. Pieces adhering to teeth sometimes cause " pinning" which makes deep scratches on the work. If this cannot be otherwise avoided it may be lessened by thoroughly chalking the file, though this may prevent it from cutting so freely. When a brush is not sufficient to remove the filings they may be shaken out by tapping the edge of the file against the vice. If this proves ineffective a file card may do the necessary, or, if working on wrought iron or steel, soft iron or a copper scorer should be used.

When, as in the smaller-sized and finer grades of teeth, dirt and grease are the chief causes of clogging, the best way to free the file is as follows :-Take a thin piece of hard sheet brass, and push it along the grooves across the file. It should be held at an angle of about 45° to the face, and may be about Jin. wide. After being pushed across 2 or 3 times, the teeth of the file make grooves in it which should exactly fit the shape of the sides of the teeth. With this the grooves may be thoroughly cleaned out, leading to a great improvement in the cutting of the file, or should the teeth of the file become so thickly clogged with filings, etc., that the ordinary file card will not clear them, it will be found that this difficulty can be overcome by picking the teeth of the file smartly all over with the end of a short brass or copper rod. In this way the delicate teeth are not injured, but the metal is hard enough to loosen the clogging matter so that it comes away easily when the file card is again used. In well-organised shops, drawers should be provided into which files may be slipped with some protective covering round them.

Most files in common use are double cut files, which means that the teeth are serrated by a diagonal cutting. Experienced workmen use single cut files sometimes to file soft metals. Very often the amateur uses a file which is too large or too small for his work. He will buy, for example, a medium size 6in. to 8in. in a medium cut or second cut. It would be far better for him to buy a 10 in. second cut, and a 6in. smooth file. The first will enable him to remove metal quickly, and the second suffices to put a good finish on the work.

Files, when new, should not have too much pressure exerted on them at each stroke. The danger of this is that some of the teeth may be broken off at the base. The proper way is to go lightly until the fine points are worn down, and then use more pressure as the teeth become blunter. A few heavy strokes at the beginning may ruin the file. New files should not be used on a weld where borax or any vitreous flux has been employed, nor should they be used on the chilled and grutty skin of castings. A worn file is best for this work, and when the hard scale has been removed, the new file can be used to finish off.