It requires a great deal of practice to file a surface flat, as there is a great tendency for the file "to rock or fulcrum" on the corners of the work and make the surface rounding or crowning. The worker should always take long strokes, not short jerky ones. Figure 131 shows the correct method of holding a file. If the file is always driven or pushed one way a series of small grooves will be cut across the work. It is always best to drive the file diagonally across the first direction to make a smoother surface. If this is done the file will always bite (cut) better, and as the marks can be seen the eye will tell when you have filed over the whole surface.
Sighting (looking) along the length of a new file will show which side is the most "bellied" (curved). This side is the best one to use.
Cast iron is harder to cut with a file than wrought iron or soft steel. A new file should never be used on rough cast iron, as the scale will dull the teeth and soon spoil the file. If the scale is not very deep it can be removed with the cutting edge of a flat file. When a file is too dull for cast iron, it may still be useful for cutting wrought iron or soft Steel Some flat files have a safe edge, i.e., a smooth edge with no teeth. Such files are used when it is necessary to file out a corner, as the safe edge prevents cutting a groove in one side of the corner when the other side is being filed. Such a file is shown in Fig. 132.
Fig. 129. - Cross-Sections of Files.
Fig. 130. - Measuring a File.
Fig. 131. - Proper Method of Using a File.
Files get clogged with chips and should be frequently cleaned with a wire brush, called a file-card. This will remove the chips, and keep the work from being scratched and grooved. File-cards are generally carried in the toolroom. When cutting cast iron with a new file, a little white chalk should be rubbed on the file; this chalk will absorb the oil and the chips will not be so likely to stick. Oil should never be used on a file for cast iron, but will sometimes make a file work better on wrought iron. It is, however, best not to use oil if only one file is available for both metals. When filing cast iron, the hand or finger should not be rubbed over the work, as the work will become greasy as a result and keep the file from cutting.
When filing finished work in the vise, the lead or copper jaws should always be used. Otherwise the vise jaw will bite into the work. The work may be given a very smooth finish by draw filing, which is simply drawing the file in a direction at right angles to its length. A single-cut, second-cut, or smooth file is best for draw filing.