The curvature or belly on a file not only enables but few teeth to be brought into action at any one turn, and thus cause it to cut more freely; but it also enables all parts of the file length to be used and worn equally. Thus in [Fig. 2238] are shown two positions of a file, one cutting at a and the other at b, these different locations being due to different levels of the file which may be given by elevating or depressing it at the handle end.
Fig. 2239.
If a file is hollow in one side of its width, and rounding on the other, as in [Fig. 2239], the hollow side is unfit for any but the roughest of work, since it will not file any kind of work true; but the rounded side is very effective for flat surfaces, since the number of teeth in action is more limited and their grip is therefore greater, while by canting the file any part of its width may be brought into action. The rounded side is especially advantageous for draw-filing (a process to be hereafter explained).
Fig. 2240.
In all cross-filing, whether performed to clean up a surface, remove a maximum of metal, or prepare the work for draw-filing, or for reducing the work to shape, the file should be given a slight lateral as well as a forward motion, and it will be found that this lateral motion is more effective if made from right to left, leaving the file marks in the direction of marks b, in [Fig. 2240], because the workman has more control over the file (especially if a large one) than when the lateral motion is from left to right; but this latter motion must be given occasionally to prevent the file from cutting deep scratches, and to keep the file surface true.