Fig. 2234.
Fig. 2235.
From the nature of the processes employed to cut the teeth of files, they are unequal in height, and as the file in addition to this varies in its straightness or warps in the process of hardening, it becomes necessary in many cases to choose for certain work files whose shape is best suited for it. Suppose, however, that files were produced whose teeth or tops or points were equal in height from end to end of the file, and it would be necessary for the workman to move the file in a true straight line in order to file a straight surface. This the most expert filers cannot accomplish. It is for this reason that hand files are made as in [Fig. 2234], being thickest in the middle m, and of a curved taper both towards the point p and the heel h, so that when applied to the work the file will bear on the work at a, [Fig. 2235], and be clear of it at b and c, which allows the file motion to deviate from a straight line without cutting away the work too much at b and c. The file curvature also enables any part of the file length to be brought into contact with the work or with any required part of the surface of the same, so as to locate or limit its action to any desired part.
If a bellied file (as this shape of file is sometimes termed) be moved in a straight line it will file flat so long as it is moved to have contact clear across the work, but if the file is concave in its length it can only cut at that part which is in contact with the edge of the work, and the latter must be filed convex.
It becomes obvious then that for flat work a bellied file must be used, and that the belly should preferably be of even sweep from end to end.
