Fig. 2236.
Fig. 2237.
But files, whatever their shape, and however evenly formed when soft, warp (as already remarked) in the hardening process, sometimes having crooks or bends in them, such as at e and d, in [Fig. 2236]. In such a file the teeth at e would perform no duty unless upon work narrower than the length of the concavity at e, while on the other side d, the extra convexity would give the file great value for work, in which particular spots only required to be filed, because the teeth at d could be brought to bear on the required spot without fear of cutting elsewhere.
If, however, we have a taper flat file, such as in [Fig. 2234], the thickness being equal from h to m, and a curved taper from m to p only, then it would be impossible to file flat unless only that part from m to p be used, because the heel h would meet the work at the same time as m, and it could not be known where the file would cut, more than that the most prominent teeth would cut the most.
An excellent method of testing the truth of a file, and of finding its high spots is to chalk a piece of board, press the file firmly to it and take several strokes and the chalk will be transferred to the highest parts of the file, showing very distinctly every hill and hollow in the teeth, even on the finest of Groubet files, and it will be found from this test that but very few of the best-made files are true, and that very great care is necessary in selecting a file for flat and true work.
Fig. 2238.