This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
Having a large number of wires to file true on the end I devised a way to do this with the use of some old worn-out and discarded files that had good cutting edges. A piece of sheet copper, about the same length as the files, was bent to fit over one edge and both sides of the file, allowing both edges to project about 1/4in. This made a guide that prevented the edge of the file from slipping off the end of the wire. The guide was held in place on the file by cutting a slit in the projecting edges, about 1/4 in. from the end, and turning these separated parts back on the file.
Ill: Guide for Using the Edge of Worn-Out Files on Small Round or Square Stock
If such a guide is fitted tightly on a file, the edges of worn-out files can be used for such work, and the file cannot slip off and mar the sides of the work. - Contributed by A. R. Drury, Hampton, Ill.