as a result machines became better and more
efficient in every way.

It had this great advantage: It taught the workman of his day how to use
the file and scraper, because both must be used conjunctively to make an
absolutely flat plate.

Contrary to general beliefs, shapers and planers do not make absolutely
accurate surfaces. The test of this is to put together two plates so
planed off. There is just enough unevenness to permit air to get between
the plates. If they were perfectly true they would exclude all air, and
it would be a difficult matter to draw them apart.

TEST OF THE MECHANIC.--To make them perfectly flat, one plate has chalk
rubbed over it, and the two plates are then rubbed together. This will
quickly show where the high spots are, and the file and scraper are then
used to cut away the metal.

[Illustration: _Fig. 46. Fig. 47. Hexagon Nut_]

In England the test of the mechanic used to be determined by his ability
to file a piece of metal flat. It was regarded as the highest art. This
is not the most desirable test at the present time, and it is recognized
that a much severer test is to file a narrow piece exactly flat, and so
that it will not have a trace of roundness, and be square from end to
end.

TEST SUGGESTIONS.--In a shop which does not have the advantage of a
planer or shaper, there are so many articles which must be filed up,
that it is interesting to know something of how the various articles are
made with a file.

To file a hexagon, or six-sided nut will be a good test with a file. To
do this a little study in geometrical lines will save a vast amount of
time. In beginning the work, measure the radius with a divider, and then
step off and make six marks equidistant from each other on the round
surface.

[Illustration: _Fig. 48. Cutting Key-way_]

USE OF THE DIVIDERS.--The distance between each of these points is equal
to the radius, or half the diameter, of the round bar. See Fig. 46,
which shows this. The marks should be scribed across the surface, as
shown in Fig. 47, where the lines show the ends of the facets of the
outside of the nut.

Do not let the file obliterate the lines at the rough cutting, but
leave enough material so you can make a good finish at the line.

CUTTING A KEY-WAY.--Another job you may have frequent occasion to
perform, is to cut a way for a key in a shaft and in a wheel hub.
Naturally, this will be first roughed out with a cold chisel narrower
than th