to use this class of tools. It is astonishing how much very hard labor
may be saved by means of a small circular saw like that shown in Fig.
1. This tool, like many others described in this series of articles,
can, in most instances, be purchased cheaper than it can be made, and
the chances are in favor of its being a more perfect article. However,
it is not so difficult to make as one might suppose. A piece of sheet
steel may be chucked upon the face plate, or on a wooden block
attached to the face plate, where it may be bored to fit the saw
mandrel, and cut in circular form by means of a suitable hand tool. It
may then be placed upon the mandrel and turned true, and it is well
enough to make it a little thinner in the middle than at the
periphery.

[Illustration: Rotary Cutting Tools.]

There are several methods of forming the teeth on a circular saw. It
may be spaced and filed, or it may be knurled, as shown in Fig. 2, and
then filed, leaving every third or fourth tooth formed by the knurl,
or it may, for some purposes, be knurled and not filed at all. Another
way of forming the teeth is to employ a hub, something like that used
in making chasers, as shown in Fig. 3, the difference between this hub
and the other one referred to, is that the thread has one straight
side corresponding with the radial side of the tooth. The blank from
which the saw is made is placed on a stud projecting from a handle
made specially for the purpose, and having a rounded end which
supports the edge of the blank, as the teeth are formed by the cutters
on the hub.

The saw, after the teeth are formed, may be hardened and tempered by
heating it slowly until it attains a cherry red, and plunging it
straight down edgewise into cool, clean water. On removing it from the
water it should be dried, and cleaned with a piece of emery paper, and
its temper drawn to a purple, over a Bunsen gas flame, over the flame
of an alcohol lamp, or over a hot plate of iron. The small saw shown
in Fig. 4 is easily made from a rod of fine steel. It is very useful
for slotting sheet brass and tubes, slotting small shafts, nicking
screws, etc. Being quite small it has the advantage of having few
teeth to keep in order, and it may be made harder than those of larger
diameter. A series of them, varying in diameter from one eighth to
three eighths of an inch, and varying considerably in thickness, will
be found very convenient.

These cutters or saws, with the e