131, where we repeat the circles _d_ and _h_, shown in Fig. 130, which
represents the inside and outside of the cylinder. We have here also
repeated the line _f_ of Fig. 130 as it cuts the cylinder in half, that
is, divides it into two segments of 180 degrees each. If we conceive of
a cylinder in which just one-half is cut away, that is, the lips are
bounded by straight radial lines, we can also conceive of the relation
and position of the parts shown in Fig. 130. The first position of which
we should take cognizance is, the tooth _D_ is moved back to the left so
as to rest on the outside of our cylinder. The cylinder is also supposed
to stand so that the lips correspond to the line _f_. On pressing the
tooth _D_ forward the incline of the tooth would attack the entrance
lip of the cylinder at just about the center of the curved impulse face,
imparting to the cylinder twenty degrees of angular motion, but the
point of the tooth at _d_ would exactly encounter the inner angle of the
exit lip, and of course the cylinder would afford no rest for the tooth;
hence, we see the importance of not cutting away too much of the half
shell of the cylinder.
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