131, three of these
degrees being absorbed for rounding and three to insure a dead rest for
the tooth when it enters the cylinder.
WHY THE ANGULAR EXTENT IS INCREASED.
Without rounding the exit lip the action of the tooth on its exit would
be entirely on the inner angle of the shell. To obviate this it is the
usual practice to increase the angular extent of the cylinder ten
degrees, as shown on the arc _o'_ between the lines _f_ and _p_, Fig.
131. Why we should allow ten degrees on the exit lip and but six degrees
on the entrance lip will be understood by observing Fig. 130, where the
radial lines _s_ and _r_ show the extent of angular motion of the
cylinder, which would be lost if the tooth commenced to act on the inner
angle and ended on the outer angle of the exit lip. This arc is a little
over six degrees, and if we add a trifle over three degrees for rounding
we would account for the ten degrees between the lines _f_ and _p_, Fig.
131. It will now be seen that the angular extent is 196 degrees.
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