the pitch circle, and though in large wheels,
working with large wheels, so that the difference between the radius of
the generating circle and that of the smallest wheel is not excessive,
it is so small as to be practically inappreciable, yet in small wheels,
working with large ones, it may form a sensible error.

[Illustration: Fig. 240.]

For showing the dimensions through the arms and hub, a sectional view of
a section of the wheel may be given, as in Figure 240, which represents
a section of a wheel, and a pinion, and on these two views all the
necessary dimensions may be marked.

[Illustration: Fig. 240 _a_. (Page 203.)]

If it is desired to draw an edge view of a wheel (which the student will
find excellent practice), the lines for the teeth may be projected from
the teeth in the side view, as in Figure 240 _a_. Thus tooth E is
projected by drawing lines from the corners A, B, C, in the side view
across the face in the edge view, as at A, B, C in the latter view, and
similar lines may be obtained in the same way for all the teeth.

When the teeth of wheels are to be cut to form in a gear-cutting
machine, the thickness of the teeth is nearly equal to the thickness of
the spaces, there being just sufficient difference to prevent the teeth
of one wheel from becoming locked in the spaces of the other; but when
the teeth are to be cast upon the wheel, the tooth thickness is made
less than the width of the space to an amount that is usually a certain
proportion of the pitch, and is termed the side clearance. In all
wheels, whether with cut or cast teeth, there is given a certain amount
of top and bottom clearance; that is to say, the points of the teeth of
one wheel do not reach to the bottom of the spaces in the other. Thus in
the Pratt and Whitney system the top and bottom clearance is one-eighth
of the pitch, while in the Brown and Sharpe system for involute teeth
the clearance is equal to one-tenth the thickness of the tooth.

In drawing bevil gear wheels, the pitch line of each tooth on each
wheel, and the surfaces of the points, as well as those at the bottom of
the spaces, must all point to a centre, as E in Figure 241, which centre
is where the axes of the shafts would meet. It is unnecessary to mark in
the correct curves for the teeth, for reasons already stated, with
reference to the curves for a spur wheel. But if it is required to do
so, the construction to find the curves is as shown in Figure 242, in