in a half
stroke cam to intersect the curved lines already drawn, as shown in
Figure 275. In practice, the sharp corner at S would be objectionable,
owing to rapid wear at this point; and hence a modification of the
dimensions for this half stroke cam would be required to obtain a larger
wearing surface at the point S, but the cam of this limit (1/2 stroke)
is correctly drawn by the process described with reference to Figure
275, the outline of the cam so constructed being shown in Figure 276.

[Illustration: Fig. 278.]

In Figure 278 is shown a cam designed to cut off the steam at
five-eighths of the piston stroke, the construction lines being given
in Figure 277, for which draw circle E and straight lines A and B, as in
the preceding example. By reference to Figure 274 it will be observed
that the diagonal line drawn through circle E at 5 is drawn from the
straight line marked 5, which intersects circle E 1, and as this
straight line 5 represents five-eighths of the stroke laid off on line
B, it determines the limit of cut-off on the five-eighths cam in Figure
277.

[Illustration: Fig. 279.]

Turning then to Figure 274, take on circle E the radius from radial line
4 to radial line 5, and mark it in Figure 277 from the vertical line
producing V'.

Now, with a radius of 18 inches, and one point of the dividers fixed at
point V, forming the intersection of the circle E with the horizontal
line B, draw the arc P. With the same radius, and one point of the
dividers fixed at point V', draw the opposite arc P'. With a radius of
10-1/2 inches from the centre X, draw the arc K 1, intersecting lines P
P', at S S. With a radius of 7-1/2 inches, draw the curved line K 2,
opposite to curved line K 1. Now, with a radius of 18 inches, and one
point of the dividers fixed alternately at S S, draw the arcs H, H, from
their intersection with the circle E, until they merge into the curved
line K 2. These curved lines embrace a cut-off cam of five-eighths
limit, shown complete in Figure 278.

[Illustration: Fig. 280.]

From the instructions already given it should be easy to understand that
the three-fourths and seven-eighths cams, shown in Figures 279, 280, 281
and 282, are drawn by taking the points of their cut-off from the same
scale shown in Figure 274, at the diagonal points 6 and 7, intersecting
circle E in that figure; and cut-off cams of intermediate limit of
cut-off can be drawn by further subdividing the stroke line B