s it is passed beneath the revolving cutter, or,
what is the same thing, at a right angle to the face of the table on
which the moulding is moved, it is obvious that the highest point C of
the moulding will be cut to shape by the point C of the cutter; and that
since the line of motion of the end of the cutter is the arc D, the
lowest part of the cutter action upon the moulding will be at point E.
It will also be obvious that as the cutter edge passes, at each point,
its length across the line A A, it forms the moulding to shape, while
all the cutting action that occurs on either side of that line is
serving simply to remove material. All that we have to consider,
therefore, is the action on line A A.

It may be observed also that the highest point C of the cutter edge must
not be less than 1/4 inch from the corner of the cutter head, which
gives room for the nut N (that holds the cutter to the head) to pass
over the top of the moulding in a 2-1/2 inch head. In proportion as the
heads are made larger, however, less clearance is necessary for the nut,
as is shown in Figure 291, the cutter edge extending to C, and therefore
nearly up to the corner of the head. Its path of motion at C is shown by
dotted arc B, which it will be observed amply clears the nut N. In
practice, however, point C is not in any size of cutter-head placed
nearer than 1/4 inch from corner X of the cutter-head.

To find the length of the cutter edge necessary to produce a given depth
of moulding, we may draw a circle _i_, Figure 292, equal in diameter to
the size of the cutter head to be used, and line A A. The highest point
of cutting edge being at _e_, and the lowest at g, then circles _d_ and
_f_ represent the line of motion of these two points; and if we mark the
cutter in, the necessary length of cutting edge on the cutter is
obviously from _a_ to _b_.

[Illustration: Fig. 291.]

[Illustration: Fig. 292.]

Now the necessary depth of cutter edge being found for any given
moulding, or part of a moulding, the curves for the edge may be found as
follows: Suppose the moulding is to be half round, as in the end view in
Figure 290. The width of the cutter must of course equal the width of
the moulding, and the length or depth of cutting edge required may be
found from the construction shown in Figure 292; hence all that remains
is to find the curve for the cutting edge. In Figure 293, let A A
represent the centre of the cutter width, its sides bein