Fig. 2182. Fig. 2183.
Suppose, for instance, that we have in the figure a piece of moulding m and a plane blade b, and the length of the cutting edge is denoted by a, [Fig. 2182]; now suppose that the blade is inclined to its line of motion (as in the case of carpenters’ planes) and stands at c, [Fig. 2183]: we then find that the cutting edge must extend to the length or depth d, and it is plain that the depth of the curve on the moulding is less than the depth of the cutting edge that produces it; the radius e being less than of d, so that if we place the cutter c upright on the moulding it will appear as shown in [Fig. 2181]. If, therefore, we are required to make a blade that will produce a given depth of moulding when moved in a straight line and presented at a given angle to the work, we must find out what shape the blade must be to produce the given shape of moulding, which we may do as follows:
Fig. 2184.
In [Fig. 2184] let a be a section of the moulding, and if the blade or knife is to stand perpendicular, as shown at b, [Fig. 2183], and if it is moved in a straight line in the direction of the length of the work, then its shape would necessarily be that shown at b, [Fig. 2184], or merely the reverse of a. In the position mentioned it could be used only as a sweep applicable to some few uses, but not adapted to cutting. To become a cutting tool it must be inclined and stand at some angle of less than 90° to its line of motion.
Fig. 2185.