After the depth of cutting edge for each member has thus been found, it remains to find the exact curve of cutting edge for each step, and, in doing this, the same scale may be used, saving much labor in this part also of the process, especially where a new piece of moulding must be inserted to repair part of an old piece that needs renewal in places only, as is often the case in railroad cars.
Fig. 2197a.
In [Fig. 2197a] we have a scale or rule constructed upon the foregoing principles, but marked to sixteenths, and it may now be shown that the same scale may be used in finding the actual curve as well as the depth of cutter edge necessary to produce the moulding of any member of it. Let the lower curves s, t, for example, represent the moulding to be produced, and the upper outline represent the blank piece of steel of which the cutter is to be made, the edges c, d being placed in line one with the other. We may then draw across both the moulding and the steel, lines such as e e, f f, g g, h h, i i, j j, all these lines being parallel to the edges c, d. To get the total depth of cutter edge for the moulding we measure with a common measuring rule the total height of the moulding, and supposing it to measure an inch, we set a pair of compasses to an inch on our cutter scale, and with them mark from the base m of the steel, the line p giving total depth of cutter edge. We next measure with a common rule the depth of member s of the moulding, and as it measures 1⁄2 inch we set the compasses to the 1⁄2 on the cutter scale, and with these compasses mark from line m line b, showing the depth of member s. In order to find the exact curve for each member, we have first to find a number of points in the curve and then mark in the curve by hand, and it is for the purpose of finding these points that the lines e e, f f, g g, h h, i i, j j, have been drawn. These lines, it may be remembered, need not be equally spaced, but they must be parallel, and as many of them may be used as convenient, because the greater their number the more correctly the curve can be drawn.
The upper edge or base line, m m, both of the steel and of the drawing, is that from which all measurements are to be taken in finding the points in the curve, which is done as follows: With an ordinary measuring rule we measure on the moulding and from line m m of the moulding as a base the length of the line f f below m m, to the curve, which in this case measures say 5⁄16 inch; we then set a pair of compasses or compass calipers to the 5⁄16 on the cutter scale, and from base m m on the cutter steel, mark, on line f f, an arc, and where the arc cuts f f is one point in the curve.
Similarly we measure on the moulding, or drawing of the moulding, the length of line g g from line m m to the moulding curve, and find that it measures, say 7⁄16 inch, hence we mark from base line m m of the steel, on line g g, arc v, distant 7⁄16 according to the cutter scale. Similar measurements are taken at each vertical line of the drawing which represents the moulding, and by means of the corresponding divisions of the cutter scale, arcs are marked on the vertical lines on the cutter steel, and where the arcs cut the vertical lines are points in the curve, and through these points the curve may be drawn by hand. We may make a cutter scale from an ordinary parallel rule, marking one end to correct inches and the other end for a cutter scale. Measurements from the moulding may then be made on one end of the rule; measurements for the cutter may be taken from the other end of the rule, and the rule may be used at the same time to draw the parallel lines e e, &c. Or, as each size of cutter head requires a different cutter scale, we may make a rule out of a piece of box or other close-grained wood, say 3⁄4 inch square, using one side for each size of cutter head. One end of each face of this rule may be marked in correct inches and parts of an inch (the divisions being thirty-seconds of an inch), and the other end may be marked as a cutter scale, the divisions being found as described with reference to [Fig. 2197].
Fig. 2198.