TEEATMENT OF THE COMSTOOK ORES. 213 



The proper curve of the face of the cam, in order that it may perform 

 the desired duty with the least friction, is the involute of a circle, the radius 

 of which is equal to the distance between the center of the cam-shaft and the 

 center of the stamp-stem. This produces a line for the face of the cam which 

 meets, better than any other, the various requirements. The bottom of the 

 tappet is constantly perpendicular to the radius of the curve of the cam ; the 

 tappet, and with it the stamp, is lifted vertically and uniformly, so that the 

 lift of the stamp is always regularly proportioned to the revolution of the 

 cam-shaft. 



The cam-curve may be constructed on paper by means of tangents, as 

 shown in Fig. 6, Plate XX. If c represents the center of the cam-shaft, and 

 c r the distance from the center of the cam-shaft to the center of the stamp- 

 stem, the circle described about c, with c r as a radius, is the developing circle 

 of the involute. The distance, representing the height to which the stamp is 

 to be lifted, is laid off upon the circumference of this circle, as from the point 

 1 ; which distance is subdivided into a convenient number of equal parts, 

 determining, as in Fig. 6, the points 2, 3, 4, * * * * * 13. From each one 

 of these points in the circle a tangent is drawn, on which is laid off a distance 

 equal to the length of arc between the point 1 and the point from which the 

 tangent is drawn. All the points thus determined in the tangent-lines are 

 points in the cam-curve, and may be connected, as shown in the figure, thus 

 producing the hne for the face of the cam. 



In practice the line of curvature is produced by cutting from a thin board 

 a circular piece, the radius of which is equal to the horizontal distance from 

 the center of the cam-shaft to the center of the stamp-stem. At a given 

 point on the periphery of the circular piece is fixed one end of a thread, 

 v/hich must have the length of the greatest desired lift of the stamp, and to 

 the other end of which is attached a pencil point. 



The circular piece, with the attached thread wound on the periphery of 

 the circle, is laid on a smooth board, on which the line is to be traced, and 

 the thread, being constantly stretched to its furthest reach, is unwound until 

 it forms a tangent to the circle at the point where the other end is attached. 

 The line described by the pencil point is the desired curve. 



Some builders slightly modify this curve, giving to the cam-arm a greater 



