The length of travel 06, Fig. 79, is supposed to be known, being fixed by some requirement of the machine to which the cam is to be applied. This distance, for a uniform motion, should be divided into any convenient number of equal parts; the more divisions, the more accurately can the cam be drawn. In this case six spaces are chosen.
The diameter of base circle D is arbitrary; and its center having been chosen, draw the original radius CO; then the radii CX and CB, limiting the arcs of rise, rest, and fall, respectively, should be drawn. As the follower must rise 1/6 of its travel while the cam rotates 1/6 of its arc, there must be as many equal divisions of the cam arc as there are of the follower travel. Hence the arc of rise OX is divided into six equal parts and the radii are drawn. Similarly, the arc of fall OB is divided into six equal parts and the radii produced.
The rotation of the follower about the cam is accomplished by setting the point of the compasses at C, and, with radius C1, striking an arc intercepting the radius corresponding to position of the follower at R1. Similarly, points R2, R3, R4, R5, and R6 are found. As the follower rests from X to B, the arc of intersection for R6 is continued to F6. For the period of fall, arcs are swung from the same points of follower travel as before, making the intersections F5, F4, F3, F2, F1.
A smooth curve is now drawn through the points of intersection, thus forming the outline of the cam. For other forms of follower than a sharp point, this line would be strictly a tangent line to the face of the follower. A pointed follower, as shown in the figure, is not a very practical form, as the point is subjected to severe wear. It is chosen for the present illustration, to afford the simplest possible cam development.
Fig. 79. Diagram for Cam with Pointed Follower Whose Path Intersects Cam Center.
The cam may be tested by laying over it a piece of tracing cloth, and tracing roughly the outline of the cam, also marking the radii and the center of rotation. A pin is now placed at the center of rotation of the cam, and the tracing cloth swung until the several radii CR1 CR2 CR3, etc., fall into the line of travel C T. If the cam is correctly developed, it will just touch the several positions of the follower in its path when the proper radius is coincident with the line of travel C T. It is very easy to make a mistake in laying out cams, especially the more complicated ones; and this rough method of proving the work should always be applied.
The face of the follower in this case is a point, and the pressure line, being the common normal between the cam and follower at point of contact, is always theoretically normal to the cam at that point.
During motion, however, the force of friction between the cam and follower would modify somewhat the direction of the pressure line, turning it so as to produce a side thrust against the follower, causing consequent chattering and possible binding in its guides. This can be minimized by ample lubrication and hardened faces; but for cams which have any considerable load to work against, a follower carrying a roll against the cam is a necessity.
The line work in cam design should be fine and accurate. A hard pencil, kept well sharpened, is necessary, and special care must be taken to get definite intersections. In order to keep the center of the cam in as good condition as possible, it is well not to continue the radii to the center, but to stop when a short distance from the center, as shown. When penciling and inking in, use a fine, continuous line, not dotted; the continuous line is more quickly made and is apt to be more accurate than the dotted line. Moreover, the cam is strictly layout work, not finished in detail, and the subsequent detail drawing of the cam should not be confused with the layout of the cam outline.