Page:Practical Treatise on Milling and Milling Machines.djvu/185

Rh Now, suppose the same gearing is retained and the spiral head is set at zero, or parallel to the surface of the table (see Fig. 70). It is apparent, also, that the axes of the index spindle and attachment spindle are parallel to one another. Therefore, as the table advances, and the blank is turned, the distance between the axes of the index spindle and attachment spindle remains the same. As a result, the periphery of the blank, if milled, is concentric or the lead is 0.

If, then, the spiral head is elevated to any angle between zero and 90° (see Fig. 71), the amount of lead given to the cam will be between that for which the machine is geared and 0. Hence it is clear that cams with a very large range of different leads can be obtained with one set of change gears, and the problem of milling the lobes of a cam is reduced to a question of finding the angle at which to set the head to obtain any given lead.

In order to illustrate the method of obtaining the correct angle, drawings of two cams to be milled, and data connected with same, are given in Figs. 72 and 73.

It is first necessary to know the lead of the lobes of a cam, that is, the amount of rise of each lobe if continued the full circumference of the cam. This can be obtained from the drawings as follows: For cams where the face is divided into hundredths, as those shown: multiply 100 by the rise of the lobe in inches and divide by the number of hundredths of circumference occupied by the lobe. For cams that are figured in degrees of circumference: multiply 360 by the rise of the lobe in inches and divide by the number of degrees of circumference occupied by the lobe. Taking Fig. 72 for example, we have a cam of one lobe which extends through 91 hundredths of the circumference and has a rise .178". Then $100×.178"⁄91$=.1956 of lobe or .196", which is near enough for all practical purposes.