Page:Practical Treatise on Milling and Milling Machines.djvu/187

Rh Fig. 73

near .29265, which is the sine of an angle of 17° and 1'. As the spiral head is not graduated closer than quarter degrees, it will be satisfactory to elevate the head just a hair over 17°; then, with the gearing for a lead of .67", a cam with a lead of .196" will be obtained.

The minute errors between the actual lead .1956" and .196", and in the sines and angles of this calculation can be safely ignored, as it is not possible in practice to work very much closer than we have outlined.

The portion of the periphery of the cam from 91 hundredths to zero, represents a clearance of the cutting tool prior to the beginning of the throw. It is usually milled to a line, or drilled, broken out, and filed.

In Fig. 73, we have a cam with two lobes, one, A, having a rise of 2.493" in 47 hundredths, and the other, B, having a rise of 2.443" in 29 hundredths. On cams such as this, where it is necessary to remove considerable stock, it is usually the practice to first outline