Page:Practical Treatise on Milling and Milling Machines.djvu/255

Rh Following down the first column we find .145-50, which embraces the required lead. Following this line across horizontally we find .1474", which is sufficiently near to .1476" for all practical purposes. At the top of the column containing .1474" is the proper combination of change gears, 24, 86, 32, and 100, and in the two columns at the right and in line with .1474" are the necessary angles; $9 1⁄2$° for spiral head, and $80 1⁄2$° for vertical milling attachment.

When the machine is already set for a given lead and it is desired to know whether another required lead can be obtained without changing the gears, proceed as follows:

Example: Machine is set with a combination of gears, 24, 72, 32, and 86, and a lead of .1080" is required.

Follow down the column of exact leads that are given under the combination of change gears for which the machine is set until .1081" is found. This is sufficiently near to .1080" for all practical purposes. Hence it is possible to obtain this lead without changing the gears, by setting the spiral head at 5° and the vertical milling attachment at 85°.

In milling cams in this way an angle of greater than 80° with the spiral head, which is the greatest angle listed in these tables, should be avoided to prevent going beyond the range of the spiral head.

A vertical spindle milling attachment with offset spindle, as shown on page 77, is preferable for this work, as it will reach nearer to the spiral head spindle when milling small cams with the head set nearly vertical.

We also manufacture an extension by the use of which the spiral head can be moved farther in on the table to bring the spiral head and vertical spindle attachment spindles nearer together. This extension is furnished on special order.

The standard end mill is of sufficient length for practically all leads on ordinary screw machine cams, for long leads usually extend over only a partial turn of the cam.

The mill should be of the same diameter as the roll to be used with the cam, and, in laying out cam, work from the centre of the roll.

The change gears that are furnished with Universal Milling Machines have the following numbers of teeth: 24 (2 gears), 28, 32, 44, 48, 56, 86 and 100.