Page:Practical Treatise on Milling and Milling Machines.djvu/188

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the approximate shape of the lobes on the blank and drill and break off the surplus stock.

Following the same method of figuring to find the lead of the lobes on this cam, we have: ${\frac {100\times 2.493''}{47}}=5.304''$ lead for lobe A, and ${\frac {100\times 2.443''}{29}}=8.424''$ lead for lobe B.

Where there are two or more lobes on a cam, the machine is geared for a lead slightly longer than the longest one required, which in this case is 8.424", then the other lobes are milled without changing the gears. Referring to the Table of Leads, we find a lead of 8.437", which is slightly larger than 8.424". This gearing is, therefore, accepted, and it is required to find the sine of the angle at which to set the head for lobe B.

${\frac {8.424}{8.437}}=.99846$ sine of angle at which to set head. Looking at a table of sines and cosines, .99846 is found to be the sine of an angle of 86° and 49'. The head is, therefore, set at a trifle over 86 3/4 °.

When lobe B has been milled, the head is set for lobe A.

${\frac {5.304}{8.437}}=.62865$ sine of an angle at which to set head. Referring again to the table of sines and cosines, we find that .62865 is very near to .62864, which is the sine of an angle of 38° and 57'. The head is, therefore, set slightly under 39° for this lobe.

The other portions of the periphery of this cam are formed up either by filing to a line before the blank is put on the milling machine or by milling to the line after the lobes have been formed.

Whenever possible, the job should be set up so that the end mill will cut on the lower side of the blank, as this brings the mill and table nearer together and makes the job more rigid. It also prevents chips from accumulating, and enables the operator to better see any lines that may be laid out on the face of the cam.

When the lead of the machine is over 2 inches the automatic feed can be used, but when the lead is less than 2 inches the job should be fed by hand, with the index crank, as shown on page 187.

By the use of the calculations just given, we have compiled tables on pages 248 to 299 that give a wide range of leads from to 20" that can be obtained with the spiral head in the manner described. These tables will be found useful, as they give all data and settings without the necessity of figuring.