Page:Practical Treatise on Milling and Milling Machines.djvu/61

Rh or minus the movement of the plate, when the plate revolves in the opposite direction to the crank. The spiral head cannot be used for cutting spirals, when it is geared for differential indexing, for when cutting spirals the head is geared to the table feed screw.

To obviate the necessity of figuring out the change gears every time a certain number of divisions is required, tables on pages 210 to 225 have been compiled. By use of these tables, all numbers obtainable by differential indexing, together with those that can be had by the plain method can be easily indexed. The tables also give the correct circle and number of holes to be indexed, graduations for setting of the index sector, and the proper change gears to use.

In order to select the proper change gears, it is first necessary to find the ratio of the required gearing between the spindle and plate. After this has been done, the correct gears can be found. The following formulae show the manner in which this gearing is calculated.


 * N = number of divisions required.
 * H = number of holes in index plate.
 * n = number of holes taken at each indexing.
 * V = ratio of gearing between index crank and spindle.
 * x = ratio of the train of gearing between the spindle and the index plate.


 * S = gear on spindle.
 * rowspan="2" | } Drivers.
 * G1 = first gear on stud.
 * }
 * }


 * G2 = second gear on stud.
 * rowspan="2" | } Driven.
 * W = gear on worm.
 * }
 * x = $HV-Nn⁄H$ if HV is greater than Nn.
 * x = $Nn-HV⁄H$ if HV is less than Nn.
 * x = $S⁄W$ (for simple gearing).
 * x = $SG_{1}⁄G_{2} W$ (for compound gearing).
 * x = ⇭⇭⇭ (for compound gearing).

V is equal to 40 on the B. & S. spiral head, and the index plates furnished have the following numbers of holes: 15, 16, 17, 18, 19, 20, 21, 23, 27, 29, 31, 33, 37, 39, 41, 43, 47, 49.

The gears furnished have the following numbers of teeth: 24 (2 gears), 28, 32, 40, 44, 48, 56, 64, 72, 86, 100.

In selecting the index circle to be used, it is best to select one with a number having factors that are contained in the change gears