Angular Indexing. With the ordinary indexing head, in which 40 turns of the index crank are required for one revolution of the work, one turn of the index crank equals 9 degrees. Hence, when one complete turn of the index crank equals 9 degrees, two holes in the 18-hole circle, or 3 holes in the 27-hole circle, must correspond to one degree. The first principle or rule for indexing for angles is therefore that two holes in the 18-hole circle or 3 holes in the 27-hole circle equals a movement of one degree of the index head spindle and the work.

Assume that an indexing movement of 35 degrees is required. One complete turn of the index crank equals 9 degrees; therefore, first divide the number of degrees for which to index, by 9, in order to find how many complete turns the index crank should make. The number of degrees left to turn after having completed the full turns are indexed by taking two holes in the 18-hole circle for each degree. In

35

=

this case, - 3%, which indicates that the index crank must be turned three 9 full revolutions, and then 8 degrees more are indexed by moving 16 holes in the 18-hole circle.

To index for 111⁄2 degrees, for example, first turn the index crank one revolution, this being a 9-degree movement. Then to index 21⁄2 degrees, move the index crank 5 holes in the 18-hole circle (4 holes for the two whole degrees and one hole for the 1⁄2 degree equals the total movement of 5 holes).

Below is shown how this calculation may be carried out to plainly indicate the movement required for this angle:

11% deg. 9 deg. + 2 deg. + 1⁄2 deg.

I turn + 4 holes +1 hole in the 18-hole circle.

Should it be required to index only degree, this may be done by using the 27hole circle. In this circle a three-hole movement equals one degree, and a one-hole movement in that circle thus equals a degree, or 20 minutes. Assume that it is required to index the work through an angle of 48 degrees 40 minutes. Below is plainly shown how this calculation may be carried out:

48 deg. 40 min. = 45 deg. +3 deg. + 40 min.

5 turns + 9 holes + 2 holes in the 27-hole circle.

Approximate Indexing for Angles.-The following general rule for approxi mate indexing of small angles is applicable to any index head requiring 40 revolutions of the index crank for one revolution of the work.

Rule: Divide 540 by the total number of minutes to be indexed. If the quotient is approximately equal to the number of holes in any index circle available, the angular movement is obtained by moving the crank one hole in this index circle; but if the quotient is not approximately equal, multiply it by any trial number which will give a product equal to the number of holes in an available index circle and move the index crank as many holes as are indicated by the trial number. (If the quotient of 540 divided by the total number of minutes is greater than the number of holes in any of the index circles, it is not possible to obtain the required movement for the angle by simple indexing.)

Example: Assume that it is required to index to an angle of 2 degrees 46 minutes. Changing this to minutes gives a total of 166 minutes. Dividing 540 by 166 we have 540 166 3.253. This quotient is next multiplied by some trial number to obtain a product which equals the number of holes in an available index circle. Multiplying by 12, we have 3.253 X 12 = 39.036. Therefore, for indexing 2 degrees 46 minutes, the 39-hole circle can be used and the index crank would be moved 12 holes.

Tables for Angular Indexing. - The table, "Angular Indexing," gives the number of turns of the index crank for indexing various angles. In the column headed, "Turns of Index Crank," the whole number (where given) indicates the number of full revolutions; the numerator of the fraction, the number of holes additional; and the denominator, the number of holes in the index circle to be used. The angular movement obtained for a movement of one hole, in various index plates is given in the table, "Angular Values of One-Hole Moves."

With the block system of

Block or Multiple Indexing for Gear Cutting. indexing, a number of teeth are indexed at one time, instead of cutting the teeth consecutively, and the gear is revolved several times before the teeth are all finished. For example, when cutting a gear having 25 teeth, the indexing mechanism is geared to index four teeth at once (see table) and the first time around, six widely separated tooth spaces are cut. The second time around, the cutter is one tooth behind the spaces previously milled. On the third indexing, the cutter has dropped back another tooth, thus finishing the gear (in this case) by indexing it around four times. The various combinations of change gears to use for block or multiple indexing are given in the accompanying table. The advantage claimed for block indexing is that the heat generated by the cutter (especially when cutting cast-iron gears of coarse pitch) is distributed more evenly about the rim and dissipated to a greater extent, thus avoiding distortion due to local heating and permitting higher speeds and feeds. The table given is intended for use with Brown & Sharpe automatic gear-cutting machines, but the gears for any other machine equipped with a similar indexing mechanism can be calculated. Assume, for example, that a gear cutter requires the following change gears for indexing a certain number of teeth: Driving gears having 20 and 30 teeth, respectively, and driven gears having 50 and 60 teeth. Then if it is desired to cut, say, every fifth tooth, multiply the fractions 20 30 5 ! In this particular instance, then, the

20

60

30 and by 5. Then, 50

=

I

60 50 I I

blank could be divided so that every fifth space would be cut, by using gears of equal size. The number of teeth in the gear, in any case, must not be a multiple of the number of teeth indexed.