(8)

which cannot be obtained by simple indexing with the index plates regularly

supplied. To obtain these divisions a differential index head is used. The

index crank is connected to the wormshaft by a train of gears instead of by

a direct coupling and with simple indexing.

The selection of these gears

involves calculations similar to those used in calculating change gear ratio

for cutting threads on a lathe.

(9)

(a)

When you must divide work into degrees or fractions of degrees by

plain indexing, remember that one turn of the index crank will rotate a

point on the circumference of the work 1/40 of a revolution.

Since there

are 360 in a circle, one turn of the index crank will revolve the

circumference of the work 1/40 of 360, or 9. Hence, in using the index

plate and fractional parts of a turn, 2 holes in a 18-hole circle equals 10,

1 hole in a 27-hole circle equals 2/3, 3 holes in a 54-hole circle equals

1/3. To determine the number of turns, and parts of a turn of the index

crank for a desired number of degrees, divide the number of degrees by 9.

The quotient will represent the number of complete turns and fractions of a

turn that you should rotate the index crank. For example, the calculation

for determining 15 when an index plate with a 54-hole circle is available,

is as follows:

or one complete turn plus 36 holes on the 54-hole circle. The calculation

for determining 13 1/2 when an index plate with an 18-hole circle is

available, is as follows:

(b)

When indexing angles are given in minutes and approximate divisions

are acceptable, movement of the index crank and the proper index plate may

be determined by the following calculations: