If an 18-hole circle is used, the fraction becomes
To apply the fraction 2/3 to the circle you choose,
4/9 × 2/2 = 8/18. For each division, turn the crank 4
convert the fraction to a fraction that has the number
of holes in the circle as a denominator. For example,
turns and 8 holes on an 18-hole circle.
if you choose the 15-hole circle, the fraction 2/3
Example 2: Index for 136 divisions.
becomes 10/15. If plate 3 happens to be on the index
head, multiply the denominator 3 by 13 to equal 39.
In order not to change the value of the original
indexing fraction, also multiply the numerator by 13.
There is a 17-hole circle, so for each division turn
the crank 5 holes on a 17-hole circle.
When setting the sector arms to space off the
required number of holes on the index circle, DO
The original indexing rotation of 6 2/3 turns becomes
NOT count the hole that the index crankpin is in.
6 26/39 turns. Thus, to mill each side of a hexagon,
you must move the index crank 6 full turns and 26
Most manufacturers provide different plates for
holes on the 39-hole circle.
indexing. Later model Brown and Sharpe index
heads use two plates with the following circle of
When there are more than 40 divisions, you may
divide both the numerator and the denominator of the
fraction by a common divisor to obtain an index circle
15, 16, 19, 23, 31, 37, 41, 43, 47
that is available. For example, if 160 divisions are
17, 18, 20, 21, 27, 29, 33, 39, 47
required, N = 160; the fraction to be used is
The standard index plate supplied with the Cincinnati
index head is provided with 11 different circles of
holes on each side.
Because there is no 160-hole circle, this fraction must
be reduced. To use a 16-hole circle, divide the
numerator and denominator by 10.
When you must divide work into degrees or
fractions of a degree by plain indexing, remember that
one turn of the index crank will rotate a point on the
Turn 4 holes on the 16-hole circle.
circumference of the work 1/40 of a revolution. Since
It is usually more convenient to reduce the
there are 360° in a circle, one turn of the index crank
original fraction to its lowest terms and then multiply
will revolve the circumference of the work 1/40 of
both terms of the fraction by a factor that will give a
360°, or 9°. Therefore, to use the index plate and
number representing a circle of holes.
fractional parts of a turn, 2 holes in an 18-hole circle
equal 1° (1/9 turn × 9°/turn), 1 hole in a 27-hole
circle equals 1/3° (1/27 turn × 9°/turn), 3 holes in a
54-hole circle equal 1/2° (1/18 turn × 9°/turn). 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
The following examples will further clarify the
example, use the following calculation to determine
use of this formula:
15° when an index plate with a 54-hole circle is
Example 1: Index for 9 divisions.