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
holes:
divide both the numerator and the denominator of the
fraction by a common divisor to obtain an index circle
Plate 1:
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
Plate 2:
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.
Side 1:
24-25-28-30-34-37-38-39-41-42-43
Because there is no 160-hole circle, this fraction must
be reduced. To use a 16-hole circle, divide the
Side 2:
46-47-49-51-53-54-57-58-59-62-66
numerator and denominator by 10.
ANGULAR INDEXING
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.
available.
7-13