Machinery Repairman

To apply the fraction 2/3 to the circle you choose,
convert the fraction to a fraction that has the number
of holes in the circle as a denominator. For example,
if you choose the 15-hole circle, the fraction 2/3
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.
The original indexing rotation of 6 2/3 turns becomes
6 26/39 turns. Thus, to mill each side of a hexagon,
you must move the index crank 6 full turns and 26
holes on the 39-hole circle.
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
that is available. For example, if 160 divisions are
required, N = 160; the fraction to be used is
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.
Turn 4 holes on the 16-hole circle.
It is usually more convenient to reduce the
original fraction to its lowest terms and then multiply
both terms of the fraction by a factor that will give a
number representing a circle of holes.
The following examples will further clarify the
use of this formula:
Example 1: Index for 9 divisions.
7-13
If an 18-hole circle is used, the fraction becomes
4/9 × 2/2 = 8/18. For each division, turn the crank 4
turns and 8 holes on an 18-hole circle.
Example 2: Index for 136 divisions.
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
NOT count the hole that the index crankpin is in.
Most manufacturers provide different plates for
indexing. Later model Brown and Sharpe index
heads use two plates with the following circle of
holes:
Plate 1: 15, 16, 19, 23, 31, 37, 41, 43, 47
Plate 2: 17, 18, 20, 21, 27, 29, 33, 39, 47
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
Side 2: 46-47-49-51-53-54-57-58-59-62-66
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
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°. Therefore, to use the index plate and
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
example, use the following calculation to determine
15° when an index plate with a 54-hole circle is
available.