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.

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

Integrated Publishing, Inc. |