or one complete turn plus 36 holes on the 54-hole

circle. The calculation to determine 13 1/2° when an

index plate with an 18-hole circle is available, is as

follows:

or 2 holes on the 20-hole circle.

Compound indexing is a combination of two plain

or one complete turn plus 9 holes on the 18-hole

indexing procedures. You will index one number of

circle.

divisions by using the standard plain indexing

method, and another by turning the index plate

When indexing angles are given in minutes, and

(leaving the crankpin engaged in the hole as set in the

approximate divisions are acceptable, you can

determine the movement of the index crank and the

first indexing operation) by a required amount. The

difference between the amount indexed in the first

proper index plate by the following calculations. To

and second operations results in the spindle turning

determine the number of minutes represented by one

the required amount for the number of divisions.

turn of the index crank, multiply the number of

Compound indexing is seldom used because (1)

degrees covered in one turn of the index crank by 60:

differential indexing is easier, (2) high-number index

9° × 60´ = 540

plates are usually available to provide any range of

divisions normally required, and (3) the computation

and actual operation are quite complicated, making it

Therefore, one turn of the index crank will rotate the

easy for errors to be introduced.

index head spindle 540 minutes.

We will briefly describe compound indexing in

The number of minutes (540) divided by the

the following example. To index 99 divisions proceed

number of minutes in the division desired gives you

as follows:

the total number of holes there should be in the index

plate used. (Moving the index crank one hole will

1. Multiply the required number of divisions by

rotate the index head spindle through the desired

the difference between the number of holes in two

number of minutes of angle.) This method of

circles selected at random. Divide this product by 40

indexing can be used only for approximate angles

(ratio of spindle to crank) times the product of the two

since ordinarily the quotient will come out in mixed

index hole circles. Assume you have selected the 27-

numbers or in numbers for which there are no index

and 33-hole circles. The resulting equation is

plates available. However, when the quotient is

nearly equal to the number of holes in an available

99 × (33 - 27) ×

99 × 6

index plate, the nearest number of holes can be used

40 × 33 × 27

40 × 33 × 27

and the error will be very small. For example the

calculation for 24 minutes would be

2. To make the solution easier, factor each term

of the equation into its lowest prime factors and

cancel where possible. For example:

or 1 hole on the 22.5-hole circle. Since the index

plate has no 22.5-hole circle, you should use a 23-hole

circle plate.

If a quotient is not approximately equal to an

The result of this process must be in the form of a

available circle of holes, multiply by any trial number

fraction as given (that is, 1 divided by some number).

that will give a product equal to the number of holes

Always try to select the two circles that have factors

in one of the available index circles. You can then

that cancel out the factors in the numerator of the

move the crank the required number of holes to give

problem. When the numerator of the resulting

fraction is greater than 1, divide it by the denominator

the desired division. For example, use the following

calculation to determine 54 minutes when an index

and use the quotient (to the nearest whole number)

plate that has a 20-hole circle is available.

instead of the denominator of the fraction.

7-14

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