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
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