Machinery Repairman

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 one complete turn plus 9 holes on the 18-hole
circle.
When indexing angles are given in minutes, and
approximate divisions are acceptable, you can
determine the movement of the index crank and the
proper index plate by the following calculations. To
determine the number of minutes represented by one
turn of the index crank, multiply the number of
degrees covered in one turn of the index crank by 60:
9° × 60´ = 540
Therefore, one turn of the index crank will rotate the
index head spindle 540 minutes.
The number of minutes (540) divided by the
number of minutes in the division desired gives you
the total number of holes there should be in the index
plate used. (Moving the index crank one hole will
rotate the index head spindle through the desired
number of minutes of angle.) This method of
indexing can be used only for approximate angles
since ordinarily the quotient will come out in mixed
numbers or in numbers for which there are no index
plates available. However, when the quotient is
nearly equal to the number of holes in an available
index plate, the nearest number of holes can be used
and the error will be very small. For example the
calculation for 24 minutes would be
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.
or 2 holes on the 20-hole circle.
COMPOUND INDEXING
Compound indexing is a combination of two plain
indexing procedures. You will index one number of
divisions by using the standard plain indexing
method, and another by turning the index plate
(leaving the crankpin engaged in the hole as set in the
first indexing operation) by a required amount. The
difference between the amount indexed in the first
and second operations results in the spindle turning
the required amount for the number of divisions.
Compound indexing is seldom used because (1)
differential indexing is easier, (2) high-number index
plates are usually available to provide any range of
divisions normally required, and (3) the computation
and actual operation are quite complicated, making it
easy for errors to be introduced.
We will briefly describe compound indexing in
the following example. To index 99 divisions proceed
as follows:
1. Multiply the required number of divisions by
the difference between the number of holes in two
circles selected at random. Divide this product by 40
(ratio of spindle to crank) times the product of the two
index hole circles. Assume you have selected the 27and
33-hole circles. The resulting equation is
99 × (33 - 27) 99 × 6
×
40 × 33 × 27 40 × 33 × 27
2. To make the solution easier, factor each term
of the equation into its lowest prime factors and
cancel where possible. For example:
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
the desired division. For example, use the following fraction is greater than 1, divide it by the denominator
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