Rapid indexing is used when a large number of
Example 3: Index for 10 divisions
duplicate parts are to be milled.
To find the number of holes to move the index
plate, divide 24 by the number of divisions required.
When the number of divisions required does not
Number of holes to move = 24/ N where
divide evenly into 40, the index crank must be moved
N = required number of divisions
a fractional part of a turn with index plates. A
Example: Indexing for a hexagon head bolt:
commonly used index head comes with three index
because a hexagon head has six flats,
plates. Each plate has six circles of holes, which we
will use as an example.
2 4 24
N = 6 = 4 holes
Plate 1:
15-16-17-18-19-20
Plate 2:
21-23-27-29-31-33
IN ANY INDEXING OPERATION, ALWAYS
Plate 3:
37-39-41-43-47-49
S T A R T COUNTING FROM THE HOLE
ADJACENT TO THE CRANKPIN. During heavy
The previous examples of using the indexing
cutting operations, clamp the spindle by the clamp
formula 40/N gave results in complete turns of the
screw to relieve strain on the index pin.
index crank. This seldom happens on the typical
indexing job. For example, indexing for 18 divisions
PLAIN INDEXING
Plain indexing, or simple indexing, is used when a
circle must be divided into more parts than is possible
The whole number indicates the complete turns of
by rapid indexing. Simple indexing requires that the
the index crank, the denominator of the fraction
represents the index circle, and the numerator
spindle be moved by turning an index crank, which
represents the number of holes to use on that circle.
turns the worm that is meshed with the worm wheel.
Because there is an 18-hole index circle, the mixed
The ratio between worm and the worm wheel is 40 to
number 2 4/18 indicates that the index crank will be
1. One turn of the index crank turns the index head
moved 2 full turns plus 4 holes on the 18-hole circle
spindle 1/40 of a complete turn. Therefore, 40 turns
that you will find on index plate 1. The sector arms
of the index crank are required to revolve the spindle
are positioned to include 4 holes and the hole in which
chuck and the job 1 complete turn. To determine the
the index crankpin is engaged. The number of holes
number of turns or fractional parts of a turn of the
(4) represents the movement of the index crank; the
index crank necessary to cut any required number of
hole that engages the index crankpin is not included.
divisions, divide 40 by the number of divisions
When the denominator of the indexing fraction is
required.
smaller or larger than the number of holes contained
in any of the index circles, change it to a number
40
representing one of the circles of holes. Do this by
Number of turns of the index crank = N
multiplying or dividing the numerator and the
denominator by the same number. For example, to
where N = number of divisions required
index for the machining of a hexagon ( N = 6):
Example 1: Index for five divisions
The denominator 3 will divide equally into the
following circles of holes, so you can use any plate
There are eight turns of the crank for each
that contains one of the circles.
division.
Plate 1:
15 and 18
Example 2: Index for eight divisions
Plate 2:
21 and 33
39
Plate 3:
7-12