Dividing Head Indexing

Indexing
Indexing is the process of evenly dividing the circumference of a circular

workpiece into equally spaced divisions, such as in cutting gear teeth, cutting
splines, milling grooves in reamers and taps, and spacing holes on a circle. The
index head of the indexing fixture is used for this purpose.
Index Head
The index head of the indexing fixture contains an indexing mechanism which is
used to control the rotation of the index head spindle to space or divide a
workpiece accurately. A simple indexing mechanism consists of a 40-tooth worm
wheel fastened to the index head spindle, a single-cut worm, a crank for turning
the wormshaft, and an index plate and sector. Since there are 40 teeth in the
worm wheel, one turn of the index crank causes the worm, and consequently, the
index head spindle to make 1/40 of a turn; so 40 turns of the index crank revolve
the spindle one full turn.
Index Plate
The indexing plate is a round plate with a series of six or more circles of equally
spaced holes; the index pin on the crank can be inserted in any hole in any circle.
With the interchangeable plates regularly furnished with most index heads, the
spacing necessary for most gears, boltheads, milling cutters, splines, and so
forth can be obtained. The following sets of plates are standard equipment:
Brown and Sharpe type consists of 3 plates of 6 circles each drilled as follows:
Plate I -15, 16, 17, 18, 19, 20 holes
Plate 2-21, 23, 27, 29, 31, 33 holes
Plate 3-37, 39, 41, 43,47,49 holes
Cincinnati type consists of one plate drilled on both sides with circles divided as
follows:
First side - 24, 25, 28, 30, 34, 37,38, 39,41,42,43 holes
Second side - 46, 47, 49, 51, 53, 54, 57, 58, 59, 62, 66 holes
Shop Indexing plate

24, 25, 30, 37, 39 and 41 holes
Sector
The sector indicates the next hole in which the pin is to be inserted and makes it
unnecessary to count holes when moving the index crank after each cut. It
consists of two radial, beveled arms which can be set at any angle to each other
and then moved together around the center of the index plate.
Plain Indexing
The following principles apply to basic indexing of workpieces:
Suppose it is desired to mill a project with eight equally spaced teeth.
Since 40 turns of the index crank will turn the spindle one full turn, l/8th of 40 or 5
turns of the crank after each cut will space the gear for 8 teeth, If it is desired to
space equally for 10 teeth, 1/10 of 40 or 4 turns would produce the correct
spacing.
The same principle applies whether or not the divisions required divide
equally into 40, For example, if it is desired to index for 6 divisions, 6 divided into
40 equals 6 2/3 turns; similarly, to index for 14 spaces, 14 divided into 40 equals
2 6/7 turns. These examples may be multiplied indefinitely and from them the
following rule is derived:
To determine the number of turns of the index crank needed to obtain one
division of any number of equal divisions on the workpiece, divide 40 by
the number of equal divisions desired (provided the worm wheel has 40
teeth, which is standard practice).
NOTE: It is good practice always to index clockwise on the plate to eliminate
backlash.
Indexing Operations
The following examples show how the index plate is used to obtain any desired
part of a whole spindle turn by plain indexing.
Example 1: Milling a hexagon.

1. Using the rule previously given, divide 40 by 6 which equals 6 2/3 turns, or
six full turns plus 2/3 of a turn or any circle whose number is divisible by 3.
2. Take the denominator which is 3 into which of the available hole circles it
can be evenly divided. In this case, 3 can be divided into the available
18-hole circle exactly 6 times. Use this result 6 as a multiplier to generate
the proportional fraction required.
40
Example:
6 = 6 32 , 2x6
3x6 = 12
18
Therefore, 6 full turns of the crank plus 12 spaces on an 18-hole circle is the
correct indexing for 6 divisions.
Example 2: Cutting a gear.

1. To cut a gear of 52 teeth, using the rule again, divide 40 by 52. This means
that less than one full turn is required for each division, 40/52 of a turn to
be exact.
2. Since a 52-hole circle is not available, 40/52 must be reduced to its lowest
term which is 10/13. Take the denominator of the lowest term 13, and
determine into which of the available hole circles it can be evenly divided.
3. In this case, 13 can be divided into a 39-hole circle exactly 3 times. Use
this result 3 as a multiplier to generate the proportional fraction required.
10x3 30
Example:
13x3 = 39
Therefore, 30 holes on a 39-hole circle is the correct indexing for 52 divisions.

When counting holes, start with the first hole ahead of the index pin.

Dividing Head Indexing

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What is indexing and what is it used for?

What are the main components of an indexing fixture?

Indexing

Indexing is the process of evenly dividing the circumference of a circular

Shop Indexing plate

Example 1: Milling a hexagon.

Example 2: Cutting a gear.

Therefore, 30 holes on a 39-hole circle is the correct indexing for 52 divisions.

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