| Plain or simple indexing can be used for machining operations when
you need a certain number of division of a circle. When angular
machining operations need to be done (Figure 1) angular indexing is
used. |
Figure 1. Holes drilled at 75 degrees to each other.
|
| By using the holes in the index plates it is possible to machine
angular surfaces. There are 360 degrees in a complete circle, one
complete revolution of the indexing head spindle is 360 degrees. If
the direct indexing plate has 24 holes, the angular spacing between
each hole is 360/24 = 15 degrees (Figure 2). |
Figure 2. The angular spacing of a 24 hole indexing plate is 15
degrees.
|
This will allow us to make any division that requires 15 degree
intervals or multiples of 15 degrees.
When a plain or simple indexing head is used, it employs a 40:1
ratio. When you turn the indexing crank 40 times, the spindle makes
one complete turn or a 360 degree revolution.
| Therefore one turn of the index crank is equal to: |
 |
Any indexing plate that is divisible by nine can be used for
angular indexing.
Example
| Calculate indexing for 45 degrees: |
 |
Five full turns of the indexing arm would give us an angle of 45
degrees.
Example
| Calculate indexing for 19 degrees: |
 |
or 2 full turns plus 1/9 additional turn. The 1/9 turn is
calculated using the simple indexing method. 1/9 could be indexed
using 6 holes in a 54 hole plate.
Angular Indexing in Minutes
| If one full turn of the index crank is equal to 9 degrees and
each degree is equal to 60 minutes, 9 degrees can be written as 9 x
60 or 540 minutes. To index by minutes apply the formula: |
|
Example
| Calculate indexing for 8 degrees 50 minutes. Convert the angle to
minutes by multiplying the degrees times 60 and adding that to the
50 minutes. |
 |
Take the converted 530 minutes and divide that by 540 minutes (9
degree converted to minutes).
| Apply the formula: |
|
| Reduce to lowest terms: |
 |
Take the lowest term denominator 54, and determine which
available hole circle that it will divide into evenly. Since the 54
hole plate is available, we don’t need to go any farther.
Therefore to index 8 degrees 50 minutes you will use 53 holes in a
54 hole circle.
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