Simple indexing is one of the most common methods used with a milling machine dividing head. It is used to divide a workpiece into equal parts so that gear teeth, splines, slots, and other equally spaced features can be cut accurately.
This page explains the simple indexing formula, shows how to choose a hole circle, and provides a practical example that can be used during machine setup.
The video below explains the same topic visually, while the text here gives the formula and calculation steps in a clear workshop format.
For a standard 40:1 dividing head, the crank movement is:
The number 40 means one full turn of the crank rotates the dividing head spindle by 1/40 of a revolution. The result may be a whole number plus a fraction, or a whole number plus a hole-circle movement.
The answer usually has two parts:
Example format: 1 turn + 8 holes on a 16-hole circle.
Suppose you need to cut a gear with 24 teeth using a 40:1 dividing head.
Step 1: Apply the formula.
40 ÷ 24 = 1.6667 turns
Step 2: Split the answer into whole turns and fraction.
1.6667 = 1 turn + 0.6667 turn
Step 3: Convert the fraction into holes.
0.6667 = 2/3
Step 4: Choose a practical hole circle.
A 15-hole circle is suitable because 2/3 of 15 = 10 holes.
Final indexing movement = 1 turn + 10 holes on a 15-hole circle.
Simple indexing is used whenever a job must be divided into equal parts with accuracy. It is a basic but very important workshop method because it helps produce consistent spacing between gear teeth or slots.
| Fraction | Example Hole Circle | Result |
|---|---|---|
| 1/2 | 16-hole circle | 8 holes |
| 1/3 | 15-hole circle | 5 holes |
| 2/3 | 15-hole circle | 10 holes |
| 1/4 | 16-hole circle | 4 holes |
What is simple indexing?
It is a dividing head method used to rotate a workpiece by equal amounts for gear cutting and other machining operations.
What dividing head ratio is commonly used?
A 40:1 dividing head is common, which means 40 crank turns equal one full spindle revolution.
Why do I need a hole circle?
The hole circle lets you convert the fractional crank movement into a practical number of holes.
Can simple indexing be used for all gear counts?
No. Some tooth counts require differential indexing when the simple indexing hole circle does not fit the result.