40:1 and 90:1

Dividing heads are closely related to rotary indexers, but with a key operational difference: Rotary indexers use a scale and show the angle of rotation (from 0 to 360 degrees), while dividing heads use pre-defined plates to rotate a fixed amount. These plates are generally circular with multiple rows of equally-spaced holes.

Using a dividing or indexing head requires you to know just how far to rotate the device for the project you're working on. Generally the indexer will jump from one hole to another, so the rotational amount is the same. However, you will need to calculate how many full rotations, and how many holes in which circular pattern to go just the rotation you need.

Fortunately, all of this math can be avoided by following the chart for your device.

This plate can be used either directly, or through a geared dividing mechanism. In direct indexing the workpiece and plate rotate in a 1-to-1 ratio, and holes are used directly. That is, a plate with 12 holes can divide the workpiece into 2, 3, 4, 6, or 12 equal segments. A dividing head incorporates an internal gear ratio (usually 40:1, 60:1, or 90:1) with the same plates. In doing so, the dividing head enables many more combinations than just direct indexing.

For example, imagine a plate with 15 equally-spaced holes and a dividing head with a 40:1 gear reduction. In direct indexing, a workpiece could be divided into 3, 5, or 15 equal segments. Using the dividing head, the same workpiece could be divided into 2, 3, 4, 5, 6, 8, 10, 12, 15, 20, 25, 30, 40, 50, 60, 75, 100, 120, 150, 200, 300, or 600 segments. Essentially, the dividing head acts as if it’s a direct indexer with 600 holes; 15 holes in the actual plate * 40:1 gear ratio. Let’s look at how some of these combinations are possible.

Ellis Dividing Head Indexing Plate #32 - own work

With a 40:1 gear ratio, you’d need to turn the crank on the dividing head 40 full turns to rotate the workpiece once. To get 6 equal divisions, you’d need 40/6 = 6+2/3 = 6.66666 turns. The 6 turns are easy, but the remaining 0.66666 needs the holes on the plate.

If we multiply the partial turn by the number of holes on the plate, we’ll know exactly how far to advance the diving head: 0.66666*15 = 10 holes. Putting these together, to divide the piece into 6 segments you would rotate the dividing head 6 turns and 10 holes.

To get 75 equal segments we’ll follow the same process: 40/75 = 0.53333 turns. Now, there is no full rotation, but 0.53333*15 = 8 holes. So if we rotate the dividing head forward by 8 holes each time, it will divide the workpiece into 75 equal segments.

A dividing head is a great add-on for a mill and opens the door to making gears, splines, bolt patterns, and much more. Most dividing heads allow you to angle the device (and your part) so you can hold your piece vertically, horizontally, or at any angle in-between. Vertical is great for drilling holes on a circle, like for a flange or connection, while horizontal helps for gear teeth and splines.

Check out our video on making a gear with the Ellis Dividing Head here: