Andrew Prestridge | September 8, 2020
What is a Dividing or Indexing Head?
Dividing Heads (often called indexing heads) are machine tools used to provide controlled and repeatable rotation to a tool or workpiece, usually as an accessory to a milling machine, grinder, or lathe. A classic and personal favorite in our machine shop is the Ellis Dividing Head, but you can also find a different manufacturer's dividing head that would fit your needs. What makes it work inside the housing is a worm and worm gear transmission. Regardless of the brand they usually come in gear ratios of 40:1 or 90:1. Electronic rotaries (with an encoder and servo, like those found in Haas mills) more often have a ratio of 60:1
Ellis Dividing Head - annotated original image
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.
Dividing Head Indexing Plates (40:1, 60:1, 90:1)
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.
Dividing Head Calculator
Ellis Dividing Head Indexing Plate #32 - own work
Dividing Head Formula
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.
Dividing Head for Milling Machines
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:
Dividing Head Summary
Rotary Indexer
Uses continuous rotation and has a scale to read the rotation angle. Subject to measurement error at each measurement
Direct Indexing
Fast rotation and simple division, but limited range of divisions per plate
Dividing Head
More complicated division, but greatest number of options per plate
30 comments
Good info Good info
Very interesting, love gear cutting.
How to order on your product
thanks for this it helps a lot
very easy dividing head calculator. good website . Thank you..
What is the indexing result for 5/8 pitch 20 teeth sprocket in 40:1 ratio
So grateful for you people Am a machinist as well
Pls i neeed indexing plate for my dividing head
im perplexed as to why it seems that you dont seem to need the outside diameter of a round blank in order to know how many teeth can be cut as you can see im new at this reguards Laurie
Such a good calculator. My old Machinery’s Handbook works in longhand using fractions. My head was spinning!
The calculator does 30 divisions @ 90:1 incorrectly! It returns TWO full turns (8 deg.) when the answer is THREE full turns (12 deg.).
Jeff – thanks for keeping us honest – we fixed the calculator! We appreciate the feedback so we can supply these useful tools.
Hello evolventdesign.com admin, Keep up the great work!
Hi evolventdesign.com administrator, Thanks for the well-researched and well-written post!
To the evolventdesign.com administrator, Thanks for the informative and well-written post!
This blog post is a comprehensive tutorial on how to use a dividing/indexing head in the machining process. Author walks reader through each stage of the installation process, detailing each component and its purpose along the way. Instructions and accompanying images are straightforward and simple to follow. In conclusion, this article is an excellent read for anyone curious about indexing and dividing heads. For More Info:-https://anotekanodizing.com/
This is exactly the article I was looking for, Thank you for publishing! It really helped me understand how to calculate turns per division.
Best aaps
absolutly brilliant, what a fantastic post…thank you….
WOW thanks for this masterpiece, clear concise information you are truly a god amongst men
Excelent calc!!! Thank you. Brilliant for small manual guys like me😊
Excelent calc!!! Thank you. Brilliant for small manual guys like me😊
I have 81teehth how can I calculate using dividing head. thanks
very very help ful after 40 years break from machining.
very very help ful after 40 years break from machining.
after a while not machining this is very helpful. thank you 🙏
Dear sir l read your articles and watched your video on cutting gears a while ago so contacted you asking if you could supply a chart with the amount of turns and holes while using a deviding head which you very kindly did unfortunately lvery carelessly have lost them whilst moving my machines and tools to an organization called the mens shed which is a group of retired guys learning new skills and keeping from under the wife’s feet, so l am shyly asking if it was at all possible if you could send the same thing again, l would be very grateful if you could see your way clear to save my backside sincerely Peter Moore.
Well over 25 yes removed from last use of indexer. Thanks for the refresher! Shop in my garage and just bought an under er. Got a job in that needed it and forgot how to use it😀
I want learn more about milling machines
Most charts for B&S dividing heads go to 382. Back in the 70s in machinist school in Saskatoon I invented a new way. Email me if you would like to know how to do 383 with the standard plates and gears. Or tell me who discovered how to do this first. Thanks
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