Dividing head – compound indexing

[BazyleParticipant @bazyle]

I don’t normally condone linking to ebay ads but this one is for your possible education not to promote the sale and it is nothing to do with me.
You have just 5 days to have a quick look at the picture for this “dividing head with 150mm chuck”.
https://www.ebay.co.uk/itm/158314288377?_skw=milling+machine&itmmeta=01M3303ZV2MAHAP3XJ5J54PP6E&hash=item24dc4454f9:g:bRoAAeSwj0lqsCdp&itmprp=enc%3AAQALAAAA8GfYFPkwiKCW4ZNSs2u11xC8gqzK7GL6RmwUD9jtO1kTptxSqCuUW4NVIMQfLQ3ipFdIcCvZDe7f0PaOFf7F7rOL4B3iZV2%2FkJb2diPrJrJUel3LvRyt2if8QMEMH1qXc6%2BIC4Ee5Mn9R2Ys3V%2FUnmWK97ksWuE9HkyGQsssf5ZaSlcfUUR8z06gXiGYQCDF5cw7%2Fswc2VLZFr4PvQ9mwxdUFJ%2BZdxLpygvzA%2F8oby94nMvNetB8h2JLBnOBRG23swm%2BJ9e1Jp6X7kf%2BIsEzIWKnxpw4lBviH4A3S%2BXfBVwShEjTNOM4jr6fkb7XUqhbTQ%3D%3D%7Ctkp%3ABk9SR-D9j-CYaA

ebay item 158314288377
What I want to point out first is the extra long secondary indexing shaft shows that it is primarily intended for spiral milling not the usual differential indexing. But that reduces the ability to cut many gears without special plates.
However look at the photo showing a plunger and lever behind the unusually thick index plate. This locks/unlocks the whole plate allowing it to be rotated to the next hole in the back (no, not the usual holes on the front). You would release this for the spiral milling instead of the main indexer which makes multistart spirals easier but this also allows compound indexing using two hole counts simultaneously. It is a rare feature; although my 4in Elliott has it I have hardly ever seen others.

[Bazyle]

On 21 September 2026 at 23:44 Bazyle Said:

…This locks/unlocks the whole plate allowing it to be rotated to the next hole in the back

…this also allows compound indexing using two hole counts simultaneously.

…although my 4in Elliott has it I have hardly ever seen others.

If you have one with the same feature, it might be useful to show a numerical example of its use. The Elliott manual does not contain tables for this feature.

On the ebay one to which you link, the rear plunger is at a fixed radius. That means it works with (only) one hole circle on the back.

Perhaps you could say with which hole circle your own one aligns.

In the Elliott manual, they call the analogous item ‘dividing plate lock’ (20a, 20b page 38 & 39, also item k page 33, but never referenced in the text).

I think both the Brown & Sharpe and Cincinnati ‘Treatise(s) on Milling’ (from the Internet Archive) have discussion of this feature.

See this post, which is one of Marv Klotz’ programs to do this work. Someone recently posted here on the forum about an updated version of these.

https://www.homemadetools.net/forum/compound-indexing-41406

[Andrew CrowParticipant @andrewcrow91475]

This feature was normal for most industrial dividing heads as not only did it facilitate spiral milling and compound indexing as already mentioned. But also differential indexing where gearing would be used to connect the drive shaft to the main spindle so that when the selector arm was turned the hole plate would rotate forward or back by a small amount to obtain divisions/angles unobtainable from normal indexing.

This feature was accomodated on the GHT dividing head by the second worm and wheel.

Andy.

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