go to the page above “Helical milling – links”

Hobnail – software for computing gear trains.

Hobnail was written by Gareth Evans specially for computing gear trains for hobbing helical gears using a modified Jacobs gear hobber. For helical gear hobbing two gear trains are needed to a very high  degree of accuracy. But it can be used perfectly well for designing any sort of gear train.

This software is written in VBA in an Excel spreadsheet. It is not available on the web.

If anyone thinks they might need a copy get in touch with me via the comment facility at the bottom of this page.

When Hobnail is first loaded it is essential that the macros are enabled.

The next step is to load onto the second page the gears that are available. If different sets are available then they can be stored on other pages and copied as required onto the second page. These numbers can be stored in any place and in any order.

 Using hobnail to design a simple gear train

At the top of the spreadsheet it says “freehand gear calculator” click this is you just want to make a single gear train for whatever purpose.

A case of this might be a gear train for helical milling. When doing this the gear set available might be very limited and the accuracy is not going to be very good. But for helical milling the required accuracy is not great. It is worth remembering that the lead required depends on the diameter of the workpiece and for this the average between the top and bottom of the cut is used.

 Designing two trains for hobbing

When hobbing it is necessary to have two gear trains. Any error in either of these trains is accumulated over a very large number of rotations of the hob. It is therefore necessary for both trains to be very accurate. It is not possible to do this with the standard sets that are supplied with a lathe or dividing head. It is necessary to have a full set from , say, 20 to 120 as would have been supplied with a gear hobbing machine before electronic means were used.

The chances of the ration being perfect are zero. This means for any particular ration required there will be two choices, one that is higher that that required and one that is lower. This is true for both trains. This does mean that choices can be made that can improve the accuracy.

The rate at which the workpiece is passed over the hob, ie, feed, does not affect the accuracy of the gear but it does affect the choices of gears that need to be used.