Harold Hall



When using an imperial lathe to cut either a metric thread or a worm wheel, the requirement is to cut what I call a complex value, typically, a 1.5mm pitch metric thread equates to 25.4/1.5. This being equal to 16.93333 tpi. Worm wheels come up with similar complex values but here the similarity ends, as whilst there will be just one required value for a metric pitch this is not the case for a worm gear.


Because of this difference, turning worm wheels and the changewheel combinations required are dealt with here with metric pitches being covered on other pages As there is though some similarity I would suggest that the viewer reads the description for for metric pitches as I have gone into greater detail regarding there being many changewheel combinations that produce the same value.


In theory, the pitch of a worm gear is not only dependent on the gear tooth size, DP or MOD, it is also dependent on the worm's helix angle which in itself is dependent on its diameter.


There are four common worm/wormwheel forms with the simplest largely confined to the home workshop, whilst the most complex is largely a method made in industry.


Method one

This uses a standard spur gear and with the two spindles at right angles as SK1a illustrates. This results in a less than ideal mesh between the two gears but will though suffice for very light duty applications, such as a simple dividing head. Making the worm gear diameter as large as possible will of course help as this reduces its helix angle. The worm gear will also mesh better where the spur gear has fewer teeth, and is as a result, smaller in diameter.


Method two

SK 1b shows how the worm is swung round to avoid the miss match but this system may be difficult to incorporate in some designs. It is though a vast improvement over method one and again would suit a dividing head, typically that in Photograph 1. Photograph 2 shows more clearly how the worm assembly has been angled to enable worm gear and spur gear to mesh better. It would also suffice for a very light duty application where it may be motorised or has frequent manual operation.


Workshop Processes

Dividing Head, shop made
Dividing Head, shop made




All pictures can be clicked on to provide a larger view