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.
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.
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.