When Cutting a Worm gear
Faced with the need to cut a worm gear I would suggest that the viewer calculates
the precise pitch required and then refers to my pages that provide the results for
all possible combinations of available gears based on gears having 20 to 75 teeth
by fives plus 21 teeth and 38 teeth gears. There are two sets of lists, one for a
lathe having an 8 TPI leadscrew, the other a 2mm pitch leadscrew.
Having worked out the pitch/tpi required for the worm gear and referred to the lists,
in most cases it will be found that there are many combinations that come sufficiently
close. The main benefit of this is that it should enable a suitable combination to
be selected even if the lathe is missing some of its changewheels. For greatest accuracy,
in most cases, it will be a three driver, three driven assembly that achieves this.
However, frequently, there will be two driver, two driven assemblies that come very
close, the viewer will of course have to decide if the application demands the more
accurate result, in many cases this will not be a requirement.
As a rough guide though, a lightly loaded and infrequently used assembly, such as
the dividing head,greater errors will be acceptable. On the other hand,a continuously
running, heavily loaded assembly, will need to be as accurate as is possible. In
this case it may be a situation for designing the assembly such that the worm gear
diameter is chosen to produce a pitch that coincides precisely with one that can
be cut on the lathe.
Using a gear box
When cutting a worm on a lathe fitted with a gearbox the comments made regarding
metric threads with a gear box still apply. This, as you can set the box to 8TPI
and set up the change wheels as though the gear box was not fitted as its input and
output rotation are identical. For more detail do see the pages regarding metric
and Imperial threads.
A simple approach
There is though a relatively simple method for achieving the majority of common DP
values. This entails fitting just a 55 tooth driver and a 35 tooth driven and using
the gearbox to establish the values required. The table shows just a few of the possibilities
but other values are easily determined. Typically, as 32 DP uses the 16TPI setting
then 18TPI would result in 36 DP.
DP WORMS VIA AN IMPERIAL GEAR BOX
DP GearBox Actual Actual
SETTING Dr Dn TPI Helix
48 24 55 35 15.2727 1.63
40 20 55 35 12.7273 1.63
32 16 55 35 10.1818 1.63
24 12 55 35 7.6364 1.63
20 10 55 35 6.3636 1.63
The actual helix angle is rather on the low side as a result a larger than normal
worm would be required if attempting to produce an error free design.
For worms to the MOD standard I would suggest that the gearbox is set to 8 TPI .
Then, the changewheels setup as chosen by reference to the pages mentioned earlier.
The brief details having been given should enable the viewer to better understand
the process of choosing changewheel combinations for cutting the complex pitches
that worm gears require. For details of the actual process of designing and producing
a worm/wormwheel assembly I would suggest that one should obtain a copy of "Gears
and Gear Cutting" in the Workshop Practice Series.