Harold Hall


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.


Workshop Processes

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   GearBox                    Actual         Actual

        SETTING  Dr  Dn     TPI              Helix

        TPI                                                 Angle

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.