I naively thought, that I had my head around Module/Metric gears. In my mind the Module equated to millimetres pitch or linear spacing. That was until I was idly remeasuring a Roxey Mouldings worm and wheel set, which I had measured up during my original online discussion. But I had understood even less at that point so I figured that I must have got it wrong. When I measured it and the worm pitch was 1.25mm according to my metric thread gauge but the accompanying gear wheel was marked as 40 tooth Which in my head should have equated to MOD 1.25.

While head scratching, I played around with the Fusion gear generator add-in until I worked out by trial and error that the worm and gear wheel were in fact Mod 0.4. At this point my mind was completely blown as everything thing that I thought I understood was in fact nonsense and it proved that I understood very little.

Thankfully when looking in the Ivan Law book on Gears and Gear Cutting, I found a table of reference for Module Gears and although I don’t pretend to understand how the term “Module” relates to anything identifiably metric* I now have something to go on when working things out.

*The nearest thing that I can find which remotely relates, is that the millimetre data under the 14.5 degrees pressure angle column, is almost equivalent to the associated Module reference number.

Further reading of the book confirms (to me at least) that the term “Module” despite being referred to as ‘Metric’, isn’t. It’s based on there being 25.4mm to an inch, which if it’s based on an inch then it isn’t metric.

I took a bit of time and copied out the table into a spreadsheet and then manipulated the spreadsheet into a layout that works better for me – Metric measurements, before imperial equivalents and the smaller modules to the top of the lists (as I doubt that I will ever use the larger modules because I am not equipped to cut such substantial gears).