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Brass Gears for Cracker steam loco

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andyf:
I reckon the diameters you quote are the Pitch Circle Diameter ("PCD") of each gear, Graham, rather than their overall (tooth-tip to tooth-tip) diameters. If the gears are Mod 0.5, their PCDs would be 20mm and 5mm. Their ODs would be 21mm and 6mm, because the PCDs lie part-way up the teeth.

If you mesh together two Mod 0.5 gears, one 40T and the other 10T and hence with PCDs of of 20mm and 5mm, their centres will be (20 + 5) divided by 2 = 12.5mm apart. Does that accord with your drawings?

Andy

Jasonb:
Seems like you have a mismatch or typo there

0.5MOD x 40T does have a 20mm PCD but 0.5Mod x 10T has a PCD of 5.5mm (6.5mm OD)so they won't mesh.

The smaller teeth numbers have to be fiddled a bit so the usual formula does not quite work. see the corrected formula here

Jason

andyf:
You're quite right, Jason; thanks. I'd forgotten the smallest toothcounts were a bit different. Figures should have been ODs of 21 and 6.5mm, PCDs of 20mm and 5.5mm, centres (20 + 5.5) divided by 2 = 12.75mm apart.

Andy

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