Turning down the OD of a gear

[Tristan Lall]

My experience is similar to Eric's: the addition of up to 0.003 in was necessitated by gearboxes being produced by hand by inexperienced machinists on milling machines of uncertain provenance. If you're CNCing the gearbox plates, you can in all likelihood get away with less. But remember that the added centre distance isn't just for position of the holes, it's also for runout of the gear and perpendicularity of the axle. (For example, in 2006, 188 had a bad EDM job that screwed up some expensive hardened gears, and necessitated a lot of rubbing compound to "solve" the runout.)

I would be interested in hearing from some of the VexPro people about the design and tolerances of their gears. How far from standard tooth profiles can they be expected to deviate at perfect form (e.g. due to profile shifting, radiusing, etc.), and how much variance is introduced in manufacturing?