On Apr 17, 7:20, Ethan Dicks wrote: There are two common ways of specifying gears. One is by diametral pitch (DP), where you specify, in effect, the distance between teeth (actually it's done by dividing the number of teeth by the pitch diameter). The other, mostly used for metric gears, is by "module", and is the reciprocal of DP, ie you divide the pitch diameter by the number of teeth. Gears that mesh will have the same DP (and, of course the same module). The trick is to find the pitch diameter -- that's the "effective" diameter of the gear wheel. If you're using DP, for normal gears the amount of the tooth above and below the pitch diameter, called the addendum and dedendum, is standardised, and it conveniently works out such that if you can accurately measure the outside diameter (OD), and you can count the number of teeth (t), you can easily work out the DP without actually measuring the pitch diameter directly: OD = (t+2) / DP therefore DP = (t+2) / OD and if you do need to know the pitch diameter, then since DP = t/p it's just p = t/DP. There's just one caveat: those formulae only apply to gears with a 20 degree pressure angle, and with more than 15 or so teeth (otherwise you need to make some corrections as the angles get more extreme). -- Pete Peter Turnbull Network Manager University of York