Is it possible to determine the pressure angle of a gear by simple measurements and if so what do I measure? Its a 24 tooth gear that is missing a few teeth, from measurements of the OD I think it's probably 16DP.
Recently I needed to figure out PA on a gear for a DaeJung engine lathe. I plugged numbers into gearspec from this link and by process of of elimination determined I had a 105T 20 degree pa, 1.5 mod gear.
Thanks, I measured around 0.477", the gearpa gave the following output.
Number of teeth on gear  ? 24 Diametral pitch of gear  ? 16
Pressure angle = 14.50 deg...Chordal span over 3 teeth = 0.4833 in Pressure angle = 20.00 deg...Chordal span over 3 teeth = 0.4823 in
I've found a copy of Machinery's Handbook with the formula that gearpa uses, but assuming the program is written correctly I'm not sure if it's going to give me any better results! Even with a brand new gear I doubt I could measure accurately to 1 thou across 3 teeth and thereby tell the difference between 14.5 and 20 deg PA.
I continue to be amazed that something that presumeably will prevent unlike gears from functioning together should be so difficult to measure! I wonder if it would actually matter in a slow-running, adjustable meshing, train such as change gears? Anyone done it?
Of course, I have seen photos of a functioning change gear made by passing a strip of shim brass between two gears to corrugate it, lapping and soldering the ends together, and epoxying it to the perimeter of a wooden disc turned to fit!