gear cutter terminology

I have to admit I've never cut a gear. I do have some gear cutters, and
I'm puzzled about one thing. Some of them say 'DP' e.g. "14DP" and some
of them say 'NDP' e.g. "12 NDP". I believe I understand what diametral
pitch is (the ratio of the number of gear teeth divided by the diameter
of the pitch circle) but I don't understand the difference between 'DP'
and 'NDP'. Is one nomenclature European or something?
Reply to
Grant Erwin
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I figured it out, enough for me anyway. What it boils down to is if there is no helix angle (i.e. if the gears are straight spur gears) then DP = NDP.
Grant Erw> I have to admit I've never cut a gear. I do have some gear cutters, and
Reply to
Grant Erwin
Exactly, they mean the same thing. jsut terms for the different gears
DP = Diametrial pitch (spur gears) NDP = Normal Diametrial Pitch (helical gears)
Cos of helix angle = DP/NDP for figuring out the angle at which to swing the table on a universal milling machine. Because when cutting a helical gear, you require to calculate both DP and NDP to find the helix angle.
# Teeth = Pitch Diameter / Diametrial Pitch
This works for spur gears (14.5 degree Pressure angle) and you need to calculate it for helical gears, the problem is that helical gears will give you a decimal number of teeth (not a whole number) and you will have to factor in the ratio of the gears in mesh (to be made, ie 4:5) to get 2 sets of gears with whole numbers that have a ratio of 4:5 , ie, 40 and 50 or 44 and 55.
i'm rambleing on here, but basically dp and ndp are both required for cutting helical gears, but in spur gear terminology, they mean the same thing.
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I have nothing to add to Stan's excellent post, but wanted to point out a good resource. Boston Gear (no relationship other than customer) runs an excellent website at
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that has a lot of useful downloadable literature about gears; not just their products. Also, they offer a free online course in "Gearology" that is pretty nice at:
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Hope this is useful. Dave
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