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.