Interesting difference between Spur and Helical gears, why?

[wjh30]

In the Lego technic universe, if I make a spur gear that is Mod 1.0, and the tooth count is divisible by 8, the spur gears will always work with the axle hole spacing of legos(8mm). I've made 16t all the way to 120t and they work perfectly. For what ever reason, with Helical gears, doing a 16t and 64t helical pair, with Mod 1.0 , the spacing did not work out. I had to use the calculator, and print this helical gear with a Mod 0.99 to get the spacing correct.
I find this very interesting, why is it this way?

[ArtF]

Hi :

  Hard to say. Its noteworthy that helicals are known for being quiet because they
mesh in more points at the same time. Their sizing then is more critical.  A bit of
error in a spur gear is likely not noticed, but a bit of error in a helical is much
more noticable and its amplified by the line of mesh being more complex.

  Its possible all your gears should be done at 99% due to the printing of them
making them a bit "squishy" in terms of exact sizing. I havent noticed this effect
but I always print my gears at 99% as early on I found it allowed a better fit
for my timing and spur gears.
  Your mod of .99 may actually be printing a mod of 1.0 , you'd never notice
on a spur, but on a helical it'd be much more acute.

Art


Art