The original implementation always took 150% of the addendum/dedendum
difference as fillet radius. For a standard full-depth system this
results in a normalized value 0f 0.375, which is pretty close to the 0.38
definded by the basic ISO rack. However, when using much shorter teeth as
e.g. required for a splined shaft, the fillet becomes way too large.
In addition, I don't understand the approximation to calculate the
distance between the gear's center and the top of the fillet yet. It was
only refactored to allow the custom fillet radii, but it retuns the same
values as the original implementation.
However, with high pressure angles, up to 45° used for splines, this
approximation comes to its limits.
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