Depth of Cut
IMService makes available a
Freeware Threading Program
which calculates thread depth. Intrigued, I decided to figure out the math
For reference, here are same basic formulas for calculating
The Area of Some Simple Polygons.
Basically, the area removed on each pass is equal. The area of a triangle is base
times width divided by 2. So the total area = (0.875p x 0.875H) / 2 = (0.875p x 0.875 x 0.866p) / 2 = 0.3315 x p x p (or 0.3315 x p squared)
Using the formula from the previous step (which preserves the angle) we get
total area = (0.875p x 0.875 x p/( 2 * tan( 30 ))) / 2 = 49 x p x p / ( 64 * 4 * tan( 30 ))
= (49 x p x p) / (256 * tan(30)). I converted the 0.875 back into 7/8 to
give integer numbers.
For a given depth d, d = base / (2 x tan(30)), so base = 2 x d x tan( 30 ), and
the area = (base x depth) / 2 = (2 x d x tan(30)) x d / 2 = d x d x tan(30),
so we can calculate d = sqrt( area / tan(30))
In this example, we're doing 5 passes, each with an equal area.
So d1 = sqrt( 1/5 of total area / tan(30)). d2 = sqrt( 2/5 of total area / tan(30)), and so on.
The thread22 program allows a Ratio value to be specified to allow the
last pass to be some percentage of the other passes. If we wanted 5 passes
with Ratio set to 0.4, then the formula for the area of pass 1 would be 1/4.4,
and the area for the 2nd pass would 2/4.4 an so on. Generalizing for pass i would give
(i / ((num_passes - 1) + ratio) x total_area) except for the last pass
which would use the total_area.