1) Determine the difference between the thread radius (3/8") and the tool radius (1/4").
2) Multiply this difference by two (because we're drawing in Dia. mode).
(3/8 - 1/4) x 2 = 1/4
3) Create a point at the start of the thread for "Z" and at the major dia. for "X". In this case, Z-3.58, X15.0.
4) Create a 1/4" dia. circle at that center point ( Z-3.58, X15.0).
5) Create a point at 3 O'Clock on that circle (Z-3.58 + .125 = Z-3.455).
6) *Select the point at 3 O'Clock and Modify, Duplicate and, 2D Rotate, 18 times @ 10Degs.
7) **Create a Thread Process for each point on the circle.
* I used 10 degs. to prove this out. Use any angle/quantity combination necessary.
** 1) On the Thread Menu make sure the Major Dia. is set to 15". 2) Hi-light the Minor Dia. field and ALT+Click the point at 3 O'Clock to input it's value into the Minor Dia's. field. 3) Hi-light the starting "Z" field and ALT+Click the same point (-3.455). 4) Hi-light the ending "Z" field and ALT+Click the same point and then modify that value to represent the end of the thread (-3.455 - 7 = -10.455). 5) Subtract a 1/2" from the Minor Dia's. value to force the tool tip to the correct diameter ( Minor Dia. value - [1/4" TNR x 2].
I alternated the thread passes from leading edge to trailing edge of the thread for each dia. step in order to make balanced cuts.