In one variant of form-milling, the single-form cutter is tilted to the helix angle of the thread and then fed radially into the blank.
From the en.wikipedia.org
The angle of rotation never repeats exactly no matter how many times the helix is rotated.
From the en.wikipedia.org
In that zone, the tube is twisted into alternating 90-degree-angle folds, so that it fairly resembles a helix.
From the sciencedaily.com
A conic helix may be defined as a spiral on a conic surface, with the distance to the apex an exponential function of the angle indicating direction from the axis.