What is the difference between helical and spiral interpolation?

I was reading your section on thread milling, and parametric programing and noticed that it said there was a difference. Thank you, Hal Linson, Uriah's Metal Works, Moline, IL

Response:

Helical interpolation is required when thread milling a straight thread. Two axes (usually X and Y) move along a circular path while the third axis (usually Z) moves along a straight path. The radius remains constant throughout the circular movement in XY.

Spiral interpolation is required when thread milling a tapered thread with a taper thread mill to avoid leaving a nasty witness mark at the beginning/end point for the thread. As the tool moves around the thread in XY, the radius of the circle being machined must decrease (or increase, depending upon the machining direction) in order to compensate for the Z axis motion.