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or misuse of techniques shown in this web page. We simply publish information
we feel will be of interest to CNC users. In all cases, the reader is totally
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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
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.