Polygon turning (also known as polygonal turning, polygon milling or polygon cutting) is a method of using a rotating cutting tool, with 1 or many cutting inserts, geared at 2x the rotation speed of the workpiece, to cut an approximation of a polygon using a lathe.
The resulting flat faces have an elliptical geometry, which more closely approximates a true flat surface as the ratio of cutting tool size to feature size increases.
The advantage of polygon turning over more conventional milling operations is its speed.
See the video below for a 3D animation.
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