Siemens SINUMERIK 828D Programming Manual page 236

Motion commands
9.6 Circular interpolation
Position of the circle plane
The position of the circle plane depends on the active plane (G17-G19).
If the tangent of the previous block does not lie in the active plane, its projection in the active
plane is used.
If the start and end points do not have the same position components perpendicular to the
active plane, a helix is produced instead of a circle.
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Programming Manual, 07/2010, 6FC5398-1BP40-0BA0
Fundamentals