Siemens SINUMERIK 840Di sl Programming Manual page 106

Positional Data
3.7 Selection of working plane (G17 to G19)
Description
It is advisable to define the working plane G17 to G19 at the beginning of the program. In the
default setting, the Z/X plane is preset for turning G18.
Turning:
For calculating the direction of rotation, the controller requires the specification of the
working plane, refer to circular interpolation G2/G3.
Machining on inclined planes
Rotate the coordinate system with ROT (see Section "Coordinate system offset") to position
the coordinate axes on the inclined surface. The working planes rotate accordingly.
Tool length compensation on inclined planes
As a general rule, the tool length compensation always refers to the fixed, non-rotated
working plane.
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Programming Manual, 11/2006, 6FC5398-1BP10-2BA0
Fundamentals