Siemens SINUMERIK 840DE Programming Manual page 326

Tool offsets
8.8 Tool radius compensation (G40, G41, G42)
A contour is deemed to be (virtually) closed if the distance between the starting point of the
first block and the end point of the second block is less than 10% of the effective
compensation radius, but not more than 1000 path increments (equals 1 mm with three
decimal places).
TRUE:
In the same situation as described above, the intersection positioned on the first partial
contour closer to the block start is selected.
Changing the direction of compensation
G41/G42, G42/G41 can be programmed without an intermediate G40.
Changing the working plane
It is not possible to change the working plane G17 to G19 when G41/G42 is active.
Changing the offset number D
The offset number D can be changed in compensation mode.
A modified tool radius is active with effect from the block, in which the new D number is
programmed.
Caution
The radius change or compensation movement is performed across the entire block and
only reaches the new equidistance at the programmed end point.
8-30
G41
G42
Programming Manual, 10.2004 Edition, 6FC5 298-7AB00-0BP1
Fundamentals