HEIDENHAIN ITNC 530 - 6-2010 DIN-ISO PROGRAMMING User Manual page 184

Contouring with radius compensation: G42 and G41
The tool moves to the right of the programmed contour.
G43
G42 The tool moves to the left of the programmed contour.
The tool center moves along the contour at a distance equal to the
radius. "Right" or "left" are to be understood as based on the
direction of tool movement along the workpiece contour. See figures.
Between two program blocks with different radius
compensations G43 and G42 you must program at least
one traversing block in the working plane without radius
compensation (that is, with G40).
The TNC does not put radius compensation into effect
until the end of the block in which it is first programmed.
You can also activate the radius compensation for
secondary axes in the working plane. Program the
secondary axes too in each following block, since
otherwise the TNC will execute the radius compensation
in the principal axis again.
In the first block in which radius compensation is activated
with G42/G41 or canceled with G40 the TNC always
positions the tool perpendicular to the programmed
starting or end position. Position the tool at a sufficient
distance from the first or last contour point to prevent the
possibility of damaging the contour.
184 Programming: Tools