HEIDENHAIN TNC 640 (34059x-02) ISO programming - Manual (Page 194)

Programming: Programming contours
6.3 Approaching and departing a contour
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TNC 640 | User's Manual for DIN/ISO Programming | 5/2013
Tangential approach and departure
With G26 (figure at center right), you can program a tangential
approach to the workpiece, and with G27 (figure at lower right) a
tangential departure. In this way you can avoid dwell marks.
Starting point and end point
The starting point and the end point lie outside the workpiece,
close to the first and last contour points. They are to be
programmed without radius compensation.
Approach
G26 is entered after the block in which the first contour
element is programmed: This will be the first block with radius
compensation
G41/G42
Departure
G27 after the block in which the last contour element
is programmed: This will be the last block with radius
compensation
G41/G42
The radius for G26 and G27 must be selected so that
the TNC can execute the circular path between the
starting point and the first contour point, as well as
the last contour point and the end point.