HEIDENHAIN TNC 620 (81760x-02) ISO programming - Manual (Page 358)

Programming: Miscellaneous functions
10.4 Miscellaneous functions for path behavior
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TNC 620 | User's ManualDIN/ISO Programming | 2/2015
Feed rate in millimeters per spindle revolution:
M136
Standard behavior
The TNC moves the tool at the programmed feed rate F in mm/min
Behavior with M136
In inch-programs, M136 is not permitted in
combination with the new alternate feed rate FU.
The spindle is not permitted to be controlled when
M136 is active.
With M136, the TNC does not move the tool in mm/min, but rather
at the programmed feed rate F in millimeters per spindle revolution.
If you change the spindle speed by using the spindle override, the
TNC changes the feed rate accordingly.
Effect
M136 becomes effective at the start of block.
You can cancel M136 by programming M137.