HEIDENHAIN TNC 620 (73498x-02) ISO programming - Manual (Page 304)

Programming: Miscellaneous functions
10.3 Miscellaneous functions for coordinate data
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TNC 620 | User's Manual for DIN/ISO Programming | 5/2013
10.3 Miscellaneous functions for coordinate
data
Programming machine-referenced coordinates: M91/
M92
Scale reference point
On the scale, a reference mark indicates the position of the scale
reference point.
Machine datum
The machine datum is required for the following tasks:
Defining the limits of traverse (software limit switches)
Approach machine-referenced positions (such as tool change
positions)
Set a workpiece datum
The distance in each axis from the scale reference point to the
machine datum is defined by the machine tool builder in a machine
parameter.
Standard behavior
The TNC references coordinates to the workpiece datum (See
"Datum setting without a 3-D touch probe", page 403).
Behavior with M91—Machine datum
If you want the coordinates in a positioning block to be referenced
to the machine datum, end the block with M91.
If you program incremental coordinates in an
M91 block, enter them with respect to the last
programmed M91 position. If no M91 position is
programmed in the active NC block, then enter the
coordinates with respect to the current tool position.
The coordinate values on the TNC screen are referenced to the
machine datum. Switch the display of coordinates in the status
display to REF (See "Status displays", page 71).