HEIDENHAIN TNC 128 (77184x-02) - Manual (Page 254)

Programming: Miscellaneous functions
9.3 Miscellaneous functions for coordinate data
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TNC 128 | User's Manual HEIDENHAIN Conversational Programming | 5/2014
9.3 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:
Define the axis traverse limits (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 286).
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 67.