HEIDENHAIN TNC 426 B User Manual page 151

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").
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.
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 section 1.4 "Status Displays").
Behavior with M92 — Additional machine datum
In addition to the machine datum, the machine tool
builder can also define an additional machine-based
position as a reference point.
For each axis, the machine tool builder defines the
distance between the machine datum and this additional
machine datum. Refer to the machine manual for more
information.
If you want the coordinates in a positioning block to be based on
the additional machine datum, end the block with M92.
Radius compensation remains the same in blocks that
are programmed with M91 or M92. The tool length,
however, is not compensated.
M91 and M92 are not effective in a tilted working plane.
If you program these M functions in a tilted plane, the
TNC will display an error message.
Effect
M91 and M92 are effective only in the blocks in which they are
programmed with M91 or M92.
M91 and M92 become effective at the start of block.
Workpiece datum
If you want the coordinates to always be referenced to the machine
datum, you can inhibit datum setting for one or more axes (see
machine parameter 7295).
If datum setting is inhibited for all axes, the TNC no longer displays
the soft key DATUM SET in the Manual Operation mode.
The figure at right shows coordinate systems with the machine
datum and workpiece datum.
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M91/M92 in the test run mode
In order to be able to graphically simulate M91/M92
movements, you need to activate working space
monitoring and display the workpiece blank
referenced to the set datum (see Chapter „12.8
Showing the Workpiece in the Working Space").
Z
Z
Y
Y
M
7 Programming: Miscellaneous functions
X
X