HEIDENHAIN ND 780 Handbuch Seite 167

Only the tools set using the same reference surface may be
changed without having to reset the datum.
If the tool table already contains tools in which the length
has been set, the reference surface should first be
established using one of them. If not, you will not be able
to switch between the new tools and the existing tools
without having to re-establish the datum. Before adding the
new tools, select one of the tools from the tool table. Touch
the tool to a reference surface and set the datum to 0.
TOOL UNIT
Enter the tool unit (inch/mm) and
cursor to the Tool Type field.
TOOL TYPE
Press TOOL TYPES soft key. Select from list of tools
and press ENTER
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