HEIDENHAIN ND 730 v.3 - Manual (Page 6)

Fundamentals
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Datum setting
The workpiece drawing is used as the basis for machining the
workpiece. To enable the dimensions in the drawing to be converted
into traverse distances of machine axes X and Z, each drawing
dimension requires a datum or reference point on the workpiece
(since a position can only be defined in relationship to another
position).
The workpiece drawing always indicates one absolute datum (the
datum for absolute dimensions). However, it may contain additional,
relative datums.
In the context of a numerical position display unit, datum setting
means bringing the workpiece and the tool into a defined position in
relation to each other and then setting the axis displays to the value
which corresponds to that position. This establishes a fixed
relationship between the actual positions of the axes and the
displayed positions.
With the ND, you can set one absolute datum point and as many as
9 relative datum points (tool datums), and store them in nonvolatile
memory.
Absolute
datum
Relative
datum
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