HEIDENHAIN ND 930 - Manual (Page 6)

Fundamentals
6
Z
X
10
5
35 30
Absolute
datum
Relative
datum
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 99
relative datum points (tool datums), and store them in nonvolatile
memory.