February 2012 20 – 323
20 Reference run
20.1 Definition
The position value (the coordinates) of an axis position is defined with respect to a freely selectable
datum. When the axes are moved, the ACTUAL position is calculated incrementally. As soon as the
machine is switched off, the reference between the axis position and the position value is lost.
Reference marks HEIDENHAIN linear encoders (except EnDat) feature one or more reference marks. The reference
marks identify an axis position at a known distance from the machine datum. The position of the datum
selected by the machine operator is defined with respect to the machine datum.
The datum and the actual position can be reproduced as soon as the reference marks are traversed.
Machine datum The machine datum is the central reference point of your machine tool.
It is defined by the machine tool builder.
The machine datum is required to ...
Define the axis traverse limits (software limit switches)
Approach machine-referenced positions (such as tool change positions)
Set a workpiece datum
Distance
between scale
reference point and
machine datum
For position encoders with distance-coded reference marks, the machine datum is defined with
respect to the scale reference point, which is at the first reference mark after the beginning of
the measuring length.
If EnDat encoders are used, the machine datum refers to the datum of the EnDat encoder.
MP960.x contains the distance between the scale reference point and the machine
datum.
Spindle preset An appropriate angle encoder or even the motor encoder in the spindle motor can be used for oriented
spindle stop.
MP3430.x contains the deviation of the reference mark from the desired spindle position.
Measuring system
Machine table
REF Value
0
10 20 30 40
Machine datum
+Z
+X
0
Workpiece
datum
Workpiece
Reference mark
REF Value
–44.985
After removing and remounting a measuring system (position, angle), MP960.x may have to be
altered.
See “Resetting the machine datum” on page 19 – 295.
You may have to reset the MP3430 after an encoder for spindle orientation was removed and
remounted or when the spindle motor was exchanged.
See “Restoring the spindle orientation” on page 19 – 299.
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