Reference Marks - HEIDENHAIN TNC 415 Technical Manual

4

2 Reference marks

By setting a datum point, a definite positional value (coordinate) is assigned to each axis position for
the machining of the workpiece. Since the actual-position value is established incrementally by the
measuring system, this correlation between axis positions and positional values must be re-
established after every power interruption.
The HEIDENHAIN linear measurement systems are therefore equipped with one or more reference
marks. On passing a reference mark a signal is generated which identifies the particular position as a
reference point. By passing over the reference marks after a power interruption, the relationships
between the axis positions and positional values (and, at the same time, the fixed machine
relationships) which were most recently determined by the datum point setting are re-established.
+Z
REF value
–44.985
Workpiece
datum
10 20 30 40
0
Workpiece
Machine table
Position encoder
Since it is often inconvenient to re-establish the reference points by traversing large distances after
switching on, HEIDENHAIN recommends the use of measuring systems with distance-coded
reference marks. With this kind of measuring system the absolute position is available after crossing
two reference marks.
Scale with distance-coded reference marks
The scale graduation consists of the line grating and a reference mark track which runs parallel to it.
The distances between any two consecutive reference marks are defined differently, so that the
absolute position of the machine slide can be determined from this distance.
Scale with one reference mark
Scale with distance-coded reference marks
4-52 TNC 407/TNC 415/TNC 425
REF value
0
+X
Reference mark
Machine datum
2 Reference marks 01.98