HEIDENHAIN TNC 426 B User Manual page 44

4.1 Fundamentals of NC
Z
Position encoders and reference marks
Y
The machine axes are equipped with position encoders that
X
register the positions of the machine table or tool. When a machine
axis moves, the corresponding position encoder generates an
electrical signal. The TNC evaluates this signal and calculates the
precise actual position of the machine axis.
If there is an interruption of power, the calculated position will no
longer correspond to the actual position of the machine slide. The
CNC can re-establish this relationship with the aid of reference
marks when power is returned. The scales of the position encoders
contain one or more reference marks that transmit a signal to the
TNC when they are crossed over. From the signal the TNC identifies
that position as the machine-axis reference point and can re-
establish the assignment of displayed positions to machine axis
positions.
Linear encoders are generally used for linear axes. Rotary tables
and tilt axes have angle encoders. If the position encoders feature
distance-coded reference marks, you only need to move each axis a
maximum of 20 mm (0.8 in.) for linear encoders, and 20° for angle
X
MP
encoders, to re-establish the assignment of the displayed positions
to machine axis positions.
X (Z,Y)
28
4 Programming: Fundamentals of NC, File Management,
Programming Aids, Pallet Management
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