HEIDENHAIN TNC 426 PB/M Technical Manual page 408

Datums
6 – 78
Servo-controlled axes:
The behavior of servo-controlled axes during "datum setting" depends on
MP7500 bit 3, bit 5, bit 7 and bit 8:
MP7500 bit 3 = 0
During "datum setting" in X, Y, and Z, the TNC saves the reference
coordinates of the tilting element needed for calculating the offset when
"tilted working plane" is active.
MP7500 bit 3 = 1
To 280 474-06:
During "datum setting" in X, Y, and Z, the datum for a rotary table C (tool axis
Z) is assumed to be 0, meaning the workpiece was aligned and datum = 0 has
been set. "Tilt working plane" refers to this 0° position. A datum cannot be set
when "tilted working plane" is active.
From 280 474-07:
During "datum setting" in X, Y, and Z, the reference coordinates of the tilting
element are assumed. If "tilted working plane" is active, datum = 0 is
assumed.
From 280 474-12 and 280 476-01:
Same as up to 280 474-06:
From 280 474-14 and 280 476-03:
Datum = 0 is assumed only for the first rotary table axis (only this axis can align
a workpiece). For all other axes, "datum setting" in X, Y, and Z is possible with
"tilted working plane" active.
MP7500 bit 5 = 0
See MP7500 bit 3 = 0
MP7500 bit 5 = 1
It can happen with titling elements with Hirth couplings that by locking the
Hirth coupling, the actual value of the encoder will no longer exactly agree with
the mechanical position of the tilting element. If this happens, the nominal
values should be used to calculate the various datums (MP7682 bit 1).
If problems continue to occur, MP7500 bit 5 should be set to 1. The tilting
angles entered in 3-D ROT are used to calculate the datums for X, Y and Z.
MP7500 bit 7 = 0
See MP7500 bit 3 = 0
MP7500 bit 7 = 1 (recommended)
During "datum setting" for X, Y and Z, the tilting angles entered in 3-D ROT
are used to calculate the datum if "tilted working plane" is active.
During "datum setting" for X, Y and Z, the reference points of the tilting axes
are used to calculate the datum if "tilted working plane" is inactive. This
allows a workpiece to be aligned, a datum to be set, "tilt working plane" to be
activated, and a new datum to be set in the "tilted working plane."
HEIDENHAIN Technical Manual TNC 426, TNC 430