Siemens SINUMERIK 840Di Function Manual page 756

Detailed Description
2.3 Coordinate systems
Axial preset offset
The "Preset" function can be used to redefine the control zero in the machine coordinate
system. The preset values act on machine axes. Axes do not move when "Preset" is active.
Note
After Preset, the reference points are invalid!
If possible do not use this function.
2.3.3 Basic coordinate system (BCS)
The basic coordinate system (BCS) consists of three mutually perpendicular axes (geometry
axes) as well as other special axes, which are not interrelated geometrically.
Machine tools without kinematic transformation
The BCS and the MCS always coincide when the BCS can be mapped onto the MCS
without kinematic transformation (e.g., TRANSMIT/face transformation, 5axis transformation
and up to three machine axes).
On such machines, machine axes and geometry axes can have the same names.
Fig. 2-15
Machine tools with kinematic transformation
The BCS and the MCS do not coincide when the BCS is mapped onto the MCS with
kinematic transformation (e.g., TRANSMIT/face transformation, 5-axis transformation or more
than three axes).
2-26
MCS=BCS without kinematic transformation
Axis Types, Coordinate Systems, Frames (K2)
Function Manual, 08/2005 Edition, 6FC5397-0BP10-0BA0