Siemens SINUMERIK 840D sl Programming Manual page 737

Figure 3-7
The sections are clearly defined: If you run through the kinematic subchain from the starting
point to the end point, the first section has the index 0, the next the index 1, and so on. The index
of the last section is then always equal to the number of orientation axes.
Correction elements
A reference can be made to a constant kinematic chain element (chain element of the type
$NK_TYPE[<n>] = "OFFSET") in each section with the $NT_CORR_ELEM_T[<n>, 0 ... 3] or
$NT_CORR_ELEM_P[<n>, 0 ... 3] system variables. The correction values that were
determined during the machine measurement are written to these elements with the
CORRTRAFO function.
Example with transformation index = 1:
● $NT_CORR_ELEM_T[1,0] = "C_AXIS_OFFSET"; Offset of the C axis (orientation axis 1) in
section 1 is defined as correction element.
● $NT_CORR_ELEM_T[1,1] = "B_AXIS_OFFSET"; Offset of the B axis (orientation axis 2) in
section 2 is defined as correction element.
● $NT_CORR_ELEM_T[1,2] = "BASE_TOOL_OFFSET"; Offset of the B axis from the tool
reference point in section 3 is defined as correction element.
The sequence of the references in $NT_CORR_ELEM_T/P[<n>, 0 ... 3] must correspond to the
sections described above, i.e. only one chain element can be in $NT_CORR_ELEM_T/P [<n>,
0] which is before the first orientation axis, etc.
The CORRTRAFO function writes the values determined by measuring the machine into the
correction elements defined in this way. The modification of the correction values is defined in
CORRTRAFO via the <Corr_Mode> parameter.
Closing a chain
If bit 7 or bit 8 are set in the $NT_CNTRL[<n>] system variable, additional constant chain
elements that establish a connection from the end point of the chain to the machine zero point
are automatically inserted internally at the end of the workpiece chain (bit 7) or before the
starting point of the tool chain (bit 8) ("close chain").
These automatically inserted elements cannot be written externally, only read (see the
$AC_TRAFO_CORR_ELEM_P/T system variables).
NC programming
Programming Manual, 12/2019, 6FC5398-2EP40-0BA0
CORRTRAFO example
3.12 Transformation with kinematic chains
Work preparation
737