Siemens SINUMERIK 840D sl Programming Manual page 213

Note
G2 and G3 are modal.
The default settings G90/G91 absolute and incremental dimensions are only valid for the
circle end point.
Per default, the center point coordinates I, J, K are entered in incremental dimensions in
relation to the circle starting point.
You can program the absolute center point dimensions in relation to the workpiece zero
block-by-block with: I=AC(...), J=AC(...), K=AC(...). One interpolation parameter I, J, K with
value 0 can be omitted, but the associated second parameter must always be specified.
Examples
Example 1: Milling
Center point data using incremental dimensions
N10 G0 X67.5 Y80.211
N20 G3 X17.203 Y38.029 I–17.5 J–30.211 F500
Center point data using absolute dimensions
N10 G0 X67.5 Y80.211
N20 G3 X17.203 Y38.029 I=AC(50) J=AC(50)
Fundamentals
Programming Manual, 09/2011, 6FC5398-1BP40-2BA0
Motion commands
9.6 Circular interpolation
213