Siemens Sinumerik 840D sl Programming Manual page 196

Motion commands
10.6 Circular interpolation
Example 2: Turning
Program code
N110 G1 X23.293 Z0 F10
N115 X40 Z-30 F0.2
N120 CT X58.146 Z-42
N125 G1 X70
Further information
Splines
In the case of splines, the tangential direction is defined by the straight line through the last
two points. In the case of A and C splines with active ENAT or EAUTO, this direction is generally
not the same as the direction at the end point of the spline.
The transition of B splines is always tangential, the tangent direction is defined as for A or C
splines and active ETAN.
Frame change
If a frame change takes place between the block defining the tangent and the CT block, the
tangent is also subjected to this change.
Limit case
If the extension of the start tangent runs through the end point, a straight line is produced
instead of a circle (limit case: circle with infinite radius). In this special case, TURN must either
not be programmed or the value must be TURN=0.
Note
When the values tend towards this limit case, circles with an unlimited radius are produced
and machining with TURN unequal 0 is generally aborted with an alarm due to violation of the
software limits.
196
Comment
; Circular-path programming with tangential transition.
Programming Manual, 01/2015, 6FC5398-1BP40-5BA2
Fundamentals