Siemens SINUMERIK 840D sl Programming Manual page 203

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 that defines 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 to 0 is generally aborted with an alarm due to violation of the
software limits.
Position of the circle plane
The position of the circle plane depends on the active plane (G17-G19).
NC programming
Programming Manual, 12/2019, 6FC5398-2EP40-0BA0
Comment
; Circular-path programming with tangential transition.
Fundamentals
2.9 Motion commands
203