Siemens SINUMERIK 840D sl Programming Manual page 173

C spline
In contrast to the akima spine, the cubic spline is continuously curved in the intermediate
points. It tends to have unexpected fluctuations however. It can be used in cases where the
interpolation points lie along an analytically calculated curve. C splines use third degree
polynomials.
The spline is not local, i.e. changes to an interpolation point can influence a large number of
blocks (with gradually decreasing effect).
Job planning
Programming Manual, 03/2006 Edition, 6FC5398-2BP10-1BA0
5.2 Spline interpolation (ASPLINE, BSPLINE, CSPLINE, BAUTO, BNAT, BTAN)
Special Motion Commands
5-9