Some background and context here.
In the past CNC machining of freeform surfaces always resulted in tool paths composed of only line segments, of which the length was defined by the maximum deviation (chordal tolerance) to the original surface.
This resulted in a huge number of small linear motions, and eventually resulted in huge gcode files, if you wanted small tolerances.
In those days, the controllers where struggling with large files, and sometimes they needed to be split, or offered in a streaming mode to the controller due to memory limitations.
Also the processing speed of the controller was usually a limitation and high feeds resulted in jerky movements as the controller had to stop sometimes to buffer some data before continuing. In machining processes, a halt is just as bad as in 3D printing.
That problem became even worse when 5-axis machining was introduced, as the complexity of the math increased even more.
Additionally the controllers tried to follow the programmed path as closely as possibly, which also was causing a lot of jerk and vibrations.
For that reason arc fitting was introduced. It reduced the code and caused less burden on the controller.
However, the pathe still contained a lot of linear elements, so motions were still not smooth.
Later NURBS (Non-Uniform Rational B-Spline) were introduced in the tool path. This gave smooth paths to the controller, resulting in smooth motions on the machine tool and usually faster motions also.
Big disadvantage was that not every controller was capable of handling it, and there was not really a standard on the format either.
Tuning the settings to the machine was a complex matter altogether, so it didn’t really breakthrough and was only used in niche areas.
Nowadays we are back to linear motions again as the controllers are now more powerful and have no real memory limitations anymore. Therefore the controller does it’s own motion planning and creates smooth motions through the given points based on the machines kinematics and dynamics.
This makes tool paths simple again and the controller, that knows all of the machine’s physics creates the optimal motion.
For this smooth motion planning, arcs are usually not allowed as they interfere with the smoothing.
Therefore the arc fitting is disabled for those advanced controllers.
Sorry for the long story, but I hope it’s making some sense now.
