I’m always amazed at how this discussion comes up repeatedly. In my (admittedly biased and elitist) opinion, there are only two scenarios where one needs to convert splines into tangent arcs (or worse polyline segments) before exporting to CAM:
The person operating the CAM system is unaware that their software can actually deal with splines directly. This occurs more often than you might think, as manufacturing people are sometimes so set in their ways that they haven’t learned any new functions of their CAM program in the last 10 years. They continue to do things the way they have done them since the beginning of CAM, as it’s always “worked for them”. There are many mechanical shops that are only used to making things that are defined by pure lines and arcs, because the whole chain of production from design to fabrication is using software and people who only understand those.
CAM systems have been able to import splines via IGES or DXF since before I started using them in the mid-90’s. Once the spline is imported, the CAM system converts the toolpath into line segments according to the tolerance determined by the toolpath programmer at the moment the toolpath is created. This is a huge advantage over pre-export conversion as you can choose the tolerance desired at the time of production and never need to ask for a new file with tighter or looser tolerances. However, this is not always possible due to either choice of CAM software (not all can deal with splines) or scenario #2 below:
The machine control is simply not able to handle the copious data in the G-Code file fast enough, either because it has an old communications link with limited rate, an old processor that can cannot calculate the blocks and accelerations fast enough, limited memory, or all of the above… In that case, there is no other choice besides pre-conversion.
IMO, if one is in the business of designing parts that are defined by free-form curves and surfaces, one should seek out suppliers who are equipped with software and hardware that can produce those parts, and not jump through hoops to try and accommodate those who can’t. Therefore both scenarios above should be considered as exceptions rather than “the rule”…