Cutting Spline Geometry — Mechanical Advantage

Splines and CNC programming software at times don't go well together. This seems to be more prevalent with software used to program plasma, laser, and waterjet cutters. It is quite common for CNC programming software to convert splines into line segments controlled by a tolerance value during the import process. The bigger the tolerance value, the larger the variation will be from the original spline profile resulting in fewer and longer line segments. The smaller the tolerance value, the smaller the variation will be from the original spline profile. However, this will result in many short line segments.

The real issue comes when the g-code is sent to the machine that will be doing the cutting. Each of these line segments will create a line of code telling the cutter where to go. The cutter will try to travel the path for each segment at the feed rate specified when the part was programmed. As the machine makes potentially thousands of short segmented moves to complete the cut that is now approximated with line segments, the machine will almost appear as if it has been damaged by the way it jerks. As the machine moves in a non-normal fashion, the program operator will have a heart attack while cancelling the cut operation as quickly as possible. As you can imagine, not a desirable situation.