Extended roughing operations to ISO 14649-11 for milling T-spline surfaces

Milling freeform surfaces using a STEP-compliant CNC machine tool highly reflect the intelligence of STEP-NC, especially when the toolpaths can be generated and adjusted online. However, in contrast to the 2.5D manufacturing features, there are very seldom entities defined in the standard ISO 14649-11 for milling freeform surfaces. Basically, there are only four freeform strategies used for finish milling a freeform surface, and no roughing operations are provided for milling a freeform surface from a raw piece to the shape before the (semi-)finishing operation. Obviously, both of the rough and finish operations are fundamental to machine a freeform surface from the raw piece. This paper extends the EXPRESS definitions for rough milling freeform surfaces, so as to enrich the related operations. Both 2.5D and freeform operations are improved, and individual operations, strategies, and technologies are added to fulfill the need of 2.5D and freeform roughing tasks respectively. The related online toolpath generation algorithms are developed and integrated into a STEP-CNC prototype. The STEP-CNC system is equipped to a three-axis machine tool and finally drives the machine tool to cut a T-spline surface.