The curvature of the NURBS curve varies along its trajectory, therefore, the commonly used feedrate-planning method, which based on the acceleration/deceleration (Acc/Dec) model, is difficult to be directly applied in CNC machining of a NURBS curve.To address this problem, a feedrate-planning method based on the critical constraint curve of the feedrate (CCC) is proposed.Firstly, the problems of existing feedrate-planning methods and their causes Poly-Fil / Stuffing are analyzed.Secondly, by considering both the curvature constraint and the kinematic constraint during the Acc/Dec process, the concept of CCC which represents the relationship between the critical feedrate-constraint value and the arc length is proposed.
Then the CCC of a NURBS curve is constructed, and it has a concise expression conforming to the Acc/Dec model.Finally, a feedrate-planning method of a NURBS curve based on CCC and the LARCH Acc/Dec model is established.In the simulation, a comparison between the proposed method and the conventional feedrate-planning method is performed, and the results show that, the proposed method can reduce the Acc/Dec time by over 40%, while little computational burden being added.The machining experimental results validate the real-time performance and stability of the proposed method, and also the machining quality is verified.
The proposed method offers an effective feedrate-planning strategy for a NURBS curve in CNC machining.