Real-time surface interpolators for multi-axis CNC machine tools.

Abstract

Free-form surfaces and curves have been widely used in CAD systems to describe the part surfaces. The current method to machine these part surfaces generates, in off-line, a cutter location file (CL file) which describes the cutter path by a sequence of tool positions. The major disadvantage of this method is that there is a trade-off between machining time and surface accuracy which depends on the size of the CL file. However, a large CL file also increases the machining time since at each line the cutting tool accelerates and decelerates. To overcome these drawbacks, a real-time surface interpolator is proposed in this dissertation. The objective of this research is to develop new interpolation algorithms for CNC machine tools that will (i) produce smoother curves and surfaces and (ii) reduce machining time with using a smaller number of part-program statements. This comprises two achievements: (1) development of the real-time three-axis interpolator for 3-D surface machining and (2) development of the real-time five-axis interpolators for ruled surface machining (flank mills) and for convex surface machining (end mills). The real-time interpolator consists of (i) tool-path planning, (ii) trajectory planning, and (iii) inverse kinematics portions. It calculates new commands based on these three portions for the control loops during one sampling time. This makes the real-time interpolator capable of generating accurate references for axes motions and eliminating acceleration and deceleration steps during machining. Therefore, it produces smoother part surfaces and requires less machining time, compared with off-line approaches. With the new types of real-time interpolators, cutting a curve or surface requires only one g-code NC instruction. Therefore, the size of a CL file is significantly reduced. These new developed interpolators are implemented on a 5-axis CNC milling machine with demonstrated surfaces produced.