Intelligent control of spark gap and discharge pulses for CNC controlled electrical discharge grinding

Although CNC controlled spark erosion machines have been widely used in tool manufacturing industry for more than 70 years, low material removal efficiency and poor workpiece surface quality have been the dominant issues, especially in eroding semi-conductive material such as PCD. Therefore, reducing material removal time while maintaining the precision of the workpiece geometry and a good surface quality of the workpiece, has always been a topic of research interest. In order to improve the erosion process efficiency, the electrical discharge pulses (sparks) and the inter-electrode gap distance need to be effectively controlled. Developing control algorithms for pulse controller and gap controller is a difficult task due to (1) Spark erosion is a stochastic and time-varying process. (2) Spark discharge occurs over a very short period of time (typically between 0.5µs to 20µs) (3) Spark erosion process involves evaporating and melting of both electrode and workpiece within a very small gap (typically between 3µm to 10µm).

The objectives of this thesis are to research and develop various novel strategies (algorithms) for monitoring and controlling discharge pulses and spark gap in a CNC controlled 3 dimensional 5 axes EDG machine with aims of (1) To improve the poor cutting edge issue of an eroded PCD tool that is normally caused by insufficient control of the heat generated by spark discharges, and (2) To improve the stability of the stochastic and time-varying erosion process, and (3) To improve the efficiency of the material removal process by maintaining the spark gap distance at an optimum level regardless of disturbances that are introduced into the system. The investigation and development outcomes of this thesis will be of a great interest to the machine tool industry for producing high performance commercial EDG spark erosion machines.

Experimental investigations of various dominating factors that affect eroded PCD tool quality is discussed in Chapter 3. A number of critical issues with the PCD tool quality (such as brittle cutting edge, PCD and WC interface undercut, and poor surface roughness) are discovered during these investigations. Effective detection methods of spark gap condition are proposed is Chapter 4 with particular emphasis on a spark erosion system that contains a rotating electrode and involves 3 dimensional PCD tool erosion. In Chapter 5 novel discharge pulse control algorithms are proposed that are particularly designed to effectively control the heat that is applied to the eroded PCD tool based on the condition of the spark gap. A multi-level hierarchical spark gap control algorithm is proposed in Chapter 6 in which each of its control level has been carefully designed to maintain the spark gap distance at an optimum level regardless of fast changing of erosion conditions or disturbance introduced into the system. Implementation of these control algorithms in embedded processors for real-time control is also discussed in this thesis. Major problems that are encountered during implementation process and how to tackle these problems are also briefly discussed.

The outcome of this research work has resulted in proposed algorithms adopted in a commercial spark erosion machine--The ANCA EDGe, 2014 Excellence in innovation award from the Australia CRC association, one international patent, one paper published in an international journal and three papers published in various international conferences.