Development of a closed loop control system for vibration assisted grinding

Abstract

Increasing demand for components made of hard and brittle materials such as glasses, steel alloys and advanced ceramics is such that conventional grinding and polishing techniques can no longer meet the requirements of today’s precision manufacturing community. However, it is essential and much needed to undertake such processes based on a scientific approach, i.e. the process to be quantitatively controlled and optimized rather than carried out in a trial-and-error manner. Vibration assisted machining been has demonstrated to reduce the amount of forces, pattern definition on a finished surface. Axial oscillation (parallel to the wheel axis) allows the grains to cut with two more faces. With vibration in two directions, the wheel exposes four times more cutting edges (two edges per oscillation) than in continuous cutting which uses only one edge, which is a great advantage over continuous cutting, Axial oscillation induces an elliptic motion leading to a lapping process, which improves the cutting efficiency as well as the quality of the finished surface. Oscillation greatly reduces the load per grain, reduces wheel wear and induces a chip-breaking effect, which is a great advantage for prevention of wheel loading. In this research, theory modelling and instrumentation for vibration assisted grinding are presented and discussed in depth. The modelling is focused on control of the amplitude of oscillation in the cutting zone which is the fulcrum of this investigation. The control system was developed using Labview 8.5 and Matlab. It was found that the application of vibration reduces the forces, increases the material removal rate and increases the G-Ratio compared to conventional grinding. The superimposition of vibration in axial direction secured better process outcomes in terms of grinding forces (average 25% improvement), surface finish quality and power consumption. In terms of frequencies it was identified that superimposing vibration at 100 Hz in this study provided the lowest forces. In addition, it was shown that at 100 Hz the desired amplitude of oscillation was achieved at the lowest driving voltage, i.e. 4V. Comparing the techniques of control systems, which were open loop, closed loop and conventional grinding, applying vibration always gave better results than with no vibration. The closed loop control in most cases was the best. Comparing oscillating in axial direction and tangential direction, the Axial gave on additional 9% reduction in grinding forces. In general the grinding with vibration assisted secured a greater depth of cut. The oscillating in Axial direction provided an additional 16 % in actual depth of cut, so Applying oscillating in axial direction showed that a significant improvement of quality could be achieved.