Fundamental Studies on Wheel Wear in ELID Grinding

Abstract

Metal bonded superabrasive grinding wheels are extensively used for machining and finishing hard and brittle materials, like mono-crystalline silicon, BK7 glass, silicon nitride, PVD hard coatings, etc, used in the electronics, optical, aerospace, nuclear and automobile industries. Electrolytic In-process Dressing (ELID) is perhaps the most popular technique for conditioning such wheels.In ELID, electrolysis forms soft and brittle anodic oxide of the metal bond of the grinding wheel. This oxide is eroded off during grinding action, exposing new sharp abrasives and shedding off old worn ones, along with grinding chips. The mechanism of wheel wear in ELID is essentially through dissolution of the metal bond and investigation of the underlying electrochemical phenomenon is the key to wheel wear predictions. This is the basic approach of the thesis, which has not been the concentration of previous researchers.The electrolytic dressing process sets aside ELID grinding from conventional grinding. Role of the electrolyte in the dressing process is first investigated. Other than electrolyte, the dressing process is also characterized by electrolytic current and thickness of anodic oxide layer. Fundamental behavior of the overall dressing process is investigated to understand the relationship of dressing conditions with oxide layer and electrolytic current and the process is modeled.The combined effect of mechanical and electrolytic action during ELID grinding is then investigated by parametric study of wheel wear in ductile regime grinding. The process showed initial and steady stages of operation. The steady stage has cyclic variations of grinding force and dressing current within specific limits such that the average value per cycle is constant. It is found that wheel wear rate in steady stage has a linear trend with a benchmark function defined from machining and dressing conditions.Brittle mode grinding experiments with coarse abrasives are carried out to find that its steady stage of grinding does not have cyclic variations of force and current, but retains a stable value. This is because the rates of oxide erosion and formation reach equilibrium and maintains a stable layer thickness of oxide. Combination of the dressing theory and an oxide erosion model is used to simulate the dressing/electrolytic current which agrees with the experimental values.Finally, an analytical and an empirical model for oxide erosion in ductile regime grinding are developed. Each of these is combined with the dressing model to simulate values of wheel wear rate and dressing current. The simulated values for steady phase of grinding agree with the experimental values. The models are verified with different types of experiments and are successful in predicting the profile of the ground component by compensating wheel wear.