Gelling of chips during vertical surface diamond grinding of BK7 glass

Abstract

n this work diamond pins of diameter 5 mm were used to produce plano surfaces on glass by vertical surface grinding on a 3-axis CNC machining centre. Chip clearance appeared to be difficult and led to the formation of stalling and zero velocity tracks, which continued to persist even after polishing. The removal of this residual roughness is almost impossible by lapping and polishing. Moreover, besides their unsightly appearance, stalling tracks reveal the formation of lateral microcracks presumably the result of indentation instead of chip flow machining or ploughing. This is an undesirable surface for lenses as it creates form errors. This problem was overcome by having a central cavity using an ingenious method to produce it. This central cavity is able to accommodate chips that not only facilitates wet grinding but dry grinding as well, thus promoting manufacturing goals put forward by Klocke and Eisenblätter [F. Klocke, G. Eisenblätter, Dry cutting. Keynote paper, Ann. CIRP 46 (2) (1997) 519–526 [1]] and Byrne et al. [G. Byrne, D. Dornfeld, B. Denkena, Advancing cutting technology. Keynote paper, Ann. CIRP 52 (2) (2003) 483–507 [2]] for the use of MQL (minimum quantity lubrication) or its elimination to give 100% dry grinding. An interesting outcome of vertical surface grinding without the creation of a central hole is the phenomenon of gelling of chips hitherto unreported in the literature. With a central hole such a phenomenon does not occur. The gelled chips are silica as evidenced by EDXA analysis. The formation of these gelled chips has been studied and a model is put forward in this paper.