Impact of central abrasive grain absence on micro-hole helical grinding

Manufacturing micro-holes in non-conductive ceramics presents significant challenges in precision machining, particularly due to the absence of central abrasive grains in micro-grinding wheels. This study investigates helical grinding under conditions where the central abrasive grain is absent, focusing on the formation of undeformed chips. It was observed that nearly all micro-grinding wheels, regardless of their manufacturing process or grain size, exhibit a central grain absence, with larger grain sizes leading to more extensive absence areas. Analysis revealed that residual patterns at the bottom of machined holes depend on the ratio of the absence zone diameter to the wheel's eccentricity. Analytical models were developed to describe the heights of cylindrical and disc-shaped residues, which were subsequently validated through kinematic simulations. The removal mechanisms for these residues differ; cylindrical residues, which cannot be removed by grinding, cause interference and should be avoided, while disc-shaped residues removal depends on the protrusion height of the first grain, influencing contact with the wheel's end face and subsequent grinding actions. Experimental validation using SiC_p/Al demonstrated that cylindrical residues create distinct ring-shaped wear marks, significantly increasing cutting forces, whereas disc-shaped residues result in hat-shaped wear marks and higher cutting forces when the first grain's protrusion is insufficient. Additionally, inadequate lubrication and chip removal can lead to chip adhesion starting from the absence zone. These findings enhance the theoretical framework of helical grinding/milling and provide valuable insights for precision machining of micro-holes in non-conductive ceramics.