Research in minimum undeformed chip thickness and size effect in micro end-milling of potassium dihydrogen phosphate crystal

Abstract

Micro-milling is a promising approach to repair the micro-defects on the surface of potassium dihydrogen phosphate (KDP) crystal. However, KDP crystal is difficult to machine due to its properties of soft-brittle and easy deliquescence. This study investigates the minimum undeformed chip thickness hm and the size effect in micro end-milling of KDP crystal by comprehensively analyzing cutting force, specific cutting force and machined surface quality. A model of the chip formation, which is capable of connecting the minimum undeformed chip thickness, the undeformed chip thickness and the periodicity of cutting force together, is developed to predict the value of minimum undeformed chip thickness. The normalized minimum undeformed chip thickness λe in terms of the ratio of the minimum undeformed chip thickness to the cutting edge radius re, is estimated to be 0.43 ≤ λe ≤ 0.48. The significantly non-proportional increase of specific cutting force indicates the existence of size effects when the ratio of feed per tooth to cutting edge radius ft/re is less than 0.7. The machined surface quality also reflects severe size effect by the phenomenon that the micro cracks and brittle pits appear on the groove base, and the value of surface roughness Ra is large when the ratio ft/re is less than 0.5. Furthermore, the surface quality deteriorates and the brittle cutting appears when the ratio ft/re is much larger than 1 which seems similar to macro-milling. The perfect machined surface with almost no ploughing effect and brittle cutting is achieved at a ratio ft/re of 0.7. Therefore, a feed per tooth, slightly larger than the minimum undeformed chip thickness but smaller than cutting edge radius, is recommended for micro-milling of KDP crystal or soft-brittle crystal.