Theoretical and experimental investigation into non-uniformity of surface generation in micro-milling

In micro-milling process, the periodically varying chip thickness with tool rotation inevitably leads to the changing effects of minimum chip thickness and ploughing on surface generation at the center and the sides of the micro-milled slots, accordingly resulting in the highly non-uniform surface quality. However, there is currently no evaluation methods and prediction models for the non-uniformity of micro-milled surface quality. In this paper, a relative standard deviation of surface roughness (RSDS) method is developed to quantitatively evaluate micro-milled surface non-uniformity, and its effectiveness is statistically demonstrated by ANOVA and SN ratio analysis. Additionally, a mathematical model that considers alignment errors, relative tool sharpness (RTS), material elastic recovery and ploughing effect is proposed to estimate surface non-uniformity. Theoretical and experimental results reveal that: (i) the variation of surface generation mechanisms can induce periodic cutting force oscillations and highly non-uniform surfaces characterized as low surface roughness in the center indicating shearing mechanism and high surface roughness on the sides suggesting ploughing effect; (ii) the resulting surface non-uniformity essentially results from the periodically varying chip thickness and its induced variation of the stochastic surface roughness dominated by the minimum chip thickness effect and ploughing; and (iii) apart from cutting speed, both feed rate and depth of cut have a pronounced influence on surface uniformity.