Determination of minimum uncut chip thickness and size effects in micro-milling of P-20 die steel using surface quality and process signal parameters

Prior to a critical value of uncut chip thickness (UCT) or the so-called minimum uncut chip thickness (MUCT), the micro milling process is affected by ploughing which causes deterioration of surface quality and reduction of tool life. Stringent surface quality is the major concern during micro milling of P-20 die steel as it has wide application in preparation of micro injection mould. Again, this MUCT value depends upon the workpiece, cutting tool material and its geometry. In this article, we tried to determine MUCT and size effects by considering the variations of surface quality parameters and process signals with the ratio of h/r(e) (UCT (h) to edge radius(r(e))). MUCT value was determined to be 0.75-1 mu m or critical h/r(e) value was found to be in the range of 0.25-0.33. Comprehensive analysis of tool wear and chip morphology with size effect have been instituted those were lacking in the earlier literatures. Curled and helical chips were obtained at feed rate higher than MUCT value, whereas the chips with arbitrary shapes were noticed at feed rates below MUCT. Lamellar structures were observed at the free surface of the chip having both inclined and vertical orientation. Edge rounding, coating delamination and built-up edge formation were observed due to size effect at feed value below MUCT.