Research on the Critical Condition of Brittle-Ductile Transition about Micro-Milling of KDP Crystal and Experimental Verification

KDP crystal is a kind of brittle material, and improper selection of processing parameter can easily result in brittle domain cutting of KDP crystal. Determining the critical cutting depth of micro-milling brittle-ductile transition of KDP crystal can guide the selection of process parameters. The elastic-plastic contact formula of blunt spherical indenter and brittle materials is used to explicate the contact of micro-cutter edge and brittle materials. Combined with indentation critical load equation, the calculation model of predicting the critical cutting depth is established. The critical cutting depth is the function of material properties of the workpiece, the tool material properties and the tool edge radius. The edge radius of CBN flat end milling cutter is measured by atomic force microscopy, and then the critical cutting depth of the KDP crystal material is calculated as 230 nm. By using this milling cutter, micro-groove milling experiments are conducted with different spindle speeds and feeds. The results demonstrate that as the spindle speed increases, the micro-groove bottom surface gradually transformed from the brittle cutting state to plastic cutting state. The critical spindle speed is 30000 rpm, and the maximum deformed cutting thickness is 227 nm, which coincides with the result of the theoretical calculation model.