MR fluid-based novel finishing process for nonplanar copper mirrors

The potential use of copper as metallic mirrors in a wide variety of industries has put an onus on producing super-finished surfaces of copper of different shapes and sizes. However, with un-controlled normal forces in traditional finishing process, it is difficult to finish copper to the order of nanometers. Moreover the diamagnetic nature of copper limits its finishing capability by several advanced magnetic-assisted finishing methods. In this research work, a novel approach of finishing nonplanar copper mirrors is explored using magnetorheological polishing fluid. A finishing tool based on electromagnet with solid rotating core and having an efficaciously slotted tip has been developed and mounted on a five-axis CNC setup. The solid spindle core and multiple slots at the tip improve the magnetic flux density, essential for finishing of diamagnetic copper workpieces, and grip the polishing fluid firmly during the finishing operation. The electromagnet-based finishing tool also aids in in-process control of finishing forces acting on the copper workpiece. During the finishing process, forces acting on the workpiece were recorded using dynamometer and analyzed to understand the material removal mechanism and the process physics associated with it. Two nonplanar copper workpieces (convex and concave samples) were finished using the new tool to produce mirror-like reflective surfaces. In two finishing cycles with varying magnetizing current, the surface roughness of two samples were reduced from initial values of 120.1 and 129.2 nm to final values of 40.3 and 43.2 nm for convex and concave samples respectively.