Laser assisted micro-milling of hard-to-machine materials

Abstract

There is a need for developing hybrid micro-manufacturing processes capable of generating three-dimensional micro-scale features in hard-to-machine materials. This paper deals with the development of the laser assisted micro-milling process for which a novel 4-axis machine has been designed and built. This paper presents the results of experiments on laser assisted micro-milling of hardened A2 tool steel (62 HRc). The dimensional accuracy of the micro-milled feature and surface finish obtained with and without laser heating are compared and discussed. Scientific explanations for the different observations are given.

Introduction

Recent innovations in micro-manufacturing are enabling the fabrication of micro- and meso-scale structures and devices in a wide range of engineering materials. One technique that has been researched extensively in recent years is mechanical micro-milling, which has distinct advantages over photolithography-based micromachining methods [1], [2]. Weule et al. [3] have demonstrated the capability of micro-milling for fabricating micro-injection molds. Other published works focus on micro-milling of soft materials like copper etc. [4], [5].

Since micro-molds and dies need to withstand repeated thermal and mechanical cycles during the molding process, a material like hardened tool steel is a natural choice for the mold/die [6]. However, the part feature accuracy and material removal rate in micro-milling of such difficult-to-machine materials are limited by the machine-tool system stiffness (especially for small foot print machines) and low flexural stiffness and strength of the micro-tools normally used. Also, rapid tool wear is a problem since it negatively impacts part feature accuracy and finish [7], [8].

One approach to overcome these limitations is to use laser heating to induce localized thermal softening as demonstrated for the micro-grooving process by Singh and Melkote [9]. By suitably controlling the laser power, spot size and speed, it is possible to produce a sufficiently large reduction in the strength of the work material and consequently, the cutting forces and tool/stage deflections [9], [10]. This paper builds upon the prior work of Singh and Melkote to develop a laser assisted micro-milling process that addresses the limitations of micro-milling of hard-to-machine materials. It should be noted that at the macro-scale, laser assisted machining of hard-to-machine ceramics has been investigated by other researchers [11]. At the micro-scale, however, very limited work has been reported on the laser assisted milling process [10].

This paper presents an experimental study of the laser assisted micro-milling process. A novel machine designed for this process is described. Experiments have been carried out on the machine to evaluate the effects of laser heating and cutting speed on the dimensional accuracy and surface finish of the grooves micro-milled in hardened A2 tool steel (62 HRc). The contributions of tool wear, tool deflection and other error sources on these responses are discussed.