Mechanical microdrilling of negative-tapered laser-predrilled holes: a new approach for burr minimization

Abstract

Despite considerable research effort being concentrated on improving hole quality, minimisation of burrs still remains a key challenge in mechanical microdrilling of ductile materials. Recently, a sequential laser-mechanical microdrilling technique, which was developed by the authors, has proved effective in improving tool life and reducing burr size. The improvement in burr size was achieved by means of laser predrilling a pilot hole before utilising a twist drill to finish the hole. This paper presents further development of the sequential laser-mechanical microdrilling process based on negative-tapered laser-drilled pilot holes. In the sequential laser-mechanical microdrilling process, although a large predrilled hole reduces the burr size, it has a negative effect on roundness and cylindricity of the hole. The results of further studies highlight that a smaller entry hole size supports the drill and suppresses tool wander which, in turn, improves hole cylindricity. Furthermore, a larger exit hole size significantly reduces burr size. Compared to pure mechanical drilling without any pilot hole, the reduction in burr size for the mechanical microdrilling of near zero tapper and negatively tapered laser-predrilled holes was observed to be 2.5 and 6 times, respectively. Moreover, the mechanical finishing of these holes does not compromise tool life and/or surface integrity. Thus, the use of a negative taper laser-predrilled hole presents a significant improvement in burr control in microdrilling of nickel-based super alloys in addition to providing a step change increase in tool life.