SIMTech technical reports (STR_V11_N1_04_MTG) Volume 11 Number 1 Jan-Mar 2010 18 Low pressure abrasive flow machining S. Y. M. Wan, W. S. Fong, C. J. Kong, D. L. Butler 1 and M. S. Tiew 1 1 School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore Abstract – The present work was motivated by a need for an abrasive flow machining process that could function at relatively low pressures using equipment commonly available in the typical machine shop. To this end, low viscous abrasive media was formulated from readily available materials, and a pneumatic shop-air driven abrasive flow machining system was designed and fabricated. We present ex- perimental results on the polishing performance of the media on small diameter holes EDM wire cut in alu- minium. Analysis of the results reveals the critical role played by the normal stress differences in deter- mining whether or not material removal and hence polishing will occur. Thus, while the media must be sufficiently viscous to suspend the abrasives, all three viscometric must be considered in characterising the efficacy of the media used in our experiments. Keywords: Abrasive flow machining, Low pressure, Normal stress difference 1 BACKGROUND Abrasive flow machining (AFM) is a non-con- ventional surface finishing process in which an abra- sive laden viscoelastic, semi-solid media is extruded over a surface to be finished. In the standard two-way flow process, as depicted in Fig. 1, a workpiece is placed between two opposing cylinders, which alternately force the abrasive media under pressure. In this illustration, the media is ex- truded through a hole in order to polish its internal surface. To finish an external surface, additional tooling is generally required to ensure that the flow gap between the external surface and the tooling is sufficiently tight for adequate abrasive action [1]. AFM has been likened to a liquid file; and per- haps its greatest advantage lies in its ability to finish (deburr, polish and radius) complex internal passages or areas that are inaccessible to more traditional methods such as mechanical honing. Abrasive media typically consist of a polyboro- siloxane, a silicone rubber (commonly known as “silly putty”) and conventional abrasives such as diamond, silicon carbide and aluminium oxide. Small quantities of plasticisers and viscosifiers are included to modify the rheology of the abrasive media [2]. Fig. 1. Standard two way flow abrasive flow machining process. Top cylinder Bottom cylinder Viscoelastic media laden with abrasives Part to be internally polished Internal surface being polished