Diamond Tool Wear when Cutting Amorphous Polymers G.P.H. Gubbels 1 , G.J.F.T. van der Beek 1 , A.L. Hoep 2 , F.L.M. Delbressine 1 (2), H. van Halewijn 3 1 Precision Engineering section, Department of Mechanical Engineering, Eindhoven University of Technology, Eindhoven, the Netherlands 2 Océ Technologies B.V., Research & Development, Venlo, the Netherlands 3 Diamond Tools Group, Research & Development, Valkenswaard, the Netherlands Abstract Precision turning of polymeric materials increases form and shape accuracies in respect to the conventional techniques, but the relatively large tool wear of the mono-crystalline diamond tools may be a problem. This paper will discus possible tool wear mechanisms that occur during diamond turning of glassy (amorphous) polymers. Special attention is given to thermal-oxidative tool wear, tribo-electric tool wear and tribo-chemical tool wear. It will be shown that tribo-electric and tribo-chemical tool wear play an important role in the precision turning process of polymers. Keywords: Wear, Polymer, Electrical discharge 1 INTRODUCTION Much research on tool wear has been performed in the past, e.g. [1] [2] [3], but only few references are found on tool wear mechanisms during diamond turning of glassy polymers [4] [5]. Evans [1] used the following classification for diamond tool wear: adhesion, abrasion, tribo-thermal and tribo-chemical. In this paper tribo- electric tool wear will be added to this list, because industry states that tool wear caused by discharging may be a problem in the lens manufacturing process. Usually more than one wear mechanism is active. But, generally only one is dominant for a certain tool/workpiece combination and a certain cutting regime. Adhesive tool wear will be excluded as a dominant diamond tool wear mechanism, since adhesive wear always occurs on the softer material (in this case the polymer) of the two adhering materials. Looking at abrasive tool wear it can be stated that this can only be excluded by chemical investigation of the polymer. It is known from the production of contact lenses [6] [7] that relatively hard silicon particles abrade the diamond tool during turning. The materials used in this study, polymethylmethacrylate (PMMA) and polycarbonate (PC), were chemically investigated by energy dispersive X-ray (EDX) analysis and they contained no elements that could cause abrasive tool wear. The next paragraphs deal more extensively on the other three possible diamond tool wear mechanisms. 2 THERMAL-OXIDATIVE TOOL WEAR Graphitisation of diamond in air is called oxidation, which occurs at approximately 800 K [8]. To investigate the possibility of thermal-oxidative tool wear to occur in diamond turning of polymers a diamond tool with encapsulated thermocouple was developed. A small diameter thermocouple (wire ø25μm) was inserted in a hole produced by excimer laser drilling. The thermocouple was fixed in the middle of the tool by a heat conducting epoxy. The thermal response of the tool was checked by a calibration procedure that involved touching the cutting edge along a slowly cooling aluminium cylinder. This way it was possible to get the difference between recorded temperature of the thermocouple and the actual temperature of the cutting edge. This was also modelled [9]. Temperature measurements are performed on tubular PMMA with a width of cut of 0.7 mm. For the temperature measurements several cutting speeds vc were used and for every cutting speed four feed rates were used (0.1, 1, 10 and 25 μm/rev). The results of the temperature measurements can be seen in figure 1. Material removal rates in the precision turning process typically have a value of less than 1 mm 3 /s. This means that during the precision turning process the temperature at the cutting edge is significantly less than the temperature at which oxidation of the diamond takes place, so no thermal-oxidative tool wear is expected. 3 TRIBO-ELECTRIC TOOL WEAR 3.1 Introduction Since glassy polymers and diamond are electric insulators, static electricity can be expected during the turning process, because of friction between tool and workpiece [10]. Scission of the polymer chains during the cutting process might also result in an additional charging Figure 1: Temperature at the cutting edge during diamond turning of PMMA.