TOOL-WORK THERMOCOUPLE TEMPERATURE MEASUREMENTS-THEORY AND IMPLEMENTATION ISSUES

Since cutting tools wear by temperature-activated mechanisms, it would be desirable to make tool temperature measurements during machinabili ty tests. However, none of the laboratory methods for measuring temper atures reported in the literature is simple and reliable enough for ro utine testing. The method which is most promising is the tool-work the rmocouple method, which yields a repeatable result which correlates re lates well with tool wear for many materials. This method is not norma lly used in machinability testing because it is not clear what tempera ture the method actually measures and because, as conventionally descr ibed, it cannot be used for roughing cuts at high cutting speeds. The purpose of this paper is to extend both the theoretical understanding and range of application of the tool-work thermocouple method. The que stion of what temperature is measured by the method is answered by ana lyzing the electrical potential distribution in a cutting tool due to a distributed interfacial emf. It is shown that in general the tool-wo rk thermocouple temperature differs from the average interfacial tempe rature, but that for tungsten carbide tools the difference is usually small. The isolation of the tool-work thermocouple circuit is also con sidered. Methods of measuring signals without introducing insulation b etween the chuck and workpiece and reducing the machining system stiff ness are described Finally, methods of minimizing measurement errors d ue to secondary junctions are discussed. Sample signals from machinabi lity tests on steels are used to illustrate significant points.