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 machinability tests. However, none of the laboratory methods for measuring temperatures reported in the literature is simple and reliable enough for routine testing. The method which is most promising is the tool-work thermocouple method, which yields a repeatable result which correlates well with tool wear for many materials. This method is not normally used in machinability testing because it is not clear what temperature the method actually measures and because, as conventionally described, 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 question of what temperature is measured by the method is answered by analyzing the electrical potential distribution in a cutting tool due to a distributed interfacial emf. It is shown that in general the tool-work thermocouple temperature differs from the average interfacial temperature, but that for tungsten carbide tools the difference is usually small. The isolation of the tool-work thermocouple circuit is also considered. Methods of measuring signals without introducing insulation between the chuck and workpiece and reducing the machining system stiffness are described. Finally, methods of minimizing measurement errors due to secondary junctions are discussed. Sample signals from machinability tests on steels are used to illustrate significant points.