Method for measuring thermo-emf of a “tool-workpiece” natural thermocouple in chip forming machining

Abstract

Abstract. The article presents a new method and a device for measuring the signal of thermoelectromotive force of a "tool -workpiece" natural thermocouple in the chip forming machining during the turning process. It is proposed to use a device of stationary current-collecting connection for fixing and measuring thermo-electromotive force signal, which allows to eliminate the influence of the transient resistance for existing constructions of movable current collectors. This circumstance makes it possible to reduce the measurement error of the signal and at the same time to increase its reliability. The review and analysis of the most similar constructive solutions have been carried out and there are also experimental data values which have been obtained when comparing the proposed device and the prototype.The processes of edge cutting machining of the workpiece with tool material are accompanied by the thermoelectromotive force (thermo-emf) generated by "tool -workpiece" natural thermocouple. It is known that the variation of the thermo-emf signal is observed due to the change in the work of the electrons liberation from the hard alloy and the processed workpiece. The value of the contact component for the thermo-emf signal of the tool-workpiece pair will be determined from the condition of equalizing the electrons electrochemical potentials. This fact gives the opportunity to use thermo-emf signal of a "toolworkpiece" as a diagnostic parameter for evaluating physico-mechanical properties of various combinations of "tool-workpiece" contact pairs for subsequent assignment and optimization of the cutting modes while processing the same workpiece with the given tool. And for this purpose, thermo-emf signal should be obtained on strictly defined test run modes, which are the same for all groups of processed and tool materials.Using the informative ability of the cutting thermoemf signal to ensure the reliability for the automated determination of the edge cutting modes is presented in many scientific works, in particular in [1][2][3][4][5].In addition to the occurrence of a parasitic component in the signal and the need for electrical insulation of the tool and the steel workpiece, the main disadvantage of thermo-emf signal application is the dependence of the transient resistance and hence, the value of the thermo-emf on the structure and type of the current collectors installed on the machine tool spindle units. Thus, the main way to increase the reliability level of the thermo-emf signal, and consequently, the reliability of the automated determination of cutting modes, is the development of a method for picking up the thermo-emf signal without the use of movable current collectors of the contact type. Let us consider the best known methods for measuring the cutting thermoemf signal.There is a method for measuring the cutting EMF [6], in which a tool or a workpiece is isolated from the machine tool mass, then the tool and the workpiece are brought into complete electrical circuit and the cutting EMF generated on ...