A new methodology for analyzing the heat transfer and thermal damage considering tool flank wear in finish hard machining

Jia-Yeh Jay Wang, Purdue University

Abstract

When hard machining is used as a finishing process, the surface quality of the work produced is critical. One of the obstacles that keeps hard machining from widespread use is thermal damage, in the form of microstructural change, in the machined surface layer. Research in the past concerning the thermally damaged layer only relates to qualitative observations and descriptions for a few special cases, which can not provide the knowledge needed for understanding the physics of heat transfer regarding the formation of thermal damage and its relationship to the cutting conditions. A new methodology for analyzing the heat transfer in finish hard machining is developed in this research. The methodology consists of a mathematical thermal model based on Green's function with a microstructure-based experimental method using orthogonal hard turning. The thermal model derived describes the heat transfer of a worn tool, with heat sources at the tool-chip interface (due to chip formation) and the tool-work interface (due to flank wear). The coupling of interface boundary conditions due to chip formation and flank wear is resolved using the proposed microstructure-based method, which is a departure from the conventionally incorrect approaches based on the assumption of constant chip formation. By utilizing the microstructural change, shown as white layers, in chips the heat entering into the chip and the work can be obtained simultaneously and therefore decoupled. By incorporating the microstructure-based method with the thermal model, the heat partition equations are solved and the temperatures, heat generated, heat partition, and the shear forces at the tool-chip and tool-work interfaces can be determined quantitatively. The results explain how an increase of flank wear, feedrate, and the rake angle towards negative facilitate the formation of the white layer. It is also shown that ceramic tools are prone to white layer formation as compared to CBN tools. In addition, the effect of flank wear on the cutting mechanics is revealed by decomposing the cutting force measurement with known interface shear forces. It is shown that the forces needed for chip formation are not constant and are reduced as the tool flank is progressively worn. The proposed methodology has the advantage of analyzing the heat transfer and the cutting mechanics of hard machining without involving: (1) the modeling of the complex mechanism of saw-toothed chip formation that is still not well understood, and (2) the interface temperature measurement that is extremely difficult to perform. The effect of cutting conditions, including cryogenic cooling, on the white layer formation in three dimensional hard turning is also studied experimentally. A statistical model that relates the thickness of the white layer and the cutting variables is established. The effect of each cutting variable on the formation of the white layer is evaluated using a relative importance index derived from the statistical model. It is found that the heat transfer analysis agrees with the result of the three dimensional experiment. Furthermore, the analysis serves as a strong base for studying the effect of cutting variables on the heat transfer, thus the thermal damage, which greatly helps the explanation on what is observed in three dimensional cutting.

Degree

Ph.D.

Advisors

Liu, Purdue University.

Subject Area

Industrial engineering|Mechanical engineering

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