A metric for defining the energy efficiency of thermally assisted machining

Abstract

A significant amount of work has been conducted on thermally assisted machining (TAM) with a great deal being focused on using a laser to deliver the thermal energy. The body of work has shown that preheating of the workpiece (usually localized heating) makes it possible to machine certain structural ceramics with a conventional cutting tool, improves the machinability of superalloys, and improves the micro-end milling of metals. A variety of metrics have been used to ascertain the impact of thermal assistance on the machining of these materials, including specific cutting energy, tool wear rate, surface roughness, residual stress, material removal rate, material removal mechanism, cost, and surface integrity. Combined, these quantities provide a good description of the process as a function of the material removal temperature. However, they do not address the question of how much thermal energy is required to achieve these temperatures and whether there is room for improving the preheating (i.e., reduce cost). This manuscript looks at the flow of energy in TAM in an attempt to determine how beneficial preheating is. Some efficiency metrics are suggested and used to study the data that have been collected to date. The total thermal energy deposited in the workpiece is compared with the theoretical minimum required to heat the removed material in order to determine what percentage of the deposited (i.e., absorbed) energy is actually used in assisting the cutting process. This enables a comparison between cutting processes (e.g., end milling and turning) and operating conditions to determine how efficiently the added thermal energy is being used. Compared to other machinability metrics the thermal energy efficiency is used to evaluate how beneficial preheating is to the machining processes studied. Four sets of data are studied: thermally assisted turning of silicon nitride and partially stabilized zirconia, and micro-end milling of 6061-T6 aluminum and 1018 steel. The specific energy for TAM of silicon nitride is compared with that for grinding of silicon nitride. It is hoped that this paper will spark a debate in the manufacturing community and provide more insight into thermally assisted manufacturing.