Temperature monitoring of milling processes using a directional-spectral thermal radiation heat transfer formulation and thermography

Abstract

Temperature is an important property to be monitored during cutting operations, such as in the milling of metals, because it affects the workpiece mechanical properties, the machining tool performance, and the process efficiency. For this reason, authors have used thermography to monitor thermal and spatial gradients, and to estimate temperature. The problem is that IR cameras native applications are mostly based on the diffuse-gray approximation, with emissivity set as a constant value. The diffuse-gray approach is a reasonable choice for opaque surfaces with emissivity approximately constant in respect to the wavelength, which is not the case in most of the time for metals under cutting operation. In this work we propose a more suitable methodology that uses radiative heat transfer directional-spectral relations to estimate cutting process temperatures during face milling of metals, using the full electrical response of commercial infrared cameras as an input. AISI H13 steel was used as the workpiece material. In the first part of the experiments, emissivity was estimated for eight different temperatures from 50 °C to 250 °C, using both directional-spectral and diffuse-gray approaches. Temperature was measured using a thermocouple type T, calibrated using a PT-100 reference from 50 °C to 250 °C. In the second part, we used the emissivity obtained in the first part to estimate the temperature for twelve different cutting conditions, again using both directional-spectral and diffuse-gray approaches. We compared the results of both methods, also with the direct application of emissivity provided in infrared cameras manuals. The temperature deviation between the different approaches were up to 41%, which demonstrates that the temperature estimation procedure affects the results substantially.