Measurement of Transient Tool Internal Temperature Fields During Hard Turning by Embedded Thin Film Sensors

This paper presents a novel approach for obtaining thermomechanical data from the close vicinity (i.e., 10s of micrometers) of the tool-workpiece interface while machining hardened steel. Arrays of micro thin film C-type thermocouples with a junction size of 5 μm × 5 μm were fabricated by standard microfabrication methods and have been successfully embedded into polycrystalline cubic boron nitride (PCBN) using a diffusion bonding technique. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) were performed to examine material interactions at the bonding interface and to determine optimal bonding parameters. Static and dynamic sensor performances have been characterized. The sensors exhibit excellent linearity up to 1300 °C, fast rise time of 150 ns, and good sensitivity. The PCBN inserts instrumented with embedded thin film C-type thermocouples were successfully applied to measure internal tool temperatures as close as 70 μm to the cutting edge while machining hardened steel workpieces at industrially relevant cutting parameters. Acquired temperature data followed theoretical trends very well. Correlations between temperature and cutting parameters have been established. The embedded micro thin film sensor array provided unprecedented temporal and spatial resolution as well as high accuracy for micro-scale transient tool-internal temperature field measurements. Tool internal temperature maps were generated from acquired data. In the frequency domain, obtained thermal data indicated the onset of regenerative machining chatter earlier and more effective than conventional force measurement by dynamometer.