Fundamental investigations on temperature development in ultra-precision turning

Abstract Ultra-precision machining represents a key technology for manufacturing optical components in medical, aerospace and automotive industry. Dedicated single crystal diamond tools enable the production of innovative optical surfaces and components with high dimensional accuracies and low surface roughness values in a wide range of airborne sensing and imaging applications concerning space telescopes, fast steering mirrors, laser communication and high-energy laser systems. Despite the high mechanical hardness of single crystal diamonds, temperature-induced wear of the diamond tools occurs during the process. In order to increase the economic efficiency of ultra-precision turning, the characterisation and interpretation of cutting temperatures are of utmost importance. According to the state-of-the-art, there are no precise methods for online temperature monitoring during the process at the cutting edge with regard to the requirements for resolution accuracy, response time and accessibility to the cutting edge. For this purpose, a dedicated cutting edge temperature measurement system based on ion-implanted boron-doped single crystal diamonds as a highly sensitive temperature sensor for ultra-precision turning was developed. To enable highly sensitive temperature measurements, ion implantation was used for partial and specific boron doping close to the cutting edge of single crystal diamond tools. Within the investigations, a resolution accuracy of 0.29 °C ≤ a R ≤ 0.39 °C could be proven for the developed cutting edge temperature measurement system. In addition, a total measurement uncertainty of u M = 0.098 °C was determined for the sensor accuracy a S in the investigated process area. For a rake angle range of 0° ≤ γ 0 ≤ −30°, reaction times of 420 ms ≤ t R ≤ 440 ms were further determined. Using the developed cutting edge temperature measurement system enables a holistic view of the temperature development during ultra-precision machining, whereby a correlation between the measured cutting temperatures and the chip formation mechanisms depending on the applied process parameters could be identified. Within the investigations, the highest measured temperatures of ϑ M = 50.18 °C and simulated maximum temperatures of ϑ S,max = 183.12 °C were determined at a cutting speed of v c = 350 m/min, a cutting depth of a p = 35 µm as well as a feed of f = 35 µm using a rake angle of γ 0 = −30°. The most uniform chips with the smoothest surfaces were identified within the chip analysis using a cutting speed of v c = 50 m/min, a cutting depth of a p = 5 µm and a feed of f = 35 µm with a measured temperature of ϑ M = 21.30 °C and a simulated temperature of ϑ S = 38.47 °C in the examined finishing area. According to the results, it was also shown that the cutting edge temperature measurement system with ion-implanted diamonds can be used for both electrically conductive and non-conductive materials. This provides the fundamentals for further research works to identify the complex temperature-induced wear behaviour of single crystal diamonds in ultra-precision turning and serves as the basis for self-optimising and self-learning ultra-precision machine tools.