Surface roughness and tool wear aspects in heat assisted high speed ductile mode end milling of soda lime glass

Abstract

Soda lime glass is used extensively in engineering application owing to its high hardness, excellent optical properties, and good corrosion and chemical resistance. However, the low fracture toughness of the material impairs its machining using conventional approaches. Nonetheless, high speed ductile regime machining enables material removal from brittle material in a ductile manner avoiding fracture. To tap this advantage high speed end milling was applied using a conventional CNC vertical milling machine fitted with a high speed attachment in order to achieve ductile mode in machining of soda lime glass. Owing to high heat generated during high speed machining (HSM), the temperature developed in the work material approaches its glass transition temperature which facilitates ductile mode machining of the brittle material. Under such temperature, soda lime glass is expected to behave in a ductile manner without causing any brittle fracture and sub surfaces damages during machining. Nevertheless, conventional HSM is not very productive as the depth of cut and feed per tooth are extremely small leading to low material removal rates. Hence, one of the objectives of heat assisted ductile mode machining were to increase the material removal rate and achieve comparable surface finish and tool wear at increased feed and depth of cut. Critical upper and lower depths of cut were identified in the preliminary experiments using vibration data acquisition and analysis system in order to ensure achievement of ductile mode machining. In the main experiments influence of cutting parameters: spindle speed, feed rate, and depth of cut on surface roughness (Ra) and tool wear were established. In these experiments the spindle speed was varied in the range of 30 000 to 50 000 rpm, feed rate in the range of 5 to 20 mm/min and depth of cut in the range of 5 to 50 µm. The heating system (soldering gun) applied to soda lime glass during machining was maintained at a constant temperature of 340°C. Actual heat developed on the workpiece surface varied depending on the feed rate from 80°C to 110°C measured using a thermal camera. Response Surface Methodology (RSM) was used to design the experiments with 3-levels full factorial. The Ra and tool wear were measured using Wyko Maho Optical Profiler measurement machine and NK measuring instrument machine respectively. Scanning Electron Microscope (SEM) was used to capture images of the work surface in high magnification. Empirical mathematical models were developed for the output responses of average surface roughness (Ra) and tool wear. Investigation using multiple response optimization of RSM showed that, to obtain minimum Ra (0.169 µm) and minimum tool wear (45.368 µm) in the experiment without heat assistance required spindle speed of 41 000 rpm in combination with low feed rate of 7.12 mm/min and low depth of cut 5 µm. On the other hand, the analysis for heat assisted machining suggested that the conditions for minimum Ra (0.24 µm) and tool wear (35.518 µm) were spindle speed of 38 000 rpm, feed rate of 20 mm/min and depth of cut of 50 µm. Thus these results show that by the application of heat in the experiment, higher feed rate and depth of cut can be used compared to the same used in experiments without heat assistance. This shows that machining with heat can increase material removal rate without sacrificing surface finish and tool life.