Publication:

Experimental investigation and modelling of ductile-regime machining of sodalime glass

Soda lime glass is a very useful engineering material. It is commonly used to manufacture products like mirrors, lenses, semiconductor, optical, bio-medical and microelectronics components due to its favorable thermal and corrosion resistance and fine chemical properties. Nevertheless, due to its low fracture toughness thus making the material brittle, machining of soda lime glass is practically impossible under normal cutting conditions. Though recent investigations have shown that machining of such a brittle material is possible in ductile mode machining condition under controlled cutting parameters and tool geometry, it remains a challenging task. This research work is focused on high-speed end milling (HSEM) of soda-lime glass using 2-flute 4 mm diameter carbide end mills in order to assess the transition between fractured and ductile-streaked of the machined work-piece surfaces, the heat generated in the cutting zone, the roughness of the machined surfaces and the flank wear of the tools. In order to study the feasibility of employing ductile mode machining concept to high-speed end milling of soda lime glass, experiments were conducted in three stages. At constant 50,000 rpm with a gradual increase in feed rate, inclined cut milling was conducted to find the critical feed rate at which three phases (plowing-ductile-brittle) transition occurred on the machined surface. Based on the critical feed rate that was obtained, inclined cut milling at different combinations of feed rates (5-20 mm/min) and spindle speeds (30,000-50,000 rpm) were performed to find the lower and upper critical depth. The spindle speeds (20,000 to 40,000 rpm), cutting depth (30 to 50 µm) and feed rate (10 to 30 mm/min) were employed to explore the effect of high cutting speed end milling parameters on the machined surface finish and assess the effect of heat generated in the cutting zone. Tool flank wear after 75 mm milling length were measured under dry condition at the cutting parameters, spindle speed (20,000 to 40,000 rpm), cutting depth (10 to 30 µm) and feed rate (5 to 20 mm/min). At 50,000 rpm, 20 mm/min was identified as a critical feed rate. From inclined cut milling, plowing and ductile phases were observed starting at the different depths, hence identified as the critical depth for all the experimental runs. This combination gives the upper and lower critical axial depth of cut, 51.943 µm and 1.558 μm respectively. Experimental results showed that HSEM using uncoated carbide tool is capable to achieve ductile surface on soda lime glass with minimum and maximum roughness values of 0.38 µm and 1.66 µm respectively. Mathematical models for the roughness parameters Ra, Rq and Rt have been developed based on Central Composite Design (CCD) and it has been found that the effect of feed rate on surface roughness is the most significant followed by the cutting speed and depth of cut. Verification tests confirm the optimal cutting condition (40,000 rpm, 10 mm/min, and 43µm) that was predicted to generate lowest Ra 0.05 µm, Rq = 0.38 µm and Rt value 5.92 µm values and MRR being 1.27 mm3/min. The tool-chip contact point temperature measured using Infra-Red (IR) thermal camera, showed that around glass transition temperature, Tg (520-650°C), ductile chips were observed on the machined surface hence facilitating the production of clean machined surface with a low roughness value. The major wear form of tool flank was abrasion, with the presences of oxidation, thermal diffusion and recast layer in some cases. Quadratic model developed for tool flank Vb based on Box-Behnken Design shows that the effect of feed rate on the wear is more significant than those of the depth of cut and spindle speed. Optimization of the results predicted that spindle speed 29331 rpm, feed rate of 5 mm/min and depth of cut 10µm, can generate Vb of 142 µm with 100 % desirability. An analytical model was developed for predicting the amount of temperature generated in the Immediate Next Removable Layer (INRL) of the soda-lime glass work-piece per unit depth of cut, ∆T ̅_INRL considering fundamental micro machining principle and materials’ physical properties. A computer program was developed using Matlab (R2014a) to simulate the analytical model to predict the amount of ∆T ̅_INRL. Simulation based on the model equation was carried out to predict the temperature in the INRL using the developed ∆T ̅_INRL model until the glass transition temperature was reached. The temperature in the tool chip interface, Ttcp within the cutting zone is also predicted based on the ∆T ̅_INRL. The measured temperature in the tool chip interface, Ttcm was compared with the simulation results and this is in reasonable agreement with the predicted Ttcp.