Development of the new fabrication system of dental prostheses using the computer assisted combined machining

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In order to reduce labor in the laboratory works and to fabricate prostheses with higher quality we already proposed the numerically controlled combined machining. Welding is a key technology to combine the separated parts fabricated casting, mechanical machining, and electric discharging machining. Therefore, we developed the numerically controlled Nd : YAG laser welder. We remodeled the oscillating battery to control the welding energy using PWM where additional pulse with lower pulse height was applied after the initial pulse to increase welding properties. We shot the single pulse to pure titanium to observe the melting properties. We found internal porosity and rough surface on the specimens since the cooling rate was too fast and molten pond solidified before the wave focused. We also found the room for the available energy to obtain good welding was restricted. On the contrary, we could reduce internal porosity and surface roughness with PWM since the solidification started afte … More r the wave of molten pond focused. In addition, freedom of available energy level increased with PWM and we expected to apply the PWM to numerically controlled welding although we found the higher amount of evaporation and deeper concavity of molten pond. Additionally, we performed the experiments to evaluate the welding shrinkage with PWM.We welded a titanium rod with the diameter of 2 mm on the titanium plate with the thickness of 3 mm and measured the shrinkage at the welding. Shrinkage was 24 mum without PWM and 6 mum with PWM.Metallurgical observation indicated larger contraction stress was produced without PWM since the both bottom and surface of molten pond solidified before the center part of the molten pond. On the contrary, smaller contraction stress and gap but deeper concavity were obtained with PWM since the bottom of the molten pond solidified prior to the surface. These findings indicated energy control with PWM reduced the residual stress at the welding spot and prevented the deformation and the occurrence of welding crack. We successfully applied the numerically controlled welding to fabricate the metal frameworks for bridge on the metal molds. Less