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

为了减少实验室劳动，提高假肢的加工质量，提出了数控复合加工方法。焊接是将分离零件组合在一起的关键技术，是铸造、机械加工和放电加工的结合。为此，我们研制了数控Nd: YAG激光焊机。我们利用PWM对振荡电池进行改造以控制焊接能量，在初始脉冲后加入较低脉冲高度的附加脉冲以提高焊接性能。我们对纯钛进行单脉冲发射，观察其熔融性能。由于冷却速度过快，熔池在波聚焦前凝固，导致试样内部出现孔隙，表面粗糙。我们还发现，获得良好焊接的可用能量空间受到限制。与此相反，PWM可以降低内部孔隙率和表面粗糙度，因为凝固是在凝固后开始的。此外，PWM增加了可用能级的自由度，我们期望将PWM应用于数控焊接，尽管我们发现了更高的蒸发量和更深的熔池凹度。此外，我们进行了实验，以评估焊接收缩PWM。我们将直径2mm的钛棒焊接在厚度为3mm的钛板上，并测量了焊接时的收缩率。无PWM收缩为24 μ m，有PWM收缩为6 μ m。金相观察表明，在没有PWM的情况下，熔池底部和表面都先于熔池中心部分凝固，产生了较大的收缩应力。相反，由于熔池底部先于表面凝固，PWM得到的收缩应力和间隙较小，但凹度较大。研究结果表明，采用PWM能量控制可以降低焊点处的残余应力，防止焊接变形和焊接裂纹的产生。在金属模具上成功地应用了数控焊接技术制造桥梁金属框架。少

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