Research projects

Fluid jet polishing is a versatile process used in super-fine finishing of complex optics and prosthetic joints. Its advantages include highly controllable sub-millimetre polishing footprints and absence of tool wear. But the very low material removal rate means that hard materials such as cemented carbides and super-alloys, used in optical surfaces and prosthetic joints, cannot be polished from a rough condition. In this research, we proposed a new method for adding finely controlled micro-bubbles into the slurry jet by ultrasonic cavitation. Acoustic and fluid dynamics simulation confirmed that under the right conditions, cavitation bubbles generated in the nozzle can reach the surface of the workpiece before collapsing. A prototype system was fabricated, and micro-bubbles could be observed at the nozzle outlet using a high-speed camera. Polishing experiments data then showed that micro-bubbles boost removal rate by up-to 380%, without causing any degradation of the surface finish.

欧米の協力の下に日本が2015年度に打上げる硬Ｘ線望遠鏡と軟Ｘ線望遠鏡を搭載したＸ線天文衛星ASTRO-Hの開発は終了し、現在は打ち上げ前の種々のテストを行っている。高性能な硬Ｘ線望遠鏡を開発するには現状の円錐近似の反射鏡ではなく、正確な回転放物面と回転双曲面からなる反射鏡が必須であり、軽量化のためには超精密な非球面金型からレプリカ法で非球面反射鏡基板を製作する方法がベストである。 従来は前加工のし易さからアルミニウム合金を超精密ダイヤモンド切削し、それに無電解ニッケルを成膜し、ダイヤモンド切削と研磨で非球面金型を製作する方法と合成石英を研削・研磨して非球面金型を製作する方法を研究開発して来た。しかし、日米欧の経済状態から次世代の硬Ｘ線望遠鏡にはより安価に非球面金型を製作する技術の開発が急務となった。特に、合成石英の非球面研磨では所用の形状を創成するのに年単位の時間が必要であった。 そこで、本年度は、前加工が極めて容易なグラファイトを金型材料として採用し、非球面切削後に炭化珪素を化学気相成長させ、その表面をこの研究で開発したShape Adaptive Grinding (SAG)工具で研削する方法を見出した。この工具は研究室内で極めて容易に製作でき、工具寿命も10時間以上あり、表面粗さ0.4nm Ra以下の面を従来法に比べ１～2桁高能率な10～100mm3/minの除去速度で加工できることを見出した。液晶用ガラスをＸ線反射鏡基板として使う場合に使用できる非球面金型をこのSAG研削のみで製作できることが判明し、次世代の高性能硬Ｘ線非球面望遠鏡の新しい製作法として注目を浴びている。 この研究により研究分担者は国際生産工学アカデミー(CIRP)より2015年8月にTaylor Medalを受賞することになった。

In fluid jet polishing (FJP), a polishing liquid is pressurized as a jet and and impinges a surface. An improved system consisting of an ultrasonic transducer and acoustic lens mounted atop the FJP nozzle cavity was tested, which boosted material removal rates by up-to 1000%. However, it was not clear whether the removal boost was due to a micro-scale or macro-scale phenomenon. A study of micro-pit formation by collapsing micro-bubbles in the near wall region determined that micro-jetting and water hammer pressure do not seem to occur in UFJP, so the micro-scale hypothesis was ruled out. A study of workpiece vibrations found that significant shear forces exist at the workpiece surface when ultrasonic vibration is applied to the FJP beam. After theoretical confirmation of the correlation between macro-scale fluid displacement and shear force, the standard erosion model in FJP was modified by adding time-averaged vibration terms. The model and experiments were finally in good agreement.

Phase1:Develop nano-cutting system and model. Phase2:Study tool-chip interaction (retardation/stagnation zone). Phase3:Study influence of crystal orientation. Phase4:Develop model of crystal orientation.

In this study, an abrasive grain homogenization method was studied and implemented. The method is based on the generation of an acoustic streaming flow inside a cylindrical cavity by ultrasonic vibration. It can be applied before injecting the disaggregated abrasive grains in a nano-blasting nozzle that is impiging a workpiece. Experiments confirmed that the agglomeration of particles with a primary particle size larger than 2μm can be well broken down. In the case of particles with a primary diameter smaller than 2μm, some agglomerates remained while the rest of the agglomerates were broken up. The process was shown to allow blasting of surfaces at conditions near the boundary between particle deposition and material removal. When abrasive grains with a primary grain diameter of 2μm were used for processing glass, the surface roughness became noticeably smaller when ultrasonic vibration was applied, under most processing conditions.