Design and Optimization of a New Micro Drill
|關鍵字:||新型微鑽針;微鑽針優化;鑽針設計;砂輪設計;新型四面鑽針;鑽針動力學;new micro drill;micro drill optimization;drill design;grinding wheel design;new four-facet drill;drill dynamics|
本研究的目的為設計與優化一種新型微鑽針，使它和一般的鑽針相比有更好的切削品質，且有更高的產率及精確率。為了達到上述目的，有四個主要的工作要做。第一，要先對新型微鑽針建立數學及幾何模型，接下來幾個重要的幾何角度將被計算，像是法前角(normal rake angle)和後角(clearance angle)，然後就可以實現切削力的預測。第二，新型微鑽針的動力學分析為透過傳遞矩陣法來決定鑽尖在切削過程中的振動及臨界轉速(critical speed)。為了更接近實際之情況，將鑽頭的柄部、斜部及鑽部三部分都模擬為旋轉型Timoshenko Beam，每個部分包含了軸承反作用力、軸向力、外施扭矩、陀螺力矩及偏心力的影響。第三，為了讓鑽尖有最小的側向位移，我們把新型微鑽針的結構做優化。最後，研製鑽針溝槽之砂輪的設計是由逆向問題來實現，其決定了微鑽針鑽部螺旋槽的形狀。
結果顯示新型四面鑽不只應用於微鑽針，也應用於正在發展的大型鑽頭。新型微鑽針有非常極端的幾何特徵，像是沿著刀刃的法前角(normal rake angle)總是大於零，沿著刀刃的後角(clearance angle)總是大於零的常數；因此，和傳統的微鑽針相比它的總推力可減少高達50.12%，總力矩可減少26.54%，所以新型微鑽針相較於一般傳統微鑽針有更長的壽命，換句話說，產率也更高。透過傳遞矩陣法，我們可以決定鑽尖在切削過程中的振動及臨界速度。其次我們透過對新型微鑽針的結構做優化，可以讓鑽尖的側向位移減少15.83%，因此它的壽命及精確度都大幅提升。從靈敏度分析顯示相對於鑽針其它部位的直徑及長度等設計參數，鑽針斜柄的長度對於鑽尖的側向振動有著很大的影響。研製鑽針開槽之砂輪幾何形狀是由逆向問題來實現，我們用Vericut software來驗證砂輪幾何形狀的精確率。
The micro drill has been widely applied in industry to drill micro holes in many devices, for example, cameras, mobile phones, computers, medical instruments, micro dies and molds, fuel injection nozzles, watches, bearings, especially printed circuit boards (PCB). Therefore, there are more and more researchers striving to improve its cutting performance to meet the development of PCB fabrication technology sector, and others. The objective of this research is to design and optimize a new micro drill to obtain better cutting quality as compared to the original one. It has not only a higher productivity but also a higher accuracy. To achieve this purpose, there are four main works. First of all, the new drill is developed by mathematical and geometrical models. Several significant geometrical factors of the micro drill, such as normal rake and clearance angles are evaluated, and then the cutting force prediction is implemented. Second, the dynamic analysis of the new micro drill is implemented by transfer matrix approach to determine the vibrations and critical speeds at the drill tip during the drilling process. The micro drill, chuck and machine spindle of the micro drilling system are modeled as rotating Timoshenko beam elements. Each element includes influences of bearing reaction force, axial force, torque, gyroscopic moments, and eccentricity. Third, the new micro drill structure is optimized to obtain the minimum lateral displacement at its tip. Finally, the design of grinding wheel design profile for the micro drill flute is carried out by the reverse problem approach. The results showed that the new four facet drill applying for not only micro drill but also macro one was developed. It has extremely great geometrical features, such as the normal rake and clearance angles along its cutting lips are always positive, and a positive constant, respectively. As a consequence its total thrust force and torque reductions are up to 50.12%, and 26.54% in comparison with the conventional one, respectively. Hence, its tool lifetime is longer, or in other words its productivity is higher. The vibrations and the first critical speeds at the drill tip were determined by the transfer matrix approach during cutting process. The structure optimization of the new micro drill reduced the lateral displacement at its tip by 15.83%. Therefore, its accuracy as well as the tool lifetime is higher. Sensitivity analysis showed that the length of the conical part connecting the drill shank has the highest influence on the lateral vibration at the drill tip compared with the diameter and lengths of other drill parts. The grinding wheel profile for the new micro drill flute was developed by the inverse problem approach. The grinding profile accuracy was validated by Vericut software.
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