Study On Hole-making Quality Of Orbital Drilling For Hard Machining Materials

Hole-making process is a significant task in the aircraft assembly industry. The machining accuracy and surface finish quality are directly related to the fatigue life of aircraft. In recent years, with the extensive application of hard-machining materials such as titanium alloy and carbon fiber reinforce plastic (CFRP), the traditional drilling technology can’t completely meet the requirements in the aviation manufacturing field. In the aircraft assembly process, how to accurately produce these tightly tolerated holes for aeronautical difficult-to-machining Materials has been a puzzle of aerospace manufacturing industry. As an innovative hole-making technology, helical milling shows great advantages compared with traditional drilling, especially for hole-making of the titanium alloy and CFRP. Because the special tool’s kinematics in helical milling process, more complex factors affect the hole-making quality compared to drilling including tool, process parameters, eccentric adjustment mechanism and the stability of the system. Therefore, these factors must be given further researches.Firstly, the factors affecting hole-making quality have been deeply studied based on the kinematics study of orbital drilling. Then orbital drilling experiments were carried out for titanium alloy and the borehole accuracy of an automatic orbital drilling system based on the robotic bore-machining platform was researched deeply in the practical aircraft assembly workshops. The impacts of the orbital drilling parameters, tool wear on borehole accuracy were investigated by the orthogonal experiments. At the same time, the continuous hole-making experiment was conducted with the typical bore diameter. According to the experimental results, a compensation method of system error and tool wear was developed and it could greatly improve the borehole accuracy based on the robotic orbital drilling system.The quality problems in hole-making process were summarized based on the helical milling experiment of CFRP/TC4DT stacks. Two serious problems were found including burrs with large size at the hole exit and the hole diameter of CFRP material was greater than the tolerance requirements. A series of experiments were designed and implemented in order to find the cause of these problems. Finally, we found that the current robotic orbital drilling system can separately machine the single titanium alloy or composite materials. However, it was difficult for the robotic orbital drilling system to make high accuracy holes on CFRP/TC4DT stacks and can’t meet with the tolerance requirement without additional reaming process. Orbital drilling of CFRP/TC4DT stacks on the platform of machine tool has a prominent advantage, but the eccentricity need to be further improved. Otherwise, the Reaming operation still need to be conducted to enhance the hole quality of CFRP/TC4DT stacks.