Conventional drilling and helical milling of aircraft stacked structures - a comparative study

Stacked structures consisting of carbon fibre reinforced polymer composites (CFRP) and titanium alloy (Ti4Al6V) are increasingly used in aircrafts nowadays due to their superior mechanical properties and reduced weight. In aircraft assembly, CFRP/Ti stacks are fastened following a multi-shot machining process which involves drilling, reaming, disassembly, deburring and reassembly operations Unfortunately, the vast difference between the mechanical and thermal properties of CFRP and metal has created enormous challenges in drilling of these materials. Other problems encountered during drilling include rapid tool wear, CFRP damage, poor surface finish and burr formation.
Helical milling (or orbital drilling) is an emerging technology and is considered as one of the best choices to perform the hole making on stacked structures due to its lower cutting forces/temperature and high efficiency. However, delamination and burr formation resulted from the helical milling process can affect the hole making quality, in addition to the geometric error (insufficient size of hole diameter). Although some research efforts have been directed to this research area, most of them are empirical and the helical milling machining mechanism and the resulting hole surface integrity / reliability are still unclear.
The aim and objectives of this project are:
1) to explore/compare the mechanisms of different hole making processes (i.e. conventional drilling and helical milling) in machining of aerospace stacked CFRP/Ti structures.
2) to establish the relationships between machining parameters and the hole geometrical accuracy, surface integrity and the material reliability (e.g. fatigue life for structures with open hole).

Fundamental characteristics of conventional drilling / helical milling process for CFRP/Ti stacks will clarify the mechanism of surface damage and thus help to inform / improve the technologies in an effort to avoid completely unacceptable occurrence in drilling or helical milling processes.

Methodology: To characterise the geometric accuracy and surface integrity of the machined holes, a range of advanced characterization facilities will be deployed, such as Scanning electron microscope, white light surface profilometer, X-ray tomography, and atomic force microscope etc. The machined surface will be further characterized for its residual stress using nanoindentation, supplemented by digital image correlation technique. The results will then be related to the fatigue test data obtained from coupons with open holes. After comprehensive performance indices for evaluating the quality of a machined hole are proposed as the criteria, the machining parameter optimization will be performed by maximizing the drilling efficiency.

This will be the first study to compare conventional drilling and helical milling in machining of aerospace structures, and to correlate the hole making process with the resulting material reliability. The project is closely in line with the EPSRC "Manufacturing the future" strategy. The outcomes of the project will provide the concrete theoretical and technical foundations and bring significant benefits to the development of automated drilling equipment and processes.