FINISH-MACHINING STRATEGIES FOR BLADED DISKS

Integrally Bladed Rotors (IBRs) or Bladed Disks (Blisks) are strategic components of compressor or turbine stages of aircraft engines. Development of manufacturing techniques and materials have aided the integration of two components, blades and the disk, which were originally manufactured separately and then assembled. A single component brings great benefits such as weight reduction, which is key the in aerospace sector. IBR components bring new challenges to the manufacturing industry due to the difficult to cut materials used, paired with complex geometries which limit the access of tooling and limits various efficient cutting strategies for the finish milling operations. Instead, a point milling strategy is commonly used to achieve drawing specifications but at a cost of machining time. Therefore, finish milling is by far the most time-consuming machining operation of IBR blades. However, many efforts from industry are directed to optimize machining times through roughing operations, which are faster to implement internally within the manufacturing engineering department, and often are not affected by fixed process approvals that are in place for the last few millimetres of material removal. This includes approval from the materials department on surface integrity modifications of the final surface, and complex approval processes with the final clients. An EngD project is an ideal scenario for the development of finish machining strategies for the reasons explained above. This thesis takes a real IBR case study as a starting point and navigates through a logical path for the development of its blade finish milling operation to provide a novel industrial optimization strategy. The research question evolves as each chapter explores different aspects of this challenging industrial problem. Initially, in chapter 2, surface integrity is explored within the typical working window (range or map of parameters selected for a given experiment), due to the relevancy of the surface integrity in the finished component. This is explored through an experimental approach which concludes surface integrity is not affected in the analysed range. Instead, chatter is identified and research efforts are then directed to improve finish machining of IBR blades through the understanding and mitigation of chatter. Chapter 3 seeks to analyse tool and component dynamics and includes a brief search into literature about process damping to understand how it might play a role in chatter mitigation. A new research line is then investigated to improve finish milling of IBR blades. A very simple concept of modifying finish milling stock is developed, using a scientific method based on Finite Element Analysis (FEA) and parametrizing the blade in order to maximize natural frequencies of interest. Once an optimized blade stock geometry has been obtained, a further literature review is carried out on chatter mitigation techniques. A knowledge gap is found in the current literature regarding time domain model for Sinusoidal Variable Spindle Speed (SVSS) model for ball end mill tools. This is observed as an opportunity to do a theoretical contribution to the predominantly experimental EngD thesis. A current time domain model has been further developed to incorporate SVSS and ball end mill geometry. Finally, implementation of variable speed in industrial environment has been researched. A further knowledge gap is identified in the implementation of variable speed in commercial milling machines, as most research up to date has been realized either theoretically or in laboratory conditions. In response to this need, a new method has been developed to be able to implement variable spindle speed and variable feed straight forwardly in a wide range of commercial milling machines. To end up with, a machine characterisation has been completed in order to identify the working window to apply the Variable Spindle Speed (VSS) method, and experimental trials have been carried out to demonstrate the capability of this approach. This thesis starts presenting a case study of IBRs with the need to improve current finish machining strategies and delivers new solutions from various perspectives, complementing each other and readily available to implement in the industry environment.