Investigation of rotary ultrasonic vibration assisted machining of Nomex honeycomb composite structures

The applications of Nomex honeycomb composite (NHC) structures in aerospace, automotive and defence sectors have been significantly increasing due to their high compressive strength, hexagonal thin-walled structure, ultra-light weight and excellent thermal resistance. Specific applications include composite sandwich structures in helicopter propellers, satellite cabins, aeroplane floors, engine cowls, wings and nacelles. Accuracy of the machined surface of NHC structures is required for adhesive bonding with face-sheets. Conventional machining processes generate machining defects in terms of tearing, damaged cell walls, burr formation, delamination and poor surface quality that result in reduction of strengths of the core structure and its bond with face sheet. Ultrasonic machining is a proven technique to overcome such machining defects and improve the surface quality of NHC structures. Novelty of this research includes the development of a three-dimensional (3D) finite element model to analyse cutting forces, chip formation and machining quality of NHC structures using disc cutter through both ultrasonic and conventional machining processes by providing feed to the workpiece instead of the cutting tool. The significant influence of machining parameters such as depth of cut, feed rate, ultrasonic amplitude and spindle speed on cutting forces was investigated numerically followed by experimental validation. Numerical model in support with experimental results show that cutting forces decrease by increasing ultrasonic amplitude and spindle speed (up to 54.74% and 62.71%, respectively), and increase with the increase of depth of cut and feed rate (up to 60% and 60.48%, respectively). It was also found that the ultrasonic machining reduces the magnitude of cutting forces as compared to conventional machining (up to 42.74%). Surface morphology analysis through scanning electron microscope also indicated improved machining quality achieved by ultrasonic machining at NHC structures' hexagonal cells, triple points and walls. A burr formation of 5% was observed during ultrasonic machining of NHC structures for F-y <= 3N, while it was found up to 10% if F-y > 3N, compared to at least 30% burr during conventional machining. To sum up, the employed methodology can be effectively applied for determining the effect of various machining parameters on cutting forces as well as surface quality, chip formation, structural integrity and dimensional accuracy of machined NHC structures during ultrasonic machining process.