FFR Radial Ultrasonic Cutting Head

A parameterised PAFEC-FE data file for the modelling of a radial ultrasonic cutting head has been constructed to enable FFR Ultrasonics to tune the patented cutting heads, at 20, 35 and 55 kHz as part of the Medlink Project (M187) in conjuction with University of Edinburgh and Smith & Nephew

It was required to redesign the blade/shaft assembly system to produce more efficient cutting. There are many modes associated with different parts of the system. Some of these are bending modes of the shaft, some bending modes of the blade tip ("frilly modes"), some axial extension modes, etc. Furthermore some modes have greater electro-mechanical coupling than others. The design strategy was to search for a "suitable cutting mode", i.e. predominantly axial, strongly coupled to the voltage drive and with an antinode at the blade tip, and not too close to "undesirable modes". The variable system parameters included axial lengths of sections, cross sectional dimensions and blend/fillet radii.

Utilising the symmetry of the structure only a half model was required. The data file was parameterised to enable the changing of dimensions and mesh densities to be as simple as possible. Restraints were applied around the perimeter of the mounting flange and on the plane of symmetry. At the intersection of circular and rectangular shaft cross-sections a ‘coupling’ plane has been defined which generates generalised constraints between the degrees of freedom of the two dissimilar surface meshes.

Initially natural frequency analyses of various blade configurations were performed, using the subspace iteration method, to enable accurate calculation of resonant frequencies and mode shapes. Additional sinusoidal response analyses were performed, where the piezoelectric ceramic rings were driven by applied voltages.

Accentuated deformed shape of 32rd mode - longitudinal
                    Contouring resultant deformation

Accentuated animated deformed shape of 32rd mode - longitudinal