Thesis defense of Xabier Badiola Aiestaran


Thesis defense of Xabier Badiola Aiestaran


Thesis defense of Xabier Badiola Aiestaran

Title of the thesis: “Slender Workpiece Cutting Process Stability Prediction and Monitoring based on Internal Signals”. Obtained the SOBRESALIENTE qualification and the CUM LAUDE and DOCTOR INTERNACIONAL mentions.



  • Title of the thesis: “Slender Workpiece Cutting Process Stability Prediction and Monitoring based on Internal Signals”.
  • Thesis directors: Pedro José Arrazola Arriola, Aitzol Iturrospe Iregui.
  • Court:
    • President: Dr. D. Philippe Lorong (Ecole Nationale Supérieure d’Arts et Métiers)
    • Vocal: Dr. D. Manuel San Juan Blanco (Universidad de Valladolid)
    • Vocal: Dra. Dña. Maite Beamurgia Bengoa (Fagor AOTEK, S. Coop.)
    • Vocal: Dr. D. Alex McCloskey Gomez (Mondragon Unibertsitatea)
    • Secretary: Dr. D. Mikel Saez de Buruaga Echeandia (Mondragon Unibertsitatea)


Chatter is commonly found when machining slender workpieces on a lathe due to the low stiffness and damping of the system. Machining accuracy is usually very low owing to large variation in stiffness along the length of the workpiece, which is a problem far the machining industry. The main contributions of this thesis are the development of (1) models far determine the chatter-free cutting conditions in slender workpiece machining and (11) a process monitoring system to detect chatter using available internal signals from the regulator. The clamping of a workpiece in the spindle of a lathe is not completely stiff, so the classical boundary conditions such as clampedfree, are not enough. The natural frequencies are usually lower than theoretical ones and varies with each machine tool and clamping method. In addition, in slender workpieces, two nearby modes usually appear due to the clamping. Far this reason a model with elastic boundary conditions is developed. In arder to identify the machine tool system parameters experimentally, a exciter tool prototype based on a pieza actuator is developed to excite the system. The workpiece modes and the system parameters far a specific clamping and machine tool have been identified by means of modal updating method. Then, a dynamic machining model has been proposed and a turning operation without tailstock is simulated with the parameters identified experimentally. The system behaviour and its stability is obtained, as well as, the effective stiffness of the system at each tool position of the workpiece. The stability of the process far a facing operation has been modelled in the state space and an eigenvalue analysis is carried out. As there are two nearby modes, workpiece chatter may occur by mode coupling in addition to regenerative. The stability of the process is determined far certain cutting conditions. The theoretical results are validated by experimental tests. To define the process stability far each cutting condition, the stability lobe diagram has been obtained. To this end, a linear time periodic stability model has been developed for slender workpieces and its stability is analysed by Floquet theory. The model takes into account both modal coupling and regenerative effect. Finally, the development and validation of a process monitoring system using internal signals available in the regulator is presented. The feasibility of different internal signals for chatter monitoring are evaluated. An Internal signal based monitoring system is implemented on a fast prototyping generic hardware and the monitoring system is validated experimentally in a lathe.