Abstract
(Englisch)
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Vibrations are the main limitation in most of machine tools. Vibrations on machines have the following main negative effects:
· Bad quality of the worked pieces and surfaces. This aspect is particularly serious in case of High Speed Milling machines or in case of grinding machines. High Speed Milling machines is becoming a very important technological process aiming at a drastic, or even elimination, of the final manual surface finishing.
· Reduced tool life. The tool of the life is strongly depending on the machine stiffness. It has been demonstrated that on rigid machines or in more rigid working areas of the same machine, the tool life is longer of the 30% or more. Due to the cost of change operation and tools, in particular the costs of the expensive coated tools used in High Speed Cutting, damping of vibrations will represent a significant economical benefit.
· Reduced machine tool life. Vibrations strongly reduce the life of the machines and increase the need of frequent maintenance services with interruption of the working activity. For instance, it can be said that the vibrations are the main causes of troubles on the spindles for High Speed Machining.
· Reduced safety. Vibrations cause fatigue effects on moving parts and tools. The reduction of the vibration is a commitment for obtaining a safe working environment.
All these factors, joined, cover very important economical consequences. So, the main objective will be vibration reduction in machine-tools (a milling machine) by means of different techniques of vibration control (active and passive damping of vibrations, and adaptive control techniques).
The machine studied is a high speed milling machine with conventional drives, it means with servomotor plus ballscrew and no linear motors (provided by HURON). In HSM the most important source of vibrations are produced by unbalance of the main spindle. These machine also incorporates an open architecture NC, so it allows to use new control techniques and algorithms, more accurate than the conventional proportional Kv-factor.
Firstly, a theoretical study of the vibrations in machine tools has been performed. This study included experimental modal analysis, cutting test and some simulations. EICN has established a procedure for vibration free-zones identification. EICN has developed a model of the kinematic chain of one axis of machine tool, which could be easily adapted to any machine tool axis.
Also different types of actuators (piezoelectric, magnetostrictive and electrodynamic) and sensors (piezoelectric, capacitive, piezoresistive…) have been analysed. Their characteristics, capabilities, advantages and disadvantages have been studied. Experimental tests have been performed for some of them to identify their dynamic behaviour.
Control strategies and algorithms have been analysed, in particular for the Huron machine : the state control. Afterwards, state control strategy was applied to the analysed machine. Some cutting tests were carried out to measure the improvement of the vibration control. The final total advantage due to the activities is an improvement of the Kv factor of about 55 %.
EICN has conceived and designed a system to measure the deflections of the tool during milling operations.
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