Research was carried out with respect to the improvement of the cutting speed and material removal rate of machine tools (a factor of 5 to 10 at frequencies around structural resonances of the subsystems). The use of one (or more) closed loop absorbers should increase the material removal rate of machine tools so that the overall time for specific machining operations should at least be reduced by 5 to 10. The methodology was tested by implementation of 3 different machine subsystems chosen as characteristic benchmarks: ram of milling machine; table of 8 pallets machining centre; spindle of milling and boring machine.
This methodology can be applied not only inside the machine tool industry but also for a lot of dynamic problems in mechanical systems involving: centreless grinding; vertical lathe stability; deburring tools; cabin stability; high speed milling machine.
In all situations, the use of a tuned vibration absorber yields improved vibration levels. Its efficiency is function of the situation. Further, an improved surface quality has been observed, but not quantified. When the absorber enabled chatter to be eliminated, a reduction of the radiated noise level of 20 dB has been measured.
Besides the reduction of chatter by a closed loop (feed back) vibration absorber, also simpler (and less expensive) solutions such as an open loop (feed forward) vibration absorber are found to yield as important improvements of the vibration level.
In 1989, the coordinator of this project was granted a feasibility award from EC-DG XIII C to study the concept of "using active elements for introducing effective damping into structures". This was achieved by means of a closed loop tuned vibration absorber.
In the mechanical engineering industry the material removal rate of machine tools if often limited by their dynamic instability, called chatter. This effect is expressed in dangerous vibration levels, which also reduce the product quality and accuracy, tool life time and wear.
A closed loop vibration consists of a classical vibration absorber (mass-spring damper system eliminating vibration at only resonance frequency by acting in opposite phase), of which the spring-damper is automatically controlled to follow the changing behaviour (and changing resonance frequencies) of the machine tool during cutting. Therefore, a closed loop absorber added to the machine will limit the vibration level and increase the material removal rate over a wide range of cutting conditions.
After its development, the absorber will be integrated and tested on three specific applications of the industrial partners :
- a milling machine
- an eight pallet machining centre
- a headstock of boring and milling machine.
Subsequently, the assessment of acquired experiences will be used to facilitate and broaden its application range inside and outside the machine tool industry : increased stability and drilling speed of boring bars, friction welders, tool holders, lightweight machine foundations, overcritical operation of papermill rolls, etc.
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