Adaptive tuned mass damper with variable mass for chatter avoidance
Introduction
The interaction between the cutting forces and the resilience response of a machine tool is the limiting factor of its performance and its precision. If the machine damping is not sufficient, the machine starts vibrating and chatter can occur. Vibrations do not only limit the performance of the machine tool but are the cause of numerous problems such as tool breakage, increased material wear or poor surface quality [1], [2], [3].
In the last decades researchers and industry dedicated a lot of effort to improve the machining stability of machine tools. While some of them improved the machining process with spindle speed variation, new tool geometry, etc., others improved the dynamic behaviour of the machine tool with additional systems or by changing the structure of the machine tool.
The approach we present here is to improve the dynamic behaviour of the machine tool deals with an adaptive tuned mass damper (ATMD) with variable mass. Since the dominant eigenfrequency of the machine tool varies with the working position of the machine, the adaption of the mass allows the adjustment of the ATMD to this eigenfrequency in order to reduce the chances of chatter occurring.
Section snippets
State of the art
Given that the approach presented in this paper focuses on improving the dynamics of machine tools, so will the following account of the state of the art. The improvement of the machine tool dynamics with additional systems can be categorized in three classes:
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passive systems without any input of energy in the vibrating system,
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semi-active or semi-passive systems, with passive properties that can be changed by energy input,
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active systems with input of energy to generate forces [4].
Passive systems
Approach
The interaction between the tool and the workpiece limits the improvement of the performance and the precision of a machine tool. This interaction depends on the FRF of the machine tool and on the cutting process. The FRF of the machine tool changes with the axis-position of the machine tool [1], [3].
In this way, the dynamic behaviour of a machine tool without any adaptive or active additional system cannot be optimal for all machining situations at all machining positions.
The approach
Design of the ATMD
For a given machine and cutting operation, one can calculate the machining stability; the result of the calculation is a stability lobe diagram, made of multiple lobes. The dominant eigenfrequency defines in the diagram the lobes and the critical depth of cut [3], [17]. The aim of the ATMD is to compensate this dominant eigenfrequency in order to increase the critical depth of cut. Nevertheless, the dominant eigenfrequency will vary with the working position of the machine. Thus, the possible
Presentation of the machine tool considered
The machine tool we used for investigating our approach of ATMD with variable mass is a LiFLEX II 766 i B2 5 axes machining centre with horizontal double spindles of the company Licon MT. The machine tool has its two spindles integrated in two Z-carriages. The Z-carriages can move in Z-direction relative to the Y-carriage on which the Z-carriages are mounted. The Y-carriage can move in Y-direction relative to the X-carriage on which the Y-carriage is mounted. Finally, the X-carriage can move in
Conclusion and outlook
This paper presented an innovative ATMD with variable mass. The adaption of the mass allows the adjustment of the ATMD to the working position of the machine tool.
We presented a procedure to design the ATMD so that it is possible to compensate the dominant eigenfrequency of the machine tool in any working position. We then commissioned the ATMD and validated our approach by comparing the dynamic behaviour of the spindle for different working positions. Finally, we validated the functioning of
Acknowledgements
We acknowledge support from the Central Innovation Program SME (Zentrales Innovationsprogramm Mittelstand – ZIM) research fund and we thank the company Licon MT for permitting our investigations.
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