| In terms of machining accuracy,surface topography and quality requirements,vibration assisted cutting technology is an effective processing method,which makes up for the shortage of traditional metal processing.During the vibration assisted cutting process,the cutting tool is excited so that the cutting tool generates a regular and stable micro-vibration,which leads to a periodic interaction between the tool and work piece.Through setting up reasonable vibration and machining parameters,this periodic tool vibration can not only effectively reduce the cutting force and cutting heat during the cutting process of difficult-to-machine material,improve the machining precision and surface integrity,but also can be used to form micro-textures in the work piece surface,improving the corresponding surface functional properties,such as drag reduction,self-cleaning and wear resistance.However,at present the study of vibration assisted machining process has not yet formed a complete systematic design theory and analysis method,the relevant theoretical research and experimental basis are still not perfect enough.In view of the reasons mentioned above,this research adopts well-developed design theory of flexure hinge which has excellent performance,building flexure hinge-based vibration assisted cutting mechanisms.The newly developed vibration assisted cutting mechanisms can be used to generate micro-textures and achieve excellent processing effect in the cutting process of difficult-to-machine materials such as stainless steel,alumina etc.The investigation will provide theoretical support and experimental basis for the progress and application of the vibration assisted machining process.(1)The principle of vibration assisted cutting and its processing characteristics are introduced.The process of micro-topography generation on work piece surface is analyzed.A corresponding mathematical simulation model is established in which the shape and motion equations of the tool and work piece are defined first,the positioning of the cutting edge and the work piece surface during the cutting process is performed afterwards,then the intersections between them are gradually iterated and the work piece surface is updated and finally generated.The work piece surface topography is further analyzed.(2)On the basis of exploring the vibration assisted cutting mechanism,using the flexure hinge design theory,two circular flexure hinge units are arranged in series and mirrored to achieve the construction of the flexure hinge based guiding mechanism.The mirror symmetrical center of this mechanism can output one-dimensional vibration displacement when it is subjected to vibration excitation.The mechanism is analyzed by theoretical model and finite element method to obtain stiffness and modal response.Through the performance test of its prototype,the theoretical analysis is verified.Furthermore,a cutting experiment was carried out to generate groove textures on cylindrical surface in order to store the lubricant and reduce the wear of the shaft-hole pair,which demonstrates the feasibility and practicality of the proposed mechanism.(3)Based on the feasibility analysis of the flexure hinge-based guiding vibrating mechanism,it can be also concluded that the newly developed mechanism does not have a displacement amplification effect,the output displacement range of which is also limited.To improve it,the symmetry centerline of the adjacent tandem type circles in the flexure hinge-based guiding structure is divided to make offset and mirrored to construct improved bridge-type flexure hinge structure.The improved bridge type structure is further compound with Z-shaped flexible beam,establishing a new type of composite bridge-type flexure hinge-based two-dimensional vibration assisted cutting mechanism.The structural composition,movement principle,statics and dynamics characteristics are analyzed during the design process.The two-dimensional vibration assisted cutting mechanism operating in a non-resonant state has a motion output amplification effect,as well as sufficient output stiffness.It can output two-dimensional vibration curve which can be controlled precisely.(4)The performance test of the composite bridge flexure hinge-based two-dimensional vibration assisted cutting mechanism is carried out to analyze its signal hysteresis,coupling performance,the output stiffness and dynamic response.The results show that the mechanism can work in the non-resonant state,with excellent anti-signal hysteresis,coupling performance and enough output stiffness.Through properly setting of the process parameters,cutting experiments are carried out to obtain three kinds of textures:strip pits,intertwined pits,and interwoven grating strips.By comparing the corresponding theoretical simulation results with.the experimentally obtained morphologies,it can be concluded that the theoretical model agrees well with the texture obtained by the experimental processing.Such consistency shows that the surface texture produced by the vibration mechanism is predictable and can be controlled.Furthermore,the hydrophobic properties characterized by contact angle of different textured surfaces are tested.It can be seen that the surface of the micro-textured array as a whole has better hydrophobicity than the ordinary surface,which further verifies the feasibility and practicality of the designed mechanism.(5)Based on the feasibility analysis of the flexure hinge-based guiding vibration mechanism and its corresponding improved two-dimensional vibration-assisted cutting mechanism of composite bridge type flexure hinge,it is obtained that the aforementioned two mechanisms have relative low operating frequency,which can not be well adapted to high-speed machining,therefore,a further improvement is made on the flexure hinge-based guided vibration mechanism.One of the two adjacent circular hinge units in the tandem type circular flexure hinge structure is rotationally transformed around its symmetrical center point to generate an angle of 60° to form a triangle-shaped bidirectional coupling structure.The tip of the triangle-shaped structure can output elliptical vibration trajectory and the two ends with the angle of 60° corresponding to the tip are connected to piezoelectric vibrator.This triangle-shaped bidirectional coupling flexure hinge-based elliptical vibration assisted cutting mechanism operating in resonant state with ultrasonic frequency can be used for high-speed machining of hard materials.Through experimental analysis and finite element simulation,the effect of ultrasonic elliptical vibration excitation signals,cutting speed,feed amount,depth of cut and other processing parameters on cutting force,chip shape and workpiece integrate are studied.The results show that the cutting resistance during ultrasonic elliptical vibration assisted cutting is smaller than that obtained by traditional cutting.Furthermore,through the force analysis of the frictional state between the tool and chip,the mechanism of the chip formation process in the ultrasonic elliptical vibration assisted cutting is demonstrated different from the traditional cutting process.The finite element analysis results of cutting resistance and the shape of the simulated chips were also verified by cutting experiments.(6)Based on the analysis of ultrasonic elliptical vibration assisted cutting mechanism of the triangle-shaped bidirectional-coupled flexure hinge,the frictional state between the cutting tool and the chip is further studied.The tool with micro-texture on the rake face is used for ultrasonic elliptical vibration cutting.The texture of the rake face is generated by laser processing.The influence of micro-texture on ultrasonic elliptical vibration cutting is studied.The preliminary conclusion that the rectangular micro-texture can improve the cutting performance is obtained.At the same time,in order to verify the feasibility of the ultrasonic bi-directional elliptical vibration assisted cutting mechanism in the machining of difficult-to-cut materials,this mechanism was used to process pure alumina crystals using polycrystalline diamond tools.The results show that compared to the traditional processing methods,the cutting force in the ultrasonic elliptical vibration assisted cutting process is significantly reduced,and the surface quality of the processed workpiece is also improved. |
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