| Ultra-precision machining(UPM),typically involving ultra-precision diamond turning(UPDT),ultra-precision raster milling(UPRM)and ultra-precision grinding(UPG),has been widely employed in national defense,military industry,aerospace and other high-tech fields.It provides nanometric surface roughness and sub-micrometric form accuracy.UPM provides an essential support and foundation for the development of manufacturing industry.The surface generation mechanism under ultra-precision machining has become the most critical point in the progress of ultra-precision machining technology for its influence in determining surface quality.It is affected by many factors,where spindle vibration(SV),tool wear,change in cutting parameters,material properties and removal mechanism are generally recognized as several key factors to determine the quality of machined surface.However,current domestic and foreign researches still exist some deficiencies.To this end,corresponding theoretical and experimental researches on the ultra-precision machining surface generation mechanism are carried out,focusing on the effects of the mentioned key factors on the surface generation under ultra-precision machining.The research work in this paper mainly includes the following three points:(1)With the effect of SV on surface generation in UPM process set as the research object,a generalized dynamic model was proposed and established to precisely describe the effect of SV on surface topography in different UPM processes,To verify the fitness of theoretical results,a series of UPDT,UPRM,UPG experiments were set up and carried out.Surface topography was detected during the experiments.It was found that in UPDT,regular patterns were generated on machined surface since constant cutting forces induced harmonic SV.In UPRM,quasi-regular patterns were produced since periodical impulsive cutting forces caused impulsive SV.In UPG,partially random patterns were formed since random impulsive cutting forces partially random impulsive SV.Results could be eventually achieved as follows: The SV system is linear and harmonic,which includes axial SV,radial SV,and coupled-tilting SV.Its dynamic responses are determined by the external excitation of cutting forces in UPM.The generalized dynamic model precisely describes the spindle vibration in the different ultra-precision machining processes mentioned above.Its theoretical analysis and experimental results of the effect on surface morphology are in good agreement.(2)With the effect of tool wear on surface generation in ultra-precision machining set as research object,machinable material,Zn-Al-Cu alloy,was milled with ultra-precision diamond tool.Phase changes of the machined Zn–Al–Cu alloy were detected by back-SEM(BSEM)and XRD techniques,nano-hardness was tested by nano-indentation,DTW characteristics was observed by SEM,surface morphology and roughness were measured by optical analysis and measurement system.The results showed that quenching and phase decomposition(namely twin phase changes)occurred simultaneously in UPRM of Zn-Al-Cu alloy under high speed cutting.External stress caused by UPRM results in phase changes,which furthermore changed material properties.The deformed thickness was even less than 100 nm under high speed cutting;flake wear of diamond tool enhanced phase changes,promoted surface hardening and increased deformation thickness,thus deteriorating surface quality,because flake wear of diamond tool caused higher external stress.On the contrary,rake wear showed the opposite situation.It could be verified in the finite element simulation in orthogonal cutting presented in this paper.(3)With the effect of ultra-precision processing parameters and material properties on surface roughness formation set as research object,,the formation mechanism of surface roughness in UPG of metallic glass was revealed,based on the UPG experiments conducted on hard material,Zr-based mental glass.A series of ultra-precision grinding experiments on Zr-based metallic glass were carried out on Precitech's Freeform L-type ultra-precision five-axis profile machine tool.The material removal mechanism of metallic glass grinding process was studied through plunge grinding.On this basis,a series of surface grinding on zirconium-based metallic glasses were carried out by orthogonal method under different cutting parameters.The surface morphology was measured and analyzed by laser interferometer,X-ray diffraction(XRD),atomic force microscopy(AFM).All the grinding experiments were conducted with coolant.XRD results showed that with coolant using in machining process,surface crystallization was absent from the machined material when zirconium-based metallic glass was machined with coolant during the ultra-precision grinding process,which was an important prerequisite to ensure the grindability of the Zr-based metallic glass.Plunge grinding clearly revealed that there involves material removal mode in the grinding of metal glass which include the ductile removal pattern and the brittle removal pattern.Critical cutting depth was the key parameter for the transformation of the two removal mechanisms;compared to the general grinding method reported in the literature,the surface roughness under the ultra-precision grinding of Zr-based BGM is greatly optimized,and its accuracy could reach an order of magnitude higher than 30 nm.The influence of cutting parameters on surface roughness formation during machining obtained through orthogonal experiment could provide technical and process reference for further improving the surface accuracy of ultra-precision grinding of metallic glass. |
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