| With the ever-increasing application and popularization of diamond turning technology, the resultant accuracy achieved with diamond turning is approaching towards the nanometric level or even more less, namely the extreme scale. For diamond turning as well known, to realize the nanometer or even sub-nanometer machining accuracy requires the ultra-precision machine tool, the associated high-resolution metrology equipments and the super-stable environment, whilst the high-accuracy diamond cutting tool is also quite crucial, especially for the rounded diamond cutting tool. Advantaged in the cost-effective setups and the simplified processes, the mechanical lapping is a conventional but the most popular method to fabricate diamond cutting tool. Resorting to the state-of-the-art of mechanical lapping, however, the cutting edge radius, namely tool sharpness, can only be sharpened down to about 70~80nm, which is far from the meeting to the development of diamond turning technology. Obviously, the 70~80nm's bottleneck has restricted the development of diamond turning technology to great extent. And as a result, how to break through such bottleneck so as to achieve a cheaper tool with a cutting edge radius of less than 50nm or even less as finished has been an urgent and challenging problem in current diamond turning field.Therefore in present work, with a view to obtain the high-accuracy rounded diamond cutting tool by means of mechanical lapping, a brittle-ductile transition theory is proposed primarily to account for the lapping mechanism of crystal diamond. As expected, a theoretical model is constructed to calculate the anisotropically critical depths of cut, which act as the switches of the dynamic brittle-ductile transition of lapped crystal diamond in different orientations and on different planes. According to the lapping theory proposed above, the material removal mechanism of lapped crystal diamond is explained systematically in both'soft'and'hard'directions. And additionally, the anisotropism of the material removal rate of crystal diamond induced by mechanical lapping is compared quantitatively with respect to the anisotropism of dynamic critical depth of cut. In terms of the dynamic critical depths of cut deduced from brittle-ductile transition theory, the lapping parameters affecting the finished cutting edge radius are analyzed in detail secondly. As a result the mechanical lapping procedure enabling of achieving a tool with a cutting edge radius of 30~55nm is established. Furthermore, the time series analysis method is introduced into this work to model the cutting edge radii's changing rules dependent on lapping time. Finally, a time series prediction model that couples with the exponential function and autoregressive equation is developed to describe the changing laws. Using this model as expected, the lapping time can be determined accurately with respect to the consumer-required cutting edge radius, which visibly avoids the unwanted over-lapping or under-lapping and reduces the expensive inspections on cutting edge. Then the production efficiency is improved and the cost is cut down in return.According to the brittle-ductile transition lapping theory thirdly, the dynamic micro mechanical strengths of crystal diamond are deduced in different directions and on different planes, including the tensile, shearing and compressive strengths. Comparing the dynamic micro mechanical strengths indicates the dynamic micro tensile strength in the'soft'direction should be employed as the designing criterion for the crystal orientation on rake and flank faces of diamond cutting tool regarding the most sharpened cutting edge to be finished, and the tensile one in the'hard'direction, however, should be selected as the designing criterion for the rake and flank faces orientation regarding the strongest resistance to wear.Moreover, being dependent on the dynamic micro mechanical strengths, the impact stresses around the cutting edge during dynamic mechanical lapping are analyzed, and therefore the ultimate sharpness of diamond cutting tool is deduced in theory, i.e. that while the (110) plane or (100) plane is oriented as the rake face, the cutting edge radius can be sharpened down to 1~6nm and 2~5nm, respectively. In terms of the boundary conditions that assumed in the theoretical deduction, an elegant lapping method, namely thermo-mechanical coupled lapping process, is developed. By this means, the validation experiments are performed finally with a tool that orients the (110) plane as rake face and the (100) plane as flank face. And the results imply the cutting edge radius has been sharpened down to 2~9nm, which is well consistent with the ultimate value deduced theoretically. Through a large number of diamond turning tests on silicon wafers fifthly, a novelty finding is demonstrated that the groove wearing mark on tool flank face contributes to the heavy wearing process. The followed in-depth investigations indicate the generation of groove wear is due to the formation of micro super hard particles, i.e. the silicon carbide and diamond-like carbon, which in return have the enough hardness to scratch and plough on flank face so as to form the groove wearing marks.In the last section, in order to optimize the geometry of diamond cutting tool, finite element simulation and general rotatory combination design methods are employed to model the quadratic regression equations of tensile and compressive residual stresses, which generate within the near surface layer of OFHC copper during machining and are considered as a function of tool geometry. And then the related analyses are carried out for tool geometries that influence the residual stresses. The results indicate that when cutting edge radius ranges from 100nm to 300nm, cutting velocity ranges from 2m/s to 10m/s, rake angle ranges from -15°to 15°and flank angle ranges from 2°to 10°, a rake angle of 5°, a clearance angle of 10°and a sharpened as possible cutting edge radius are the optimum geometry for a diamond cutting tool to cut the ductile materials.
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