Study On Ultra-precision Turning Technology Of Cleartran ZnS Crystals

In recent years, with the ever-increasing application or requirements in space science and technology, the national defense and military fields, they have been of particular concern in many countries that the preparation and properties of infrared materials and the optical components manufacturing etc. In addition, these research achievements and infrared optical products will effectively promote the development of civilian technology, like computers and optical communication industry, so that they can bring more broad application and enormous economic benefits. However, since the single point diamond turning technology(SPDT) becomes widely used in engineering, people are eager to adopt such a precision, high efficiency, low cost and simple processing method to fabricate a variety of complex optical components, and then to make up for the weakness of traditional grinding and polishing process. Allegedly the infrared materials like Zinc sulfide(Zn S) crystal currently has been able to conduct a more sophisticated manufacture by SPDT in other country. But in view of its important military scientific value, the process has heavily security features, and then the specific forms, equipments and conditions of processing and other detail issues are reported more rarely. Therefore, we have to solve these materials independently key technical problems in their ultra precision machining. And it has a very important theoretical and practical significance to explore a novel diamond turning approach, which is adapted to cutting process of the soft brittle polycrystalline material like Zn S crystals.Thus, in this work it is the core content to explore an effective method of SPDT of cleartran Zn S crystals and study its specific manufacturing process. The above contents are further introduced and summarized in detail as following:Firstly, not only study the mechanical properties and cone pit morphology of Zn S crystal by Vickers indentation tests, but also obtain the theoretical prediction values of critical load conditions when the material occurs crack propagation. In order to fulfill such modeling goal, the dimensional analysis and nano-indentation method are adopted, which is to establish the stress-strain curve of Zn S crystal based on the data of diamond indenter load-displacement relation. At last, we sum up lots of characteristic parameters by previous tests, which prepares the conditions for subsequent theoretical and simulation on the turning process of brittle materials.Secondly, we establish a newly developed CUCT prediction model of brittle material cutting based on independent oscillator(IO) model and equivalent crystal theory(ECT). Then, it is well verified by oblique fly-cutting experiments, in which the depth of scratching varies with different tool cutting position. The CUCT model reveals well the relationship between the tool edge radius and the minimum cutting thickness of brittle material. Furthermore, we also establish the ultra precision oblique turning model of brittle material by systematically analysis the process of diamond turning, in which they are all coupled with material properties, tool geometrical characteristics and machining parameters of oblique turning. In according to the newly proposed “brittle-ductile coupled material removal” critical cutting conditions, we predict the critical feed rate, cutting depth and tool rake angle, and prefer the nose radius and the oblique turning angle by the cutting model and brittle-ductile transition depth. Thus, the oblique diamond turning model provides a theoretical foundation for revealing the brittle-ductile mode removal mechanism of soft brittle materials.Thirdly, according to the constitutive relations of Zn S crystal from the results of nano-indentation tests, we perform the simulation analysis on brittle materials cutting process under conditions of dynamic impact by the nonlinear FEM software LS-DYNA. Then, it is revealed that the relationship between the process parameters and material removal performance based on the maximum shear stress theory and the results are in good agreement with the experiment observations. In addition, we also optimize the each process parameter by the specific method of SWOT, and obtain their order of priority about effect degree for surface quality, as follows: 1) rank angle of the tool 2) cutting depth 3) tool nose radius 4) oblique turning angle of the tool 5) tool feed rate. These valuable conclusions become the roots of following experimental design and the process optimization.Finally, In order to verify the foregoing theoretical and simulation results, a systematic study of the effect of various process conditions to improve the finished surface quality is conducted by single factor experimental method. Then, we summarize and analysis the optimum turning process for Zn S crystal according to the proposed optimization goals, which includes these main factors: suppressing or eliminating the pits and spots on the finished surface, reducing the tool wear and surface roughness. Furthermore, we have better completed the optimization process of the Zn S crystal in consideration of other factors, such as tool wear, impurities inner the crystal and choice of the cutting fluid etc. At last, an ultra smooth mirror(Sa=7.3nm) of Zn S crystal is reached successful by the optimum diamond turning process, which has been able to meet most of the engineering requirements.