Experimental Study On Vitrified Bond CBN Wheel Based On Quick-point Grinding

Ultra-high speed grinding is regarded as "the highest peak of modern grinding technology" and listed as one of main development directions in 21st century. In 1994, ultra-high speed point grinding is developed by Junker Company in Germany. It is a new application form of ultra-high speed grinding. Ultra-high speed point grinding is an advanced processing technique which includes flexible numerical control machining technology, CBN super-hard abrasives and ultra-high speed grinding and so on. Vitrified bond CBN wheel is an important part of quick-point grinding, and called as high performance grinding wheel. Its characteristics are high speed, high efficient, high precision, and low cost, low environment pollution. Now, it is a development focus all over the world.In the paper, by using grinding mechanism of ultra-high speed point grinding and structural characteristics of ultra-high speed vitrified CBN wheel, and according to grinding theory and preparation principle of compose material, the key technologies of all parts of CBN wheel and manufacturing processes are systematically studied. Eventually, a vitrified bond CBN wheel is developed. Grinding performance of vitrified bond CBN wheel is tested on experiment table of quick-point grinding. The main works of the paper includes the following aspects:(1) The characteristics, key technologies and development status at home and abroad of vitrified bond CBN wheel and quick-point grinding are discussed. Research background and meaning of the thesis are introduced.(2) By undeformed chip thickness and contact arc length between grinding wheel and workpiece, the paper discusses grinding mechanism of ultra-high speed point grinding based on variable angleα&β. The theory mathematics models such as ultra-high speed point grinding force, grinding temperature and surface roughness are educed according to ultra-high speed point grinding mechanism. By MATLAB simulation, the effect of variable angleα&βand some grinding parameters on grinding force, temperature and surface roughness were analyzed.(3) Considering the security of CBN wheel, stress distribution characteristics of high speed and ultra-high speed grinding wheel are presented. Taking the distributing radial expansion volume of wheel base as objective function, selection of wheel hub shape, material, size and optimization of section shape are analyzed by using ANSYS software.(4) In order to develop an excellent ultra-high speed point grinding wheel, the paper does some comparative experiments for five kinds of home-made CBN abrasives and three kinds of foreign CBN abrasives, and analyzes their normal temperature characteristics, high temperature characteristics and resistant to corrosion. Different abrasive materials and grain sizes are tentatively selected to meet different workpiece material and machining requirements. Three kinds of vitrified bonds are configured based on request of low temperature high intensity vitrified bonds. These bonds’component, refractoriness, coefficient of thermal expansion, strength, mineral phase and micro-topography are verified by experiments. At last, the kind of low-temperature high-strength vitrified bond is selected.(5) The segment of CBN wheel plays the most important role on grinding wheel performance. Thus, considering the strength of the segment and its forming process, the size of wheel segment of ultra-high speed point grinding wheel withΦ370mm,vs=200m/s is optimized by ANSYS software. Considering the security of wheel, the minimum segment strength is obtained according to strength of segment optimized. Afterward, the wheel sample configured by different abrasives and bonds selected before are sintered. By the measurements of strength, grain size, concentration and micro-topography of the abrasive and bond are determined for samples with high strength.(6) Based on the experiment research before, the ultra-high speed point grinding CBN wheel is made according to its manufacture techniques. And the performances of the CBN wheel are tested on NEU ultra-high speed point grinding test-bed and high speed camshaft grinder made by Beijing Mechanism College. Experimentation shows that point grinding force decreases with the wheel speed increases, and with workpiece speed, grinding depth, longitudinal feeding speed increases, the grinding force increases too; the grinding temperature decreases with the wheel speed, and with workpiece speed, grinding depth, longitudinal feeding speed increases, the grinding temperature increases too; the grinding forece and temperature decreases with level variable angle increases, but surface roughness decreases. Whenα&β=±5°, although the surface roughness value (Ra) increases, but surface of workpiece present residual compress stress and without white layer and phase transformations; whenα&β=±0°, although the surface roughness value(Ra) decreases, but surface of workpiece present residual tensile stress and having white layer and phase transformations. When taking workpiece surface roughness value Ra≤0.4μm and workpiece surface without residual stress as grinding blunt standard, the grinding ratio (G) of ultra-high speed point grinding wheel is 170 in experiment. The performance of the wheel approach importation grinding wheel level in production.(7) The paper analyzed wear mechanism of ultra-high vitrified bond CBN wheel. Dressing experimentation of vitrified bond CBN wheel is completed. The experimentation indicates that wheel topography is best when velocity ratio j=0.428 between the diamond rotary and CBN wheel.