Study On Form Accuracy And Surface Quality Of Hard Sphere Grinding

Metallic hard sealing spherical valve is most widely used in petroleumchemical engineering, coal liquification engineering, nuclear powerengineering and other important industrial areas. This spherical valve mustresist high temperature, high pressure, corrosion and wear for bad workingconditions. As a key part of the metallic hard sealing spherical valve, thesphere must have a high hardness (HRC≥60) and high precision to work inharsh environment. The sphericity error of the sphere SΦ≤0.02mm (DN≤12in.), and the sphere surface roughness Ra≤0.2m. To obtain a high surfacehardness, WC Co, Ni60, STL20and other hard alloys are sprayed onto thesphere substrate to reach a coating depth of0.40.6mm. The sphere must beground after coating to meet the requirements of the form accuracy andsurface roughness. In this dissertation, the error of non swing and swingsphere grinding machine which will affect the sphere form error is studied;secondly, the surface roughness and texture is analyzed; thirdly, the surfacedamage is studied; finally, the grinding parameter of hard sphere grinding isoptimized by NSGA Ⅱ genetic algorithm.In the grinding process by using non swing sphere grinding machine(NSSGM), the normal vertical error between the cup wheel rotational axisand sphere center will cause the sphere shape appears low in middle part andhigh in around of the mid concaved shape. An adjustable depth of cut spheregrinding (ADCSG) is introduced based on swing sphere grinding machine(SSGM) to reduce this form error. The sphericity error can reduce to11μmby using ADCSG. In the grinding process by using SSGM, the horizontallongitudinal error between the rotational axis and sphere center will cause thesphere shape appears high in middle part and low in around of themid convexed shape. A back up sphere grinding is proposed for the sphericalshape like mid convexed, and the sphericity error can reduce to9μm byusing this method. NSSGM and SSGM surface texture model are built by using coordinatetransformation. The effect of grinding parameters to surface texture andsurface roughness is analyzed by simulation, and the modeling andsimulation results are tested through surface roughness experiment. Thesimulation and experimental results showed main speed ratio k (k=ns/nw) ofsphere grinding using NSSGM should not to be an integer. The density oftrajectories increases with the increase of k, and the surface roughnessdecreases with the increase of k. When the grinding speed ratio k of SSGM isequal to Z_k+0.5(Z_kis an integer), the grinding stripes in the two directionswill cross, and the surface roughness is minimum.A dynamic grinding force threshold based fuzzy adaptive controlalgorithm (DFFC) is introduced to avoid the workpiece to be scratched.Surface grinding burn and crack are researched by using SEM. The influenceof grinding parameters on the surface grinding burn is analyzed. Workpiecematerial is mainly to be removed by brittle fracture. There is not large andlong macroscopic crack on the workpiece surface, but there are a lot ofmicrocracks. Surface residual stress is measured by X ray stress analyzer. Itis show that workpiece surface residual stress is compressive residual stress,and it is between300to500MPa. This can benefit for the sphere to be used.Hard sphere grinding parameters is optimized by NSGA Ⅱ geneticalgorithm for the maximum material removal rate and minimum surfaceroughness as target and n_s,n_w,a_pand v_bas variables. The experimental resultsshowed NSGA Ⅱ genetic algorithm can make the material removal rateincreases to55mm~3/min, and the surface roughness decreases to0.1μm. It isbetter than orthogonal experiment. NSGA Ⅱ genetic algorithm can quicklyand efficiently obtain high quality and inspiring optimized results. It caneffectively solve the optimization of the hard sphere grinding.