| The pipe thread comb cutter is a necessary tool for machining high-precision oil casing threads.The tooth profile error of the comb directly affects the sealing performance of the threaded connection of the oil casing.The coarse bronze bonded diamond wheel is often used to process the comb cutter.It is very difficult to dress the formed wheel after the initial machining or blunt grinding.In order to solve the problems of low dressing efficiency,poor dressing accuracy and surface morphology of coarse-grained bronze bonded diamond wheel,a composite dressing method of laser rough truing,electrical discharge grinding semi precision truing,roller grinding precision truing and silica powder mixed electrical discharge dressing method were proposed.The corresponding dressing experiment platform was built,and the precision dressing experiment of formed diamond wheel was carried out systematically.The composite dressing technology has been deeply studied from the following aspects: material removal mechanism,numerical calculation analysis,process parameter optimization,experimental comparison and exploration.The specific research content of this paper includes the following five aspects.1.Aiming at the laser tangential incidence truing of arc-shaped diamond grinding wheel,the material removal mechanism of laser ablation and the main factors affecting the profile accuracy of arc-shaped diamond wheel are analyzed.Then,the shape and energy distribution of laser beam are theoretically modeled and analyzed to reveal the cause of "light blocking" phenomenon in truing.Taking the roundness error of the arc-shaped grinding wheel as the optimization objective,the kinematic parameters of the machine tool in truing are optimized.In the truing experiment of the arc-shaped grinding wheel with a radius of 13 mm,the final arc-shaped profile radius obtained after laser truing is 13.133 mm,the dimensional error is 113 μm,and the roundness error is 19.34 μm.The larger size error is caused by the "light blocking" phenomenon in truing.The results of Raman detection of the abrasive particles on the surface of the grinding wheel show that the laser truing results in a certain degree of graphitization.2.It is the first time to use electrical discharge grinding(EDG)method to dressing metal bonded diamond wheel.The material removal mechanism of EDG dressing is deeply analyzed.A three-dimensional heat conduction model was established,and the temperature field distribution of bronze and diamond in the single pulse discharge was obtained,and the removal amount of bronze bond after the single pulse discharge was simulated.The thermal effect of electrical discharge on diamond abrasive particles is analyzed,and the thermal softening depth of diamond abrasive particles is used to guide the selection of grinding depth in dressing experiment.The influence of kinematic parameters(wheel speed,tool wheel speed,feed speed,grinding depth)on grinding wheel dressing profile accuracy and tool wheel wear depth in EDG dressing is investigated.The PV value of grinding wheel profile error obtained by rough dressing is 25.43 μm.The PV value of grinding wheel profile error obtained by precision dressing is 11.97 μm.After dressing,the abrasive grains on the surface of the grinding wheel have been partially removed and have good contour.3.The tangential oblique line interpolation rotation method was used to dress the small circular arc diamond roller.By analyzing the dressing mechanism of this method,it is found that the size of the dressing roller arc-shaped profile can be adjusted by controlling the feed rate of the tool wheel.The prediction models of B-axis rotation center error and U/V axis tool setting error on roller arc profile error are established,and the relationship between profile error and dressing results is revealed.The dressing process of small circular arc roller with radius of 0.18 mm is explored,and the segmental circular arc compensation dressing method is proposed to improve the dressing accuracy of circular arc profile.The experimental results show that the roundness error has been reduced from 19 μm to 4 μm,and the dimensional error has also been reduced from 13 μm to 1 μm.4.A compound truing method of laser rough truing,electrical discharge grinding semi-precision truing,roller grinding precision truing is proposed and tested,and a concave wheel profile with high precision is obtained.A truing scheme is proposed to remove the rectangle first and then the whole profile layer by layer,which is used in laser rough truing.Affected by the "light blocking" in laser truing,the maximum error of oblique line profile reaches 462 μm.In the semi precision truing of EDG,the truing scheme of first segmenting,then removing layer by layer by side is adopted.After semi-precision truing,the angle error of oblique line profile is within 1°3’ and the dimension error of arc is within 60 μm.The profile precision of the concave grinding wheel is higher after the roller grinding method is used.The overlap between the oblique line and the circular arc has a good transition,and the dimensional errors of the oblique line and the circular arc are within the allowable error range.However,there is almost no protruding height of the abrasive grains after precision truing,so it is necessary to continue the sharpening to give full play to the excellent grinding performance of the concave wheel.5.The sharpening effect of powder mixed electrical discharge technology on coarse-grained diamond grinding wheel was investigated.The effects of compressed air(gas phase state),deionized water mist(gas-liquid two-phase mixed state)and mixed powder atomizing medium(gas-liquid solid three-phase mixed state)on the discharge process are compared and explored,and the relationship between the discharge gap is verified by experiments.The results show that the discharge gap in the mixed powder atomizing medium is the largest,and the medium is easier to be broken down,which is beneficial to the improvement of grinding wheel dressing efficiency.Then,the diamond grinding wheel dressing experiments in different discharge mediums were carried out,and it was found that the dressing effect was the best in the mixed powder atomizing medium.The dressing efficiency in mixed powder atomizing medium is 30.43% and 114.29% higher than that in deionized water mist and compressed air respectively.The lowest surface roughness can be obtained by grinding the workpiece with the grinding wheel sharpened in the mixed powder atomizing medium.The concave diamond wheel after compound truing is subject to sharpening test in powder mixed atomizing medium.The average protrusion height of the abrasive particles on the surface of the wheel after dressing is 26.42 μm,which can play a good role in the grinding performance of the wheel.The grinding comb blade has been tested to meet its design size and quality requirements. |
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