In the conventional grinding process, the specific pressure and friction are large, the temperature of the grinding zone is high, and the surface of the workpiece is often subjected to residual tensile stress due to thermoplastic deformation. Rapid forced cooling of the surface of the material being processed can reduce the temperature of the ground surface on the one hand. Track Spot Light,Track Light Fixtures,Track Light Spotlight,Led Track Spotlight Foshan Brinno Technology Co.,Ltd , https://www.brinnolighting.com
Strong Cold Grinding Mechanism In the conventional grinding process, the specific pressure and friction are large, the grinding zone temperature is high, and the workpiece surface often generates residual tensile stress due to thermoplastic deformation. The rapid forced cooling of the surface of the material to be processed can reduce the temperature of the ground surface and reduce the plastic deformation and grinding burn phenomenon. On the other hand, a pre-formed cold shrinkage is formed on the surface of the material. The stress field, which is nonlinearly superimposed with the stress field formed by force and heat during grinding, suppresses the generation of residual tensile stress and can lead to the generation of residual compressive stress. Forced cooling grinding (referred to as forced cooling grinding) is based on this mechanism.
The strong cold grinding test uses liquid nitrogen as the cooling medium. The liquid nitrogen temperature is -176 ° C, which is a by-product of the chemical industry. It is non-toxic, non-polluting and easy to obtain. It is a good cooling medium. It is sprayed on the surface of the material to be processed, and a large amount of heat can be quickly absorbed by the volatilization of nitrogen, so that the surface temperature of the workpiece under the cooling field cover is drastically lowered, the surface of the workpiece is cooled and contracted, the brittleness of the workpiece material is increased, and the plastic deformation during processing is alleviated. . Compared with the case of conventional grinding, the effect of rapid forced cooling shrinkage during intense cold grinding is concentrated on the outermost layer of the material to be processed, and the inner material below the outermost layer is relatively cold and the cooling rate is slow. This results in uneven elastoplastic deformation of the surface of the workpiece relative to the inner layer. After the strong cooling effect is removed, the large relative temperature difference (temperature rise) causes the surface layer of the workpiece (relative to the inner layer) to expand, resulting in the generation of residual compressive stress on the machined surface. After conventional grinding, the surface temperature of the workpiece is lowered and contracted, resulting in the generation of residual tensile stress on the machined surface.
In the case of strong cold grinding, the direction and location of the liquid nitrogen injection may be the surface area to be processed and the processed surface area. The injection into the area to be processed is used to cool the surface layer of the material to be processed to form a cold-shrinkage prestressed field; the injection into the processed area accelerates the cooling process of the workpiece, so that the outermost layer is rapidly cooled and the inner layer forms a large gradient temperature field. . Both methods produce residual compressive stress due to surface expansion during subsequent recovery to normal temperature.
Strong cold grinding experiment
The degree of cooling of the surface layer of the workpiece and the state of the cold shrinkage prestressing state are related to the liquid nitrogen flow rate q, the relative moving speed of the workpiece and the liquid nitrogen nozzle (the workpiece moving speed vw in the test), the thermal characteristics of the material to be processed, and the like, and the flow rate q, Control of the speed vw and the amount of grinding to achieve active control of the residual stress state of the machined surface.
In the test of this paper, the liquid nitrogen flow rate is set to q=1000mm3/s (the liquid nitrogen flow rate should be set so that the heat absorbed by the fully vaporized surface of the action surface satisfies the requirements for cooling degree, and the minimum is also guaranteed to be absorbed. The heat is greater than the grinding heat). The liquid nitrogen nozzle has a rectangular shape of B×L=7×3 mm, which ensures a stable cooling field formed by the liquid nitrogen sprayed to cover the entire grinding area. The nozzle is as close as possible to the grinding area, and the closer the distance, the better the cooling effect. The moving speed of the nozzle relative to the surface of the workpiece affects the formation of the shrinkage prestressing field on the surface of the workpiece. There are different optimization values ​​for different processing materials and processing conditions. The moving speed of the workpiece is too small, the workpiece material is sufficiently cooled, and it cannot be formed in the surface layer. The cold shrinkage prestressing field of the layer; if the velocity vw is too large, the surface layer is too late to cool, and it is difficult to form a cooling field. In both cases, the control of residual stress on the machined surface cannot be achieved by strong cold grinding.
The test was carried out on a M7125 surface grinder using GZ60ZRA300×40×75 grinding wheel. The surface speed of the grinding wheel was vs=1320m/min, the moving speed of the workpiece was vw=7.2m/min, and the lateral feed rate was f=0.3~0.4mm/ Single stroke, grinding depth ap=0.04mm, grinding width b=7mm.
The test material was 45 steel (annealed state), and the test piece size was 100×100×15 mm rectangular plate.
Conclusion and analysis
1. Strong cold grinding can obtain residual compressive stress on the machined surface or reduce residual tensile stress.
2. The forced cooling of the workpiece material during the grinding process increases the brittleness of the material, reduces the plastic deformation of the material during the grinding process, and has a certain effect on reducing the surface roughness of the machined surface. The surface roughness Ra values ​​in Tests 1 and 2 were reduced from 0.22 μm and 1.1 μm to 0.15 μm and 0.80 μm, respectively. At the same time, the occurrence of grinding burns is also avoided.
3. The strong cold grinding process is simple and easy to use. The manual control of liquid nitrogen flow rate is difficult to control the cooling effect; using the sensor temperature measurement, the liquid nitrogen flow rate can be controlled by the temperature controller, and a good cooling effect can be achieved, thereby realizing the active control of the residual stress nature and size.
4. The environment around the cooling field also has an effect on the cooling effect. Keeping the air flowing relatively static can improve the cooling effect. However, the smoke generated by the vaporization of liquid nitrogen will affect the operator's observation of the grinding zone and should be eliminated in time.
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