What are the causes of grinding burns?Focus on these four areas
In the machining process, the machining surface temperature will rise due to the cutting heat in the workpiece processing area.When the temperature exceeds the critical point of the microstructure change, the microstructure change will occur.For the general cutting, cutting heat is most of the chip away, the impact is not serious.
But for grinding and blind, because of its cutting heat generated per unit area dozens of times larger than the general machining, so the surface temperature can be as high as 1000 ℃ or so, will inevitably cause the change of the microstructure of surface layer, the surface hardness decreased, with residual tensile stress and cracks, which makes the service life of the workpiece greatly reduced, this phenomenon is called grinding burns.When grinding burns occur, the surface layer of the workpiece often appears yellow, brown, purple, blue and other burn colors, which are the oxide film color of the workpiece surface caused by instantaneous high temperature.
Factors affecting grinding burns include:
1. Grinding dosage
Mainly including grinding depth, workpiece longitudinal feed and workpiece speed.When the grinding depth increases, the indicated temperature of the workpiece and the temperature at different depths under the surface will increase, and the grinding burns will increase, so the grinding depth should not be too large;With the increase of the longitudinal feed of the workpiece, the contact time between the grinding wheel and the workpiece surface is reduced, the heat dissipation condition is improved, and the grinding burn is reduced.Although increasing the workpiece speed increases the temperature of the grinding area, but because the heat source action time decreases, the metallographic structure has no time to change, in general, it can reduce the grinding burns.
For the increase of roughness caused by increasing feed and workpiece speed, it is generally compensated by increasing the grinding wheel speed and wider grinding wheel.
2. Cooling method
The use of cutting fluid to remove the heat from grinding can avoid burns, but the current cooling method is less effective because the cutting fluid fails to enter the grinding area.
In order to allow the cutting fluid to enter the grinding zone and play a cooling role, the main methods used at present include internal cooling method, jet method, discontinuous grinding method and ancient oil sand wheel, etc.The internal cooling method is to introduce the cutting fluid into the central cavity of the grinding wheel through the hollow spindle of the grinding wheel. Due to the porosity of the grinding wheel, when the grinding wheel rotates at a high speed, the powerful centrifugal force will throw the cutting fluid around the gap of the grinding wheel, so that the grinding area can be cooled directly.
3. Workpiece materials
The higher the hardness of the workpiece material, the more heat generated by grinding;But the material is too soft, it is easy to plug the grinding wheel, but the surface temperature rose sharply.
The strength of workpiece material can be divided into high temperature strength and room temperature strength.The higher the high temperature strength, the more power consumed during grinding.In at room temperature, for example, the strength of the 45 steel than 20 crmo alloy steel of high strength 65 n/was, but at 600 ℃, the intensity of the latter is higher than the former 180 n/was, therefore, 20 crmo steel grinding heat is bigger than 45 steel
The greater the toughness of the workpiece material, the greater the grinding force required, and the more heat.
Materials with low thermal conductivity, such as bearing steel and high speed steel, are more likely to produce metallographic changes in grinding.
4. Selection of grinding wheel
The grinding wheel with too high hardness has too strong binding force and poor self-sharpening, which will increase the grinding force and easily cause grinding burns. Therefore, the softer grinding wheel is often selected.Improve the hardness, toughness and strength of grinding wheel particles, help to maintain the sharpness and self-sharpening of the blade tip, so as to prevent grinding burns.Diamond abrasive because of its strength, hardness are relatively high, and in the case of no cutting fluid, its friction coefficient is only 0.05, relatively speaking, the most difficult to produce grinding burn, is an ideal abrasive.
Grinding wheel binders shall be of certain elastic materials, such as resins.In this way, when the grinding force is increased for some reason, the abrasive particles can produce certain elastic concessions and reduce the cutting depth.At the same time, due to the poor heat resistance of the resin, the bonding performance decreased significantly at high temperature, and the abrasive particles were easy to fall off.These all help to avoid grinding burns.
Coarse - grained grinding wheel can not only reduce the heat output, but also avoid the blockage of grinding wheel when grinding soft and plastic materials.