The grinding crack (black spot) produced by the vibration grinder is not formed by sudden break, but appears sporadically on the surface of the workpiece. Although the grinding cracks, the novice is still indistinguishable. The crack of grinding fluid treated with special method is not deep, the general depth is only 0.05~0.25mm.
The causes of crack in vibration grinding machine may have the following kinds: artifacts have internal stress exceeds the limit of fracture surface layer, the grinding or heat treatment and machining because the residual mechanical stress and thermal stress in the surface part. Due to the grinding of the stress which has just been well balanced, the residual stress exceeds the strength of the workpiece, and the grinding cracks are produced by some.
Among all the reasons, "grinding cracks" is the key to the problem. The larger problem is the stress caused by the heat of the vibration grinding machine. Because of the grinding heat, the local temperature on the surface of the workpiece rises rapidly, and this part will be tempered or other heat treatment. Due to the change of internal structure and surface shrinkage, cracks are produced under the action of tensile stress.
1. Examples of the relationship between the feed amount and residual stress of the grinding wheel.
The tensile stress will gradually increase with the increase of the feed force of the grinding wheel, which will gradually approach the tensile strength of the workpiece material. When the tensile strength of the material is exceeded, the vibration grinding machine will crack.
(2) the compressive stress will not change, because the scale and the experimental conditions of different so can't compare, but almost unchanged is turning back to 0.05 mm, residual tensile stress is larger, even if it cuts deep residual tensile stress to also won't big is too big. It is generally believed that this is the cause of the grinding.
2. An example of the residual stress after grinding is measured by changing the feed amount of the grinding wheel of the vibration grinding machine.
The larger the feed amount of the grinding wheel, the deeper the residual stress is.
(2) the surface residual stress as the tensile stress in the direction of the effect of grinding at the same time, also can be in the form of pressure acting on the grinding direction of vertical direction, and the more deep, to internal stress will be sharply reduced.
In the direction of grinding and the vertical direction, it first becomes the compressive stress and then suddenly becomes the same tensile stress as the grinding direction. When it reaches a larger value, it gradually decreases and eventually becomes a small compressive stress.
The hardness of the grinding wheel of the vibration grinding machine is related to the tensile strength of the grinding wheel, and the hardness is higher in G, H, I and J. The higher the hardness is, the greater the residual stress is.
The speed of the grinding wheel (circular velocity) affects the residual stress. Once the speed is over 1500m/min, the residual stress will increase sharply.
In addition, because of the different materials of the workpiece, there is also the difference between the cracking of the vibration grinding machine and the grinding crack.