The microscopic unevenness of the ground surface depends mainly on the following three aspects:
1 The residual peaks and troughs of the abrasive grain scratches are mainly affected by the geometrical relationship (grain size, abrasive grain distribution, height deviation, etc.) of the abrasive grains on the abrasive belt.
2 Vibration of the process system.
3 Metal deformation and micro-melting under the action of grinding temperature. These micro-melted and softened metals are applied to the machined surface during cutting or extrusion to increase the roughness.
At the same grinding speed, as the particle size becomes finer, the roughness is lowered because the scratches and ridges generated in the fine abrasive grain processing are correspondingly small. In addition, the abrasive grains of the same particle size have different roughness during different life periods.
The flexibility of the abrasive belt substrate also has an effect on the roughness. The greater the flexibility, the lower the roughness achieved by the abrasive belt. The Ra value processed with the abrasive tape of the animal adhesive is smaller than that of the synthetic resin adhesive tape. Thickening and stiffening of the belt joint will also increase the roughness. Therefore, it is better to use a soft, flat and uniform thickness of the abrasive belt for fine polishing.
Impact of the contact wheel
The hardness of the contact wheel (ie Shore hardness) is linear with the roughness of the abrasive belt grinding. The harder the contact wheel, the greater the surface roughness of the machined surface; the smaller the contrary.
In general, the larger the diameter of the contact wheel, the larger the contact area during processing and the smaller the surface roughness of the machined surface. The effect of the groove on the surface of the contact wheel, the angle and size of the groove on the roughness is: the smooth surface contact wheel has less roughness than the part processed by the grooved surface contact wheel, and the imbalance of the contact wheel and the roundness error will cause The vibration during grinding increases the surface roughness of the machined surface, so the dynamic balance of the contact wheel and the outer circle dressing are important measures for obtaining a low-roughness machined surface.
Effect of grinding fluid
The use of grinding fluid is one of the important means to improve the surface roughness of abrasive belt grinding. The lower the viscosity of the lubricating oil, the lower the surface roughness obtained. Grinding fluids containing oils are more advantageous than aqueous solutions for reducing the roughness. At lower belt speeds, there is little difference in the surface roughness of the different grinding fluids. However, when the belt speed is increased, the effect of the grinding fluid on the surface roughness is very significant. Edible oil is more advantageous for improving roughness than other synthetic fluids.
Effect of grinding amount
The amount of grinding is not only related to the grinding efficiency of the abrasive belt, but also has a great influence on the surface roughness of the machined surface. By selecting the appropriate amount of grinding, not only can a lower surface roughness be obtained, but also a combination of higher grinding accuracy and higher production efficiency can be obtained. The grinding amount mainly refers to the belt speed, workpiece speed, late depth, feed rate and the like.
As the speed of the abrasive belt increases, the number of abrasive grains entering the grinding zone increases per unit time, the cutting depth of a single abrasive grain becomes smaller, the deformation of the metal surface is small, and the contact time between the workpiece and the abrasive grain is shortened, and the metal grinding is reduced. The height of the bulge formed by the plough and the thermoplastic deformation, and thus the roughness value is reduced.
As the feed rate increases, the system vibration increases, and the surface area of Cao Liu increases, resulting in an increase in the surface roughness of the machined surface. When the feed rate reaches a certain level, the surface roughness increases sharply with the increase of the feed rate, so the belt must use a small feed amount during the fine grinding.
The effect of grinding depth on surface roughness is relatively small. As the grinding depth increases, the roughness increases slightly. This is because the roughness value is much smaller than the grinding depth, but the increased grinding depth increases the wear of the belt, which reduces the grinding ability and causes the system to vibrate and the roughness increases.
The effect of the change in workpiece speed on the roughness is also similar to the effect of the grinding depth. When the workpiece speed is low, the roughness value is small.
In summary, the choice of abrasive belt grinding should take into account the requirements of both grinding efficiency and surface quality.
The difference in grinding methods also affects the surface roughness. The smoothing is smaller than the roughness of the back grinding, and the free grinding is smaller than the contact wheel grinding.
The sand density of the abrasive belt abrasives also has an effect on the surface roughness, which is similar to the effect of the abrasive belt grinding speed. The densely packed abrasive belt of the same particle size has a smaller surface roughness than the spalled abrasive belt.
In terms of machine tools, the stability of the entire grinding process system, the grinding head rotation accuracy and dynamic balance are all prerequisites for the lower roughness of the belt grinding. This is the first consideration that must be considered when selecting and using abrasive belt grinding.
In addition, the lateral vibration of the belt during the grinding process also has a significant effect on the surface roughness. As the vibration frequency increases, the surface roughness decreases. This is because the lateral vibration causes the surface of the abrasive belt to form a complex and non-repetitive grinding path, that is, a mesh pattern, which is advantageous for reducing the surface roughness. Normally, after adding lateral vibration, the surface roughness can be reduced by about 30%.
In summary, a single-layer abrasive belt cannot change its cutting edge shape and performance by modification, but can only control its surface roughness in a relatively small range. Therefore, when selecting the abrasive belt, the surface roughness should be improved step by step according to the original surface roughness before the workpiece processing and the final required roughness, with different abrasive belt combinations. This is one of the effective ways to improve the surface quality without affecting the processing efficiency. The sand belt preparation of the multi-grinding belt grinding machine is carried out according to this principle.