Discussion on blockage of grinding wheel in grinding process
Grinding is a widely used metal cutting method, mainly processing hard materials that are difficult to cut with traditional tools and materials with high surface quality and dimensional accuracy requirements. With the emergence of a large number of new materials and the application of parts precision and quality requirements, the growth rate of grinding processing applications exceeds other traditional processing methods. In the grinding process, the size, shape and distribution of the abrasive particles play an important role in the process. However, when processing ductile metals, the grinding wheel will be blocked and passivated sharply, which will shorten the life of the grinding wheel. In order to avoid the blocking of the grinding wheel, passivation and adverse effects. It is very necessary to analyze the mechanism and cause of the blockage of the grinding wheel.
1. Formation of wear debris
The grinding process is to obtain a relatively high surface quality and accuracy by removing a certain amount of workpiece material. The commonly used tool is a grinding wheel. The grinding wheel is a loose body formed by compacting, drying and sintering abrasives and binders. A single abrasive grain is a tiny cutting edge with a large negative rake angle and a blunt edge radius. The high-speed abrasive particles cut into the workpiece after sliding and ploughing. The cutting layer material has obvious sliding along the shear surface to form short and thin chips, which are heated to a high temperature in the grinding zone , And then oxidized and melted, and solidified into a particle sphere, there are some branches on the surface of the sphere, which is a main form of wear debris. When grinding the acid-resistant stainless steel Cr20Ni24Si4Ti, a large number of spherical wear debris was found, along with band-shaped, nodular wear debris and ash. Many of these debris will fill the pores of the grinding wheel and adhere to the surrounding abrasives, causing the grinding wheel Clogging leads to decreased grinding accuracy, burns the workpiece, and shortens the life of the grinding wheel.
2. Analysis of the type and mechanism of wheel blockage
2.1 Types of wheel blockage
The types of wheel blockage include embedded type, adhesive type and mixed type. Embedded type plugging is the clogging state of abrasive debris in the gaps of the working surface of the grinding wheel; adhesive type plugging is the clogging state of abrasive particles and bonding agent; mixed type plugging is both embedded type and adhesive type. Blocked.
2.2 Mechanism analysis of embedded blockage
External factors: Grinding has a very important feature, generally Fy/Fz is greater than 2-10, the harder the workpiece material, the smaller the plasticity, the greater the ratio, and the grinding debris in the grinding zone will be affected by the strong positive pressure. The machine squeezed into the gap on the surface of the grinding wheel. The wear debris slides out along the front of the abrasive particles. There are several layers of wear debris accumulated in the local area in front of the abrasive particles. With the high-speed rotation of the grinding wheel, an airflow vortex area is formed behind the abrasive particles. The air pressure in the vortex area is significantly reduced. Under the action, part of the abrasive debris is attached to the back of the abrasive grains, forming a dependent blockage of the flank face of the abrasive grains, and most of the attachments are ash and particles.
The effect of the electrostatic field: a small electric field composed of the grinding wheel and the workpiece is formed in some small areas of the grinding area. Under the action of the electric field, some of the grinding debris will show polarity. According to the principle of opposite sex attraction, the polarity of the grinding wheel is opposite. The wear debris is adsorbed on the working surface of the grinding wheel. With the help of greater mechanical pressure between the grinding wheel and the workpiece, the grinding debris that has been adsorbed on the surface of the grinding wheel can be stably embedded between the gaps on the surface of the grinding wheel.
2.3 Mechanism analysis of adhesive blockage
Melting bonding: During the grinding process, most of the input power is converted into grinding heat, so that the grinding point temperature is as high as 1200k. The grinding debris is rapidly oxidized in air to form low-melting metal oxides, which are heated at high temperatures in the grinding zone. It is in a molten or micro-melted state, covering the surface of the grinding wheel. When this part of the surface of the grinding wheel participates in grinding again, it is squeezed or strengthened under the action of the grinding force, which increases the affinity and adhesion with the grinding wheel, and some are The extrusion adheres to the raised groove surface of the workpiece surface. Through multiple random grinding, many grinding debris adhere to the surrounding abrasive particles, which increases the grinding force and the temperature rises. This causes a vicious cycle and aggravates clogging until the abrasive particles break or fall off.
Chemical bonding: The chemical affinity between different elements is another important reason for the blocking of bonding. The abrasive particles and the material to be ground are in contact at high temperature. The temperature factor increases their mobility and affinity. When certain conditions are met, a chemical reaction is caused, causing the abrasive particles and debris to form a loss of cutting ability on the surface of the grinding wheel. Crystal.
3. The influence of the grinding wheel itself on clogging
3.1 Abrasive types
The degree of clogging of different grinding wheels is very different. From the perspective of reducing the degree of clogging and improving the grinding effect, different types of abrasives should be used for different workpiece materials. If the selected abrasive cannot adapt to the grinding performance of the workpiece material, it will easily cause a sharp blockage and make the processing unable to proceed normally. For example, when using corundum abrasives to grind iron-carbon alloys, carbon forms a thin oxide film with oxygen in the air, which can effectively prevent the chemical affinity between the workpiece and the abrasives. However, when grinding titanium alloys, blockages More serious. In some factories, the grinding wheel on the grinder is not changed for a long time and can grind everything, which seems to be economical and convenient, but in fact it loses efficiency and precision.
3.2 Abrasive particle size
The particle size of the abrasive has a certain effect on the clogging of the grinding wheel. Generally speaking, the fine particle size is easier to cause the clogging phenomenon than the coarse particle size. Because the pore volume and the cross-sectional area of the grinding debris of the fine-grained grinding wheel are small, the number of cutting edges of the fine-grained grinding wheel is increased, and the chips are also more. Coupled with the increase in the grinding temperature and other reasons, the fine-grained grinding wheel is easy to cut in the range of smaller cutting times. Blocked. With the increase in the number of cuts, the cutting depth of coarse-grained grinding wheels is larger than that of fine-grained grinding wheels, the abrasive cutting edge wear is greater, and the grinding temperature rises, and the chip fusion in the pores increases. After a certain number of times, the clogging amount of the coarse-grained grinding wheel will exceed the clogging amount of the fine-grained grinding wheel. For semi-finishing and fine grinding, the cutting amount is small, the temperature is low, and the blockage is light, so choose a fine grinding wheel; the rough grinding has a large cutting amount, high temperature, and there are more debris and sintered materials blocking the gap. Choose a coarse grinding wheel.
3.3 Hardness of grinding wheel
The hardness of the grinding wheel refers to the difficulty of the abrasive particles falling off, which is guaranteed by the strength of the binder. The higher the strength of the binder, the greater the hardness of the grinding wheel, and the more blunt abrasive grains will wear. The more severe the friction and extrusion of the workpiece before the abrasive grains fall off, the more easily the abrasive particles can be mechanically filled into the gaps of the grinding wheel. At the same time, it is accompanied by more frictional heat, which provides a sinter for the adhesive blockage. Therefore, the hardness of the grinding wheel has a greater influence on the amount of clogging. The harder the grinding wheel, the greater the amount of clogging. Under normal circumstances, the hardness of the grinding wheel is selected as G?H, in some difficult-to-machine materials, the hardness of D?0 is also adopted.
3.4 Grinding wheel organization
The structure of the grinding wheel reflects the proportional relationship between the abrasive, the binder and the pores. The denser the structure of the grinding wheel, the greater the number of abrasive particles in the work, the smaller the distance between the cutting edges, and the easier the grinding wheel will be blocked. The average blockage of the grinding wheel with 45% abrasive is half less than the average blockage with 49.2% abrasive; the blockage with the grinding wheel with 53% abrasive is twice as high as that of the grinding wheel with 49.2% abrasive. When grinding difficult-to-machine materials, you should choose the grinding wheels with organization No. 7 to 9. The big hole grinding wheel has better effect.
4. Influence of grinding conditions
4.1 Grinding wheel linear speed
The increase in the linear speed of the grinding wheel reduces the maximum depth of cut of the abrasive particles, reduces the cross-sectional area of the chip, and increases the number of cuts and the heat of grinding. Both factors increase the amount of blockage, but when the linear speed of the grinding wheel reaches a certain level (such as Up to 50m/s or more) The clogging amount of the grinding wheel is greatly reduced. When grinding stainless steel and high-temperature alloys in production, the 50m/s grinding wheel speed is 30% to 100% less than the 30m/s grinding wheel. Therefore, when grinding difficult-to-grind materials, either a speed lower than 20m/s or a speed higher than 50m/s is used. The grinding speed selected between them is very unfavorable to the blockage of the grinding wheel. For various workpiece materials, each has a certain critical grinding wheel speed value with a small blockage.
4.2 Workpiece speed
The influence of the speed of the workpiece on the degree of blockage of the grinding wheel is closely related to other factors in the cutting conditions. The linear speed of the workpiece is doubled, and the blockage of the grinding wheel is tripled. This is because the higher the workpiece speed, the shallower the penetration depth of the abrasive grains, and the smaller the cross-sectional area of the chips, which is equivalent to the hardening of the grinding wheel, which is likely to cause the grinding wheel to block.
4.3 Grinding method
Pitch grinding is more clogged than longitudinal grinding. When cutting into grinding, the contact area between the grinding wheel and the workpiece is large, and the abrasive cutting edge has to be rubbed several times on the same wear mark. It is difficult for the grinding fluid to enter the grinding area, and the heat is high during grinding, which is easy to cause blockage. The side edge of the grinding wheel that is in contact with the workpiece material during longitudinal grinding, when the wear surface increases to a certain extent, the abrasive grains are broken and fractured under the action of the grinding force to realize self-sharpening. Most abrasive particles can work in a sharp state, so that the grinding force and grinding heat are relatively low. At the same time, a considerable part of the area affected by the grinding force and grinding heat can be discharged out of the workpiece along the longitudinal grinding direction, so the possibility of chemical adhesion is reduced.
4.4 Radial cutting amount
The influence of the radial cutting amount on the blockage of the grinding wheel shows a hump trend. When the radial cutting amount is small (ap<0.01mm), clogging occurs. As the cutting amount increases, the average clogging amount also increases. When the cutting amount increases to a certain extent (ap=0.03mm), the clogging amount is again It shows a decreasing trend, and then as the cutting volume continues to increase (reaching ap=0.04mm), the blockage volume rises sharply. When grinding hard-to-grind materials, controlling the last radial cut is essential to improve the surface quality and accuracy of the workpiece.
4.5 Grinding temperature
During grinding, all factors that increase the grinding heat and increase the grinding temperature will aggravate the blockage of the grinding wheel. The main form of blockage is adhesive blockage, and of course it is also accompanied by diffusion blockage.
4.6 Grinding wheel dressing speed
The dressing speed of the grinding wheel has a significant effect on the clogging. When the dressing speed of the grinding wheel is low, the working surface of the grinding wheel is flat, and the effective number of sharpening per unit area increases, so that the cross-sectional area of the chip becomes smaller and the cutting quantity increases, so it is easy to cause clogging. When the dressing speed of the grinding wheel is high, the working surface of the grinding wheel becomes thicker, the number of effective abrasive particles decreases, and there are recesses on the surface of the grinding wheel, which play the role of pores. The chips are easily washed away and the sintered material is easy to fall.
4.7 Grinding fluid
Different grinding fluids have a great influence on the grinding effect. At present, the common emulsion contains a large amount of mineral oil and oily additives. After dilution, it becomes an oil-in-water milky white liquid. Its specific heat capacity and thermal conductivity are small. It is easy to cause adhesive wear and diffuse wear between the grinding wheel and the workpiece, block the grinding wheel, increase the grinding force, and finally cause premature crushing and shedding of abrasive particles, which reduces the grinding ratio. The selection of fine grinding fluid plays an important role in improving the grinding performance.
Grinding wheel blockage is a common phenomenon in grinding. No matter how reasonable the processing conditions are selected, it is impossible to completely prevent blockage, but the degree is different. The type of grinding wheel and processing conditions have a great influence on the blockage of the grinding wheel, but the most important thing is the physical and mechanical properties of the processed material and the presence or absence of grinding fluid.