Grinding is a widely used metal cutting method, mainly processing hard materials that are difficult to be cut by traditional tools and materials that require high surface quality and dimensional accuracy. With the emergence of a large number of new materials and the use of parts accuracy, quality requirements, grinding processing applications have increased more than other traditional processing methods. In grinding process, the size, shape and distribution of abrasive particles play an important role in the processing. However, in the processing of ductile metal, the grinding wheel is sharply blocked and passivated, which leads to the shortening of the life of the grinding wheel. In order to avoid the adverse effects of the grinding wheel clogging and passivation and production, it is necessary to analyze the mechanism and cause of the grinding wheel clogging.
1. Formation of abrasive chips
The grinding process is to obtain higher surface quality and accuracy by cutting out a certain amount of workpiece material, and the commonly used tool is the grinding wheel. The grinding wheel is a loose body made of abrasive and bond by compact, drying and sintering, and its single abrasive particle is a tiny cutting edge, with a large negative front Angle and blunt radius of the cutting edge. High-speed motion abrasive after sliding, ploughing into the workpiece, the cutting layer material has obvious sliding along the shear surface to form short and thin chips, these chips in the grinding zone is heated to a very high temperature, and then be oxidized and melted, and solidified into a particle sphere, there are some fork branches on the sphere surface, is a main form of grinding chips. When grinding the acid-resistant stainless steel Cr20Ni24Si4Ti, it is found that a large number of spherical grinding chips, accompanied by ribbon, knobby grinding chips and ash, many parts of these grinding chips will be filled into the stomata of the grinding wheel, attached to the surrounding of the abrasive, causing the clogging of the grinding wheel, resulting in reduced grinding accuracy, burn the workpiece, and shorten the life of the grinding wheel.
2. Type and mechanism analysis of grinding wheel clogging
2.1 Types of grinding wheel clogging
The types of grinding wheel plugging are embedded type, adhesive type and mixed type. Embedded-type plugging is the plugging state where the abrasive chip is inserted in the gap of the working surface of the grinding wheel. Adhesive plugging is the plugging state in which the abrasive chips are fused on the abrasive particles and the binder. The mixed type of plugging is both embedded type plugging and adhesive type plugging.
2.2 Mechanism analysis of embedded-type plugging
External factors: Grinding processing has a very important feature, the general Fy/Fz is greater than 2 ~ 10, the harder the workpiece material, the smaller the plasticity, the larger the ratio, the grinding area under the strong positive pressure, the grinding chips are mechanically squeezed into the gap on the surface of the grinding wheel. The abrasive chips slide out along the front of the abrasive grains, and several layers of abrasive chips accumulate in the local area in front of the abrasive grains. Under the action of high-speed rotation of the grinding wheel, the airflow vortex area is formed behind the abrasive grains, and the air pressure in the vortex area is significantly reduced. Under the action of negative pressure, some of the abrasive chips are attached to the back of the abrasive grains, forming an adhesive blockage on the knife surface behind the abrasive grains, and the attachments are mostly ash and particles.
The effect of electrostatic field: a small electric field composed of grinding wheel and workpiece is formed in some small areas of the grinding zone. Under the action of the electric field, some grinding chips will show polarity. According to the principle of opposites, the grinding chips with the opposite polarity of the grinding wheel will be adsorbed on the working surface of the grinding wheel. By means of the large mechanical pressure between the grinding wheel and the workpiece, the grinding dust adsorbed on the surface of the grinding wheel can be stably embedded in the gap between the surface of the grinding wheel.
2.3 Mechanism analysis of adhesive blockage
Melting bond: In 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 chips are oxidized rapidly in the air, forming metal oxides with low melting point. The high temperature heating in the grinding zone is melting or slightly melting, covering the surface of the grinding wheel. When this part of the surface of the grinding wheel is involved in grinding again, it is squeezed or strengthened under the action of grinding force. The affinity and adhesion with the grinding wheel are increased, and some are extruded and adhered to the groove surface of the workpiece surface. Through multiple random grinding, a lot of grinding chips are adhered to around the particle, which increases the grinding force and increases the temperature, resulting in a vicious cycle that intensifies the blockage until the particle is broken or falls off.
Chemical bonding: The chemical affinity between different elements is another important reason for adhesive plugging. The abrasive particles and the grinding material are in contact at high temperature, and the temperature factor enhances their activity and affinity. When certain conditions are met, the chemical reaction will be caused, causing the abrasive particles and grinding chips to form a crystal on the surface of the grinding wheel that loses the cutting ability.
3. The influence of the grinding wheel itself on the blockage
3.1 Types of abrasives
Different grinding wheel blockage degree is very different, from reducing the degree of blockage, improve the grinding effect, different workpiece materials, should choose different types of abrasive. If the selected abrasive can not adapt to the grinding performance of the workpiece material, it is easy to produce sharp blockage, so that the processing can not proceed normally. If the iron carbon alloy is ground with corundum abrasive, the carbon generates a thin oxide film with oxygen in the air, which can effectively prevent the chemical affinity between the workpiece and the abrasive, but if the titanium alloy is ground, the blockage is much more serious. In some factories, the grinding wheel on the grinding machine is not changed for a long time, and it can grind everything, which seems to be saving and convenient, but actually loses efficiency and accuracy.
3.2 Abrasive particle size
Abrasive particle size has a certain effect on the grinding wheel clogging, and generally fine grain size is easier to clogging than coarse-grain size. Because the pore volume and cross-sectional area of the fine grain grinding wheel are small, the number of cutting edges of the fine grain grinding wheel increases, the chip is also more, and the grinding temperature rises, the fine grain grinding wheel is easy to block in the range of small cutting times. With the increase of cutting times, coarse-grained grinding wheel compared with fine-grained grinding wheel, the cutting depth is larger, the abrasive cutting edge wear is larger, and the grinding temperature rises, the chip fuse in the pore will increase. After a certain number of times, the amount of blockage of coarse-grained grinding wheel exceeds that of fine-grained grinding wheel. Semi-fine grinding and fine grinding, small cut, low temperature, light blockage, choose fine sand wheel; Coarse grinding cut large, high temperature, blocked in the gap of the grinding chips, fused matter, choose coarse grinding wheel.
3.3 Hardness of grinding wheel
The hardness of the grinding wheel refers to the difficulty of the abrasive particle 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, the more dull the abrasive particles, the more serious the friction and extrusion of the workpiece before the abrasive particles fall off, the more easily the abrasive chips are filled into the grinding wheel gap, and the friction heat is also accompanied by more friction heat, which provides the fuse for the adhesive plugging. Therefore, the hardness of the grinding wheel has a greater impact on the amount of blockage, and the harder the grinding wheel, the greater the amount of blockage. Under normal circumstances, the hardness of the grinding wheel is G ~ H, and the hardness of D ~ 0 is also used in some difficult materials.
3.4 Grinding Wheel Structure
The structure of grinding wheel reflects the proportional relationship among abrasive, binder and air hole. The denser the tissue of the grinding wheel, the more abrasive particles are worked, the smaller the distance between the cutting edges, and the grinding wheel is more easily blocked. A wheel containing 45% abrasive has less than half the average amount of clogging than a wheel containing 49.2% abrasive particles; A grinding wheel containing 53% abrasive grinds the workpiece with twice as much clogging as a wheel containing 49.2% abrasive. When grinding difficult materials, the grinding wheel with organization number 7 to 9 should be selected, and the grinding wheel with large porosity has a better effect.
4. The influence of grinding conditions
4. 1 grinding wheel line speed
The increase of grinding wheel line speed reduces the maximum cutting depth of abrasive particles, reduces the cross-sectional area of chips, and increases the cutting times and grinding heat. These two factors both increase the amount of plugging, but when the grinding wheel line speed is up to a certain degree (such as up to 50m/s), the amount of plugging of the grinding wheel is greatly reduced. When grinding stainless steel and superalloy in production, the speed of 50m/s grinding wheel is 30% to 100% less than that of 30m/s grinding wheel. Therefore, when grinding difficult materials, either a speed of less than 20m/s or a speed of higher than 50m/s is used, and the grinding speed between them is very unfavorable to the blockage of the grinding wheel. For various workpiece materials, there is a certain critical grinding wheel speed value with small plugging amount.
4. 2 Workpiece speed
The influence of workpiece speed on the degree of grinding wheel plugging is closely related to other factors in cutting conditions. The workpiece line speed is doubled, and the amount of grinding wheel plugging is increased by three times. This is because the higher the workpiece speed, the shallower the abrasive cutting depth, the smaller the cross-sectional area of the chip, which is equivalent to the hardening of the grinding wheel characteristics, so it is easy to cause the grinding wheel blockage.
4. 3 Grinding methods
Insert grinding is more clogging than longitudinal grinding. When cutting into grinding, the contact area between the grinding wheel and the workpiece is large, the abrasive cutting edge should be rubbed several times on the same grinding mark, the grinding fluid is difficult to enter the grinding area, and the heat is high during grinding, which is easy to cause blockage. The longitudinal grinding contact with the workpiece material is one side edge of the grinding wheel. When the wear surface increases to a certain extent, the abrasive particles are broken and fractured under the action of grinding force to achieve 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 zone affected by grinding force and grinding heat can be discharged to the workpiece along the longitudinal grinding direction, so the possibility of chemical adhesion is reduced.
4.4 Radial cut
The influence of radial cut on grinding wheel plugging shows a hump trend. When the radial cut quantity is small (ap<0.01mm), the blockage phenomenon occurs. With the increase of the cut quantity, the average plug quantity also increases. When the cut quantity increases to a certain extent (ap =0.03mm), the plug quantity shows a decreasing trend, and then increases sharply with the continuous increase of the cut quantity (ap =0.04mm). When grinding difficult materials, it is very important to control the last radial cut to improve the surface quality and accuracy of the workpiece.
4.5 Grinding temperature
When grinding, any factor that increases the grinding heat and causes the grinding temperature to rise will aggravate the blockage of the grinding wheel, and the blockage form is mainly bonded blockage, and of course, it is also accompanied by diffusion blockage.
4.6 Grinding wheel dressing speed
When the grinding wheel dressing speed is low, the working surface of the grinding wheel is flat, and the number of effective grinding tools per unit area increases, which makes the cross-sectional area of chips smaller and the number of cutting increases, so it is easy to produce blockage. When the grinding wheel dressing speed is high, the working surface of the grinding wheel becomes thicker, the number of effective abrasive particles decreases, and there is a concave on the surface of the grinding wheel, which plays a role of porosity, and the chips are easy to be washed away and the fused material is easy to fall off.
4.7 Grinding fluid
Different grinding fluids have a great impact on the grinding effect. At present, the commonly used emulsion, containing a large amount of mineral oil and oily additives, is an oil-in-water milky liquid after dilution. Its specific heat capacity and thermal conductivity are small, and it is easy to cause adhesion wear and diffusion wear between the grinding wheel and the workpiece in the process of intense friction, so that the grinding wheel is blocked and the grinding force is increased. Finally, the abrasive particles are broken and fall off prematurely, and the grinding ratio is reduced. Selecting excellent grinding fluid plays an important role in improving grinding performance.
5. Conclusion
Grinding wheel clogging is a common phenomenon in grinding processing, no matter how reasonable the processing conditions are selected, it is impossible to completely prevent clogging, but the degree is different. Grinding wheel type and processing conditions have a great impact on grinding wheel plugging, but the most important is the physical and mechanical properties of the processed material and whether there is grinding fluid.
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