Grinding is a long history, widely used metal cutting method. At home, currently mainly used in traditional tools difficult to cut hard materials and precision, high surface quality requirements of the parts of the processing. With the emergence of a large number of new materials and applications as well as scientific and technological development brought about by the right part accuracy and quality of the new requirements, the growth rate of grinding applications than any other conventional processing methods. Overseas, the grinding process has been widely used in the blank directly processed in many ways to replace the traditional method of cutting, grinding machine number has reached about 60% of the total. Grinding, not only the abrasive size, shape and distribution of the processing plays an important role in the processing of ductile metals often occurs when a sharp wheel blocking passivation, resulting in a premature end wheel life, to avoid clogging wheel passivation and The resulting adverse effects of clogging wheel mechanism, the process is necessary.
First, the formation of debris
The grinding process is a multi-factor complex, multi-variable process of interaction, which object is achieved by removal of the workpiece material is given a certain amount of high surface quality and precision. Is a grinding wheel abrasive compacts binding agent by drying and sintering the porous body, which is a tiny single abrasive cutting edge, a large negative rake angle and radius of the rounded edge. After a high-speed movement of grain swipe, cut into the workpiece after plowing. Layer material with a clear cut along the shear surface formed after a short slip and thin chips, debris in the grinding zone which is heated to a high temperature (such as the steel material up to 1200K or more), then oxidation and melting, and then solidified into spherical particles, the surface of the sphere there is some tine, this spherical debris is a major form of debris. When grinding steel Cr20Ni24Si4Ti by scanning electron microscopy and found that a large number of spherical debris, of course, accompanied Ribbon, debris and ash-like section, a number of these parts will wear debris filled into the pores wheel, attached to the abrasive around, causing clogging wheel, resulting in decline in precision grinding, burn the workpiece and shorten the wheel life.
Second, the type of wheel and mechanism blocking
Wheel types are embedded plug, attachment type, adhesive type, mixed type. Embedded debris clogging is embedded in the wheel at the work surface pores clogged state. Abrasive type attachment plug is attached by a temporary power after the abrasive cutting surface of the cutting edge a clogging. Adhesion type plug is melted and adhered to the abrasive grinding the cutting edges projecting on weeks or adhesive. Hybrid is more than three types of plug at a small part of the collection or set of layers.
Embedded and dependent-type blocking mechanism
Embedded type attachment plug and debris are mechanically filled voids generated in the wheel clogging. Fill power comes from two aspects, one is external, one internal, related to the physical, electrical, thermal and other factors.
External factors: grinding a very important feature, the radial grind component is greater than the tangential component Fy Fz, Fy / Fz ≥ 2 ~ 10, more hard workpiece materials, plastic smaller, Fy / Fz ratio greater such debris in the grinding zone of the strong positive pressure, is mechanically squeezed into the gap in the wheel surface. From the microscopic analysis of abrasive wear debris along the front slide, abrasive local area in front of several layers of debris piled in the back of abrasive, high-speed rotation of the wheel, the formation of a vortex flow area, vortex area significantly reduced air pressure in the negative pressure, so that part of abrasive wear debris attached to the back, forming abrasive flank dependent blockage, dependent mostly ash and particulate matter.
Electrostatic field: the relative velocity of the grinding wheel and the workpiece is much larger than V V sand workers, ordinary grinder V s = 3 ~ 50m / s, the speed of high-speed grinder grinding maturity value is 50 ~ 80 / s, foreign trials speed of 200m / s ~ 250m / s, the workpiece speed 1.5m / s or less. Relative movement of the grinding wheel and work in the grinding area, the grinding wheel and the workpiece surface will occur due to electron escape reasons arranged according to certain rules of charge. Meanwhile, the grinding region of the gas will lead to shock effect due to high temperature discharge, the neutral gas is ionized into positive ions and electrons. Some small areas in the grinding area formed by the grinding wheel and the workpiece composed of small farms in the electric field, with neutral atoms, positive ions, electrons, impurities, dust, not only the neutral atoms are ionized in the process, there are ion and electron recombination process. Under the action of the electric field, the polarity of some debris will be presented, based on the principle opposites attract, and the wheel opposite polarity debris is adsorbed on the surface of the wheel work. Since the electric field strength is small, so the adsorption force is also very weak, wear debris in the wheel surface is not solid, but the grinding wheel and the workpiece by means of mechanical pressure between the larger, so that has been adsorbed on the surface of the grinding wheel wear debris can be stably embedded wheel in a gap between surfaces.
The mechanism of adhesion-type plug
Most of the grinding process the input power into the grinding heat, the grinding point temperature up to 1200K or more, at such a high temperature under the action of grinding the first case of air quickly oxidized to form low melting point metal oxide, followed by the metal oxide was heated at high temperature in the grinding zone melting state, covering the surface of the wheel when the wheel again on the surface of this part in the grinding, the grinding force of the, some pushed open, some enhancement, increased affinity and adhesion of the grinding wheel, as well as the adhesion of the workpiece surface is pressed ridge groove surface. Through multiple random grinding, abrasive four weeks will be a lot of debris adhesion, the grinding force increases, the temperature rises, thus creating a vicious cycle, increasing congestion, until the grains broken or fall off, which is melted adhesive .
Chemical affinity between the different elements are bonded blockage is another important reason. And the grinding abrasive material in the heat exposure, the temperature factors to enhance their activity, increased affinity, when have certain conditions, leads to chemical reactions, the abrasive particles and debris on the wheel surface ability to produce a loss of cutting crystals. Such as corundum grinding wheel titanium alloy, wear debris quickly adhere abrasive grains, and have spread to the surrounding and growth trends, clearing debris, there are still some residual abrasive material adhered around They are oxygen, titanium, aluminum complex compound described this process undergoes a chemical reaction equation is 3Ti 2Al2O3 = 3TiO2 4Al, TiO2-based product with some free aluminum molecules such as silicon carbide wheel switch, plug will reduce the , the surface quality of the workpiece to be ground is also improved, because the affinity of titanium and silicon carbide is small, only scattered abrasive surface with some adhesive material, the adhesive material into the grinding zone again, most of the friction , falling under compression.
Three, wheel their impact on congestion
Abrasive
The abrasive material of different chemical affinity with different workpiece, grinding at different temperatures, different grinding force, in order to reduce congestion level, different workpiece materials, should be used in different types of abrasives. Corundum abrasive grinding with iron-carbon alloy, carbon and oxygen in the air to generate a thin oxide film can effectively prevent the workpiece and the chemical affinity between the abrasive effect, but grinding titanium, is much more serious congestion the. The thermal stability for clogging the abrasive has a significant impact, thermal stability, good thermal stability than the abrasive clogging much lighter. Such as cubic boron nitride abrasive grinding with titanium, the grinding efficiency than Baiyu Gang abrasive increase several times.
Abrasive particle size
The same premise in the organization, the more fine abrasive, grinding wheel grit size units within the perimeter more the number, the more uniform the number of stomata have more time, but the smaller the volume of a single pore in the same grinding parameters, the fine Wheel prone to clogging.
In the semi-fine grinding and fine grinding, cut more often, cut a small amount of low temperature, light blockage, often choose fine grinding wheel. In the coarse grinding, cut into large, grinding, high temperature, wear debris clogging the pores, sintered materials and more, should be selected coarse grinding wheel.
Binder and hardness
The hardness of abrasive grinding means ease of shedding, guaranteed by the strength of the binder, they blocked the grinding wheel greater impact. The higher adhesive strength, wheel hardness greater, the amount of the more blunt abrasive, abrasive workpiece before shedding scratch, squeezing even more serious debris easier mechanically filled into the pores to wheel, wheel debris in the gap exacerbated the friction wheel on the workpiece material, extrusion, while the debris can be strengthened in this process, this process is accompanied generate more friction heat, friction heat is provided frit bonding material blockage. Therefore, the higher the hardness wheel, blocking more serious. So grinding hard workpiece materials should be selected softer wheel.
Wheel Organization
Organization reflects abrasive wheel, binder, the ratio between the pores, the more dense tissue, the smaller the percentage porosity, the smaller the distance between the cutting edge, the grinding wheel is more easily blocked. Containing 53% of the abrasive grit grinding wheel of 49.2% compared with the average amount to be twice as high congestion, with 45% abrasive grit grinding wheel of 49.2% compared with an average less than half the amount of blockage. In the grinding difficult materials should be chosen organizational No. 7-8 of the wheel.
Fourth, the impact of grinding conditions
Wheel speed
Wheel speed is comparatively complex, when wheel from 28.8m / s up to 33.6m / s when the speed is only increased by 16%, while the blockage increased threefold. Because the wheel speed increase in the maximum depth of cut abrasive grains decreases, the chip cross-sectional area decreases, while the number of cutting and grinding heat increases, so that clogging increases. But when the grinding wheel speed is high to a certain extent (eg up 50m / s or more), the amount of clogging wheel but greatly reduced. Therefore, the selection of grinding wheel speed is best to avoid 20m / s to 50m / s this speed.
Workpiece speed
Experiments show that the work twice as fast, wheel blockage increased threefold. Because the higher the speed the workpiece, abrasive load greater, the more shallow depth of cut grain, chip cross-sectional area becomes small, when the grinding thickness increases, and glazing increase, increase the grinding wheel layer extrusion of the workpiece equivalent wheel features harden, thus exacerbate wheel clogged.
Grinding mode
In the grinding way, all wheel and workpiece increases the contact area of the grinding wheel will exacerbate congestion. This is because the contact area of the wheel and the workpiece, the cutting edge of abrasive wear scar at once on the same scratch the workpiece on the grinding wheel layer enhanced intensified cooling fluid into the grinding zone and difficult, grinding more heat, high temperature , to create conditions for the blockage, easy to produce chemical adhesion clogging and blockage embedding. End mill grinding than weeks as easy to plug, horizontal than vertical plunge grinding grinding congested.
Radial depth of cut
Radial depth of cut of the grinding wheel was clogged hump trend. When the radial depth of cut is small, (ap <0.01mm) clogging phenomenon, with cut increases, the average amount of clogging increases to a certain extent when the large cut (ap = 0.03mm), the plug volume and Movement decreasing trend, then continue to increase as the amount of cut (up to ap = 0.04mm), the block volume is rising sharply.
Grinding fluid
Different grinding fluid grinding effect a great impact on the current generic emulsion contains a lot of mineral oils and oil additives, oil in water was diluted milky white liquid, it is the specific heat capacity and thermal conductivity, in the intense friction process very easy cause adhesion between wheel and workpiece wear and diffusion wear, clogging of the grinding wheel, grinding force increases, the final cause crushing and abrasive premature shedding, the grinding ratio decreases. Therefore, the choice of excellent grinding fluid to improve grinding performance has an important role.
In short, the wheel size, hardness, structure, wheel speed, the speed of the workpiece, the grinding method, grinding fluid, cutting depth and the grinding process and the grinding result of phenomena important parameters. Therefore, the impact of various factors such as wheel blockage analysis study on the grinding parameters and other parameters for univariate and multivariate experiments to establish optimal rational grinding parameters and summed up the law, is an effective way to guide the production, but also grinding technology should focus on one of the elements.
