The choice of porous grinding wheel
At the time of grinding, the porous grinding wheel has the advantages of being not easily blocked, high durability, and strong cutting ability. Suitable for coarse and fine grinding of non-metallic materials such as soft metals and plastics, rubber and leather. At the same time, it has the characteristics of fast heat dissipation, so it has a good effect in grinding some heat-sensitive materials, thin-walled workpieces and dry-grinding processes (such as sharp-edged carbide tools and machine tools, etc.).
The method of manufacturing the porous grinding wheel and the general ceramic binder grinding wheel is basically the same. The difference is that a certain amount of a pore-enhancing agent is added to the furnish, and it is completely volatilized or burned before the sintering of the grinding wheel, thereby generating atmospheric pores.
The selective features of the porous grinding wheel are that:
The abrasive is generally selected from carbides and corundum. For example, black carbide (C), green silicon carbide (GC) and white corundum (WA) are commonly used. These abrasives have high hardness and good efficiency;
Better brittleness and sharpness, with good thermal and electrical conductivity;
Abrasive particle size (36#~180#);
Binder (ceramic binder);
Hardness (G~M levels);
Shape (flat, cup, bowl or dish, etc.);
Pore size (about 0.7 to 1.4 mm).
In general, the wheel will be marked with a row of values at the factory. The general order is the shape code, size, abrasive, grain size, hardness, tissue number, binder, and maximum allowable line speed.
Such as "P400 × 40 × 127WA60L5V35" alphanumeric specific analysis is as follows:
"P" indicates that the shape of the grinding wheel is a parallel grinding wheel.
“400×40×127” represents the size of the grinding wheel (outer diameter×thickness×aperture).
“WA” indicates that the abrasive material for the grinding wheel is a white corundum grinding wheel.
"60" indicates the size of the grindstone, which is 60# (partial fine grinding). "L" indicates that the hardness of the grinding wheel is medium-soft.
"5" indicates the organization number of the grinding wheel and it is denser. "V" indicates the bonding agent of the grinding wheel. It is a ceramic bonding agent and is relatively brittle.
"35" indicates the wheel speed and the 35m/s speed is medium.
First, the principle of selection of grinding wheel
1. When grinding steel,use corundum type grinding wheel;while grinding hard cast iron, hard alloy and non-ferrous metal, use silicon carbide grinding wheel.
2. When grinding soft and hard materials, choose hard and soft grinding wheels respectively.
3. When grinding soft and tough materials, choose coarse abrasives (such as 12-36#); when grinding hard and brittle materials, use fine abrasives (such as 46-100#).
4. When required grinding surface roughness is low, choose fine abrasive particles. When required metal removal rate is high, choose coarse abrasive particles.
5. When the quality of the surface to be machined is good, use a grinding wheel made of resin or rubber binder, and if the maximum metal removal rate is required, use a ceramic binder grinding wheel.
Second, Hardness of commonly used grinding wheel
There are two arguments about abrasive hardness:
First, abrasive hardness refers to the binding agent’s resistance to abrasive particles separating from the abrasive surface, or refers to the difficulty of the particles separating from abrasive tool surface.
Second, the hardness of the grinding tool refers to the integrated strength of the bonding agent and the abrasive particles in the work due to the resistance of the external force from the surface of the grinding wheel.
Essential factors in choosing the wheel - see how much you know about hardness?
Abrasive hardness reflects the ability of binders to hold abrasive particles, not the hardness of the abrasive particle itself.
When selecting the hardness of the grinding tool, the basic principle is to ensure that the grinding tool has a proper self-sharpening ability during the grinding process to avoid excessive wear of the grinding tool and to ensure that no excessive grinding temperature is generated during grinding.
As mentioned earlier, the hardness of the grinding tools is related to the amount of the bonding agent. The higher the hardness of the grinding tools, the greater the amount of bonding agent, the thicker the bonding agent bridge, and the greater the holding power of the binding agent on the abrasive, granules can withstand large grinding forces without breaking or falling off. On the contrary, when the hardness of the abrasive tool is low, the holding force of the binding agent on the abrasive particles is small, and the abrasive particles are easily broken or fall off. Therefore, if the hardness of the abrasive tool is selected too high, not only the blunt abrasive particles are not easily broken or fall off and the cutting ability is lost, but also the friction between the abrasive tool and the workpiece is increased, and the surface of the workpiece is prone to heat and burns. In order to remove the blunt abrasive grains in time, it is necessary to dress the abrasive tools frequently, resulting in a lot of wear of the abrasive tools. If the hardness of the abrasive tool is chosen to be too soft, the abrasive particles will fall off when they are sharp, causing unnecessary wear. At the same time, the abrasive wear is too fast, the wear on the work surface is extremely uneven, and it also affects the machining accuracy of the workpiece.
To sum up, only by properly selecting the hardness of the grinding tool, can it maintain its normal grinding state and meet the processing needs. In particular, when sharpening certain tools, even if the hardness of the grinding tool is single grade smaller , it will affect the sharpening quality. The effect of the hardness of the grinding tool is very important.
When selecting the hardness of the grinding tool, the most basic method is:
The hardness of the workpiece is high, the hardness of the grinding tool choose low; the hardness of the workpiece is low, and the hardness of the grinding tool choose high. Because the hardness of the workpiece is low, the pressure on the abrasive particles cut into the workpiece is relatively small, the abrasive particles are not easy to wear, so that the abrasive particles will not be broken or fall off before dulling, so the choice of high hardness Some abrasive tools are more appropriate; conversely, when the workpiece hardness is high, the pressure on the abrasive particles cut into the workpiece is relatively large and dull. The grinding tool with softer hardness can produce self-sharpening in time to maintain the grinding performance of the abrasive tool. However, when the workpiece material is softer and the toughness is large (such as soft bronze, brass, etc., since the cutting is easy to clog the grinding tool, it is preferable to use a grinding tool with a relatively coarse grain and a softer hardness for processing.
The hardness of the grinding tool is also an important factor affecting the temperature of the grinding area. Grinding workpieces with low thermal conductivity (such as alloy steel), due to the relatively high surface temperature of the workpiece, it is often prone to burns, cracks, at this time we should select the grinding wheel with low hardness, loose structure, but also to strengthen Cooling, so as to effectively avoid the burn of the workpiece. Similarly, when grinding thin workpieces, it is also necessary to use grinding wheels with looser texture and lower hardness. When grinding the outer circle of a thin-walled hollow workpiece, the hardness of the grinding wheel is lower than when the solid workpiece is ground, which is also to prevent the deformation of the workpiece due to the increase of the grinding temperature.
When selecting the abrasive hardness, generally consider the following:
When the area of contact between the grinding tool and the workpiece is large, the hardness of the grinding tool should be selected to be lower in order to avoid excessive heating of the workpiece and affect the grinding quality. For example, the hardness of the grinding tool used for the vertical shaft grinding is low; the hardness of the grinding wheel used in the flat grinding and internal soft grinding is lower than the hardness of the grinding wheel used for cylindrical grinding. However, when grinding fine and long internal holes, because of the low speed of the grinding wheel, the grinding wheel is easy to wear and cause the workpiece to have a taper (trumpet), so the hardness of the grinding wheel is higher than that of the internal cylindrical grinding. In the same way, grinding of small-aperture workpieces can use harder grinding wheels, and grinding of large-aperture workpieces should use softer grinding wheels.
When grinding intermittent surfaces and casting burrs, hard grade or super hard grade grinding wheels should be used. When grinding heavy billets, hard grade or super hard grade grinding wheels should also be selected to prevent the grinding wheels from being worn too quickly.
The diamond grinding tools (grinding wheels or oil stones) used for dressing are often used with super hard grade grinding tools because they require high hardness due to the high pressure during dressing.
Heavy-duty grinding machines and rigid grinding machines, because they have less vibration during grinding, the abrasive particles are not easily destroyed, so the grinding wheel with lower hardness can be used.
When the outer circle cuts into the grinding, in order to avoid the burn of the workpiece, the hardness of the grinding wheel should be lower than that in the axial feeding.
The automatic knife grinder can be used with a softer grinding wheel than a manual knife grinder.
The smaller the surface roughness requirements and the more accurate the workpiece size requirements, the lower hardness grinding wheel should be selected to avoid too much grinding heat and the surface of the workpiece is bad. For example, ultra-soft stage mirror grinding of resin binder grinding wheel can grind a surface with a roughness Rz of 0.05 μm. However, for a general grinding wheel, the hardness is higher. Otherwise, the work surface of the grinding wheel will be uneven. Wear affects the machining accuracy of the workpiece.
Scratches on the surface of the workpiece are often related to improper selection of the hardness of the grinding tool. When the hardness of the abrasive tool is too low, the abrasive particles are apt to fall off, so that due to the effect of squeezing or rubbing, the detached abrasive particles can scratch the surface of the workpiece. Therefore, the hardness of the abrasive tool should be appropriately increased at this time.
When dry grinding, the workpiece is easy to generate heat. It is better to use soft wheels with 1~2 grades lower than wet grinding.
When the production efficiency is relatively high, some softer grinding wheels can be used to facilitate sharpening of the grinding wheel and reduce the number of dressings. However, the wear of the grinding wheel will increase correspondingly. Therefore, comprehensive analysis and comparison shall be made on technical and economic indicators.
In high-speed grinding, when the feed rate is constant, the chips cut by the abrasive grains become thinner, the cutting forces of the abrasive grains are reduced correspondingly, and the wear of the grinding wheel is slower. In order to improve the self-sharpening of the grinding wheel, Its hardness is 1 to 2 softer than that of Putuo Grinding, which is the case during high-speed finish grinding. Similarly, for some unbalanced workpieces (such as crankshafts, etc.), since the workpiece speed during grinding cannot be too high, the hardness of the grinding wheel must also be selected to be low so as not to burn the workpiece. High-speed grinding with the main purpose of improving cutting efficiency is to increase the cutting feed, and the grinding force on the abrasive grains increases. In order to ensure that the abrasive particles do not fall off prematurely, the hardness of the grinding wheel should be 1 to 2 orders of magnitude lower than that of ordinary grinding.
When grinding steel balls (balls), super hard grade grinding wheels should be selected; generally cutting the workpiece, the hardness of the grinding wheel should be selected to medium hard grade.
When grinding cemented carbide and high-speed steel tools, the grinding wheel with J~G hardness should be selected.
During the forming grinding, in order to maintain the correct geometry of the workpiece, the wear of the grinding wheel should not be too large, so the hardness of the grinding wheel should be higher.