In the machining of parts, the final machining process with grinding can reach 80%, that is, the surface quality of most parts is obtained by grinding. In the grinding process, it often causes grinding burns of the workpiece, which greatly reduces the surface quality of the workpiece. Severe grinding burns double the service life of the parts, and sometimes they cannot be used at all. A very important question in.

1. Overview of grinding burn
What is a grinding burn? During grinding, due to the instantaneous high temperature (generally 400-1500 ℃) in the grinding zone, the surface structure of the workpiece changes locally, and some parts of the surface appear discoloration (that is, the color of the oxide film) -Yellow, brown, purple, blue, etc.), this phenomenon is called grinding burn
Grinding burns can be distinguished according to appearance, nature and depth of surface microstructure changes. According to the appearance of the burn, there are general burns (the entire surface of the part is burned), patchy burns (scattered burn spots on the surface), linear burns (the entire surface of the part has linear burns), and linear burns are the most common in production. Burns.
According to the nature of the surface microstructure change, there are tempered burns-when the temperature of the surface layer of the workpiece does not exceed the phase transformation temperature Ac3, but exceeds the transformation temperature of martensite, the martensite will be transformed into tempered flex with lower hardness Body or sorbite; quenching burn-when the temperature of the surface layer of the workpiece exceeds the phase transformation temperature Ac3, martensite is transformed into austenite, and there is sufficient coolant at this time, the surface layer will be rapidly cooled to form secondary quenched martensite , The hardness is higher than tempered martensite, but very thin (only a few microns), and the lower layer is tempered sorbite and troostite with lower hardness; annealing burn-when the temperature of the surface layer of the workpiece exceeds the phase transformation temperature Ac3, the The tensite transforms into austenite, and if there is no coolant at this time, the surface hardness drops sharply, and the surface layer of the workpiece is annealed. This can easily occur during dry grinding.

According to the different depth of surface microstructure changes, burns can be divided into shallow burns (depth to 0.005mm), medium burns (depth to 0.001mm), and deep burns (depth of more than 0.01mm).

2、According to the different depth of surface microstructure changes, burns can be divided into shallow burns (depth to 0.005mm), medium burns (depth to 0.001mm), and deep burns (depth of more than 0.01mm).

2. Factors affecting grinding burns and solutions
In the grinding process, the main cause of burns is that the temperature of the grinding zone is too high. In order to reduce the temperature of the grinding zone, the two ways of reducing the generation of grinding heat and accelerating the transfer of grinding heat can be started.
2.1. Reasonably select the amount of grinding
In order to select the amount of grinding reasonably, the relationship between the surface temperature of the grinding zone and the amount of grinding must first be analyzed. Now take the surface grinding as an example, according to the actual processing situation, the grinding zone is simplified to a continuous uniform heat source AB only to the workpiece The calculation problem of the semi-infinite thermal field conducted under the surface (see Figure 1). Through the theoretical analysis and calculation of the temperature field, it can be concluded that the heating value of the continuous uniform heat source AB in the grinding zone per unit time is Q = C0VI0.2ap0.35V sand 0.35f-0.3 (C0 is a constant). From the above relationship between the surface temperature of the grinding zone and the grinding amount, it can be seen that:
(1) Grinding depth ap When the grinding depth ap increases, the heat generation will increase, which will increase the surface temperature of the workpiece and increase the degree of burn. Therefore, ap cannot be selected too much.
(2) Transverse feed rate f When the inverse feed rate f increases, the surface temperature of the grinding zone decreases instead, and grinding burns decrease. The reason is that the increase in f makes the contact time of the grinding wheel and the surface of the workpiece relatively reduced, so the heat action time is reduced, and the heat dissipation conditions are improved. In order to compensate for the increase in the surface roughness caused by the increase in the lateral feed, a wider Of the grinding wheel.

(3) Workpiece speed vI When the workpiece speed vI increases, the surface temperature of the grinding zone rises. However, further research found that the greater the vI, the greater the temperature gradient near the grinding surface, as shown in Table 1.
This is because although vI increases, the heat action time is reduced, that is, although the temperature in the grinding zone is high, but the surface of the workpiece is too late to burn, the grinding zone is effectively cooled. According to the data in Table 1, the choice is more Large vI can reduce burns on the ground surface, while increasing productivity.

Of course, increasing vI will increase the surface roughness. In order to compensate for this defect, the grinding wheel speed can be increased accordingly. In fact, according to relevant studies, if vI increases by 3 times, resulting in an increase in surface roughness, only a 39% increase in v sand is required. Can be compensated. Practice has proved that increasing vI and v sand at the same time can avoid grinding burns. Figure 2 shows the critical ratio curve of vI and v sand without burns when grinding 18CrNiWA steel. The lower right of the graph is the dangerous zone (zone I) that is prone to burns, and the upper left of the curve is the safe zone (zone Ⅱ) .
2.2 Reasonable selection of workpiece materials
The influence of the workpiece material on the temperature of the grinding zone mainly depends on its hardness, strength, toughness and thermal conductivity. The higher the hardness of the workpiece, the greater the heat in the grinding zone, but the material is too soft and it is easy to block the grinding wheel, which will increase the temperature of the machined surface; the higher the strength of the workpiece, the more power is consumed during grinding and the more heat is generated; the workpiece The greater the toughness, the greater the grinding force and the greater the heat generation. In addition, materials with poor thermal conductivity, such as heat-resistant steel, bearing steel, stainless steel, etc., are prone to burns during grinding.
2.3 Correct choice of grinding wheel
In the grinding process, the cutting force per unit cutting section is about (1×105～2×105)N/mm2, which has exceeded the strength limit of the material dozens of times, and the cutting force is obviously not caused by the processing. It is caused by the strength resistance of the material, but it is caused by the unreasonable cutting during the grinding process.

As a result, improving the cutting process, reducing power consumption and reducing the temperature of the grinding zone are very promising.

According to the analysis of the grinding process, the grinding is carried out by each abrasive grain on the grinding wheel. The cutting thickness of each abrasive grain is usually in the range of (0.2～0.02) μm, and the radius of curvature of the abrasive grain ρ is generally 8 μm, which is much larger. Regarding the cutting thickness, in most cases, the metal of the cutting layer is only squeezed and not removed. This layer of metal is only squeezed by a large number of abrasive grains and fatigued and peeled off, so most of the cutting resistance is friction. If the abrasive grains are sharp, the friction will be greatly reduced, and the sharpness of the abrasive grains depends on the hardness and strength of the abrasive grains. Therefore, increasing the hardness and strength of the abrasive is an important direction to improve the grinding conditions of the grinding wheel.

When the diamond abrasive wheel is grinding cemented carbide, due to the hardness and strength of the abrasive particles, the sharp tip of the tool improves the grinding conditions, thereby reducing the grinding force and the temperature of the friction zone. In addition, diamond and metal are in no way The friction coefficient in the case of lubricating fluid is extremely low, so no burns are caused.
At present, CBN grinding wheel is also widely used. It has good thermal stability, low grinding temperature, and its hardness and strength are second only to diamond, and the grinding force is small, and it can grind high surface quality. In addition, the use of a certain elastic bond, such as rubber, resin, etc., can also improve the grinding conditions of the grinding wheel. When the chip force increases due to some reason, such a bond can make the abrasive grains of the grinding wheel produce a certain elastic retreat, so that the chip depth can be automatically reduced, and burns can be avoided.

2.4 Reasonable use of cooling methods
In the grinding process, due to the strong air flow layer generated on the surface of the high-speed rotating grinding wheel, it is difficult for the coolant to enter the grinding area, and a large amount of it is poured on the processed surface that has left the grinding area, and burns have already occurred at this time. So reasonable use Cooling methods are necessary. Here are several effective cooling methods:
1Using high pressure and large flow cooling.
This not only enhances the cooling effect, but also washes the surface of the grinding wheel so that the gaps are not easily blocked by cutting. To prevent the coolant from splashing, the machine tool must be equipped with a protective cover.
2 Install a coolant nozzle with an air baffle on the grinding wheel.
In order to reduce the effect of the high-pressure attached airflow on the surface of the high-speed rotating grinding wheel, a coolant nozzle with an air baffle as shown in Figure 3 can be installed, so that the coolant can be injected into the grinding area smoothly.
3 Realize internal cooling.
As the pores on the grinding wheel can seep water, the coolant can be sent into the grinding area under the action of centrifugal force through the inside of the grinding wheel. In order to prevent the gap of the grinding wheel from being blocked by the coolant, special attention should be paid to the cleanliness of the coolant when using internal cooling for grinding.
4 Right-angle nozzles.
The design of the right-angle nozzle is simple, but the effect is outstanding. It has been widely used for high-speed grinding. The right-angle nozzle basically eliminates the effect of air flow on the grinding fluid in a wide range of grinding wheel speed V = (40～150)m/s Blocking effects.
5 Dipping the grinding wheel.
Infiltrate the liquid solid lubricant into the pores of the grinding wheel and cover the surface of the abrasive particles. During grinding, the abrasive particles and chips are formed as well as the abrasive particles and the machined surface.
6 Self-acceleration method.

Using the principle of hydrodynamic pressure, the grinding fluid obtains a speed equivalent to the circumferential speed of the grinding wheel, thereby generating a synchronous cooling effect.

7 Low temperature compressed air cooling method.
In the heat exchanger, the compressed air is cooled to (-110°C) with liquid nitrogen (-192°C), and then cooled from the nozzle to the grinding zone. In order to prevent the moisture in the air from freezing in the heat exchanger tubes when the temperature drops, an air dryer must be used. This is a new technology that replaces the traditional grinding fluid cooling method.
Grinding burns often occur in production and processing. In addition to the above measures, attention should be paid to heat treatment and strict compliance with process discipline.