Introduction Gear transmission is the most important mechanical transmission in modern machinery. Gear is an important basic component of mechanical products. Grinding is a finishing process for gear machining. Grinding teeth can not only correct the errors caused by gear pre-machining, but also significantly improve the overall accuracy of the gearbox.
With the increasing demand for precision gears, more and more grinding gear grinding machines have been replaced by forming gear grinding machines, which has significantly improved the grinding gear efficiency. However, in the process of production and use, the tooth surface burn problem of the profile grinding machine has appeared more and more, especially for the nickel-containing materials such as 17CrNiMo6 and 20CrNi2MoA, which are more prominent, which seriously affects the gear quality and service life.
To this end, this paper analyzes and discusses the causes, influencing factors and preventive measures of grinding tooth burns.
1 Grinding tooth burns 1.1 Causes of grinding burns Inevitably, a large amount of grinding heat is generated during the grinding process, part of which is carried away by the cooling liquid, and the other part is transmitted into the shallow surface layer of the processing gear, and the gear is quickly made The surface temperature rises. When the grinding heat is generated in a large amount, a tempering layer is formed in the shallow surface of the tooth surface. When the grinding is particularly abnormal, the phase change or even the melting temperature can be reached, and the secondary quenching layer is formed by the chilling of the cooling liquid, thereby forming a grinding machine. Tooth burns. Grinding cracks are also formed when the grinding tensile stress generated and acting on the surface of the tooth surface of the tooth surface exceeds the brittle breaking strength of the material.
1.2 Determination of grinding tooth burn The main process parameters of the grinding process on the formation of burns can be qualitatively or quantitatively evaluated by four methods.
(1) Erosion test method, according to GB/T17879-1999 "Erosion test of surface tempering after gear grinding", after being eroded with 3~5 nitric acid solution, it is divided into light according to the change of color of tempering area from light to heavy. There are 5 levels of B, C, D, and E. According to the maximum percentage of tempering surface area, there are 3 levels of 1, 2, and 3. The degree of wear burns was determined from the maximum percentage of tempering color and tempering area, respectively.
(2) The metallographic method is to determine the depth of the structural change layer by the metallographic analysis and measurement of the microhardness method, and is also a commonly used method in actual production.
(3) Analytical test method, which is evaluated by the coefficient K=Q/F. Where Q is the amount of metal ground by the grinding wheel per unit time (unit grinding amount of volume metal), and F is the contact area of ​​the grinding wheel with the tooth surface.
In this way, a certain amount of metallographic analysis is to be completed. Based on these data, the relationship between the coefficient K and the depth of the tissue change layer is established to determine the depth of the tooth burn.
(4) Analytical method, according to the heat wave permeation equation [2] h'=ατπLnTmaxTper where h'--the depth of the damage layer α--the temperature conduction coefficient, α=λcrλ--the heat transfer coefficient c--unit Heat capacity γ --- density of material to be processed τ --- heat dissipation time Tmax --- the highest contact temperature in the grinding zone Tper --- allowable temperature (no martensite decomposition at depth h') The method is based on the analysis of many parameters of the grinding tooth thermal process under certain assumptions, and a large amount of calculation must be completed.
The first method is mainly to judge the grinding tooth burn from the surface reaction. It is the simplest and most convenient test method and one of the commonly used methods in production practice. The second method is to deeply analyze the influencing factors such as the hardness influence and tissue change of the grinding tooth burn by the metallographic analysis of the tooth burn layer. Through the first two methods, the grinding tooth burn can be comprehensively analyzed from the surface to the tissue, and is also a commonly used test method in actual production. The three or four methods calculate the depth of the grinding tooth from a theoretical point of view, which can fully complement the grinding tooth burn.
1.3 Hardness analysis of grinding tooth burns The hardness curves of 17CrNiMo6 and 20CrNi2MoA materials after grinding by a profile grinding machine include the original curve before grinding and the two typical burn types after grinding: mild tempering Burns and secondary quenching and tempering severe burns.
3 is the original hardness curve when the teeth are not ground. It is found that the surface hardness of Case 1 and Case 2 is significantly different from the original hardness.
The first case of the grinding tooth burn is the case shown in Fig. 1. After the material 1 is ground, a tempering layer is formed at a distance of 0.1 mm from the surface, so that the hardness is remarkably lowered. According to GB/T17879-1999 "Erosion test of surface tempering after gear grinding", it was found that the surface was dark gray or dark after pickling with 5 nitric acid solution. It belongs to grade B mild tempering, and the tempering area accounts for about 25.
In actual use, it is found that the gear tooth surface is formed by Hertz stress perpendicular to the tooth surface and develops along the tooth width direction. In addition, along the tooth height direction, adjacent cracks develop into a cross-web shape, and the tooth surface layer forms " “Island†finally caused spalling along the transition zone of the carburized layer, which significantly reduced the life of the gear.
In the second case, the case 2, the material 2 is subjected to secondary quenching after grinding, and a hardened layer is formed at a distance of 0.1 mm from the surface, so that the surface hardness is remarkably improved and a large tensile stress is formed. After 12 hours of grinding, it was found that there were severe cracks on the tooth surface and wrinkling in the severe place.
According to GB/T17879-1999 "Erosion test of surface tempering after gear grinding", it was found to be white blocky area after pickling with 5 nitric acid solution, surrounded by black tempering surface, which belongs to class E severe overheating, and tempering area accounts for 80 or more. .
Metallographic analysis revealed that the white block region was a secondary quenched untempered martensite with a very shallow depth (about 0.1 mm), and cracks were generated under stress during the placement. This type of gear will collapse during use and its life will be seriously degraded.
2 Factors affecting the grinding of the tooth 2.1 The influence of the grinding method of the machine tool on the grinding of the tooth The current grinding equipment is mainly divided into two categories: the grinding machine and the forming gear grinding machine. Due to the different grinding principles, the grinding method is different, which determines the tendency of grinding teeth to burn. Fig. 2 shows two different feeding modes of the gear grinding machine and the forming gear grinding machine. The left picture shows the tangential feed, also called the equidistant feed, which is the main feed mode of the gear grinding machine. Radial feed is the main feed method for forming gear grinding machines.
The grinding principle of the gear grinding machine is the principle of gear and rack meshing.
The imaginary grinding wheel is a rack that allows the machined gear to reciprocate on the imaginary rack. The running-in process consists of a combination of a straight line and a circular motion. The different speeds of the motion are controlled by a hanging wheel or a computer PC. Synthesize an involute curve. The most common method of grinding is to machine one tooth face of the same tooth groove and then machine the other tooth face. Due to the line contact, the grinding process produces extremely high temperatures, and because the contact area between the grinding wheel and the tooth surface is small, the pressure formed on the tooth surface is large, and the tensile stress on the surface layer is also large, so the processing method is the easiest. Cracks are generated.
However, it has a large stroke during grinding, and the general coarse grinding is 90 m/min. There is a time difference between the meshing lines, which is favorable for heat dissipation. Therefore, "exhibition into grinding" is not easy to produce such overheated burns of surface tempering.
The profile grinding machine is different from the developed gear grinding machine, and its feed is perpendicular to the workpiece, ie radial feed. The shape of the forming wheel is equal to the final tooth shape, but the shape of the tooth groove is narrow before it is processed (because of the amount of grinding). According to the principle of the gear, when the grinding wheel is away from the gear, the pressure angle of the grinding wheel and the gear is smaller. The position is first contacted, not the pressure angle of the grinding wheel and the gear design coincides, so the part above the gear index circle does not participate in the cutting during the previous grinding process. At the lower end of the indexing circle, especially the upper end of the over-arc of the hob is removed. As the radial feed increases relative to the lower end of the gear, the upper end increases, so that different metal removals are formed with the same radial feed. The forming mill is mostly double-sided grinding, that is, the grinding wheel simultaneously cuts the left and right tooth surfaces of one tooth groove, the stroke feeding is slow, the general rough grinding is 2500 mm/min, and the feed amount is large, and the same tooth groove is continuously ground. It is cut several times, so that the contact marks between the grinding wheel and the tooth surface are formed into two faces. Because it is continuous grinding, it is extremely unfavorable for heat dissipation and the entry of coolant. Therefore, the profile grinding machine is prone to cause surface tempering and secondary quenching of the tooth surface. However, since its grinding is balanced on both sides and the contact area is large, the tensile stress on the tooth surface is small, so cracks are less likely to occur.
The above analysis shows that the grinding principle of the profile grinding machine determines the tendency of grinding burns.
2.2 Influence of grinding wheel and grinding parameters on grinding tooth burn In practice, it is found that different grinding wheels produce different effects in the grinding process under the same grinding process. Therefore, in the selection of the grinding wheel, the softer grinding wheel should be selected as much as possible under the premise of ensuring the roughness of the tooth surface. However, due to the different grinding principles, the grinding wheel of the profile grinding machine is slightly higher than the hardness of the grinding wheel of the gear grinding machine. Because the shape of the grinding wheel of the profile grinding machine is more complicated, the cutting amount is different at each point, and it needs better shape retention performance.
2.2.1 Influence of hardness and particle size of grinding wheel Titanium alloy steel was ground with green silicon carbide grinding wheel of different particle size to compare the grinding temperature. The test results show that the soft grinding temperature of the grinding wheel is low, the reason is that the soft grinding wheel has better The self-sharpness, the abrasive grains on the working surface of the grinding wheel are often in a sharp state, and the energy generated by friction and plastic deformation is less, so the temperature of the harder grinding wheel is lower. The coarse grinding temperature of the grinding wheel is low because the number of abrasive grains per unit area on the working surface of the coarse grinding wheel is small, and the contact area with the workpiece is also small, so the grinding heat generated per unit time is also small, and the natural grinding temperature is low. It is low.
2.2.2 Influence of grinding wheel type (1) CBN grinding wheel grinding test result Cycle number Grinding wheel linear speed feed rate Stroke speed Trimming times Grinding wheel removal amount Spindle load dressing amount SG grinding wheel test result Cycle number Grinding wheel linear speed Infeed amount Stroke speed Trimming tooth number trimming Remarks 125m/s0.033mm2500mm/min60.035mm225m/s0.037mm2500mm/min60.035mm325m/s0.037mm2500mm/min60.035mm425m/s0.02mm1500mm/min160.015mm(3) Ordinary corundum grinding wheel test result cycle number grinding wheel linear speed feed rate stroke speed Dressing number of trimming volume Remarks 125m/s0.015mm2200mm/min30.035mm225m/s0.015mm2000mm/min60.015mm327m/s0.01mm1200mm/min160.015mm Test proves: CBN grinding wheel is higher in grinding performance than ordinary steel jade grinding wheel, CBN grinding wheel There is almost no loss during grinding (excellent shape retention). The CBN grinding wheel has a good cutting edge angle and is not easy to wear, so the compressive stress on the gear is very small, and it is not easy to cause grinding cracks and burns.
The advantages of the SG grinding wheel are: good cutting performance and high production efficiency. From the current conservative experiment, the efficiency is 40 higher than that of ordinary corundum, and the peak single crystal corundum grinding wheel is increased by 30, and it is not easy to produce burns.
2.3 The influence of the moving speed of the stroke on the grinding tooth burns Under the premise of maintaining the metal removal amount per unit time, the stroke speed can be appropriately accelerated, the cutting amount can be reduced, the burn surface can be effectively improved, and the grinding machine can be improved. Surface finish. However, this method of forming a gear grinding machine can effectively improve the burn surface, but it will reduce the surface finish.
2.4 Influence of coolant and cooling system on grinding tooth burn In the grinding process, the instantaneous temperature of the contact zone between the grinding wheel and the workpiece can reach above 960 °C. Therefore, the position of the cooling oil pipe determines the possibility of grinding tooth burn. Normally, the coolant is directly sprayed into the grinding area, but the instantaneous heat generated in the grinding area atomizes the cooling liquid, forming a lean area in the grinding area, so that the cooling liquid cannot achieve the cooling effect, and the grinding tooth burn is increased. possibility.
The improved coolant avoids direct injection in the grinding area, and the blockage of the grinding hole is considered. The division of the two sets of cooling oil pipes is as follows: one of the groups is directly injected into the grinding wheel to participate in the cutting part to flush the iron stuck on the grinding wheel. Chips, the jet direction of this group is opposite to the direction of rotation of the grinding wheel. The other group is sprayed in the grinding area, which is the same as the direction of rotation of the grinding wheel, which ensures that the cooling fluid reaches the grinding area through the centrifugal force of the grinding wheel, and the cooling effect is achieved. Tests have shown that the improved cooling system has a significant improvement in tooth burns.
2.5 The impact of fixtures on the grinding of the teeth The stiffness of the fixtures and auxiliary tools is also a key factor in determining the burn of the teeth. Conventional tooling rarely considers the stiffness of the mandrel. During the grinding process, the instantaneous feeding of the grinding wheel causes the workpiece to bear a large axial force, and the feeding of the grinding wheel causes the workpiece to be in an unstable state, so that the uniformity of the set feed amount cannot be ensured, thereby causing the grinding tooth to be made. The possibility of burns is greatly increased.
2.6 Influence of auxiliary process on grinding tooth burn Before grinding the tooth, the carburizing and quenching gear should be sanded or sandblasted, which can effectively remove the oxide scale of the tooth surface, reduce the possibility of impurities blocking the grinding wheel, and effectively reduce the possibility of grinding tooth burn. In addition, the cleanliness of the cooling oil can also be improved.
2.7 The influence of other factors on the grinding of the grinding wheel on the balance of the grinding wheel to achieve excellence, careful operation and regulation. Because the grinding wheel is rotating at high speed, every tiny shake will cause serious burns on the tooth surface.
Both the grinding gear and the forming gear grinding machine must maintain the sharpness of the finished diamond pen and diamond roller. To trim the sharp grinding wheel. If the diamond pen has more than 0.3mm wear, or the diamond wheel has more than 0.02mm wear, it indicates that the diamond (pen) has been passivated and needs to be replaced in time. The dressing speed of the diamond pen and the diamond wheel must be moved quickly during the roughing to ensure the sharpness of the grinding wheel.
3 The gear grinding of the articulated gear is objective. Through the discussion of the factors of grinding tooth burn, some factors affecting the grinding tooth burn are found, which are constantly improved and perfected in the production process, which can effectively improve the status of grinding tooth burn, especially It is the choice of the type of grinding wheel that can significantly improve. However, the tests in this paper are still limited. It is still impossible to fully summarize the performance of all grinding wheels and the matching grinding parameters. More experimental work is needed.
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