Grinding is a common machining process. It has many similarities with milling machining parts, both involve removing materials from the workpiece. Milling is the process of removing materials from a workpiece using a rotating tool. And grinding is the process of removing materials from a workpiece using a grinding wheel. This is a grinding process that can be used to form various shapes and features on the workpiece.
In order to obtain better surface quality and higher precision, most parts will be machined by grinding. But there are often many regularly arranged cracks on the surface which is basically perpendicular to the grinding direction during grinding, which affects not only the appearance of the parts, but also the quality of the parts directly. It has a negative impact on the on-time delivery of parts.
Common methods of cylindrical grinding
Grinding is a comprehensive process in which the abrasive particles on the surface of the grinding wheel act on the cutting, trenching and sliding off the workpiece surface. Many particles participate in the cutting movement at the same time, and the extremely thin and fine chips can be removed, so the machining precision is high and the surface roughness is small. As a finishing method, grinding has been widely used in production. At present, there are three common grinding methods: cylindrical grinding, inner hole grinding and surface grinding.
Cylindrical grinding can be carried out on an ordinary cylindrical grinder, a universal cylindrical grinder or a center less grinder, usually as the finishing of semi-precision turning.
In practical application, there are four common methods of cylindrical grinding.
(1) in longitudinal grinding, the workpiece makes a circumferential feed movement, and at the same time, it moves longitudinally with the worktable, so that the grinding wheel can grind out all surfaces. At the end of each longitudinal stroke or reciprocating stroke, the grinding wheel makes a lateral feed to gradually grind away the grinding allowance. It can grind very long surface with good grinding quality. In particular, longitudinal grinding is generally used in single-piece, small-batch production and fine grinding.
(2) the cross grinding method (cut-in grinding method) the workpiece has no longitudinal feed movement. Use a grinding wheel that is wider than the surface to be ground (or as wide as the grinding surface) and feed laterally to the workpiece at a slow feed rate until all machining allowance is worn out. The cross grinding method is mainly used to grind the outer circular surface with short length and the workpiece with steps on both sides.
(3) the characteristic of the deep grinding method is that all the grinding allowance (usually 0.2~0.6 mm in diameter) is ground in a longitudinal knife. During grinding, the circumferential feed speed and longitudinal feed speed of the workpiece are very slow, and the front end of the grinding wheel is trimmed to a stepped or tapered shape. The productivity of the deep grinding method is about twice as high as that of the longitudinal grinding method, the precision can reach IT6 grade, and the surface roughness value Ra is 0.4~0.8 mm. However, the dressing of the grinding wheel is more complex, which is only suitable for mass production and grinding allow the grinding wheel to cross a large distance between the two ends of the machined surface.
(4) the center less cylindrical grinding workpiece is placed between the grinding wheel and the guide wheel, and there is a bracket below. The grinding wheel (also known as the working grinding wheel) rotates and plays a cutting role. The guide wheel is a rubber bond grinding wheel with very fine abrasive grains. The friction between the workpiece and the guide wheel is large, so that the workpiece rotates at a linear speed close to the guide wheel. Center less cylindrical grinding is carried out on a center less cylindrical grinder. The productivity of the center less cylindrical grinder is very high, but the adjustment is complex, so the coaxiality error between the hole and the outer circle of sleeve parts can not be corrected, and the parts with longer axial grooves can not be ground to prevent the outer circle from producing large roundness error. Therefore, center less cylindrical grinding is mostly used for mass production of slender optical shafts, shaft pins, small sets and other parts.
Basic knowledge of grinding crack
In general, parts meet the technical requirements by means of heat treatment, followed by turning or grinding to get finished parts that meet the requirements. In particular, the surface of the parts after chemical heat treatment and quenching must be machined, in which cylindrical grinding is a commonly used processing process. However, grinding cracks often occur due to improper processing, which generally occur when the stress is too large or not tempered in time after quenching, and the tempering is not sufficient (insufficient) or too many retained austerity. Grinding cracks will occur even under normal conditions.
The grinding crack can be detected by a naked eye or magnetic powder on the grinding surface of the parts. It is found that there are usually three types of cracks: the cracks are distributed in the shape of a small network, the cracks are in the shape of thin strips and cross with the feed direction of the grinding wheel, and are distributed in fine dots, similar to worms, etc. It can be cut perpendicular to the direction of the crack for analysis, and it is generally found that there is a black tempering layer on the surface. If the grinding pressure is too high, a white hardening layer will be formed. The depth of the crack is in the tempering layer, perpendicular to the surface crack and crossing at the sharp corner.
In the final grinding process of the quenched parts, if the grinding parameters are improperly selected, such as the grinding wheel is too hard, the grinding quantity is too large and the cooling condition is poor, the grinding wheel heats up with the generation of grinding heat in the grinding process. The surface shrinks for the first time at 100 ℃, but the interior of the part is still in a state of expansion, so the surface is subjected to tensile stress and cracks occur perpendicular to the grinding feed direction. It is parallel and linear, and the depth is in 0.1~0.2 mm.
When the grinding temperature of the surface reaches 300 ℃, the second shrinkage will occur and the surface will show a reticular crack. the crack is detected by thermal acid etching method, because its microstructure is Tautening. Therefore, the occurrence of grinding cracks can be completely avoided by strictly controlling the grinding process parameters, fully cooling the surface of the parts when they should be cooled, and correcting the grinding wheel in time.
The grinding cracks are characterized by thin and shallow cracks, which are cracked or arranged regularly in the shape of radiation, perpendicular to the grinding direction, and seriously reticulated cracks.
The cause of grinding crack
1) when the grinding temperature reaches 150-200 ℃, the martensite decomposes and the volume of the surface of the part shrinks, but the interior is not heated, which makes the surface crack under tensile stress. When the grinding temperature is above 200 ℃, the surface layer becomes sorbite or troainite structure, the surface layer shrinks again, the tensile stress of the surface layer exceeds the brittle fracture resistance, and the surface of the part appears cracking.
2) when the surface temperature after grinding is 800 ℃ 900 ℃, the temperature rising rate reaches 600 ℃ / s. If the cooling is not enough, the heat generated by grinding is enough to make the thin layer of the surface of the part reheat to austenite state, and then quenching treatment is carried out again. Quenched martensite is formed, resulting in volume expansion.
3) the retained austenite in the surface layer is transformed into brittle martensite. Due to the increase of the surface temperature of the parts due to the heat generated by grinding, the grinding tensile stress, microstructure stress and thermal stress together lead to the grinding crack. After analysis, after determining whether the crack is a grinding crack, it is necessary to further judge and determine whether it is caused by improper grinding technology of the grinding wheel, poor structure of the parts or insufficient tempering. In order to make an in-depth inspection of the structure near the crack, the first is whether there is a coarse cementite network, the second is whether there is too much retained austenite, whether there are surface grinding burns and so on.
Precautionary measures.
The original microstructure states of the material also has a certain influence on the grinding quality. If more than three grades of reticulated carbides appear in the structure, the cracks produced in the grinding process are also reticulated. In addition, the different microstructure of the material after heat treatment has different effects on grinding annealing and grinding cracks. The general rule is that the most carbides in the structure, the worse the thermal conductivity, the higher the hardness, the more serious the brittleness of the parts, the more serious the tendency of grinding cracking. On the other hand, there are more retained austerity structure in the parts, the hardness of the surface of the parts is high, and grinding cracks are easy to occur. In the process of grinding, in addition to the factors such as material and heat treatment process, the change of grinding conditions and process parameters may lead to grinding cracks.
Conclusion
Grinding plays an important role in machining. The machining of the quenched workpiece surface has higher dimensional accuracy and surface roughness requirements, which is mainly guaranteed by grinding. The causes of grinding crack are complex, and there are many factors, such as physical properties, chemical composition, blank defects, improper heat treatment and improper processing methods, which will lead to cracks during grindings. The measures to eliminate grinding cracks are mainly through replacing grinding wheel, increasing dressing frequency of grinding wheel, improving cooling performance and improving machining parameters.
