Machining technology of valve seat and guide hole

Machining technology of cylinder head valve seat and guide hole

cylinder head is an important component of internal combustion engine, and its machining accuracy directly affects the working performance of the engine. When the engine is working, the combustible gas is ignited after being compressed in the combustion chamber of the cylinder head, causing the valve seat to bear a high thermal load and mechanical load. This requires both high wear resistance and good sealing of the valve seat. If the center of the valve rod shifts during operation, it will not only cause harmful heat conduction and rapid wear of the valve rod and guide hole, but also cause an increase in fuel consumption. Therefore, high requirements are put forward for the machining accuracy of valve seat and guide hole, especially for the coaxiality between the working cone of valve seat and guide hole

for gasoline engines, the coaxiality tolerance is specified as 0 025mm, but for diesel engine, it is only 0 015mm (in the combustion chamber, the compression ratio of diesel combustible gas is 0.5 times greater than that of gasoline). In mass production, in order to maintain such a tolerance stably, in addition to the need to optimize the processing workers' use of recycled waste plant fibers, waste wood powder in wood processing and used recycled plastics, which is beneficial to the full utilization of the environment and resources, and has the advantages of wood and plastics, the selection of positioning datum, the reasonable structure and accuracy of special cutters and fine boring heads are of great significance

the machining of valve seat and guide hole is the key technology of cylinder head machining. For a long time, many modular machine tools and tool manufacturers at home and abroad, such as Dalian Modular Machine Tool Research Institute, ex-cell-o, ALFING, Grob, H ü ler Hille, Ernst Krause & Co and KOMET, Plansee, Beck, MAPAL and other tool factories, have attached great importance to the development of such technical equipment and special tools. In recent years, great progress has been made in the development of special tools, which plays a positive role in improving machining accuracy, tool durability and machining efficiency

bottom hole machining of valve seat and guide hole

Figure 1: special tool equipped with cemented carbide rod

Figure 2: fine machining of valve seat and guide hole (left), fine machining of valve seat working cone and guide hole (right)

bottom hole accuracy of valve seat and guide hole is an important factor that directly affects the final machining accuracy of valve seat and guide hole. Because the coaxiality error of the bottom hole (generally less than 0..05mm) will cause uneven distribution of the finishing allowance of the valve seat and guide hole, thus affecting the final machining accuracy

in order to ensure the coaxiality tolerance of the valve seat and the bottom hole of the conduit hole, many manufacturers use special composite tools and process them in three processes: drilling and expanding, semi precision boring, and precision boring. In fine boring, in order to enhance the rigidity of slender boring bars, most of them use cemented carbide boring bars (Fig. 1), but there are also back guide supports (Fig. 2). Since the elastic modulus of cemented carbide (E = 500000 n/mm -630000 n/mm) is about 3 times larger than that of steel (E = 200000 n/mm), the boring bar made of cemented carbide can obtain better rigidity (r = 3ei/L). Adopting the back guide support method can also enhance the rigidity of the boring bar, but in order to ensure the support effect, the support guide sleeve of the back guide and the center of the boring bar should maintain a high enough coaxiality, which is also complex in structure

positioning of cylinder head

when finishing the working cone surface and guide hole of valve seat, most of them are positioned by the joint surface with the cylinder block and the two locating pin holes on the plane. This once widely used positioning method of one side and two pins, because the position error between the fixture positioning pin and the valve seat, the guide hole and the position error between the adjacent valve seat (and the adjacent guide hole) will cause the offset of the machining allowance. In the final finishing, the reamer with poor rigidity will also produce the machining offset, so this positioning method can not always achieve the specified accuracy

therefore, in order to ensure the machining accuracy, it is necessary to reduce the positioning error to improve the machining allowance. The research results of the project are of great significance to the uniformity of preventive maintenance resonance of high-grade asphalt pavement in our province to reduce the generation of waste mixtures and carbon emissions. Otherwise, it is more reasonable that the valve seat and conduit hole should be processed twice

the cylinder head adopts the plane and the outer circle of the guide hole for positioning, which can significantly reduce the above positioning error (right in Figure 2). With this positioning method, the positioning guide sleeve of the fixture and the spindle of the machine tool should maintain a strict coaxiality to ensure the uniform distribution of machining allowance. In this way, the valve seat and guide hole can reach the specified tolerance only by one-time processing. However, the disadvantage is that only one valve seat and conduit hole can be processed at one station. Compared with the positioning method of one side and two pins, the productivity is lower, that is, under the condition of maintaining the same productivity, a certain number of processing stations need to be added, thus increasing the length of the production line

special tool for machining valve seat and guide hole

Figure 3: Machining of valve working cone

Figure 4: special tool for machining valve seat and guide hole

using a special tool to simultaneously process valve seat and guide hole is conducive to improving coaxiality. Generally, two forming processes, spot facer and turning, are used to machine the working cone of the valve seat. The inclination angle of the countersink blade should be equal to the half cone angle of the working cone of the valve seat. During machining, because the valve seat is made of hardened material (HRC), the wear of the blade is faster, and the worn contour of the blade will be copied on the sealing cone, thus affecting the sealing performance of the valve seat during operation. However, its advantage is that the structure of the tool and the control of the cutting motion of the tool are relatively simple, and the machining efficiency is also high

machining the working conical surface of the valve seat with turning process can avoid the defects during spot facing. When machining the valve seat, it is generally necessary to machine the end face, 75 ° cone and 45 ° working cone of the valve seat. The first two surfaces are machined to obtain a working cone with constant width. Figure 4 shows the special tools for machining the valve seat and guide hole. The sliding plate tool holder arranged obliquely on the special tools is used for turning the working cone of the valve seat, and the three fixed tools are used for machining the end face, 45 ° and 75 ° cone of the valve seat. The reamer for processing the conduit hole is guided by the guide sleeve installed at the end face of the special tool. In order to ensure the coaxiality between the guide sleeve and the machine tool spindle, the guide sleeve can be ground after the guide sleeve is inserted into the tool body, so as to eliminate the manufacturing error and assembly error of components. The guide rail of the special tool body is manufactured accurately according to the angle of the working cone of the valve seat, so the machining accuracy and repetition accuracy of the working cone can be guaranteed

before machining, the two push rods that push the sliding plate and the reamer to move are at the starting position, so that the turning tool and the reamer are at the initial position of machining (the reamer only extends a few millimeters out of the guide sleeve). At the beginning of processing, the fixed tool first spot facers the valve seat and end face (cutter II) and chamfers (cutter edges III and IV) (Fig. 3). Then these cutters retreat by about 0.2mm, so that the cutting edges II, III and IV are separated from the machining surface, and the turning tool installed on the inclined sliding plate is in the position to be machined. At this time, the sliding plate is fed by pushing the outer push rod, and the working conical surface of the valve seat is precision machined. After machining, when the sliding plate moves to the end position, adjust the speed of the special tool according to the reaming speed, and the inner push rod pushes the reamer to realize the reaming of the conduit hole. When the reaming is finished, the internal software system displays the experimental force and displacement in real time, and the push rod returns and the tool returns to the starting position

if the CBN tool is used to turn the working cone of the valve seat, due to the high speed (2000r/min-3600r/min), the centrifugal force generated during processing will affect the machining accuracy. Therefore, the balance sliding plate should be used for this special tool

with the entry of high-speed machining centers into the field of automobile production, more and more cylinder heads have turned to machining centers for processing. Because it is difficult for the machining center to set up an oil cylinder to push the reamer to realize feeding at the rear end of the spindle, it can only seek other ways. MAPAL and Beck have developed special tools suitable for machining centers controlled by centrifugal force (Fig. 5). When machining the valve seat and guide hole, the tool first countersinks the conical surface of the valve seat at the speed of 1000r/min. After machining, the tool retreats by about 0.2mm, and then the tool speed increases to 5000r/min. At this time, the piston moves radially outward depending on the centrifugal force generated, squeezing the oil in the oil chamber, and with the help of the control of the valve, the reamer is driven to process the guide hole at a constant feed speed. When the conduit hole is processed, the speed of the special cutter will be reduced to 1000r/min, and the reamer will automatically return to the starting position

Figure 5: special tool for reaming feed driven by centrifugal force

Figure 6: comprehensive accuracy measurement of valve seat and guide hole

tools used for finishing guide hole are either single edge reamers or multi edge reamers. Because the single edge reamer relies on two guide strips arranged on the cutter body to guide and support the cutting force in the reaming process, it is less sensitive to the uneven machining allowance, which is conducive to improving the coaxiality of the machining of the working cone of the valve seat and the guide hole

with multi blade reamers, due to the high feed speed, the processing time of the guide hole can generally be shortened by about 75% compared with that of a single blade, but this reamer is sensitive to uneven machining allowance, which will affect the coaxiality of the working cone of the valve seat and the processing of the guide hole. At present, from the machining technology level of modular machine tools on the automatic line, the use of single edge reamers is conducive to ensuring machining accuracy

spindle components

according to experience, the machining accuracy of valve seat and guide hole is largely related to the rigidity and accuracy of spindle components. Therefore, the radial runout of the spindle components should be less than 2mm, and the end face runout should not be greater than 1.5mm. The size of these deviations directly determines the processing roundness of the working cone of the valve seat, which also affects the sealing performance of the valve seat

most spindle components use triple (or double) angular contact ball bearings as front bearings, and double angular contact ball bearings as rear bearings. The accuracy grade of bearings generally adopts P4

automatic detection of machining accuracy of valve seat and guide hole

in order to monitor the machining quality of cylinder head on the automatic line, a measuring station is usually set behind the finishing station. Figure 6 shows the automatic measuring device for the comprehensive accuracy of the cylinder head valve seat and guide hole. The device uses four pneumatic measuring heads to measure four valve seats and guide holes at the same time. An annular nozzle with a gap width of about 50mm is set on the measuring head measuring the working cone of the valve seat, and the mandrel measuring the conduit hole is also equipped with a measuring nozzle. When the mandrel is introduced into the guide hole at low speed, the working cone of the valve seat is automatically centered and aligned, which is achieved through specially designed rolling bearings. With the help of the spring, a certain adhesion force is applied to the measuring head for measuring the roundness of the working cone of the valve seat, so that the measuring head rests on the working cone. When the compressed air is connected for measurement, it can be measured by the amount of gas discharged from the annular nozzle:

roundness of the working cone of the valve seat

roundness of the guide hole

runout error of the working cone of the valve seat facing the guide hole

from the analysis of various processing technologies of the valve seat and guide hole, the following ways to improve its processing accuracy can be proposed:

in order to improve the processing coaxiality of the working cone of the valve seat and the guide hole, Ensure that the machining allowance of the conduit hole is evenly distributed, which can be achieved by positioning the joint surface of the cylinder head and the outer circle of the conduit