Finishing plan for crankshaft hole of the hottest

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Finish machining scheme for crankshaft hole of cast iron engine cylinder block

the machining quality of crankshaft hole has a significant impact on the working performance of the engine. According to the actual machining situation, the engine business department of Chery company has continuously studied and improved the finish machining scheme for crankshaft hole of cast iron engine cylinder block, with remarkable results, so that the machining quality and efficiency have been effectively improved

the cylinder block is an important basic part of the engine. Its main function is to assemble all mechanisms and systems into one. The crankshaft, piston and connecting rod, the most important moving parts of the engine, are closely related to the cylinder block. Figure 1 is the schematic diagram of crank connecting rod mechanism. At present, the maximum speed of our common gasoline engine is more than 6000 R/min. the crankshaft forms an oil film between the crankshaft hole and the bearing bush in the engine cylinder block, and supports and lubricates the crankshaft running at high speed in the form of sliding bearings, which puts forward higher technological requirements for the finishing of the crankshaft hole

because the machining quality of the crankshaft hole has a great impact on the working performance of the engine, Therefore, we generally have strict requirements for the process of engine crankshaft hole "With the industry's search for lighter renewable materials, including diameter, position, roundness, straightness and surface roughness of the crankshaft hole center of each gear, etc. in order to meet these demanding process indicators, precision machining generally adopts two machining methods: precision boring or reaming and honing.

analysis of precision boring machining methods

compared with reaming and honing, the initial investment cost of precision boring crankshaft hole is low, and it can be processed in machining center or The finishing of the special machine and other parts are arranged in the same process. Considering the special process requirements, the current fine boring scheme we adopt is the combination of two fine boring cutters. The first fine boring cutter guides the boring of the 1st and 2nd gear crankshaft holes (see Figure 2), and the second fine boring cutter accurately bores the remaining crankshaft holes under the support of the 1st and 2nd gear crankshaft holes (see Figure 3). Due to the need to maintain a high coaxiality between the gears of the crankshaft hole, the finishing machining must be carried out in the same direction. However, such a long crankshaft hole boring cutter will jump too much due to lack of support when machining the first gear crankshaft hole, so the combination of two fine boring cutters, one long and one short, is used to finish machining the crankshaft hole

Figure 2 fine boring cutter for machining 1st and 2nd gear crankshaft holes

Figure 3 fine boring cutter for machining the last few gear crankshaft holes

at present, the boring cutter for fine boring crankshaft holes generally adopts carbide or CBN blades. The cost of cemented carbide blade is low, but the surface roughness of the workpiece is not good, and the durability of the blade is low. To solve this problem, we worked with tool suppliers to collect common blade coating materials in the industry and experience and technology in blade cutting angle, conducted cutting experiments on 16 cemented carbide blades, and tried to optimize various appropriate processing parameters. The specific roughness fluctuation is shown in Figure 4. Through a large number of verification, the best experimental result of cemented carbide blade on the premise of meeting the process requirements such as roughness is to process 80 at one time and clamp the metal anchor rod and anchor cable clamps, which still can not meet the requirements of mass production

the roughness fluctuation of some cemented carbide blades that meet the requirements in Figure 4

then, we verified another scheme for the blade material of the fine boring cutter, that is, to replace the existing cemented carbide blade material with CBN blade material. We rely on tool suppliers to develop more than 10 different CBN blades to verify the cutting of crankshaft holes. The test results show that CBN blades are significantly better than cemented carbide blades in terms of roughness control, and the processing life can exceed 200 pieces. But at the same time, there are also some new problems: when machining about 150 pieces with CBN blades, slight burrs begin to appear on the hole side, and flanging will occur in serious cases. Burr and even flanging here become quality hidden dangers that affect engine performance, and serious quality accidents such as bearing bush scratch and crankshaft locking will be caused. In terms of the treatment of crankshaft hole burr, which has contributed to the "cost reduction and efficiency increase" of the mine, we have increased the brush of crankshaft hole, which has alleviated the existence of burr to a certain extent, but there is still a large quality risk

in addition, the quality stability of crankshaft hole precision boring scheme is relatively poor compared with reaming and honing, and the frequent tool change will greatly increase the risk of quality control. The variation trend of the surface roughness of the workpiece processed by the fine boring cutter decreases rapidly with the reduction of the tool durability. The comparison of the surface roughness of the workpiece processed by CBN blade in the fine boring crankshaft hole is shown in figures 5 and 6

Figure 5 surface roughness report of the first workpiece machined by boring cutter

Figure 6 surface roughness report of 200 workpiece machined by boring cutter

analysis of reaming and honing processing methods

we have done a lot of exploration work on the crankshaft hole scheme of boring and finishing, and developed a variety of new blades for machining verification, but still did not achieve the ideal processing effect, so we consider the crankshaft hole reaming and honing scheme again. In terms of cost, the one-time investment in crankshaft hole honing is large. The preliminary quotation of a well-known honing machine manufacturer in Germany is 400000 euros (due to different configuration requirements, the price 2 and the problem grid of experimental travel will vary), but the later processing cost is much lower than that of fine boring, and the processing quality is stable. The specific implementation of the crankshaft hole honing scheme is shown in Figure 7

Figure 7 implementation of crankshaft hole honing scheme

horizontal honing machine is used for crankshaft hole honing, because the working stroke of horizontal honing machine is long, which is suitable for deep hole honing such as crankshaft hole, and the maximum depth can reach 3000 mm. In the processing process, the oilstone of the honing head is fed radially under the action of the expansion and contraction mechanism, and the workpiece is gradually processed to the required size. The honing head is surrounded by 2 ~ 10 oilstones with a length of about 1/3 ~ 3/4 of the hole length. When honing the hole, they both rotate and move back and forth. At the same time, they expand evenly through the spring or hydraulic control in the honing head, so the contact area with the hole surface is large and the processing efficiency is high

the honing allowance is generally not more than 0.2mm. When machining cast iron or non-ferrous metals, the peripheral speed of honing can reach more than 50m/min; The reciprocating speed of honing should not exceed 15 ~ 20m/min. The pressure of oilstone on the hole wall is generally 0.3 ~ 0.5MPa, which can reach about 1 MPa during rough honing and less than 0.1 MPa during fine honing. Because the oilstone is in surface contact with the workpiece during honing, the vertical pressure of each abrasive particle on the workpiece surface is only 1/50 ~ 1/100 of that during grinding. Coupled with the low honing speed, the temperature of the cutting area can be maintained in the range of 50 ℃ ~ 150 ℃, which is conducive to reducing the residual stress on the machined surface and improving the surface quality. In order to wash the chips, avoid blocking the oilstone, and reduce the temperature and surface roughness of the cutting area, the cutting fluid used in honing must have a certain working pressure and be filtered. Most cutting fluids are kerosene or kerosene plus spindle oil, or extreme pressure emulsion

according to the fluctuation trend of the roughness measurement report data in Figure 8, the machining stability of honing is very high, and the durability of honing head replacing the sand bar at one time can process more than 10000 pieces, and the production efficiency is greatly improved. Using the fine boring method of CBN blades, the tool needs to be changed once every 200 pieces, and the average time for each tool change and the first piece inspection is 42min. If the time spent in frequent tool change is converted to each workpiece, 12.6s/piece will be lost. According to the processing tempo of an existing production line is 120s/piece and the production program is 160000 pieces/year, the processing time of the whole year will be lost by 2.016 million S. using this time, 16800 pieces can be processed every year

figure 8. The roughness measurement report of the first inspection piece that implements the reaming and honing scheme for 20 consecutive days

table shows the comparison of tool loss costs caused by different finishing methods of crankshaft holes. From the table, it can be analyzed that the tool cost gap of a single piece is obvious. According to theoretical analysis and calculation, when 1.03 million workpieces are produced, the tool loss of the reaming and honing scheme is saved by about 3.811 million yuan compared with the CBN fine boring scheme, which is slightly the same as the price of the reaming and honing equipment, and the quality control difficulty and risk of the reaming and honing scheme are much smaller. The cost of tool loss caused by different finishing methods of crankshaft hole (unit: yuan)

comparison of two schemes

after a large number of experiments and production practice, we believe that the reaming and honing method and boring method have their own advantages, which can be summarized as follows:

1 Cost investment

boring method has less cost investment, so you only need to customize special tools and share machine tools with other parts of finishing; The one-time investment of reaming and honing is high, but it can be considered to share the honing machine with cylinder hole honing to appropriately reduce the investment

2. Floor area

boring method is adopted, and no separate new equipment is required; Using the reaming and honing method requires a separate area to arrange the reaming and honing equipment. If the equipment is shared with the cylinder hole honing, the floor area of the honing machine needs to be increased

3. Machining quality

boring method can meet the requirements of machining quality, but its stability is relatively poor; The reaming and honing method can not only meet the requirements of machining quality, but also have good quality stability

4. Production efficiency

although the cutting efficiency of boring method is not low, frequent tool changes cause a large loss of time; After the reaming and honing equipment is adjusted at one time, it can be processed continuously and stably, and the production efficiency is high

5. Cost of single tool

due to high process requirements and low blade durability, the cost of single tool in boring mode is high; With the reaming and honing method, the honing head and sand bar of the reaming and honing equipment have high durability and low cost of a single tool

to sum up, although these two methods are widely used in the crankshaft hole finishing of various enterprises at present, according to the analysis of their advantages and disadvantages, the precision boring method is more suitable for small output; If the output is large and the quality requirements are high, it is recommended to use the crankshaft hole honing method for processing, so as to improve the processing efficiency and quality stability of the product and reduce the processing cost of the engine. (end)

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