BMW is no stranger to automobiles. The fame of BMW automobiles is inseparable from its "heart" - the manufacturing strength of its engines. Today, we found a set of design drawings of the BMW engine, and also provided a detailed explanation of the engine processing technology. With these, would you machining technology experts consider building one?
Let's first take a look at a complete section of BMW's China factory-the entire engine block manufacturing and assembly process:
▲Real shot of the cylinder production line
From the video and historical experience, we can summarize the following key points to ensure the accuracy of the complex production process of the cylinder: using a machining center with automatic compensation function, and using special fixtures for one-time positioning and clamping, selection of key process tools, Optimizing the CNC and honing machine programs of the machining center, ensuring production conditions of constant temperature and humidity, etc. Let us take a closer look at an example below.
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Example: Cylinder Dimensional Characteristics Requirements
The figure below is a schematic diagram of the final product dimensions of an engine, including the finishing dimensions of the cylinder block top surface, cylinder bore, and crankshaft bore. The relevant requirements are as follows:
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▲Schematic diagram of the final product size of the engine 1
Dimensions of the top surface of the cylinder block: The dimensional tolerance of the distance from the top surface of the cylinder block to the crankshaft hole is 0.08mm, the roughness of the top surface is Rmax12.5 μm, the parallelism of the top surface to the crankshaft hole is 0.05mm, and the flatness of the top surface is 0.05 mm.
Cylinder bore size: The cylinder bore diameter tolerance is 0~0.015mm, the cylinder bore verticality requirement to the crankshaft bore is 0.05 mm/150mm, the cylinder bore processing position accuracy is 0.2mm in diameter, the cylinder bore cylindricity accuracy requirement is 0.01mm, the cylinder bore The hole roughness is Rz2~5μm.
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▲Schematic diagram of the final product dimensions of the engine 2
Cylinder crankshaft hole size: The roughness of the crankshaft hole is Rz10 μm, the diameter tolerance is 0~0.018 mm, the crankshaft hole position accuracy requires a diameter of 0.2 mm, a roundness of 0.005mm, and a cylindricity of 0.005mm. The crankshaft hole is No. 2, The coaxiality of the 3rd and 4th gears is 0.008 mm.
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Various process controls for top surface finishing
1. The clamping and positioning of the cylinder body is designed according to the product drawings. Since the drawing size datum is the bottom surface and the two locating pin holes on the bottom surface, and this type of datum has been processed in the previous process, the positioning datum for processing the top surface of the cylinder is the bottom surface and the two locating pin holes on the bottom surface. This eliminates the problem caused by datum conversion. Accuracy error.
2. Due to the large space size of the cylinder, if the clamping is not tight during the processing, the cylinder will shake slightly, which will seriously affect the processing accuracy. Therefore, it is necessary to design several self-locking auxiliary support points and clamping points on the fixture, and they should Ensure that the clamping force of the cylinder is uniform and prevent uneven clamping force and stress release after processing from affecting the flatness of the cylinder.
3. The machining center has added a 3-point gas inspection method. If the clamping is not in place, it will alarm in time, ensuring the stability of product processing.
4. Cubic boron nitride (CBN) blades and reamers are used for fine milling and precision hole processing on the top surface of the cylinder. These tools have long service life and have the advantages of high machining accuracy, small surface roughness parameter values, and high productivity. If carbide blades are used, the roughness processing requirements cannot be guaranteed under mass production conditions. As long as the coated carbide blades are processed more than 100 pieces, roughness will easily occur. However, the effect of using CBN blades is better. A lot, more than 200 pieces can be processed.
5. Control the cutting parameters of the machining center, optimize the CNC tool path, and appropriately increase the cutting speed, which can reduce the excessive flatness of the front and rear end surfaces of the cylinder caused by the uneven force on the top surface of the cylinder due to the uneven force of the tool when entering and exiting the cylinder. .
6. After the large plate is milled, use the CNC control program and add a large brush to scrape around the top surface of the cylinder along the milling cutter processing route to remove cylinder burrs.
7. Workshop temperature and humidity control. Control and set a constant temperature of 20±2℃ and a relative humidity of 40%~60% to reduce the impact of thermal expansion and contraction on the finishing of the cylinder.
The parallelism, flatness, and roughness inspection report after finishing the cylinder top surface is as follows.
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▲Cylinder top surface size inspection report
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Various process controls for cylinder bore finishing
Cylinder bore finishing size requirements are high, which is the focus of process manufacturing control. In order to ensure the quality of cylinder bore honing, the cylinder bore diameter size tolerance needs to be controlled to ±0.01 mm before honing. That is, only a 20 μm tolerance zone is allowed to pass the gas inspection before it can be entered into the honing machine for reaming honing. Before machining, the cylinder hole needs to be finely bored to the predetermined size in the machining center. The processing process is as shown in the figure.
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▲Processing process diagram
The precision boring tool adopts an internal cooling structure to ensure that the heat generated during the cutting process is taken away in time, thereby reducing the temperature of local processing and improving the surface quality of the cylinder bore. According to the comparison of the final product drawings, after the cylinder bore diameter is finely bored in the pre-honing process, a machining allowance of 0.04~0.05mm is reserved for the subsequent honing machine, the roughness is controlled at Rz10~20μm, and the cylindricity is controlled at 0.015mm. The positioning diameter of 0.2mm and the verticality of 0.05mm/150mm are consistent with the product drawings, that is, the roughness, diameter, and cylindricity of the cylinder hole are controlled by reaming while ensuring the positioning and verticality. After the cylinder bore is precision bored, the cylinder bore diameter is detected by online gas inspection equipment, and qualified products enter the honing machine through a motorized roller table. Although the one-time investment in imported cylinder bore reaming and honing equipment (such as a well-known honing machine in Germany) is relatively large, the subsequent processing costs are extremely low and the processing quality is stable.
Use vertical honing machine for processing. During the processing, the whetstone of the honing head is fed radially under the expansion and contraction mechanism, and the workpiece is gradually processed to the required size. There are 6 to 9 hinged honing sand bars on the outer periphery of the honing head. Six can be used for rough honing. Nine honing rods are used, and the length is about 1/3~2/3 of the length of the cylinder hole. When honing, the reciprocating speed is controlled at 25~35 m/min, and the honing efficiency is high. The greater the reciprocating acceleration of honing, the smaller the arc transition area formed during reversing, and the higher the quality of the honing texture.
The pressure of the honing stone on the hole wall is controlled at 0.3~0.5MPa. The pressure of the honing stone directly affects the surface quality of the workpiece, the wear amount of the oilstone, the dimensional accuracy and surface roughness of the workpiece.
After the cylinder bore is finely bored, rough honing and fine honing can meet the product size requirements. After honing, a 20°~30° honing pattern will be produced, which has a good effect on the lubricant storage of the oil film in the cylinder bore. The diameter, position, cylindricity, verticality and roughness inspection reports of the cylinder bore after honing are as follows.
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Various process controls for crankshaft hole finishing
The maximum speed of commonly used gasoline engines can reach 6000r/min, which places high demands on crankshaft hole technology. In order to ensure the quality of crankshaft hole honing, the crankshaft hole diameter size tolerance needs to be controlled to ±0.015mm before honing, that is, only a tolerance of 0.03mm is allowed. After the belt passes the gas inspection, it can go into the honing machine for platform honing. Before the crankshaft hole goes into honing, the crankshaft hole needs to be finely bored to a predetermined size in the machining center. The processing process diagram is shown in the figure.
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▲Crankshaft hole process diagram
The precision boring tool of the machining center adopts an internal cooling structure. Since the depth of the crankshaft hole is large, reaching 300mm, the requirements for the boring tool are also high. CBN blades are used, and their structure is shown in the figure.
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▲Structure diagram of boring tool
The selection of the tool is very important. The tool has 4 semi-finish boring inserts and 6 fine boring inserts. During the machining process, the guide hole is first enlarged by the blade and most of the allowance is cut away, and then the semi-finish boring blade is bored to a diameter of 48.79mm, leaving a 0.2mm allowance for the blade with a diameter of 48.995mm for cutting; the crankshaft hole is first bored The positions of the 1st and 2nd gears, and then rotate the turntable to boring the 5th, 4th and 3rd gears of the crankshaft hole on the other side. According to the comparison of the final product drawings, after the cylinder crankshaft hole diameter is finely bored in the pre-honing process, a machining allowance of 0.04~0.05mm is reserved for honing on the subsequent honing machine platform.
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Compared with ordinary honed crankshaft holes, the crankshaft hole with platform texture is shortened by 1/3~1/2, the service life is increased by 10%~20%, the torque is increased by 5%, and the oil consumption is reduced by 50%~60%. The platform honing roughness is controlled at Rz10~20μm, the cylindricity is controlled at 0.01mm, while the position diameter is 0.2mm, the roundness is 0.01mm, and the coaxiality is 0.008mm. The product size requirements are also met through platform honing.
In order to ensure dimensional accuracy, the crankshaft hole is honed in place in one step, that is, from the first gear platform to the fifth gear, while rotating and reciprocating. The diameter, roundness, position, coaxiality, cylindricity, and roughness inspection report of the cylinder crankshaft hole after honing is shown in the figure.
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▲Crankshaft hole size inspection report
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For the above fine machining of the top surface of the engine block, cylinder bore, and crankshaft bore, the self-locking fixture and auxiliary support design are adopted, advanced tool materials and tool combination structures are selected, the processing temperature and humidity are controlled, and the processing process parameters and chip margin are well controlled. and other process measures to ensure product finishing accuracy requirements, thereby ensuring product assembly and mass production performance.
