Before the 1970s, the rough machining method of the engine crankshaft was to turn the crankshaft main journal and connecting rod journal on a multi-tool lathe. This method has low processing accuracy, poor flexibility, low process quality stability, and is prone to large internal stress, making it difficult to achieve a reasonable processing allowance. After rough machining, stress relief and tempering is generally required to release stress. Therefore, rough machining needs to leave a large machining allowance for the subsequent finishing process to remove the bending deformation. Crankshaft finishing uses ordinary grinding technology, generally using MQ8260 crankshaft grinder rough grinding-semi-finish grinding-finish grinding-polishing. Usually manual operation results in unstable processing quality and high scrap rate.
From the 1970s to the 1980s, CNC turning and CNC external milling were used for crankshaft rough machining, and the processing conditions improved. Finishing is still dominated by ordinary grinder grinding technology.
In the mid-1980s, the CNC internal milling process appeared again. The performance index of CNC internal milling is higher than that of CNC external milling. Especially for forged steel crankshafts, internal milling is more conducive to chip breaking. The finishing process mostly uses semi-automatic crankshaft grinders, and the headstock and tailstock are driven synchronously, which improves the processing accuracy to a certain extent.
From 1985 to 1990, the crankshaft turning and turning-turning processes were developed. This process has the advantages of high precision and high efficiency. It is especially suitable for the side of the balance weight that does not need to be processed and the journal has sinking. For crankshafts with grooves (including axial undercut grooves), the crankshaft can be directly fine-grinded after processing, eliminating the need for rough grinding. A small amount of CNC grinding machine grinding technology has been used for crankshaft finishing, and dimensional consistency has been improved.
In the mid-1990s, CNC high-speed external milling was developed. For crankshafts that need to be processed on the side of the balance weight, the production efficiency is higher than CNC turning, CNC internal milling, and lathe-turn pulling. In addition, CNC turning-turning process requires two processes to process the connecting rod journal, while CNC high-speed external milling can be completed in only one process. It has the following advantages: high cutting speed (up to 350m/min), short cutting time, and process The cycle time is shorter, the cutting force is smaller, the workpiece temperature rise is lower, the tool life is longer, the number of tool changes is less, the machining accuracy is higher, and the flexibility is better. Therefore, CNC high-speed external milling will be the development direction of rough machining of crankshaft main journal and connecting rod journal. CNC grinding machines are used for finishing, and control devices such as hydrostatic spindle, hydrostatic guide rail, hydrostatic feed screw (grinding wheel headstock) and linear grating closed-loop control are used to ensure that the dimensional tolerances and geometric tolerances are reliably guaranteed. CNC abrasive belt polishing machines are also widely used for super-finishing. The surface roughness of the crankshaft journal after super-finishing is at least improved by one level of precision.
CBN high-speed grinding developed in the 1990s. The crankshaft grinder produced by the British company LANDIS has a grinding speed as high as 120m/s. It only takes a few minutes to complete the process from rough to fine grinding in one clamping using the peeling method. This will lead to a new situation in which grinding replaces other rough machining processes.
After entering the 21st century, composite processing technology has entered the crankshaft manufacturing industry. Composite machine tools should have process integration functions and multiple processing integration functions. The horizontal turning and milling compound machining center produced by the Austrian WFL company (Figure 3 is the M40G model) can "clamp once and process all" before the crankshaft is hardened. The processed crankshaft can be directly transferred to the finishing process; in terms of crankshaft finishing, CBN CNC grinding machines with integrated processes have also appeared, that is, all crankshaft main journals and connecting rod journals can be ground in one clamping (oscillation tracking grinding).
It can be seen from the above evolution that the crankshaft processing technology is developing in the direction of high speed, high efficiency and compounding. At present, the more popular rough machining process is that the main journal adopts turning-turning process and high-speed external milling, and the connecting rod neck adopts high-speed follow-up external milling, all of which are dry cutting; finishing machining is processed by CNC grinder, with automatic feeding, automatic It has functions such as correction of the grinding wheel, automatic size and roundness compensation, automatic indexing and electronic synchronous drive at both ends. The main journal and connecting rod neck can be clamped and grinded at one time; for super-finishing, a CNC belt polishing machine with a size control device is used.
Introduction to the performance of typical crankshaft machining advanced equipment CNC high-speed follow-up external milling: The performance of a high-speed follow-up external milling machine model VDF315OM-4 is introduced.
This machine tool is a flexible CNC milling machine specially designed and manufactured by the German BOEHRINGER company for automobile engine crankshafts. This equipment applies workpiece rotation and milling cutter feed servo linkage control technology, and can be clamped at one time without changing the crankshaft rotation center. It can follow the connection of the milled crankshaft. Rod journal. It adopts an integrated composite material structure bed and is driven by electronic synchronous rotation at both ends of the workpiece. It has the characteristics of dry cutting, high machining precision, and high cutting efficiency. Using the SIEMENS840DCNC control system, the equipment operating instructions are displayed on the human-machine interface by inputting the basic information of the part. The processing program can be automatically generated based on the parameters, and various crankshafts with a length of 450 to 700mm and a rotation diameter within 380mm can be processed. The diameter error of the connecting rod journal is ±0.02mm.
CNC turning-turning machine tool: This equipment can complete all concentric circle turning in one setting, and complete turning-turning (car side end face) processing on the same machine tool, with high processing efficiency. The use of special chucks and tool systems can also achieve flexible processing, and the machine tool maintenance is simple and the maintenance cost is low. It is especially suitable for crankshafts that do not need to be processed on the side of the balance weight and have undercut grooves in the journal. Among them, the broaching process can be replaced by the efficient comb tool (Figure 5) turning process. The comb tool processing is usually placed at the last step of the process, and high-speed finishing is achieved through trace radial feed and longitudinal turning.
Crankshaft thrust surface turning and rolling machine: This equipment is used to finish turning and rolling the crankshaft thrust surface, and has the following technical features: rolling and polishing the thrust surface and online measurement, rolling polishing instead Grinding processing and turning processing can be performed at the same time. A rolling polishing device is installed on the cutter head to obtain higher precision. At present, the equipment with better performance includes the crankshaft thrust surface rolling machine of the German company HEGENSCHEID.
CNC crankshaft grinder: Take the swing tracking series grinder of Erwin Junker Machine Manufacturing Co., Ltd. (JUNKER) in Germany as an example. This equipment uses a CBN grinding wheel for high-speed machining and an oil-cooled crankshaft. The combination is suitable for processing automobile engine crankshafts with reliable quality. The main functions are: detecting and correcting the journal roundness and size during the machining process; a control system with a "learning function", additional automatic compensation for roundness deviation and interference amount. The interference amount that can be compensated is: temperature, Mechanical and power influences, changes in grinding allowance, changes in materials and metallographic structures, the machinability of the grinding wheel, and the wear status of the machine tool; grinding the main journal and connecting rod journal are clamped at one time, and the theoretical deviation is zero ; Plunge grinding and swing grinding; To support "sensitive workpieces", an automatic centering three-point steady rest is used on the spindle; CNC-controlled coolant supply ensures long-term use of the grinding area; Use of static pressure circle Type guide rails, no creeping phenomenon, ensuring long-lasting high accuracy (X-axis guide rails, feed screws, thrust bearings); shock-absorbing and anti-torsion machine bed, cast from mineral synthetic materials, has good shock-absorbing and bending resistance Function; Grinding spindle is suitable for grinding up to 140m/s.
From the advanced equipment introduced above, we can see one thing in common, which is high speed, high efficiency and flexibility, which is suitable for today's development trend of multiple varieties and small batches of products. Since crankshaft machining is different from ordinary machining, special tools must be used for many processes, such as internal milling, turning-turning and high-speed external milling introduced above. All special tools are used. The tool bodies of these tools are complex to manufacture and the price is also high. Relatively expensive. If the product variant involves a change in the crankshaft structure, a new cutter body must be used to process the crankshaft, which will affect the product development cycle and manufacturing costs, ultimately leading to a lack of competitiveness in the product. Now Walter and other tool manufacturers have developed flexible special tools for crankshaft manufacturing-modular tools. The product development cycle is greatly shortened and manufacturing costs are reduced.
Discussion on Crankshaft Agile and Flexible Production Line (AFTL) Plan Currently, most domestic car crankshaft production lines are high-speed flexible production lines FTL (FlexibleTransferLine). The characteristic of this production line is that it can not only process crankshafts of the same series, but also process variant products. , replacement products and new products, truly have the meaning of flexibility. In order to further improve the production efficiency of high-speed flexible production lines and adapt to the market faster, the next step of FTL's development is the agile flexible production line AFTL (Agile Flexible Transfer Line). Its main purpose is:
To meet the changing needs of the market.
Not only meet the requirements of current products, but also consider future market needs.
Meet the needs of production methods. It can meet the production needs of modern engines of "multiple varieties, large and medium batches, high efficiency and low cost".
In line with the principles of "lean production". Put an end to waste and seek profits with minimum investment and maximum return.
Due to the unique structure of the engine crankshaft itself, the author believes that the crankshaft AFTL should have the following characteristics: It is composed of a high-speed machining center and efficient special machine tools (including a small number of combined machine tools). The machine tools are arranged according to the process flow and connected by automatic conveying devices, and are produced using flexible fixtures and efficient special tools. In order to prevent the entire production line from shutting down due to equipment failure in key processes, parallel equipment can be added to meet the needs of mass production. The following is the process flow (only the metal processing part):
Milling the end face, determining the total length, drilling the mass center hole, turning the large and small end cylindrical → milling the main journal and shoulder → milling the connecting rod neck and shoulder → Turning the main journal and countersunk groove → Turning the connecting rod neck and countersunk groove → Gun drilling the oil hole → Cleaning → Fillet rolling → Flange drilling and tapping → Precision grinding the main journal (CBN) → Precision grinding the connecting rod neck (CBN)→Bevel cutting and grinding the small end→Bevel cutting and grinding the flange end→turning the rolling thrust surface and milling the keyway→dynamic balancing→abrasive belt polishing of the spindle, connecting rod and flange outer diameter→cleaning, cooling→detection and classification.
Several issues regarding the above-mentioned process flow are discussed as follows: Selection of crankshaft mass center hole and geometric center hole.
The quality of the blank is good, the machining allowance is small and the machining allowance is evenly distributed. At this time, the mass center hole of the crankshaft basically coincides with the geometric center hole, so there is no need to spend a high amount of money to purchase mass centering equipment and directly drill the geometric center hole. ,li>The quality of the blank is poor, the machining allowance is large and the machining allowance is unevenly distributed, so the quality center hole should be given priority. Due to the large initial imbalance, if the geometric center hole is drilled, the mass distribution will be uneven and the moment of inertia will be large, which will damage the accuracy of subsequent processing equipment. Furthermore, when the geometric center hole is used for dynamic balancing, the initial imbalance may exceed the requirements of the balancing machine and cannot be balanced. In this case mass centering machines should be preferred.
Principles for the rational selection of crankshaft roughing machine tools. The importance of advanced metal processing equipment in crankshaft manufacturing is undoubted. It can reliably ensure dimensional accuracy and consistency, adapt to the requirements of production rhythm, and improve the overall level of craftsmanship. However, we cannot adopt the "use principle", and it does not mean that the more advanced the equipment, the better. It should comply with the following three principles: 1) It conforms to the principle of craftsmanship and, combined with the product structure, can meet the requirements of dimensional accuracy and consistency. 2) In line with the principle of economy, the form of bidding is used to reduce costs. 3) Comply with the principles of equipment management and maintainability, inspect the quality of after-sales service of the equipment manufacturer, and whether the equipment wearing parts can be purchased at any time, etc.
Reasonable combination Domestic crankshaft manufacturing companies have some misunderstandings about the introduction of equipment. For example, they believe that the more advanced and expensive the equipment, the better. In fact, if used improperly, advanced equipment will not play its due role and cause waste. The following takes the reasonable combination of CNC high-speed external milling, CNC internal milling, and CNC turning-turning as an example.
The side of the crankshaft balance block needs to be processed. CNC internal milling or CNC high-speed external milling should be used first for the main journal processing. CNC high-speed external milling should be used for the processing of the connecting rod neck. If the blank is a forged steel blank, CNC internal milling is more conducive to chip breaking. It is not advisable to use CNC turning-turning. Since the side of the balance weight is turned intermittently and the crankshaft speed is very high (about 1000r/m), the tool collapse phenomenon is serious.
The side of the crankshaft balance block does not need to be processed. It is more reasonable to use a CNC lathe for main journal processing, and the machining accuracy is high. Since the connecting rod journal axis is not on the same center line, such as a six-turn crankshaft, there are some troubles in lathe-turn machining, so CNC high-speed external milling is more reasonable.
For crankshafts with undercut grooves in the journal, CNC turning and turning shows its superiority. If there are undercut grooves in the axial direction, CNC high-speed external milling and CNC internal milling cannot process it, while turning -The car can be processed.
The above equipment should use independent dual cutterheads, modular tool systems, etc. to achieve flexible processing.
Crankshaft fillet rolling strengthening The crankshaft fillet rolling strengthening is mainly to improve the fatigue strength of the crankshaft.
According to statistical data, the service life of a ductile iron crankshaft can be increased by 120-230% after fillet rolling; the service life of a forged steel crankshaft can be increased by 70-130% after fillet rolling. Therefore, this strengthening method is highly valued by various crankshaft manufacturers. At present, almost all foreign car crankshafts adopt the rolling strengthening process. When using this equipment, attention should be paid to flexibility to adapt to the processing of different products.
Crankshaft abrasive belt polishing uses abrasive belt polishing to polish the main journal, connecting rod neck, flange, fillet and thrust surface at the same time. The fillet and thrust surface are polished by the axial movement of the crankshaft. The surface roughness after polishing is at least improved by one level of accuracy. In order to process crankshafts of various varieties and modifications, independent polishing heads can be used to process in multiple steps to achieve flexibility.
Crankshaft cleaning: The crankshaft is usually cleaned twice. The first cleaning is arranged after the gun drills the oil hole to remove the iron filings in the oil hole and the lubricating oil on the crankshaft surface to provide clean water for the next process. Semi-finished product. The second cleaning is arranged after the abrasive belt polishing, and a special fixed-point cleaning machine is used to clean the oil holes, flange screw holes, etc. with a special nozzle.
For the finishing process of crankshaft, single grinding wheel or independent double grinding wheel CBN CNC grinder should be used for the crankshaft grinding main journal and connecting rod neck process. It is not easy to use multi-grinding wheel integrated grinder. Although it is efficient, it cannot be adapted. Multiple varieties of flexibility required.