In 1970s and 1980s, CNC turning and CNC external milling were used in crankshaft rough machining, which improved the machining conditions. Finishing is still a grinding process based on ordinary grinder.
In the mid-1980s, NC internal milling reappeared, and its performance index was higher than that of NC external milling, especially for forged steel crankshaft, which was more conducive to chip breaking. Semi-automatic crankshaft grinder is mostly used in finishing process, and the headstock and tailstock are driven synchronously, which improves the machining accuracy to a certain extent.
From 1985 to 1990, crank turning and turning-turning technology were developed. This process has the advantages of high precision and high efficiency, and is especially suitable for crankshafts with countersunk grooves (including axial countersunk grooves) on the side of the balance weight. The crankshaft can be directly finished after machining, which saves the rough grinding process. A small number of CNC grinders are used in crankshaft finishing, and the consistency of dimensions has been improved.
In the mid-1990s, CNC high-speed external milling was developed, which was more efficient than CNC turning, CNC internal milling and vehicle-to-vehicle milling. In addition, the NC turning-turning process needs two processes to process the connecting rod journal, and the NC high-speed external milling can be completed in only one process, which has the following advantages: high cutting speed (up to 350m/min), short cutting time, short process cycle time, small cutting force, low temperature rise of the workpiece, long tool life, few tool changes, high machining accuracy and good flexibility. Therefore, CNC high-speed external milling will be the development direction of rough machining of crankshaft journals and connecting rod journals. CNC grinding machine is used for finishing, and hydrostatic spindle, hydrostatic guide rail, hydrostatic feed screw (grinding wheel head frame), linear grating closed-loop control and other control devices are used to ensure the dimensional tolerance and shape tolerance reliably. CNC abrasive belt polishing machine is widely used in ultra-precision machining, and the surface roughness of crankshaft journal after ultra-precision machining is improved by at least one grade.
CBN high-speed grinding developed in 1990s. Crankshaft grinder produced by LANDIS Company in England has a grinding speed as high as 1.20m/s, and it takes only a few minutes to finish grinding from blank to finish machining by peeling. This will lead to a new situation in which grinding replaces other rough machining processes.
After entering 2 1 century, compound machining technology has entered the crankshaft manufacturing industry. The compound machine tool should have the functions of process integration and multi-process integration. The horizontal turning and milling compound machining center (M40G model in Figure 3) produced by WFL company in Austria can be "clamped once and machined completely" before the crankshaft hardens, and the machined crankshaft can be directly transferred to the finishing process. In the aspect of crankshaft finishing, CBN CNC grinding machine with integrated technology has also appeared, that is, grinding all crankshaft journals and connecting rod journals at one time (swing tracking grinding).
It can be seen from the above evolution that the machining technology of crankshaft is developing in the direction of high speed, high efficiency and compounding. At present, the popular rough machining technology is turning-turning-drawing technology of main journal and high-speed external milling, and the high-speed follow-up external milling of connecting journal is dry cutting. Finishing is processed by CNC grinding machine, which has the functions of automatic feed, automatic correction of grinding wheel, automatic compensation of size and roundness, automatic indexing and electronic synchronous drive at both ends. Both the spindle journal and the connecting rod journal can be clamped and ground at one time; Ultra-precision machining adopts CNC abrasive belt polishing machine with size control device.
Brief introduction of CNC high-speed external milling performance of typical advanced crankshaft processing equipment: This paper introduces the performance of a VDF3 15OM-4 high-speed external milling machine. This machine tool is a flexible CNC milling machine specially designed and manufactured by Boehringer Company in Germany for automobile engine crankshaft. The equipment applies the servo linkage control technology of workpiece rotation and milling cutter feed, which can follow up and mill the crankshaft connecting rod journal at one time without changing the crankshaft rotation center. The lathe bed with integrated composite structure is driven by electronic synchronous rotation at both ends of the workpiece, which has the characteristics of dry cutting, high machining accuracy and high cutting efficiency. By using siem nuced dcnc control system, the basic parameters of the parts can be input in the man-machine interface to automatically generate a machining program, which can machine various crankshafts with a length of 450 ~ 700 mm and a rotary diameter of less than 380mm, and the diameter error of the connecting rod journal is 0.02 mm ..
CNC Lathe-Lathe Machine Tool: This equipment can turn all concentric circles at one time, and finish lathe-lathe (lathe side end face) machining on the same machine tool, with high machining efficiency, flexible machining with special chuck and cutter system, and low maintenance cost of the machine tool, which is especially suitable for crankshafts that do not need to machine the side face of the balance weight and the countersunk groove of the journal. Broaching process can be replaced by efficient comb-cutter turning process (Figure 5), and comb-cutter machining is usually placed in the last step of this process, and high-speed finishing can be realized through a small amount of radial feed and longitudinal turning.
Crankshaft thrust surface turning machine: This equipment is used for finish turning and hobbing the crankshaft thrust surface, and has the following technical characteristics: hobbing and polishing the thrust surface and measuring it online, instead of grinding, turning at the same time, and installing a hobbing and polishing device on the cutter head to obtain higher accuracy. At present, the equipment with better performance is the special machine for turning and rolling the crankshaft thrust surface of HEGENSCHEID company in Germany.
CNC crankshaft grinder: Take Erwin JUNKER's swing tracking series grinder as an example. The equipment combines high-speed machining CBN grinding wheel with oil-cooled crankshaft, and is suitable for machining reliable automobile engine crankshaft. Mainly manifested in: detecting and correcting the roundness and size of the journal during machining; The control system with "learning function" supplements the automatic compensation of roundness deviation and interference. Compensatable disturbances include: temperature, mechanical and dynamic influences, changes in grinding allowance, changes in materials and metallographic structure, machinability of grinding wheels and wear of machine tools; Grinding the main journal and connecting rod journal once, the theoretical deviation is zero; Cut-in grinding and swing grinding; For the support of "sensitive workpiece", a three-point center frame with automatic centering is adopted on the main shaft; The supply of cooling liquid controlled by CNC ensures the lasting consumption of grinding area; Adopt static pressure circular guide rail, no crawling phenomenon, to ensure lasting high precision (X-axis guide rail, feed screw, thrust bearing); The damping and torsional bed is made of mineral synthetic materials, which has good damping and bending functions; The grinding wheel shaft is suitable for grinding up to140m/s.
From the above-mentioned advanced equipment, we can see one thing in common, that is, high speed, high efficiency and flexibility, which is suitable for the development trend of many varieties and small batches of products today. Because crankshaft machining is different from ordinary machining, many processes must use special tools, such as internal milling, turning-turning, high-speed external milling and so on, which are all in use. The bodies of these tools are complex and expensive to manufacture. If the product modification involves the change of crankshaft structure, it will lead to the need to use a new tool body to process the crankshaft, which will affect the product development cycle and manufacturing cost, and eventually lead to the lack of competitiveness of the product. Now tool manufacturers such as Walter have developed flexible special tools for crankshaft manufacturing-modular tools. Greatly shorten the product development cycle and reduce the manufacturing cost.
Discussion on the Scheme of Agile Flexible Crankshaft Production Line (AFTL) At present, domestic car crankshaft production lines are mostly high-speed flexible production lines (FTL), which are characterized by processing not only the same series of crankshafts, but also variant products, substitute products and new products, thus achieving real flexibility. In order to further improve the production efficiency of high-speed flexible production line and adapt to the market faster, the next development of FTL is AFTL(AgileFlexibleTransferLine). Its main purposes are:
Meet the changing needs of the market. It not only meets the requirements of current products, but also tests the future market demand.
Meet the needs of the mode of production. It can meet the production requirements of "multiple varieties, large and medium batch, high efficiency and low cost" of modern engines.
Based on the principle of "lean production". Put an end to waste and seek profits with the least investment and the greatest return.
Because of the particularity of engine crankshaft structure, the author thinks that crankshaft AFTL should have the following characteristics: it is composed of high-speed machining center and high-efficiency special machine tools (including a small number of combined machine tools). Machine tools are arranged according to the process flow, connected by automatic conveying devices, and produced by using flexible fixtures and efficient special tools. In order to prevent the failure of key process equipment from causing the whole line to stop production, parallel equipment can be added to meet the needs of mass production. The following is the process flow (metal processing only):
Milling the end face, fixing the total length, drilling the center hole of mass, turning the excircle of large and small heads → milling the main journal and shaft shoulder → turning the main journal and countersunk head → turning the connecting journal and countersunk head → gun drilling the oil hole → cleaning → rounding → flange drilling and tapping → fine grinding the main journal (CBN)→ fine grinding the connecting journal (CBN)→ oblique cutting and grinding the flange end →
Several problems in the above process are discussed: the selection of crankshaft mass center hole and geometric center hole.
The blank quality is good, the machining allowance is small, and the machining allowance is evenly distributed. At this time, the mass center hole of crankshaft basically coincides with the geometric center hole, so it is not necessary to spend a lot of money to buy quality centering equipment, and directly drill the geometric center hole. Li> blank has poor quality, large machining allowance and uneven distribution of machining allowance, so the quality center hole should be preferred. Because of the large initial unbalance, 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 the subsequent processing equipment. Furthermore, when the geometric center hole is used, the initial unbalance may exceed the requirements of the balancing machine and cannot be balanced. In this case, high-quality centering machine should be preferred.
Reasonable selection and selection principle of crankshaft roughing machine tool The importance of advanced metal processing equipment in crankshaft manufacturing is beyond doubt. It can reliably ensure dimensional accuracy and consistency, meet the requirements of production rhythm and improve the overall process level. However, the principle of "bringing in" cannot be adopted, and the more advanced the equipment, the better. The following three principles should be followed: 1) Conform to the process principle and meet the requirements of dimensional accuracy and consistency in combination with the product structure. 2) Based on the principle of economy, reduce the cost through bidding. 3) Conform to the principles of equipment manageability and maintainability, and inspect the after-sales service quality of equipment manufacturers and whether the wearing parts of equipment can be purchased at any time.
Reasonable combination There are some misunderstandings about imported equipment in domestic crankshaft manufacturing enterprises, for example, the more advanced and expensive the equipment, the better. In fact, if used improperly, advanced equipment will not play its due role, resulting in waste. Let's take the reasonable combination of CNC high-speed external milling, CNC internal milling and CNC lathe-lathe as an example to introduce it.
The side face of crankshaft balance weight needs to be machined, and CNC internal milling or CNC high-speed external milling is preferred for the machining of main journal, and CNC high-speed external milling is adopted for the machining of connecting rod journal. If the blank is forged steel blank, NC internal milling is more conducive to chip breaking. It is not suitable to use CNC lathe-lathe, because the side of the balance weight rotates intermittently, the crankshaft speed is very high (about 1000r/m), and the phenomenon of tool collapse is very serious.
There is no need to machine the side of crankshaft balance weight, so it is more reasonable to choose CNC lathe-lathe to machine the main journal with high machining accuracy. Because the axis of connecting rod neck is not on a central line, such as six-crank crankshaft, turning-turning processing is somewhat troublesome, and CNC high-speed external milling is more reasonable.
There is a root notch on the journal of the crankshaft. At this time, CNC lathe-lathe drawing shows its advantages. If there is undercut in the axial direction, CNC high-speed external milling and CNC internal milling cannot be processed, but the lathe-lathe diagram can be processed.
The above equipment should adopt independent double cutter heads and modular cutter system to realize flexible machining.
Crankshaft fillet rolling strengthening Crankshaft fillet rolling strengthening is mainly to improve the fatigue strength of crankshaft. According to statistics, the service life of nodular cast iron crankshaft can be increased by 120-230% after fillet rolling. The service life of forged steel crankshaft after fillet rolling can be increased by 70- 130%. Therefore, this strengthening method is highly valued by crankshaft manufacturers. At present, almost all foreign automobile crankshafts adopt rolling strengthening technology. This kind of equipment should be flexible to adapt to the processing of different products.
Crankshaft abrasive belt polishing can polish the main journal, connecting rod journal, flange, fillet and thrust surface at the same time, and fillet and thrust surface can be polished by axial movement of crankshaft. After polishing, the surface roughness is improved by at least one level of accuracy. In order to realize the machining of multi-variety and variant crankshafts, independent polishing head and multi-step machining can be used to realize flexibility.
Crankshaft cleaning Crankshaft usually needs to be cleaned twice. The first cleaning is arranged after the gun drills the oil hole, so as to remove the iron filings in the oil hole and the lubricating oil on the crankshaft surface and provide clean semi-finished products for the next working procedure. After polishing the abrasive belt, arrange a second cleaning, use a special cleaning machine for fixed-point positioning, and use a special nozzle to clean the oil hole and flange screw hole.
CBN CNC grinder with single grinding wheel and independent double grinding wheels should be selected for the finishing of crankshaft main journal and connecting rod journal, and it is not easy to choose multi-grinding wheel integrated grinder. Although the efficiency is high, it can not meet the needs of multi-variety flexibility.