Evolution of crankshaft machining process

1. Prior to the 1970s, engine crankshaft roughing was processed using a multi-tool lathe to crank the crankshaft main journal and the connecting rod journal. In this way, the processing precision is low, the flexibility is poor, the process quality is low, and it is easy to generate large internal stress, and it is difficult to achieve a reasonable machining allowance. After roughing, it is generally necessary to perform stress relief tempering to release stress. Therefore, roughing requires a large machining allowance for the subsequent finishing process to remove the amount of bending deformation. Crankshaft finishing uses a common grinding process, generally using MQ8260 crankshaft grinding machine rough grinding - semi-finishing - fine grinding - polishing. Usually by manual operation, the processing quality is unstable and the scrap rate is high.
2. From the 1970s to the 1980s, crankshaft roughing was CNC turning and CNC milling, and the machining conditions improved. Finishing is still dominated by ordinary grinding machines.
3. In the mid-1980s, the CNC internal milling process appeared. The CNC internal milling processing performance index is higher than the CNC external milling processing, especially for the forged steel crankshaft, and the internal milling is more conducive to chip breaking. The semi-automatic crankshaft grinding machine is used in the finishing process, and the head frame and the tailstock are synchronously driven, and the machining accuracy is improved.
   

   Figure 1 Double cutter car - car pull

   Figure 2 Double cutter high speed milling

4. From 1985 to 1990, the crankshaft pulling and car-car pulling process was developed (Fig. 1 is a double-cutter car-car pull). The process has the advantages of high precision and high efficiency, and is especially suitable for the side of the balance block without processing. The crankshaft has a crankshaft with undercut grooves (including axial undercut grooves), and the crankshaft can be directly ground after machining, eliminating the rough grinding process. Crankshaft finishing has been carried out with a small amount of CNC grinding machine, and the dimensional consistency has been improved.
5. In the mid-1990s, CNC high-speed external milling was developed (Fig. 2 is double-cutter high-speed external milling), which is more efficient than CNC turning, CNC internal milling, and car-car pulling. Be high. In addition, the CNC car-car pull process requires two steps of the connecting rod journal. CNC high-speed external milling can be completed in one process, with the following advantages: high cutting speed (up to 350m/min), short cutting time, and process Short cycle time, low cutting force, low workpiece temperature rise, high tool life, fewer tool changes, higher machining accuracy and better flexibility. Therefore, CNC high-speed external milling will be the development direction of crankshaft main journal and connecting rod journal roughing. Finishing uses CNC grinding machines, using static pressure spindles, hydrostatic guides, static pressure feed screws (wheel heads) and linear grating closed-loop control and other control devices to ensure reliable tolerances and dimensional tolerances. The CNC grinding belt polishing machine is also widely used for super-precision machining, and the surface roughness of the crankshaft journal after super-finishing is improved by at least one level of precision.
   High-speed grinding of CBN developed in the 1990s. The crankshaft grinding machine produced by LANDIS in the United Kingdom has a grinding speed of up to 120 m/s. It takes only a few minutes to complete the grinding from the blank to the fine grinding with the suede method. This will lead to a new situation in which grinding replaces other roughing processes.
6. After entering the 21st century, the composite processing technology has entered the crankshaft manufacturing industry. Composite machine tools should have process integration capabilities and multiple process integration functions. The horizontal turning and milling combined machining center (Fig. 3 is M40G type) produced by WFL of Austria can “clamp and process all the time” before the crankshaft is hardened. The processed crankshaft can be directly transferred into the finishing process; in terms of crankshaft finishing, There is also a process-integrated CBN CNC grinding machine that grinds all crankshaft main journals and connecting rod journals (oscillation tracking grinding) in one clamping.

Figure 3 M40G turning and milling composite machining center

It can be seen from the above evolution that the processing technology of the crankshaft is developing in the direction of high speed, high efficiency and compounding. At present, the more popular roughing process is the car-car pull process and high-speed outer milling of the main journal, the high-speed follow-up milling of the connecting rod neck, all adopt dry cutting; the finishing is processed by CNC grinding machine, with automatic feeding and automatic Correct the grinding wheel, automatic compensation of size and roundness, automatic indexing and electronic synchronous drive at both ends. The main journal and the connecting rod neck can be completely ground in one clamping; the super finishing machine uses a CNC belt polishing machine with a size control device.

Introduction to the performance of typical crankshaft machining advanced equipment

Figure 4 CNC high speed follower milling machine

Figure 5 comb blade

CNC High Speed ​​Follower Milling (Fig. 4): The performance of a high speed servo milling machine model VDF 315 OM-4 is now presented. The machine tool is a flexible CNC milling machine designed and manufactured by BOEHRINGER of Germany for the crankshaft of automobile engine. The machine uses the workpiece rotary and milling cutter feed servo linkage control technology, which can clamp the crankshaft without changing the crankshaft rotation center. Rod journal. It adopts integrated composite structure bed, electronically synchronous rotary drive at both ends of the workpiece, featuring dry cutting, high machining precision and high cutting efficiency; using SIEMENS 840D CNC control system, equipment operation manual on the man-machine interface, through input parts The basic parameters can automatically generate the machining program. It can process various crankshafts with a length of 450-700mm and a rotary diameter of 380mm. The diameter of the connecting rod journal diameter is ±0.02mm.

CNC car-car pull machine : This equipment can complete all concentric turning in one setting, and complete the car-car pull (vehicle side end face) processing on the same machine tool, with high processing efficiency, through the use of special chuck and tool system. It can also realize flexible processing, and the maintenance of the machine tool is simple and the maintenance cost is also low. It is especially suitable for the crankshaft where the side of the balance block does not need to be processed and the journal has an undercut groove. The broaching process can be replaced by a high-efficiency comb (Fig. 5) turning process. The combing process is usually carried out at the last step of the process, and high-speed finishing is achieved by a small amount of radial feed and longitudinal turning.

1. Crankshaft thrust surface rolling machine (Fig. 6): This equipment is used for rolling the crankshaft thrust surface and has the following technical features: rolling and polishing thrust surface and on-line measurement, rolling polishing instead of grinding It can be turned at the same time and equipped with a rolling polishing device on the cutter head for higher precision. At present, the equipment with better performance includes the crankshaft thrust surface rolling special machine of HEGENSCHEID Company of Germany.
2. CNC Crankshaft Grinding Machine : Take the example of JUNKER's oscillating tracking series grinding machine, which uses a combination of CBN grinding wheel for high-speed machining and oil-cooled crankshaft for machining automotive engines. The crankshaft is of reliable quality. The main performances are: detecting and correcting the roundness and size of the journal during the machining process; the control system with “learning function”, with automatic compensation for roundness deviation and interference, the amount of interference that can be compensated is: temperature, Mechanical and dynamic effects, changes in grinding allowance, changes in material and metallographic structure, machinability of the grinding wheel, wear of the machine tool; grinding of the main journal and the connecting rod journal at one time, the theoretical deviation is zero Cut-in grinding and oscillating grinding; support for “sensitive workpieces”, automatic center-to-center three-point center frame on the spindle; CNC-controlled coolant supply guarantees long-lasting use of the grinding area; Type guide rail, no creeping phenomenon, ensuring long-lasting high precision (X-axis guide rail, feed screw, thrust bearing); shock-absorbing and torsion-resistant bed, cast from mineral synthetic material, with good shock absorption and bending resistance Function; the grinding wheel shaft is suitable for grinding up to 140m/s. Figure 7 is a CNC crankshaft grinder of the type "JUCRANK 6000/50-50". The grinding machine has four CBN grinding wheels, each of which can be independently ground. One clamping can grind all the main journal and connecting rod neck.
   

   Figure 6 Pushing surface fine car rolling machine

   Figure 7 Front and rear shape of the JUCRANK 6000/50-50 CBN grinding machine

From the several advanced equipments introduced above, we can see that one thing in common is high-speed and high-efficiency flexible, which is suitable for the development trend of many varieties and small batches of today's products. Since crankshaft machining is different from ordinary machining, many processes must use special tools, such as the internal milling, car-car pull and high-speed outer milling described above, all of which use special tools. The cutters are complex and cost-effective. More expensive. If the product variant involves changes in the crankshaft structure, the new tool body must be used to machine the crankshaft, which affects the product development cycle and manufacturing costs, and ultimately leads to a lack of competitiveness. Tool manufacturers such as Walter have now developed flexible tools for the manufacture of crankshafts – modular (Figure 8). The product development cycle is greatly shortened and the manufacturing cost is reduced.

Figure 8 Modular Crankshaft Tooling System

Discussion on the Program of Crankshaft Agile Flexible Production Line (AFTL)

At present, the domestic car crankshaft production line is mostly high-speed flexible production line FTL (Flexible Transfer Line). This production line is characterized by not only processing the same series of crankshafts, but also processing variant products, replacement products and new products, which are truly flexible. In order to further improve the production efficiency of the high-speed flexible production line and adapt to the market faster, the next step of FTL development is the Agile Flexible Transfer Line (AFTL). Its main purpose is to:

1. Meet the needs of market changes. Not only meet the requirements of current products, but also consider the future market demand.
2. Meet the needs of production methods. It can meet the production needs of modern engines "multi-variety, large and medium-volume, high efficiency, low cost".
3. Meet the principle of “lean production”. Eliminate waste and make profits with the least investment and maximum return.

Due to the special structure of the engine crankshaft, the author believes that the crankshaft AFTL should have the following characteristics: it consists of a high-speed machining center and a high-efficiency special machine tool (including a small number of combined machine tools). The machine tools are arranged according to the process flow and connected by automatic conveyors, using flexible fixtures and efficient special tools. In order to prevent the failure of key process equipment and cause the whole line to be discontinued, parallel equipment can be added to meet the needs of mass production. The following is the process flow (metal processing only):

Milling end face, fixed total length, drill quality center hole, car size end outer circle→milling spindle neck and shoulder shoulder→milling connecting rod neck and shoulder shoulder→carriage main journal and sinking groove→car pull connecting rod neck and sinking groove→gun Drill hole → cleaning → fillet rolling → flange drilling tapping → fine grinding spindle neck (CBN) → fine grinding rod neck (CBN) → oblique cutting small end → oblique cutting flange end → rolling Thrust surface, milling keyway → dynamic balance → abrasive belt polishing spindle, connecting rod and flange outer diameter → cleaning, cooling → detection classification.

Several questions about the above process flow are discussed as follows:

1. Selection of the center hole of the crankshaft and the geometric center hole.
1. The blank quality is good, the machining allowance is small and the machining allowance is evenly distributed. At this time, the center hole of the center of the crankshaft substantially coincides with the geometric center hole, so that the geometric center hole can be directly drilled without costing to purchase a mass centering device. ,li>The quality of the blank is poor, the machining allowance is large and the machining allowance is unevenly distributed. The center hole is preferred. Because the initial unbalance is large, if the geometric center hole is drilled, the mass distribution is not uniform, the moment of inertia is large, and the accuracy of the subsequent processing equipment is damaged. Furthermore, with a geometric center hole, the initial imbalance may exceed the balancer requirements and cannot be balanced during dynamic balancing. In this case, the quality centering machine should be preferred.
2. Reasonable selection of crankshaft roughing machine tools
1. Selection principle
   The importance of advanced metal processing equipment in the manufacture of crankshafts is undoubtedly reliable, ensuring dimensional accuracy and consistency, adapting to the requirements of production tempo, and improving the overall process level. However, it is not possible to adopt "takenism", nor the more advanced the equipment, the three principles should be met: 1) Comply with the principle of craftsmanship, combined with product structure, can meet the requirements of dimensional accuracy and consistency. 2) In line with the principle of economy, use the form of bidding to reduce costs. 3) In line with the principle of equipment management and maintainability, inspect the quality of after-sales service of equipment manufacturers, whether the wearing parts of equipment can be purchased at any time.
2. Reasonable combination
   Domestic crankshaft manufacturers have some misunderstandings about the introduction of equipment, such as the more advanced and expensive equipment. In fact, if used improperly, advanced equipment will not play its due role and cause waste. The following is an example of a reasonable combination of CNC high-speed external milling, CNC internal milling, and CNC-car pulling.

   1. The side of the crankshaft balance block needs to be machined. The spindle neck machining should be CNC internal milling or CNC high-speed external milling. The processing of the connecting rod neck is CNC high-speed external milling. If the blank is a forged steel blank, CNC internal milling is more conducive to chip breaking. It is not advisable to use CNC car-car pull. Since the side of the balance block is intermittently turned, the crankshaft speed is very high (about 1000r/m), and the phenomenon of chipping is very serious.
   2. The side of the crankshaft balance block does not need to be machined, and the spindle neck processing is more reasonable with the CNC car-car pull, and the machining precision is high. Since the axis of the connecting rod neck is not on a center line, such as the six-cranked crankshaft, there is some trouble in the car-car drawing process, and CNC high-speed outer milling is more reasonable.
   3. The journal has a crankshaft with a slotted groove. At this time, the CNC-car pull shows its superiority. If there is an undercut in the axial direction, CNC high-speed outer milling and CNC inner milling cannot be processed, and the car-car pull can be processed.
   The above equipment should be flexible machining by using independent double cutterheads and modular cutter systems.
3. Crankshaft fillet roll strengthening
The rounded roll strengthening of the crankshaft is mainly to improve the fatigue strength of the crankshaft. According to statistics, the life of the ductile iron crankshaft can be increased by 120-230% after being rounded, and the life of the forged steel crankshaft can be increased by 70-130% after being rounded. Therefore, this strengthening method is highly valued by the crankshaft manufacturers. At present, almost all of the foreign car crankshafts adopt the rolling strengthening process. The use of such equipment should be flexible to accommodate the processing of different products.
4. Crankshaft belt polishing
   The abrasive belt polishing can simultaneously polish the main journal, the connecting rod neck, the flange, the rounded corner and the push surface, and the crankshaft axially moves to realize the polishing of the rounded corner and the thrust surface. The surface roughness after polishing is increased by at least one level of precision. In order to realize the processing of multi-variable and modified products of the crankshaft, it is possible to realize the flexibility by using an independent polishing head and multi-step processing.
5. Crankshaft cleaning
   The crankshaft is usually cleaned twice. The first cleaning is arranged after the oil hole of the gun to remove the iron filings in the oil hole and the lubricating oil on the surface of the crankshaft to provide a clean semi-finished product for the next process. After the second cleaning is arranged, the special cleaning machine for fixed point positioning is used to clean the oil hole and the flange screw hole with a special nozzle.
6. Crankshaft finishing
   Crankshaft finishing grinding neck and fine grinding rod neck process should use single grinding wheel, independent double grinding wheel CBN CNC grinding machine, it is not easy to use multi-grinding wheel integrated grinding machine, although the efficiency is high, but can not meet the needs of multi-variety flexibility.

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