Efficient rough machining of engine crankshaft journal

Efficient rough machining of engine crankshaft journal

Figure 1 Multi-tool wheel turning-turning and pulling

At present, crankshaft processing technology has developed towards high-speed, high-efficiency, and compounding. The more widely used processing techniques are internal milling, turning-turning and high-speed external milling. Since various processes have their advantages and disadvantages and applicable fields, different process methods should be selected according to the structure of the product or combination, so as to ensure the high quality and efficient production of the product.

The crankshaft is one of the most important parts in the engine, and it is also the part that bears the greatest load in the engine. When the engine is working, the linear reciprocating motion of the piston and connecting rod is converted into rotary motion through the crankshaft and output power. The crankshaft has to withstand huge thermal shock loads and the combined effects of various forces when working, so the crankshaft needs to have a higher strength. Because the crankshaft works under high speed and high alternating load, it has strict technical conditions, form and position tolerances and dimensional accuracy, such as material, journal surface hardness, dynamic balance and bending resistance. Claim. However, the crankshaft has a complex structure and poor rigidity, especially a crankshaft with a small overlap system between the main journal and the connecting rod journal, which brings difficulties to processing.

The shape and position tolerance, dimensional accuracy and technical requirements of the crankshaft are mainly concentrated on the main journal and connecting rod journal of the crankshaft. Therefore, the processing technology of these two journals constitutes the main content of the crankshaft processing technology. Below we focus on the rough machining process of the crankshaft journal.

Rough machining technology of crankshaft journal

The crankshaft journal rough machining technology was mainly multi-tool turning before the 1970s, including: multi-tool turning of the intermediate transmission full journal for main journal turning, which can simultaneously perform multi-tool machining on each main journal; used for connecting rods The double-head drive profiling multi-tool turning of journal turning can turn the journals of the connecting rods in each phase or multi-cut turning the journals of the same-phase connecting rods through eccentric clamping. Due to the low machining accuracy and high cutting force, this cutting method is easy to cause deformation of the workpiece, increased stress, and poor flexibility, etc. It has basically not been adopted at present

With the advent of CNC lathes, it is possible to use CNC lathes for rough machining of journals. This process eliminates the shortcomings of multi-tool turning, but it is intermittent cutting when machining the side of the crank, which is detrimental to the life of the tool and therefore limits the increase in cutting consumption. Because this process is single-cut cutting, the production efficiency is low, and it is rarely used in the rough machining of connecting rod journals and main journals, but it is still used in the rough machining of other journals of large and small crankshafts. .

In the 1960s and 1970s, companies such as Heller of Germany and Komatsu of Japan developed the external milling process of the journal. At that time, the rotational speeds of the external milling tool and the workpiece were both high. The workpiece rotated to achieve circumferential feed while the tool rotated and slowly fed radially to perform circumferential milling on the journal. Later, German Berliner and other companies introduced the internal milling process, which has two process methods: workpiece rotation and workpiece immobilization. Internal milling is superior to external milling in terms of cutter head clamping force and cutting stability. Therefore, external milling process is gradually replaced by internal milling. Due to the unique advantages of internal milling, especially the workpiece can be kept still during the machining process. Through the use of high-rigidity support, the rigidity of the workpiece during the machining process can be enhanced, and CNC control can be used. For the rough machining of the neck, internal milling is still the first choice. Especially for forged steel crankshafts, internal milling is more conducive to chip breaking.

In the 1980s, with the development and popularization of CNC technology, CNC cart and cart-car technology were quickly promoted. These two processes have high machining accuracy and production efficiency, and can realize the machining of undercut grooves. At present, they are widely used in the rough machining of the crankshaft of small and medium-displacement engines, and the rough grinding process can be omitted, and the journal directly enters the fine grinding. However, the connecting rod neck and the main shaft journal cannot be processed in the same process. They must be completed on two machine tools respectively. Because of the different phase angles, the connecting rod neck processing needs to be installed multiple times to realize the journal processing with different phase angles. Figure 1 shows the multi-tool wheel turning-turning process.

In the 1990s, CNC high-speed external milling was developed. In this process, a disc milling cutter equipped with multiple teeth rotates at a high speed and is fed to the machining size along the radial direction. The workpiece rotates slowly for a circle for circular feed to complete the machining of a journal. This process can use two cutter heads to process the connecting rod journal and the main journal respectively. The cutting speed of the tool is fast. The connecting rod journals with different phase angles can be installed in one installation to complete the processing, which saves the time of workpiece clamping and loading and unloading. It has high production efficiency and flexibility, so it has been increasingly used in the rough machining of crankshaft journals. Figure 2 shows the high-speed external milling with dual-cutter heads.

Analysis of the characteristics of several processing methods

Crankshaft processing technology has developed in the direction of high speed, high efficiency and compounding. At present, the three processes that are widely used are internal milling, turning-turning and high-speed external milling. Its different characteristics and advantages are mainly manifested in:

1. Car-car pull

Turning-turning and pulling uses a disc-shaped turning-turning tool to process the journal and thrust surface. Its characteristics include:

(1) Only one tool is in contact with the workpiece at a moment, and the metal cut by each blade under the high-speed rotation of the workpiece is very thin, about 0.4～0.6mm, so the cutting force is small, the bending deformation of the workpiece is also small, and the cutting is stable and processing High precision and good surface precision.

(2) In the cycle of processing a workpiece, each knife only contacts the workpiece once, so the tool life is long, and each blade can process 4 000 to 5 000 workpieces.

(3) The whole processing is completed with multiple blades. The blade layout can be optimized according to different workpieces, materials and uses, and the most suitable tool material can be selected.

(4) The CNC system of the car-carriage equipment not only controls the rotation and radial coordinates, but also controls the axial coordinates, and can process workpieces with axial undercut grooves.

The turning-turning machine tool has high production efficiency, good processing accuracy, strong flexibility, high degree of automation and short tool change time. After processing, the crankshaft can be directly subjected to fine grinding, eliminating the need for rough grinding. However, if the side of the balance weight is machined, when the broach rapidly advances and approaches the crankshaft, the side of the balance weight of the crankshaft is processed first. Since the side of the balance weight is an intermittent surface, the machining is intermittent cutting. The continuous surface is only when the flange of the crankshaft reaches the outer circle of the journal. The crankshaft rotates at a higher speed, and the tool is subjected to greater impact. It was quickly broken. This is the problem that exists when the crankshaft that needs to be processed on the side of the balance weight is processed by turning.

At present, when turning and turning the crankshaft balance weight side and journal, foreign countries have adopted the method of increasing the number of blades to solve the problem, which will lead to abnormal tool consumption and increase in tool costs. Especially when the quality of the blank is not good and the machining allowance is large, the side of the balance weight of the crankshaft should not be processed by the turning-drawing process.

2. High-speed external milling

The advantages of CNC high-speed crankshaft external milling process are: high cutting speed (up to 350m/min), short cutting time, short process cycle time, small cutting force, low workpiece temperature rise, high tool life, fewer tool changes, and The processing precision is high, and the flexibility is better.

When high-speed external milling is used, the workpiece rotates slowly with circumferential feed, while the external disc milling cutter performs high-speed cutting and radial feed. The slow motion of the workpiece reduces the dynamic interference effect of the unbalanced mass of the workpiece. The disc cutter performs high-speed cutting, and the cutting force of each blade is small, and better accuracy can be obtained. The cutter head is fed radially to a predetermined depth, and the workpiece rotates slowly one circle to complete the machining of a journal. The machine tool adopts CNC control. Through a fast and independent tool control and corresponding program adjustment, connecting rod journals with different phase angles of crankshafts with different cylinder numbers can be machined on one machine tool. The double-cutter high-speed external milling machine tool can simultaneously use one cutter to mill the main journal and the other cutter to mill the connecting rod journal, which is an efficient and highly flexible process method.

3. Internal milling

The cutting process of internal milling is set inside the annular cutter head, and the material is removed by rotating the machining tool around the fixed workpiece. The position of the workpiece is fixed during the whole process, and the supporting condition is good. This rigidity condition enables the workpiece to absorb large cutting forces and withstand high material removal rate. Therefore, internal milling is more suitable for rough machining of workpieces that require a large amount of material removal. In commercial vehicles or large engines, crankshafts and crankshafts have large machining surfaces. Under the circumstances, it is more common.

The production efficiency of internal milling is relatively high, and the use of multi-tool milling can complete the processing of all main journals in one clamping. In the processing of connecting rod journals, material removal is relatively small, and high-speed external milling is often used instead of internal milling.

The comparison of machining accuracy of different machining processes is shown in the table.

Selection of Crankshaft Rough Machining Process

By analyzing the characteristics of the three widely used process methods, it can be seen that each process has its advantages and disadvantages and applicable fields, and different process methods should be selected according to the structure or combination of the product.

1. When the side of the crankshaft balance weight needs to be machined, CNC internal milling or CNC high-speed external milling should be preferred for main journal machining, and CNC high-speed external milling should be used for connecting rod neck machining. If the blank is a forged steel blank, CNC internal milling is more conducive to chip breaking. The CNC turning-turning process should not be used, because the side of the balance weight is turned intermittently, the crankshaft speed is very high (about 1 000r/min), and the phenomenon of tool collapse is more serious.

2. If the side of the crankshaft balance weight does not need to be processed, CNC turning-turning is used for the main journal processing, which has high machining accuracy. Since the connecting rod neck axis is not on the same center line, turning-turning processing will increase the number of machine tools. CNC high-speed external milling or internal milling can be used.

3. For crankshafts with undercut grooves in the journal, CNC turning-turning can show its superiority. If the crankshaft has axial undercutting grooves, CNC high-speed external milling and CNC internal milling cannot be processed, and turning-turning should be used. Processing.

The above equipment should adopt independent double cutter heads and modular cutter system to realize flexible processing while improving production efficiency.

Current Status of Domestic Crankshaft Journal Rough Machining Equipment

In recent years, the technical level of my country's equipment manufacturing plants has been greatly developed and advanced, and some equipment required for rough machining of engine crankshafts has been developed, such as CNC crankshaft internal milling machine tools, CNC crankshaft oil hole drilling machine tools and so on. However, from the perspective of engine manufacturers, the current domestic equipment is still unable to meet the requirements of modern manufacturing in terms of accuracy, efficiency and reliability. Therefore, currently in large-scale engine production plants, the proportion of imported equipment is relatively high. The gap of domestic equipment is mainly manifested in: the same type of machine tools, there is a gap in performance; some domestic equipment required for advanced manufacturing processes is not yet mature, such as turning-turning machine tools for crankshaft journal processing; domestic equipment is in The production efficiency and reliability are not as good as foreign equipment, and the accuracy cannot meet the requirements of the product. It is difficult to pass the Cmk value acceptance; domestic equipment has a high failure rate, with an MTBF (machine tool capability index) of about 500h, while foreign equipment is above 1,000h.

In view of the importance of the engine in the whole vehicle, and the complex structure and high precision of the main parts of the engine, the gap between the domestic equipment in the above aspects is more prominent. Therefore, it is still very necessary to improve the technical level of my country's automobile manufacturing and the ability of domestic independent manufacturing equipment by improving the key technology and equipment of engine manufacturing, and to completely change the current situation of mainly importing equipment in the main engine parts production line.