Measures to solve the convex point problem of the

2022-08-05
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The piston pin hole is one of the most critical parts of the piston. As the quality control point of piston machining, the precision boring pin hole process has a direct impact on the performance of the piston

the pin hole diameter of car piston is small, generally 14 ~ 21mm; The dimensional tolerance requirements are strict, generally 0.004 ~ 0.006mm; The requirements for shape accuracy are high. Generally, the cylindricity of pin hole is 0.0015mm; The surface roughness is required to be high, generally Ra0.2 ~ 0.4; And most product pin holes require grouping. The above factors increase the machining difficulty of the piston pin hole of the car to varying degrees

the Great Wall gw413ef piston and NAC N4 piston produced by our company had difficulty in assembling the piston pin. In addition, some passenger car pistons with pin seat oil hole also have different forms of piston pin hole failure due to convex points at the intersection of pin seat oil hole and pin hole. As shown in Figure 1, there is a bump at the intersection of the pin seat oil hole and the pin hole, which destroys the lubricating oil film between the piston pin and the pin hole. The dry friction generates a lot of heat, which eventually leads to the melting and adhesion of this part

Figure 1 pin hole failure caused by the convex point at the intersection of pin seat oil hole and pin hole

the above problems are attributed to the convex point of piston pin hole. For this, our company has established a special project team to analyze the causes and put forward feasible solutions

cause analysis

through careful research and analysis, considering various factors affecting pin hole processing, such as machine tool, tooling fixture, piston material, metallography, heat treatment and environment, we draw the following conclusions:

1 In terms of machine tools and tooling

(1) most of the boring bars used by our company are made of 45ps steel, which has low rigidity. The diameter of the boring bar used for processing car pistons is small, and it will vibrate under the combined action of bending, torsion and compression loads during processing

(2) the piston inner block of some cars is relatively narrow, and its width is less than the width of the pin hole on one side. During boring, there is the phenomenon that two rough and fine knives eat at the same time, resulting in a boss near the edge of the inner cavity of the pin hole

(3) the bench drill for processing pin seat oil hole has low precision and slow speed, and most of them are processed manually, so the pin seat hole cannot achieve high processing quality

2. Piston material

piston material, metallography, heat treatment and other reasons cause deformation, alkali emission (in the pin hole) and other defects of the finished piston after long-term storage, resulting in difficulties in pin penetration of the piston pin hole

3. In terms of processing

(1) in the process of seat hole processing, if the bit grinding quality is poor and the bit is not sharp, the stress around the seat hole may be concentrated, and these stresses will be released in the subsequent process, thus forming bumps

(2) during fine hole machining, the pin seat oil hole is cut intermittently. Due to the rigidity of the boring bar and the quality of the tool, it is easy to form bumps at the outlet of the pin seat oil hole

(3) when rolling the pin hole, although the convex point formed during processing can be pressed off temporarily, it only makes the point elastically deformed, and the convex point will slowly rebound in the subsequent storage process. According to the test results of NAC N4 and great wall gw413ef pistons, this rebound will be clearly reflected in one month

(4) because the production line now adopts one flow production, there is no WIP between processes, which also causes that the processing stress of the rough processing process cannot be fully released, which has a certain impact on the quality of finishing

(5) the pin hole is processed by dry cutting without forced cooling, which also has a certain impact on the processing quality

4. Environmental problems

(1) during production, the production line is processed at normal temperature, which is greatly affected by the ambient temperature, and the temperature difference control is difficult to master

(2) the piston pin hole is tested at 20 ℃ constant temperature, but the assembly site of the main engine factory is at normal temperature. When the temperature is low in winter, the expansion coefficient of the piston pin is smaller than that of the piston, which will also have a certain impact on the assembly of the piston pin

solutions

based on the comprehensive analysis of the above reasons and the current situation of the company, we have formulated corresponding solutions, as follows:

1 Process document improvement

the tool change frequency of the pin seat oil hole drilling process is unified to 200 pieces/time; The frequency of fine tool change in the pin hole precision boring process is unified as 10000 pieces/time (Set 3 to calculate the theoretical reference data of density equipment and tools under normal conditions). At the same time, the processing of the pin seat hole is arranged after the rough boring of the pin hole, which can eliminate part of the stress around the pin seat hole

2. process equipment improvement

mainly refers to the improvement of the fine boring bar: the purpose is to increase the rigidity and rotation accuracy of the boring bar. A large number of tests have proved that, under the same processing conditions, cemented carbide is a boring bar material with excellent comprehensive performance for processing small diameter and high-precision holes. The vibration resistance of cemented carbide (YG6X) boring bar is much better than 45x steel boring bar, which can reduce the surface roughness and have no vibration marks on the processing surface. Figure 2 shows the carbide boring bar currently used by our company. Considering that the carbide boring bar is difficult to process, it is necessary to regularly check whether there is oil leakage at the main machine and oil source. A section of t7a cutter head bar is welded at the front end of the boring bar to facilitate tool clamping. The boring bar is composed of three parts: cutter head bar (material: t7a), cutter bar (material: YG6X) and cutter head (material: 40Cr). Requirements for heat treatment of cutter head: 30 ~ 35hrc, blue surface, 0.05 ~ 0.08mm interference between cutter rod and cutter head, and the welding quality must be ensured at the brazed part of cutter head rod and cutter head rod (the rotating speed of cutter head is usually composed of motor, reducer and bearing, etc. 3000r/min, if the desoldering is serious). If there is interference between two cutters in the pin hole machining of some products, it is necessary to design an extended boring bar to ensure that the rough cutter and the fine cutter do not eat at the same time

Fig. 2 cemented carbide boring bar (1-cutter head bar; 2-Cutter bar; 3-cutter head)

3. tool improvement

fine hole coarse cutter is changed from the current cemented carbide cutter to PCD cutter. PCD tools have the characteristics of low friction coefficient, high thermal conductivity and high hardness. They can grind sharp cutting edges, greatly reduce the cutting force, improve the precision of cutting, and reduce the requirements for the rigidity of precision machining machine tools. They not only improve the machining quality of rough tools, reduce the frequency of tool change, but also improve the machining efficiency and prolong the service life of tools. Considering the processing requirements, the specification of PCD rough cutter is φ 6mm × 15mm(R=0.2mm)。 Figure 3 is the schematic diagram of PCD rough cutter. PCD rough cutter rake angle is 5 °, cutter rake angle α The complex back angle is adopted, which is 12 ° and 15 ° respectively, and the tool edge bandwidth is 0.2 ~ 0.3mm. When the front angle is determined, the smaller first rear angle can make the wedge angle β The tool strength increases with the increase of; The increase of the second rake angle is beneficial to reduce the friction and interference between the rake face and the inner surface of the pin hole. The main deflection angle and auxiliary deflection angle are symmetrically distributed, and the tool tip arc r=0.2mm. Because the PCD boring tool bears a very small cutting depth, the cutting part of the main and auxiliary cutting edges of the tool is actually borne by the tool tip arc (with the function of polishing at the same time)

Figure 3 PCD rough cutter

4 Temperature control

although the influence of temperature can not directly cause the formation of convex points in the processing of car piston pin hole, the difference of temperature will directly affect the assembly of car piston, resulting in the difficulty of car piston assembly. Therefore, we have taken necessary measures in temperature control. We have conducted temperature correction tests on a large number of piston pin holes of cars. According to the basic data provided, eelcee, in cooperation with South Korean auto parts manufacturer duckyang Industrial Co., Ltd. and MS autotech, has developed relevant control measures and summarized the law that the diameter of piston pin holes of cars changes with temperature

the formula for the correction value of pin hole diameter is:

measured value of pin hole =6.68 ×× Diameter of measured pin hole × (20 ℃ - actual temperature)

for example, for the size of pin hole, the requirement is φ See table for temperature correction of 18 +0.014 +0.008 piston during machining. Temperature correction of piston pin hole diameter of a car (unit: mm)

5 In terms of detection

when the precision measuring room detects a piston with a pin seat hole in the pin hole, in order to accurately reflect the roundness of the pin hole, it is necessary to detect the roundness of the central section through the pin seat hole, cover the pin seat hole with transparent tape during measurement, and the tape cutting specification: a square piece not greater than the seat hole diameter +2mm

conclusion

we have conducted a large number of tests on the above measures, which have successfully avoided the recurrence of the piston pin hole convex problem, and have been successfully applied to mass production. In the future, with the low emission of engine and the continuous improvement of power, economy and reliability, the requirements for piston machining accuracy will be higher and higher. While summarizing our experience, we are also committed to continuously improving the piston processing level and making unremitting efforts to enhance the overall competitiveness of the company. (end)

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