Abstract: This paper introduces a new method of using a stepping motor as the driving source and a combination of a soft profile and a hard profile to realize the turning system of the convex and convex elliptical piston skirt. The system is controlled by a computer and compensates for machining errors, which effectively improves the machining accuracy and efficiency and achieves good results.
The piston is an important component of the engine, and the quality directly affects the performance of the engine. Its shape has a great influence on the fuel economy, reliability, life, emissions and noise of the system. Considering the influencing factors such as the gas pressure, the piston deformation caused by the cylinder pressure and the thermal deformation of the piston itself, the piston skirt is widely used in the convex and convex elliptical surface. This profile has the following characteristics: the cross section is elliptical or elliptical, and its ellipticity varies along the axial direction of the piston (Fig. 1a); the profile of the longitudinal section is a centrally convex curve (Fig. 1b).
The piston is designed in such a shape that the deformation caused by factors such as thermal expansion during the operation of the piston can be expanded in the direction of the pin of the piston, so that the cross section of the skirt becomes approximately circular, so that a wider and more uniform operation can be obtained when the piston operates. The bearing surface also makes the skirt of the piston fit better with the cylinder wall, which reduces the air gap of the matching cylinder, helps to reduce the specific pressure of the piston skirt, reduces the impact of the piston on the cylinder wall, and reduces the engine running time. noise. In addition, the wedge-shaped oil gap between the convex skirt and the cylinder wall can improve the lubrication and improve the centering of the piston, which greatly reduces the wear of the parts and improves the working performance of the piston. Service life. At present, the convex-convex elliptical surface has been widely adopted as a well-recognized excellent profile of the piston skirt.
For the turning of such complex profiles, the existing machining methods mainly include hard mastering and soft mastering (CNC machining).
The hard mastering process is that the piston and the piston have the same shape as the female master, and the main shaft of the machine is coaxially mounted on the machine tool spindle, and then the profiling device is used to manufacture the piston of the same shape as the female master. This processing method has the characteristics of good processing stability, and the inconvenience is that the mold is difficult to manufacture, the processing cycle is long, and the cost is high. At the same time, the dynamic characteristics of the mechanical structure make the spindle speed of the machine tool limited, which has a greater impact on the production efficiency of the piston.
Soft mastering is programmed according to the profile of the piston, and the programmed program is used as a master. The high-frequency micro-displacement feed mechanism of the computer-controlled tool is used to process the piston. This type of processing has greater flexibility in the modification of the piston. At present, domestic actuators relying on soft mold processing generally have problems such as poor linearity and dynamic characteristics, and large influence by sudden load, which cannot guarantee the processing accuracy well.
In this paper, the surface forming method adopts the decomposition design method, and the processing of the convex-convex elliptical shaped surface is decomposed into several groups of motion synthesis. Taking advantage of the advantages of numerical control unit and mechanical mechanism, a new type of medium-convex variable elliptical piston skirt turning system was designed and designed by combining soft and hard molds to realize high-precision and high-efficiency machining of piston skirt.
Stepper motors are widely used in the machine tool industry. In recent years, as the stepping motor adopts a refinement circuit, the step angle is getting smaller and smaller, and the corner precision is also higher and higher. In addition, the characteristics of the moment frequency, the maximum speed and the maximum torque performance are also improved. Based on the characteristics of stepper motor with high precision, small inertia and favorable digital control, we adopt the system structure shown in Figure 2 to realize the convex-elliptical surface processing of the piston.
The whole system is composed of the main control CPU, stepper motor, photoelectric encoder, master system and lever system. It adopts the existing NC machine tool as the mother body, and uses the stepping motor as the main driving source of the system. It drives the elliptical model to rotate at high speed and realizes the high-frequency reciprocating feed of the tool through the lever structure. The main control CPU processes the feedback signal provided by the photoelectric encoder and other sensors in real time, continuously adjusts the rotation of the stepping motor, and also maintains communication with the CNC machine tool to ensure the motion matching of the entire machining system, thereby achieving the centering convexity. Processing of elliptical piston skirts. In order to save machine space, the lever system was deliberately designed to be right angled.
According to the kinematics of the machine tool, the convex-elliptical profiled surface of the piston skirt is formed, and the machining of the convex-convex elliptical surface of the piston skirt can be decomposed into three groups of motions during turning, namely:
Figure 3 shows the basic principle of the turning machining system. It matches the above three groups of movements according to a certain motion relationship, so that the movement of the spindle in the system, the movement of the model system, the movement of the lever system and the movement of the tool controlled by the CNC machine tool are maintained. relationship. That is, the movement (1) realizes the elliptical section of the piston, the movement (2) realizes the elliptical deformation of the piston, and the movement (3) realizes the convex surface of the piston. In this way, a piston profile with a convex ellipse can be machined.
The method proposed in this paper combines the current status of piston machining at home and abroad, adopts a new processing method combining soft and hard mastering technology, and designs a turning machining system that realizes the convex elliptical profile in the piston skirt. The system has the following characteristics:
1 Introduction
 (a) Ellipticity of any section of the T(z)-piston |  (b) A(z)-shaped profile of the piston skirt |
| Figure 1 convex elliptical piston shape | |
The piston is an important component of the engine, and the quality directly affects the performance of the engine. Its shape has a great influence on the fuel economy, reliability, life, emissions and noise of the system. Considering the influencing factors such as the gas pressure, the piston deformation caused by the cylinder pressure and the thermal deformation of the piston itself, the piston skirt is widely used in the convex and convex elliptical surface. This profile has the following characteristics: the cross section is elliptical or elliptical, and its ellipticity varies along the axial direction of the piston (Fig. 1a); the profile of the longitudinal section is a centrally convex curve (Fig. 1b).
The piston is designed in such a shape that the deformation caused by factors such as thermal expansion during the operation of the piston can be expanded in the direction of the pin of the piston, so that the cross section of the skirt becomes approximately circular, so that a wider and more uniform operation can be obtained when the piston operates. The bearing surface also makes the skirt of the piston fit better with the cylinder wall, which reduces the air gap of the matching cylinder, helps to reduce the specific pressure of the piston skirt, reduces the impact of the piston on the cylinder wall, and reduces the engine running time. noise. In addition, the wedge-shaped oil gap between the convex skirt and the cylinder wall can improve the lubrication and improve the centering of the piston, which greatly reduces the wear of the parts and improves the working performance of the piston. Service life. At present, the convex-convex elliptical surface has been widely adopted as a well-recognized excellent profile of the piston skirt.
For the turning of such complex profiles, the existing machining methods mainly include hard mastering and soft mastering (CNC machining).
The hard mastering process is that the piston and the piston have the same shape as the female master, and the main shaft of the machine is coaxially mounted on the machine tool spindle, and then the profiling device is used to manufacture the piston of the same shape as the female master. This processing method has the characteristics of good processing stability, and the inconvenience is that the mold is difficult to manufacture, the processing cycle is long, and the cost is high. At the same time, the dynamic characteristics of the mechanical structure make the spindle speed of the machine tool limited, which has a greater impact on the production efficiency of the piston.
Soft mastering is programmed according to the profile of the piston, and the programmed program is used as a master. The high-frequency micro-displacement feed mechanism of the computer-controlled tool is used to process the piston. This type of processing has greater flexibility in the modification of the piston. At present, domestic actuators relying on soft mold processing generally have problems such as poor linearity and dynamic characteristics, and large influence by sudden load, which cannot guarantee the processing accuracy well.
In this paper, the surface forming method adopts the decomposition design method, and the processing of the convex-convex elliptical shaped surface is decomposed into several groups of motion synthesis. Taking advantage of the advantages of numerical control unit and mechanical mechanism, a new type of medium-convex variable elliptical piston skirt turning system was designed and designed by combining soft and hard molds to realize high-precision and high-efficiency machining of piston skirt.
2 medium convex elliptical piston turning machining system
 1,12. Photoelectric encoder 2. Spindle 3. Machining part 4. Tool 5. Large carriage 6. Right angle lever 7. Small carriage 8, 10, 14, 15. Stepper motor 9, 13. Stepper motor driver 11. Elliptical model Figure 2 convex elliptical piston turning machining system |
Stepper motors are widely used in the machine tool industry. In recent years, as the stepping motor adopts a refinement circuit, the step angle is getting smaller and smaller, and the corner precision is also higher and higher. In addition, the characteristics of the moment frequency, the maximum speed and the maximum torque performance are also improved. Based on the characteristics of stepper motor with high precision, small inertia and favorable digital control, we adopt the system structure shown in Figure 2 to realize the convex-elliptical surface processing of the piston.
The whole system is composed of the main control CPU, stepper motor, photoelectric encoder, master system and lever system. It adopts the existing NC machine tool as the mother body, and uses the stepping motor as the main driving source of the system. It drives the elliptical model to rotate at high speed and realizes the high-frequency reciprocating feed of the tool through the lever structure. The main control CPU processes the feedback signal provided by the photoelectric encoder and other sensors in real time, continuously adjusts the rotation of the stepping motor, and also maintains communication with the CNC machine tool to ensure the motion matching of the entire machining system, thereby achieving the centering convexity. Processing of elliptical piston skirts. In order to save machine space, the lever system was deliberately designed to be right angled.
3 Principle of convex elliptical forming in the piston skirt
Figure 3 Basic principles of the turning system |
According to the kinematics of the machine tool, the convex-elliptical profiled surface of the piston skirt is formed, and the machining of the convex-convex elliptical surface of the piston skirt can be decomposed into three groups of motions during turning, namely:
- The radial feed motion of the tool maintains the radial reciprocating linear motion frequency of the tool twice the rotational frequency of the piston to form an elliptical cross section of the piston.
- Axial relative movement between the tool and the lever system When the tool is fed in the axial direction, the relative movement of the tool and the lever is controlled according to the curve of the ellipticity of the piston skirt, and the radial reciprocating stroke of the tool is changed to form a piston. The elliptical section of the skirt.
- CNC machine-controlled tool-added radial motion CNC machine tool mainly controls the tool to give the tool an additional radial movement when feeding along the axial direction to achieve the convex line of the piston skirt.
Figure 3 shows the basic principle of the turning machining system. It matches the above three groups of movements according to a certain motion relationship, so that the movement of the spindle in the system, the movement of the model system, the movement of the lever system and the movement of the tool controlled by the CNC machine tool are maintained. relationship. That is, the movement (1) realizes the elliptical section of the piston, the movement (2) realizes the elliptical deformation of the piston, and the movement (3) realizes the convex surface of the piston. In this way, a piston profile with a convex ellipse can be machined.
- Mechanism of realization of elliptical skirt ellipse
- Forming mechanism of convex line
The processing of the ellipse is realized by the combination of two sets of machining motions, one is the high-frequency reciprocating linear motion of the tool, and the other is the axial relative motion between the tool and the lever system. The former controls the movement frequency of the tool, and the latter controls the movement range of the tool. The superposition of the two sets of motions can make the length of the short axis of the elliptical section of the piston skirt constant during the machining process, and the length of the long axis changes with the elliptic curve in the direction of the piston axis. Processing of the ellipse.
In the realization process, since the cross section of the piston is an ellipse, when the turning processing mode is adopted, the turning motion of the tool relative to the main shaft reciprocates linearly. Each time the piston is rotated, the tool must be fed back and forth twice. It can be concluded that the feed frequency of the tool is twice the rotation frequency of the spindle. The higher the spindle speed, the higher the reciprocating frequency of the tool, and the greater the speed and acceleration of the tool. At the same time, since the difference between the length of the long and short axes of the elliptical section of the piston skirt is generally less than 2 mm, the radial movement of the tool is small. Thus, the motion of the tool is actually a high frequency micro-displacement motion.
In order to realize the high-frequency micro-displacement feed of the tool, after considering the advantages of the hard master, the principle shown in Fig. 4 is adopted, and the stepping motor is used as the driving source, and the elliptical model is rotated at a high speed by a lever structure. Feeds the high frequency micro-displacement of the tool. In order to avoid the uncoordinated frequency of the tool reciprocating motion and the spindle rotation frequency during the machining process, the processed piston is deformed. The system is equipped with a photoelectric encoder on the main shaft and the master, and the output signal of the photoelectric encoder is used as the output signal. The feedback signal is quickly screened by the main control CPU, and the control signal frequency of the stepping motor driving source is continuously changed to adjust the rotation speed of the stepping motor, and the position synchronization of the elliptical master and the main shaft is realized to ensure the phase of the workpiece and the elliptical model. consistency.
 Figure 4 Schematic diagram of the elliptical processing |
The synchronization of the elliptical master and the main axis is the key to the realization of the elliptical section, and it is also the premise of realizing the ellipse. In the principle of the elliptical machining shown in FIG. 4, the actuator converts the rotation of the stepping motor into the swing of the lever through the elliptical master, thereby pushing the cutter to perform high-frequency micro-displacement linear reciprocating motion. In the figure, if the relative position between the cutter and the lever is kept unchanged, the processed piston skirt should be an elliptical cylinder, that is, the elliptical shapes of the piston sections are equal in size. In order to be able to machine the elliptical piston profile, the rotation of the other stepper motor 15 can be controlled by the master PCU to drive the ball screw (Fig. 2) so that the lever has an axial movement relative to the knife. In this way, the distance L between the tool and the fulcrum of the lever can be controlled to change the radial travel of the tool. The grating displacement sensor in Figure 3 provides the system with a feedback signal for the tool displacement, ensuring that the machined ellipse conforms to a given ellipticity curve. It should also be pointed out here that during the machining process, when the contact between the lever and the elliptical cam is at the elliptical long axis position, the lever must always remain in a horizontal state, the purpose of which is to ensure that when the tool is moved relative to the lever The length of the short axis of the ellipse of each section of the piston is always constant. As the relative position of the tool and the lever constantly changes, only the long axis length of the ellipse. Therefore, with such a structure, the entire control process for forming the ellipse can be simplified, and the processing of the cross section of the piston with arbitrarily varying ellipticity can be realized.
The elliptical model lever mechanism driven by the stepping motor realizes the processing of the ellipse, making full use of the advantages of the lever and the hard master. The whole system can not only obtain a large rigidity and a large enough thrust, but also the dynamic characteristics and followability of the system during high-frequency vibration, making the system motion stable and reliable. In addition, the system also integrates the advantages of the soft master, and can easily change the distance between the tool and the lever (lever arm L) by controlling the movement of the stepping motor by the master CPU, so that the profiles of different elliptic curves can be processed. Flexible implementation of various types of profile pistons.
Usually, the convex line of the piston is designed with a list data, and the radial position change of the line is only a series of discrete points. In order to determine the amount of radial position change at each section of the piston axial direction, a certain finite point must be fitted to a certain curve to generate a curve of the convex line in the piston. There are many methods for fitting processing, such as cubic spline fitting method, equal error straight line approximation method, Gaussian fitting method, polynomial fitting method, and the like. By fitting the process, it is possible to find the radial relative position of the tool to the workpiece at any skirt length.
Since the system uses the existing CNC machine as the parent, the system itself has the ability to directly process the convex line. According to the curve obtained by fitting, the contour line can be automatically programmed in the computer according to the NC instruction rules to generate an NC program for the convex line in the machining. Then, through the communication with the CNC machine CPU, the programmed program is transmitted to the program storage unit of the CNC to realize the processing of the convex line in the piston skirt. During the NC machining process, the corresponding relationship with the machined elliptical section should be maintained, so that the convex line can be accurately superimposed on each cross section of the piston to complete the processing of the convex ellipse in the piston skirt.
4 Conclusion
The method proposed in this paper combines the current status of piston machining at home and abroad, adopts a new processing method combining soft and hard mastering technology, and designs a turning machining system that realizes the convex elliptical profile in the piston skirt. The system has the following characteristics:
- Make full use of the advantages of the soft master. For workpieces with different piston shapes, whether it is adjusting the convex line of the piston profile or adjusting the elliptic curve of the piston, just adjust the control program in the main control CPU to achieve The piston is modified so that the system has greater flexibility.
- The processing system has the characteristics of simple structure, large output force and good processing stability.
- The motion error in the system can be compensated by the improvement of the software to improve the machining accuracy.
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