When the aircraft landing gear is renovated, if the chrome layer of the front pivot pin is worn beyond the standard, it will need to be repaired in depth. After the chromium layer is removed, it needs to be re-plated with chromium. The size of the newly plated chromium layer will basically exceed the repair scope, which requires Perform grinding and finishing to restore the size of the chromium layer to the design range. Mainly introduces several grinding methods to restore the size of the chromium layer.
PART 1
Preface
As the main load-bearing structure of the aircraft and the main force-bearing structure during take-off and landing, the landing gear will be corroded or damaged to varying degrees after long-term use or multiple take-offs and landings. Even if it is plated The chromium-layered main landing gear front pivot pin has greatly enhanced wear and corrosion resistance. During renovation, it was also found that the chromium layer on the pin still showed varying degrees of wear, and the base body of some front pivot pins even showed signs of corrosion. . Therefore, a rebuild would require a deep repair of the front pivot pin followed by a chrome layer restoration. An ordinary cylindrical grinder can be used to restore the size of the cylindrical direct pin part of the front pivot pin, but for the dimensional restoration of the spherical chrome layer, a special grinding method is required to meet the needs. At present, there are mainly the following grinding methods for spherical chromium layers: shaped grinding wheel processing, bowl-shaped grinding wheel processing, forming method, centerless grinding method and programmed processing method.
The difficulty in processing the spherical chromium layer is the high hardness of the chromium layer (57~62HRC), which requires the use of a grinding wheel with a lower particle size while ensuring the surface roughness after grinding. Because the shape is spherical, it is difficult to meet the demand using ordinary cylindrical grinding equipment.
PART 2
Form grinding wheel processing method
The forming grinding wheel processing method (see Figure 1) is a common method for finishing spherical surfaces in the early days. It mainly modifies the grinding surface of the grinding wheel to the spherical arc surface of the spherical surface. It must be ensured that the width of the grinding wheel is wider than the width of the spherical surface, otherwise it will not be possible to grind the spherical surface. Shave the entire sphere. Moreover, the diamond pen for grinding wheel dressing must be sharp enough so that the grinding wheel can be trimmed to fit the spherical surface.
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Figure 1 Schematic diagram of forming grinding wheel processing method
Because during the grinding process, the diameter of the chromium layer on the spherical surface is constantly shrinking, and the grinding wheel is also constantly consumed. Therefore, from a micro perspective, the contact surface is not an arc surface, resulting in a larger local diameter of the ground spherical surface. or smaller. And when the grinding wheel is repeatedly dressed, the repeated positioning accuracy of the equipment will also be superimposed on the dressing of the grinding wheel, resulting in the trimmed grinding wheel contact surface not being a true arc surface, which will also affect the sphericity of the ground spherical surface.
After the grinding wheel is disassembled and then reinstalled, if the removal amount when dressing the grinding wheel does not reach the depth of the original arc, the arc contact surface of the trimmed grinding wheel will also produce a slight misalignment [1]. This is the dressing setting after reinstallation. It is caused by the error between the starting point of the grinding wheel and the starting point of the original arc.
Therefore, the forming grinding wheel processing method is most suitable for processing spherical surfaces with a uniform diameter in batches, and at the same time, try not to assemble and disassemble the grinding wheel to avoid increasing the starting position error of the grinding wheel dressing.
PART 3
Bowl grinding wheel processing method
The bowl-shaped grinding wheel processing method (see Figure 2) is relatively simple, the processed spherical surface has high precision, and the dressing of the grinding wheel is also convenient. When processing, ensure that the diameter of the bowl-shaped grinding wheel is wider than the width of the spherical surface to be processed. It uses the arc of the grinding wheel bowl mouth to contact the arc surface of the workpiece, and the grinding wheel and the workpiece rotate at the same time to grind the spherical surface. This method does not require special equipment and grinding wheel shaping and dressing tools. Because the grinding wheel is in arc contact with the spherical surface, even if there is local wear, it will not affect the grinding quality of the spherical surface. But it can only be used on special spherical grinder equipment, because general cylindrical grinder equipment cannot properly install bowl-shaped grinding wheels. If this method is used to grind the front pivot pin of the landing gear, special cylindrical grinding machine equipment is required.
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Figure 2 Schematic diagram of bowl-shaped grinding wheel processing method
PART 4
development method
The generation method uses a flat grinding wheel. The outer circular surface of the trimmed flat grinding wheel is brought into contact with the surface of the workpiece to be ground. The workpiece rotates around the axis and rotates left and right at a certain angle around the spherical diameter perpendicular to the axis. At this time, it rotates at high speed. The grinding wheel surface will grind the contact surface brightly [2], as shown in Figure 3. Moreover, the same grinding wheel can be used to grind spherical surfaces with different spherical diameters. The dressing requirements of the grinding wheel are the same as those of ordinary cylindrical grinding. However, the principle of this grinding method also belongs to the grinding of a special spherical grinding equipment. The equipment needs to be able to rotate the workpiece left and right around the spherical diameter direction perpendicular to the axis. This method cannot be directly used on a cylindrical grinding machine.
PART 5
Centerless grinding method
The centerless grinding method (see Figure 4) has a certain similarity with the formed grinding wheel processing method. It mainly modifies the grinding wheel into an arc surface with the same diameter as the spherical surface. However, centerless grinding requires that the grinding guide wheel also be trimmed into a spherical surface. The size of the spherical diameter, a circular space that is the same as the spherical diameter is formed between the centerless grinding wheel and the guide wheel, which is used to grind the spherical workpiece. The biggest advantage of centerless grinding compared to formed grinding wheel grinding is that it does not require precise clamping. The spherical pin is placed between the grinding wheel and the guide wheel and the grinding will be centered automatically. The effect after grinding is also greatly affected by the arc shape of the grinding wheel. , if the arc trimmed by the grinding wheel is not round enough, it will easily lead to a large local deviation of the spherical diameter.
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Figure 4 Schematic diagram of centerless grinding processing method
PART 6
programming method
Dress the grinding wheel into an arc surface, set the tool compensation in the equipment, and use CNC two-axis interpolation to run the arc program to grind the spherical workpiece. Because the grinding wheel is constantly worn during the grinding process, the arc angle of the grinding wheel is also constantly getting smaller. It is necessary to constantly trim the arc surface of the grinding wheel. At the same time, the compensation of the program tool [3] is required to ensure the roundness of the grinding wheel. The arc surface follows an arc path to ensure the accuracy of the spherical surface ground by the grinding wheel. The spherical surface ground by this method (see Figure 5) will also have different local spherical diameters. At the same time, there are certain requirements for the width of the grinding wheel and whether there are straight shanks at both ends of the spherical surface.
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Figure 5: Schematic diagram of program processing method
PART 7
Conclusion
Each of the above processing methods has its own advantages and disadvantages, and some grinding processing methods require special machine tools to meet the requirements. For ordinary cylindrical grinders, if they need to be modified for grinding spherical surfaces, the forming grinding wheel processing method can be used, which is easy to operate and has a high success rate. For the actual situation, it is necessary to select an appropriate method based on the actual needs of the workshop and the equipment it has to grind and finish the ball pin to restore the size of the chromium layer to the design requirements.
