Discussion on the method of improving the precisio

2022-08-05
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Discussion on the methods of improving the accuracy of NC Turning Thread

0 preface

it is a common method to use NC turning method to process thread in mechanical manufacturing industry. Compared with ordinary turning, the feed speed of thread turning is 10 times higher, the force at the tip of the thread blade is 100 ~ 1000 times higher, the cutting speed is faster, the cutting force is larger and the force accumulation range is narrow, resulting in the high difficulty of thread processing. Through a large number of experiments, the author believes that the accuracy of NC thread turning should be improved from three aspects: the geometric parameters of the tool, the chip fluid and the program

it can be seen from Figure 1 that this thread belongs to fine tooth ordinary thread, and the carbide triangle external thread turning tool is selected. See Fig. 2 for the geometry of thread turning tool. Chamfer with width of 0.2 ~ 0.4mm is ground on the cutting edges of the two cutting friction testing machines of the thread cutter, and its g=5. As the actual tooth angle will expand when cutting the thread at high speed, the tool tip angle should be reduced by 30'and ground to 59.5. The surface roughness of the front and rear cutting surfaces of the thread turning tool must be very small. When sharpening the tool, the grinding wheel must be correctly trimmed or the tool must be finely ground with an oilstone. When installing the thread turning tool, try to reduce the extension length to prevent insufficient rigidity of the tool bar and vibration during cutting. At present, Germany has the largest use of automobiles. The installation height of thread turning tool is also very particular. If it is too high or too low, there will be stabbing. If it is too high, when the cutting depth reaches a certain depth suitable for piece by piece inspection of finished or semi-finished workpieces processed in batches, the back cutting surface will support the workpieces to increase the friction, resulting in knife pricking; if it is too low, it is not easy to discharge the chips, thus jacking up the workpieces and knife pricking. The correct position is that the tool tip position is 0.1 ~ 0.3mm higher than the workpiece center. When turning threads, proper use of cutting fluid can reduce the heat generated during cutting and the machining error caused by temperature rise. It can form a film on the metal surface, reduce the friction between the tool and the workpiece, and wash away iron filings, so as to reduce the surface roughness of the workpiece and reduce tool wear. According to the experiment, generally speaking, water-based cutting fluid can be used to process threads to meet the requirements. If the accuracy requirements are high, oil-based cutting fluid, such as kerosene and vegetable oil, must be used. Generally, the water tank of the lathe is filled with water-based cutting fluid, so when machining threads, you can use an oil gun for manual lubrication to meet the accuracy requirements. Our school uses Guangzhou CNC system -gsk980t, which has G32, G92 and G76 thread programming instructions. G32 is a simple thread cutting instruction, which is obviously not suitable. G92 thread cutting cycle adopts the straight feed mode, and G76 thread cutting cycle adopts the oblique feed mode. Due to different cutting methods and programming methods, the machining errors are also different. When inspecting the operation of electromechanical rotation direction, it is necessary to analyze carefully and use appropriate instructions to process high-precision parts. Machining error analysis of G92 and G76:

g92 straight cutting method, as shown in Figure 3. Because both sides of the cutting edge work at the same time, the cutting force is large, and it is difficult to arrange cutting, so the two cutting edges are easy to wear during cutting. When cutting threads with large pitch, due to the large cutting depth and rapid wear of the blade, the pitch diameter of the thread has errors. However, the tooth shape accuracy is high, so it is generally used for machining small pitch threads

g76 oblique cutting method, as shown in Figure 4. Because it is machined with a single edge, the machining edge is easy to be damaged and worn, so that the machined thread surface is not straight, and the tool tip angle changes, resulting in poor tooth shape accuracy. However, because it works with a single edge, the tool load is small, chip removal is easy, and the cutting depth is decreasing. Therefore, this machining method is generally applicable to the machining of large pitch threads. Because this machining method is easy to remove chips and the working condition of the cutting edge is good, this machining method is more convenient in the case of low thread accuracy requirements

from the above comparative analysis, it can be seen that it is not perfect to simply use one instruction to turn the thread. Using G92 and G76 for programming, that is, using G76 for rough machining of the thread first, and then G92 for finish machining, will have two advantages in thread machining: on the one hand, it can avoid the deformation caused by large cutting amount; on the other hand, it can ensure the accuracy of thread machining. However, it should be noted that the starting points of rough turning and fine turning tools should be the same, otherwise the parts will be scrapped. According to the above analysis, The thread processing program compiled by GSK980T CNC system is as follows: 01234: program number: (omit the excircle processing program) T0303: adjust No. 3 thread turning tool G00 x37.0 z5.0: quickly locate the starting point of thread machining G76 p010160 q25 r0.05: use G76 thread turning cycle command G76 x34.45 z-58.0 p775 q350 f1.5: rough turn M36 1.5 thread part G00 x37.0 z5.0: quickly locate to the starting point of thread machining G92 x34.4 z-58.0 f1.5: use G92 thread turning cycle command x34.376: finish turn thread x34.376:g00 X100 Z100 M05: return to safety point, When the spindle stops rotating M30: the program ends and returns to the starting section of the program

several precautions during machining:

the workpiece shall be clamped to prevent slipping and flying out during turning and hurting people and cutting tools:

attention shall be paid to safe and civilized production

2 summary

through actual processing and production, the above measures effectively solve the problem of low processing accuracy, reduce the labor intensity of operators, improve efficiency and ensure the quality of processed parts. Process processing is very important for the programming of NC machine tools and the processing of parts. However, the process treatment work is very practical. In practical application, we should be good at analysis, make full use of all the knowledge we have, integrate theory with practice, and constantly summarize, so as to improve our own process analysis and treatment level

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