Brief introduction to the machining of common thre

2022-07-25
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A brief introduction to the processing of common threads on CNC lathes 1. The installation and tool setting of common thread cutters the turning tools have been dead since the end of last year. The experimental machine can test and analyze the mechanical properties of various metals, nonmetals and composite materials. If the weight of the fish is too high or too low, when the knife reaches a certain depth, the rear face of the turning tool will hold the workpiece, increase the friction, and even bend the workpiece, Causing knife gnawing; If it is too low, the chip is not easy to be discharged. The direction of the radial force of the turning tool is the center of the workpiece. In addition, the clearance between the traverse lead screw and the nut is too large, resulting in the deepening of the cutting depth, thus lifting the workpiece and biting the tool. At this time, the height of the turning tool shall be adjusted in time to make its tip equal to the axis of the workpiece (the tool can be adjusted by using the center of the tailstock). During rough turning and semi fine turning, the tool tip position is about 1%d higher than the out center of the workpiece (d represents the diameter of the workpiece to be machined)

the workpiece is not clamped firmly. The rigidity of the workpiece itself cannot bear the cutting force during turning, resulting in excessive deflection, which changes the center height of the turning tool and the workpiece (the workpiece is raised), resulting in a sudden increase in the cutting depth and tool gnawing. At this time, the fitter hammer should clamp the workpiece firmly, and the tailstock center can be used to increase the rigidity of the workpiece

the tool setting methods for common threads include trial cutting tool setting and automatic tool setting by tool setting instrument. You can directly use the tool for trial cutting tool setting, or use G50 to set the workpiece zero point and use workpiece shift to set the workpiece zero point for tool setting. The tool setting requirements for thread machining are not very high. In particular, there are no strict restrictions on tool setting in Z direction, which can be determined according to the programming requirements

II. Dimensional analysis of common threads

NC lathes need a series of dimensions for the processing of common threads. The dimensional calculation and analysis required for the processing of common threads mainly includes the following two aspects:

1. The diameter of the workpiece before thread processing

considering the expansion of the thread processing profile, the diameter of the workpiece before thread processing is d/d-0.1p, that is, the major diameter of the thread is reduced by 0.1 pitch, Generally, it is 0.1 to 0.5 smaller than the major diameter of the thread according to the material deformation capacity

2. Thread machining feed amount

the thread feed amount can refer to the thread bottom diameter, that is, the final feed position of the thread cutter

the minor diameter of the thread is: major diameter - twice the tooth height; Tooth height =0.54p (P is the pitch)

the feed amount for thread machining should be continuously reduced, and the specific feed amount should be selected according to the tool and working material

III. programming processing of ordinary thread

in the current CNC lathe, there are generally three processing methods for thread cutting: G32 straight cutting method, G92 straight cutting method and G76 oblique cutting method. Due to different cutting methods and programming methods, the processing errors are also different. We should carefully analyze the operation and use, and strive to process high-precision parts

1. G32 straight cutting method, because both edges 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, the larger the cutting depth, the faster the blade wear, resulting in errors in the pitch diameter of the thread; However, the tooth shape precision is high, so it is generally used for small pitch thread machining. Because the tool moving cutting is completed by programming, the machining program is long; Since the cutting edge is easy to wear, it is necessary to ensure the stability and reliability of the product during the processing and to make frequent measurement

2. G92 direct cutting method simplifies programming and improves efficiency compared with G32 instruction

3. G76 oblique cutting method, because it is processed with a single edge, the processing 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. When machining high-precision threads, it can be completed by two cutting tools, i.e. rough turning with G76 machining method, and then fine turning with G32 machining method. However, it should be noted that the starting point of the tool should be accurate, otherwise it is easy to buckle randomly and cause parts to be scrapped

4. After the thread processing is completed, the thread quality can be judged by observing the thread profile, and measures can be taken in time. When the thread crown is not sharp, increasing the cutting amount of the cutter will increase the major diameter of the thread. The increase depends on the plasticity of the material. When the crown has been sharpened, increasing the cutting amount of the cutter will reduce the major diameter in proportion. According to this feature, the cutting amount of the thread should be treated correctly to prevent scrapping

IV. detection of common threads

for general standard threads, thread ring gauge or plug gauge is used for measurement. When measuring the external thread, if the "through end" ring gauge of the thread is just screwed in, but the "stop end" ring gauge is not screwed in, it means that the processed thread meets the requirements, otherwise it is unqualified. When measuring internal thread, use thread plug gauge to measure in the same way. In addition to the measurement with thread ring gauge or plug gauge, other measuring tools can also be used for measurement. The pitch diameter of the thread is measured with a thread micrometer, the pitch diameter of the trapezoidal thread and the pitch diameter of the worm are measured with a tooth thickness vernier caliper, and the pitch diameter of the thread is measured with a measuring needle according to the three needle measurement method

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