Thread milling is a popular thread processing method in manufacturing in developed countries. It is a new method for thread machining using the helical interpolation function of CNC machine tools. Compared with the traditional thread turning tool or thread tapping, die tapping and buckle, the main advantages are as follows: 1 The same thread milling cutter can process left-hand thread or right-hand thread with the same pitch diameter; 2 same thread The milling cutter can machine the thread to any tolerance zone by modifying the tool radius compensation value; 3 Since the thread milling cutter has no guiding part, the precise depth of the thread can be machined near the bottom of the thread without the need of the undercut or the transition buckle; Or damage does not affect the quality of the part - the damaged part of the tool can be easily removed from the workpiece.

With the widespread popularity of CNC machine tools in China, this thread processing method has gradually been adopted and applied in China. However, it is limited to the machining of large-diameter threads, the processing of small-diameter threads, especially internal threads, and the use of thread milling. Small hole threading is generally performed by tapping, but when machining small hole threads on some ultra high strength steels, it is almost impossible to complete the machining with taps. Because the tap must have enough torque to tap the thread, the tap can't withstand the torque applied by the threading process when tapping the thread on some ultra-high-strength steel. Especially in the case of extremely high material hardness, the tap is also extremely wearable. Once the tap is slightly worn, the torsion force during attack is also sharply increased, which also accelerates the wear of the tap, and such a vicious cycle will eventually lead to breakage of the tap.

By virtue of the many advantages of thread milling, the thread milling method is adopted when machining the threaded hole of the flange joint of an ultra high strength steel pressure vessel, and good economic benefits are obtained. The following is a brief introduction to this process and process.

1 Features of parts and difficulties in processing

The part is a large pressure vessel, and a blind hole thread of 60-M16×1.5-6H (uniform) is to be processed at one end of the flange joint. The material has a tensile strength of 1,800 MPa and a hardness of 52 HRC, which is extremely difficult to process. These materials are not only high in strength but also extremely hard. At the beginning of the tap tapping, only the imported solid carbide with TiAlN coated taps can be used for processing. However, the price of such a tap is extremely expensive (2,500 yuan / support), and the life is not high (only 20 holes / support), more importantly, the tap is often broken during the process, the removal is difficult, and the thread is easily damaged when taken out. Affect product quality.

figure 1

2 tool selection

Tool selection should be based on the principle of meeting machining requirements, increasing tool life and machining efficiency. 1 Tool geometry: total length 70mm, diameter D2 = ? 14mm. Tool length After considering the clamping part and the cutting part, the tool overhang should be as short as possible, the tool diameter should be as large as possible, slightly smaller than the thread bottom hole to increase the rigidity of the tool and reduce the vibration during machining; 2 Material: Solid carbide with TiAlN coating. This kind of tool not only has excellent wear resistance, but also has high strength. It is suitable for materials with extremely high processing strength and hardness. 3 Tool teeth: Z=6 teeth. The more the number of teeth, the larger the feed rate and the higher the processing efficiency.

3 processing methods and parameter selection

figure 2

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