Turning trapezoidal thread by the hottest layer me

2022-08-02
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Layered turning of trapezoidal thread

introduction

trapezoidal thread is larger in pitch and profile than triangular thread, and has high precision. The surface roughness value on both sides of the profile is small, resulting in deep cutting, fast tool walking, large cutting allowance and large cutting resistance when turning trapezoidal thread. This has led to the causes and troubleshooting methods of this kind of fault caused by trapezoid: (1) the turning of the long thread of the push rod of the key switch is very difficult, which is difficult for students to master in the lathe skill training, and it is easy to produce the phenomenon of cutting, which makes students nervous and afraid. In many years of lathe worker practice teaching, through continuous exploration, summary and improvement, we have also had a certain understanding of the turning of trapezoidal thread. Let's explore which turning method is more suitable for beginners to understand, learn and master

1 selection of turning method for trapezoidal thread

when turning trapezoidal thread, high-speed steel tools are usually used for low-speed turning. Generally, there are four feeding methods for low-speed turning trapezoidal thread as shown in Figure 1: straight cutting method, left-right cutting method, turning straight groove method and turning step groove method. 2. The problem of experimental travel. Generally, the straight forward method is only applicable to turning trapezoidal threads with small pitch (p4mm), and the left-right cutting method, straight groove turning method and step groove turning method are often used. Let's explore these turning methods:

Figure 1 four feed methods for trapezoidal thread turning

straight feed method: the straight feed method is also called the grooving method, as shown in Figure 1 (a). When turning the thread, only the middle carriage is used for transverse (perpendicular to the guide rail) feed, and the thread turning is completed in several strokes. Although this method can obtain a relatively correct tooth profile, and the operation is very simple, because the three cutting edges of the tool participate in cutting at the same time, the vibration is relatively large, the tooth side is easy to pull out burrs, it is not easy to obtain good surface quality, and it is easy to produce the phenomenon of cutting. Therefore, it is only applicable to the turning of trapezoidal thread with small pitch:

left and right cutting method: when turning trapezoidal thread with left and right cutting method, In addition to using the scale of the middle carriage to control the transverse feed of the turning tool, the scale of the small carriage is also used to control the left and right micro feed of the turning tool until all the teeth are turned, as shown in Figure 1 (b). When using the left-right cutting method to turn the thread, because one of the two main cutting edges of the turning tool is cutting on one side, the simultaneous cutting of three edges is avoided, so it is not easy to produce the phenomenon of cutting. In addition, low speed (v=4 ~ 7m/min) shall be selected as much as possible during finish turning, and cutting fluid shall be poured, generally good surface quality can be obtained. In the actual operation process, students should control the left and right feed rate and observe the chip condition at the same time according to their actual experience. When the discharged chip is very thin, finishing can be used to make the thread surface smooth and clean with high precision. However, the operation of the left-right cutting method is more complex. When the small carriage is fed slightly from the left to the right, it is easy to make mistakes due to the influence of the empty stroke. In addition, the middle carriage and the small carriage feed at the same time. The size and proportion of the feed amount of both are not fixed, the cutting amount of each tool is not easy to control, and the tooth shape is not easy to turn. Therefore, the left-right cutting method has high requirements on the operator's proficiency and cutting skills, and is not suitable for beginners to learn and master:

turning straight groove method: when turning trapezoidal threads with straight groove method, generally select the rectangular thread turning tool with the tool head width slightly less than the tooth bottom width, and use the transverse straight cutting method to rough turn the threads to the small diameter (with 0.2 ~ 0.3mm allowance on each side), and then use the fine turning tool for finishing, as shown in Figure 1 (c). This method is simple, easy to understand and master, but when turning trapezoidal threads with large pitch, the tool is easy to break due to its long and narrow head and insufficient strength; the cutting groove is deep and the chip removal is not smooth, resulting in the accumulated chip "smashing" the head: the feed rate is small and the cutting speed is low, Therefore, it is difficult to meet the turning needs of trapezoidal threads:

turning step groove method: in order to reduce the damage of the tool head during turning with the "straight groove method", we can use the turning step groove method, as shown in Figure 1 (d). This method also uses a rectangular thread turning tool to cut grooves, but instead of directly cutting to the small diameter, it is divided into several tools to cut into stepped grooves, and finally it is trimmed to the specified size with a fine turning tool. In this way, the cutting and chip removal is smooth and the method is simple, but it is not easy to align the spiral straight groove when changing the tool, so it is difficult to ensure the correct tooth shape and easy to produce tooth inversion

to sum up: in addition to the straight forward method, the other three turning methods can reduce or avoid the simultaneous cutting of three edges to varying degrees, so that the chip removal is smoother, the stress and heating conditions of the tool tip are improved, so that vibration and tool pricking are not easy to occur, and the cutting parameters can be increased to improve the surface quality of the thread. Therefore, left-right cutting method, turning straight groove method and turning step groove method have been widely used. However, for beginners, the above three turning methods are difficult to master and cumbersome to operate, and need to be made easier and simpler. Through years of teaching and practice, it is found that "layered turning trapezoidal thread" can make students understand and master it easily

2 "layering method" for turning trapezoidal thread

"layering method" for turning trapezoidal thread is actually a comprehensive application of straight forward method and left-right cutting method. When turning trapezoidal threads with large pitch, the "layered method" usually does not cut the trapezoidal grooves at one time, but divides the grooves into several layers (each layer is about 1 ~ 2mm deep) and converts them into several shallow trapezoidal grooves for cutting, thus reducing the turning difficulty. The cutting of each layer adopts the turning method of first straight cutting and then left and right cutting. Since the groove depth is unchanged during left and right cutting, the tool only needs to feed longitudinally (along the guide rail direction) to the left or right (as shown in Figure 2), so it is much simpler and easier to operate than the left and right cutting method mentioned above

Figure 2 ladder thread turning by layered method

Table 1 calculation formula and parameter value of 38% thread completed by ladder coal

name code calculation formula and parameter value/mm crown clearance ac0.5 major diameter D nominal diameter (.3750) pitch diameter d2d2=d-0.5p= 118 minor diameter d3d3=d-2h3= 5370 tooth height H3 h3=0.5p+ac=3.5 tooth top width ff=0.366p=2.196 tooth bottom width ww=0.366p-0.536ac=1.928

the calculation formula and parameter values of trapezoidal thread are listed in Table 1. Here is turning tr36 × E as an example, this paper introduces the tool selection for turning trapezoidal thread by "layered method": "layered method" for turning trapezoidal thread: the coarse turning tool and fine turning tool used for turning trapezoidal thread by "layered method" are basically the same as other processing methods, except that the tool head width of the coarse turning tool (W tool =1.2 ~ 1.5mm) is less than the tooth bottom width (w=1.928mm), and the tool tip angle (er=29 ~ 29 30') is slightly less than the tooth angle of trapezoidal thread (a=30)

2) "layered method" operation steps for turning trapezoidal thread:

· rough and fine turning the major diameter (.3750) of trapezoidal thread, and the chamfer is 15 ° to the end face. Here, a trimming allowance of about 0.15mm can also be reserved for the major diameter of the thread, so that it can be trimmed when the tooth top is found to be torn and deformed after the fine turning of the thread

· use the trapezoidal thread rough turning tool to turn to about 1/3 of the depth of the tooth groove (h1=1mm), because the cutting depth is small and the cutting force is small, generally there will be no vibration and stabbing, as shown in Fig. 2 and Fig. 3 (a). At this time, the middle carriage stops feeding and performs transverse feeding (the turning tool enters the original cutting depth each time). Only a small carriage is used to make the turning tool feed slightly to the left or right. The feeding amount is about 0.2 ~ 0.4mm. The number of feeding depends on the specific situation. The tooth groove is widened at a faster speed, as shown in Figure 2 and figure (b). After widening, the crown width F'(F' is about 2.5mm) shall be greater than the theoretical calculation value f (f=2.196mm), so as to ensure that about 0.15mm fine turning allowance is left on both sides of the thread

Fig. 3 steps of "layered method" turning of trapezoidal thread

· return the turning tool head to the middle position of the first layer to widen the tooth groove (just return the small carriage to half of the number of borrowed slots), then use the straight forward method to cut the second layer, and turn it to about 2/3 of the depth of the tooth groove [h2=2mm, as shown in Fig. 3 (c)], then stop the horizontal feeding of the middle carriage, and use the left and right cutting method to widen the tooth groove [as shown in Fig. 3 (d)]. When widening the alveolar, the two alveolar sides of the second layer should coincide with the first layer. Be careful not to turn to the side of the first layer of alveolar again, otherwise the fine turning allowance of the crown may be insufficient

· repeat the above steps, continue to turn to the third layer (tooth height h3=3mm) and the fourth layer (tooth height h4=3.5mm, d3=.5370) with straight forward method and left-right cutting method, and then widen the alveolar (Fig. 2). The turning times of the "layered method" can be two, three, or even more times, depending on the size of the pitch and the strength of the turning tool

· use a fine turning tool to finish turn the left and right tooth sides of the thread respectively [Fig. 3 (E)]. Generally, finish turn one side of the tooth groove first, and then the other side of the tooth groove. At the same time, ensure the technical requirements such as the dimensional accuracy of the pitch diameter of the thread and the surface roughness of the two tooth sides. From the above processing process, it can be seen that the "layered method" for turning trapezoidal thread has the following advantages: relatively simple operation, easy to understand and master: it basically overcomes the problems of three-sided cutting, difficult chip removal, easy cutting, etc.; it can obtain a clearer tooth profile, increase the cutting amount to improve the production efficiency, and easily ensure the dimensional accuracy and obtain a better surface roughness

2 conclusion

in practical teaching, the "layered method", which is easy to understand and master, has been fully affirmed and praised. Teachers can explain and teach the turning method to students more vividly and intuitively, and students can generally understand and master this turning method faster and easier, which greatly reduces the teaching difficulty and intensity of the subject of trapezoidal thread turning. However, in the production practice, the turning of trapezoidal thread is quite complex. In the turning process, one method should not be used only to turn, but should be well understood. Therefore, only when students master and master various turning methods can they flexibly use them in the turning process and complete the turning of trapezoidal thread with high efficiency, high precision and high quality. (end)

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