Straight-forward method: Straight-forward method is also called grooving method, as shown in figure 1(a). When turning threads, only the middle carriage is fed transversely (perpendicular to the guide rail direction), and thread turning is completed in several strokes. Although this method can obtain more correct tooth profile and the operation is very simple, the vibration is relatively large because the three cutting edges of the tool participate in cutting at the same time, and the burr is easy to be pulled out from the tooth side, so it is not easy to obtain better surface quality and it is easy to cause tool sticking. Therefore, it is only suitable for turning trapezoidal threads with small pitch:

Left-right cutting method: When turning trapezoidal thread by left-right cutting method, the horizontal feed of the turning tool is controlled by the scale of the middle carriage, and the left-right micro feed of the turning tool is controlled by the scale of the small carriage until all the teeth are turned around, as shown in figure 1(b). When turning threads by the left-right cutting method, one of the two main cutting edges of the turning tool cuts on one side, which avoids the simultaneous cutting of the three edges, so it is not easy to cause the tool sticking phenomenon. In addition, when finishing, try to choose low speed (V = 4 ~ 7m/min) and pour cutting fluid, which can generally obtain good surface quality. In students' actual operation, we should observe the chip situation while controlling the left and right feed according to actual experience. When the discharged chips are very thin, finishing can be used to make the thread surface smooth and high precision. However, the operation of the left-right cutting method is more complicated, and the small carriage is easily affected by the idle stroke and makes mistakes. Moreover, the middle drag plate and the small drag plate feed at the same time, and the feed amount and proportion of the two are not fixed, so the cutting amount of each tool is not easy to control, and the tooth profile is not easy to cut clearly. Therefore, the left-right cutting method requires the operator's proficiency and cutting skills, which is not suitable for beginners to learn and master:

Straight groove turning method: generally, when turning trapezoidal thread with straight groove turning method, rectangular thread turning tool with bit width slightly smaller than the width of tooth groove bottom is selected, and the thread is roughly turned to a small diameter size (with a margin of 0.2 ~ 0.3 mm on each side) by transverse straight groove turning method, and then trimmed with fine turning tool, as shown in figure 1(c). This method is simple and easy to understand and master. However, when turning large pitch trapezoidal thread, the tool is easy to break because of its long and narrow head and insufficient strength. The cutting groove is deep and the chip discharge is not smooth, which causes the accumulated chips to "break" the machine head. The feed is small and the cutting speed is low, so it is difficult to meet the turning needs of trapezoidal threads.

Turning step groove method: In order to reduce the damage degree of cutter head when turning with "straight groove method", we can adopt turning step groove method, as shown in figure 1(d). This method also uses rectangular thread turning tool to cut the groove, but it is not directly cut to the small diameter size, but divided into several knives to cut the stepped groove, and finally trimmed to the specified size with fine turning tool. In this way, the chip removal is smoother and the method is simpler, but it is difficult to align the spiral straight groove when changing the tool, it is difficult to ensure the correct tooth profile, and it is easy to cause the phenomenon of tooth inversion.

To sum up, except for the straight-ahead method, the other three turning methods can reduce or avoid cutting three sides at the same time to varying degrees, making the chip removal smoother, improving the stress and heat of the tool tip, making it difficult to vibrate and stick the tool, increasing the cutting capacity and improving the surface quality of the thread. Therefore, the left-right cutting method, turning straight groove method and turning stepped groove method have been widely used. However, for beginners, the above three ways of turning are difficult to master and complicated to operate, which needs to be simplified. Through years of teaching and practice, it is found that "turning trapezoidal thread layer by layer" can make students understand and master it easily.