When we went to Yi Tuo factory for internship, I saw a master tapping, but there was no positioning and clamping device. Why?

The master's words are right. In general, when tapping threads below M6, no fixture is used, which can achieve better results. Because of the strength of holding the workpiece, you can grasp the strength at any time (the vocabulary on the table is called adaptive control of strength), but this effect can not be achieved with fixtures.

Tapping above M8 requires a fixture, and sometimes even an extension rod.

Most of the knowledge in books can only play a guiding role in work. (the more common meaning of paying attention to guidance is that you can only refer to it and get a general understanding. I don't understand that guidance is a program and an unshakable constitution. This kind of guidance can also be understood as the meaning of superior guidance, because most of them don't understand any specific problems, but only work out the "rough technology" when the work must be completed and when the products should be replaced, and the specific "detailed technology" still needs to be determined by the technicians themselves. Sadly, at present, we don't pay attention to these "meticulous" engineers and technicians. We should know that many of the early scientific theories came from practical work, and many of them were engineers. If those people are put into today's society, at least half of them can be academicians of the Chinese Academy of Sciences and the Chinese Academy of Engineering. So many so-called high-tech technologies can't be realized at all. Even if we can design them ourselves, we can't process them. It's a little too much to talk about this You will have a personal experience after you really work for a period of time. In short, the book comes from the end, and it feels very shallow.