How did Lu Ban invent the ruler?

One of the signs of Lu Ban's invention is that he can correctly draw a right triangle, also called a class ruler, and can tell craftsmen which sizes are irregular and which ones are unlucky according to the rules of divination (geomantic omen). These rulers are still available in Hong Kong today. Lu Ban attached great importance to the invention of the saw. Or inspired by a blade of grass with a toothed blade that cuts your finger, or seeing a cricket cut and eat food with its sharp teeth and leave. Anyway, most of them are described like this. Luban and craftsmen were asked to cut down a lot of wood. After chopping for several days, they were exhausted and their axes were dull. At this time, Lu Ban suddenly cut his finger by a blade of grass. He immediately thought: It must be a good way to make a tool for cutting wood. He chose a bamboo and cut a row of teeth on its edge with an axe. This new saw cuts bark easily. When he sawed the tree horizontally back and forth, the soft bamboo teeth were quickly polished. However, this proves the principle that a saw can cut wood. So Lu Ban put down his hand and went to the blacksmith to prepare an iron plate as hard and sharp as an axe, and then made it into a tooth shape. With this first-hand saw blade, Luban can use it to cut wood accurately and effortlessly on the wooden frame.