How does the milling machine fix the workpiece, only the milling cutter moves? Isn't that feeding motion just the horizontal and vertical motion of the workpiece?

What you are talking about is a traditional milling method, which is commonly called "plane milling". In plane milling, the workpiece is clamped and fixed on the worktable of the milling machine, and the milling cutter is processed by the rotation of the milling head.

Feed movement refers to the movement of the workpiece relative to the milling cutter during milling, including the movement of the workpiece in the transverse, longitudinal and vertical directions. These movements are controlled by different axes of the milling machine, which are usually called "X axis", "Y axis" and "Z axis".

In the milling process, the milling cutter moves along the surface of the workpiece and removes a certain amount of material. At the same time, the workpiece also moves on the X, Y and Z axes to keep the distance between the milling cutter and the workpiece surface unchanged, thus producing the required shape and size.

In a word, the feed movement in milling machine processing refers to the relative movement of the workpiece, not the movement of the milling cutter.