CNC machine tools are essential tools in modern manufacturing technology. This paper begins with the concept and research direction of human-machine interface (HMI) design, fully considering human factors, and conducts a preliminary exploration of the design principles of HMI for CNC machine tools. A CNC machine tool is a high-tech mechatronics product, a programmable general-purpose machining equipment composed of a CNC device, servo drive device, machine tool body, and other auxiliary devices. It is widely used in various fields of the manufacturing industry. Compared with general-purpose and special-purpose machine tools, CNC machine tools are best suited for machining parts with complex structures and high precision requirements, as well as for the production of multi-variety, small-batch parts with frequent product updates and short production cycles. Modern CNC machine tools are developing towards higher speed, higher precision, intelligence, multi-functionality, and higher reliability. The design of the HMI for CNC machine tools directly affects their work efficiency and operational comfort. A good HMI is simple and effective to operate, and has a guiding function, making users feel happy and increasing their interest, thereby improving efficiency. A human-machine interface (HMI) is the operating method for humans to interact with machines, that is, the medium through which users and machines exchange information, including information input and output. Human-computer interface (HCI) design should be studied by integrating knowledge from multiple disciplines such as psychology, ergonomics, computational linguistics, art design, intelligent human-computer interfaces, sociology, and anthropology. Its development trend is also moving towards greater humanization and high technology. Some scholars believe that HCI design can be understood as both broad and narrow definitions. In a narrow sense, HCI refers to the human-computer interface in a computer system, also known as the human-computer interface or user interface. It is an interdisciplinary research field combining computer science with psychology, graphic arts, cognitive science, and ergonomics. It serves as the medium for transmitting and exchanging information between humans and computers, and is the comprehensive operating environment provided by the computer system to the user. From a broad perspective, HCI primarily studies the rationality of the human-machine relationship. The "human" in HCI refers to the person as the main working entity, including operators and decision-makers. Human physiological characteristics, psychological characteristics, and adaptability are all important research directions. The "machine" in HCI refers to the totality of all objects controlled by humans, including all products and engineering systems operated and used by humans. Designing a machine that meets human requirements and conforms to human characteristics is an important issue in human-machine interface design. The human-machine interface of CNC machine tools can be divided into software human-machine interface and hardware human-machine interface. 1. Design principles of CNC machine tool software interface Software plays a very important role in CNC machine tools, and its user interface directly affects the working efficiency and effect of CNC machine tools. The design principles of CNC machine tool software interface can be summarized as follows: (1) Consistency principle. The consistency principle of software human-machine interface mainly refers to the similarity of interface appearance, layout, human-machine interaction method and information display format between different systems and within the application system. The consistency principle helps users learn the operation of the machine tool and reduces the error rate during use. (2) Provide information feedback. Information feedback refers to the machine tool's response to the user's operation. It can indicate whether the user's operation is accepted by the machine tool, whether it is correct, and what the effect of the operation is. Feedback methods can be text, graphics and sound, etc. (3) Maintain the rationality of the interface space layout. The layout of the interface space should be simple and clear. Designers should design interfaces that are convenient for users based on their understanding of the actual operating objects. For example, in the graphical interface of the CNC5000V200 CNC drilling machine control system software, there are three independent processes: OPERATE, PROGRAM, and CONTROL. Each process serves the system operator, programmer, and supervisor, respectively. Each process has its own menu line, and the menus under the menu line are drop-down menus with their own sub-menus. Sub-menus can be selected to proceed to the next step. (4) Make reasonable use of colors and graphics. Using colors on the interface can better prompt operations and also alleviate the visual fatigue of the operator. The number of colors used should generally not exceed 3 to 5. For example, in the graphical interface of the CNC5000V200 CNC drilling machine control system software, the highlight represents the activated process. The activated process has its own color and is displayed as the foreground of the screen. OPERATE corresponds to green, PROGRAM corresponds to dark blue, and CONTROL corresponds to light blue. Graphics have the advantages of being intuitive, vivid, and information-rich. Using graphics in the interface can enhance the comprehensibility and ease of use of the operation. (5) Choose a suitable font and size. In an interface, it is best not to have too many fonts, and it is even more unsuitable to choose fonts that are too complex or weak. The simpler and clearer the font, the better the recognizability. For example, the height-to-width ratio of the characters can be 2:1 or 1:1 for clear recognition. 2. Design principles of CNC machine tool hardware interface Since CNC machine tools use digital technology to realize the direct control of the execution sequence and displacement of the machine tool, the gearbox structure of the traditional machine tool has been eliminated or partially eliminated, and the mechanical structure has been greatly simplified. The design principles of the hardware human-machine interface of CNC machine tools can be summarized as follows: (1) Design principles of machine tool appearance color. The machine tool is fixed and the working atmosphere is calm. Therefore, the color should not be too stimulating or exciting, nor too dull. It should make the operator feel happy when working. Generally, colors with low purity and high brightness are preferred. Colors with stimulating and exciting effects should not be used in large areas, but there should be appropriate contrast effects. Generally, decorative color strips, panel colors, and warning colors are used to contrast with the main body. For large machine tools, colors that are too light should not be used. Colors with a slightly neutral gray tone can produce a strong and stable visual sense. At the same time, multi-color configurations can be used to avoid the overall tone being too dark, so as to achieve a stable yet vivid and harmonious effect. Some machine tool shapes are vertically long and have a towering and unstable feeling. In order to achieve visual stability, lines or color strips can be used to divide the bed horizontally. The illusion of division can be used to adjust the visual sense of size proportion, thereby increasing the sense of stability. (2) Design principles of machine tool appearance scale and proportion The scale and proportion between the main components of the machine tool and between them and the entire machine tool structure are crucial to the machine tool styling design. For example, when determining the basic dimensions of the machine tool, the principle of "golden ratio" was used. The ratio of the height to the length of the machine tool is 0.1618, which makes the outline of the machine tool more compact and the size relationship of each part of the machine tool more coordinated, thus increasing the aesthetic appeal of the machine tool. The machine tool uses the horizontal line as the benchmark for the overall layout, and each part is divided by the horizontal line. The horizontal line should not be messy, irregular, or lacking in hierarchy. It should have strict hierarchy, uniformity, and coordination to increase the precision, regularity, and stability of the machine tool. The vertical dividing lines of the machine tool are not many, which makes the horizontal division of the machine tool more prominent. The vertical dividing lines should be simple, regular, and elegant. In addition, the surface of the machine tool should not have too many concave and convex parts to prevent the accumulation of oil and dust, which is difficult to clean. (3) Layout principles of the machine tool worktable and control panel The height of the main worktable should be designed according to the operation requirements, based on the operation requirements of standing, sitting, and sitting/standing postures. On the worktable, the design of manual actuators such as handwheels and cranks should be based on their function and usage, considering their size, shape, category, and location. Control buttons and displays on the control panel should be divided into functional areas, meeting the requirements of display-control compatibility. Their position, height, and arrangement should conform to ergonomics, facilitating operation and observation. For example, displays should ideally be placed within a 0-30° downward angle of the user's horizontal line of sight and within 15° to the left and right of the user's vertical line of sight. The colors of buttons on the control panel should be categorized according to their purpose. For example, red indicates danger, stop, or prohibition, and can be used for automatic alarm buttons and stop buttons on lathes. Green indicates safety, normal operation, etc., such as the machine tool's start button. On the worktable, on or beside the actuators, their functions should be labeled with text or symbols, preferably in white or black.