The basics of CNC machine tools

Basic Components

The basic components of a Computer Numerical Control (CNC) machine tool include the machining program carrier, CNC device, servo drive system, machine tool body, and other auxiliary devices. The following is a brief explanation of the basic working principles of each component.

Machining Program Carrier

When a CNC machine tool is in operation, it does not require direct manual operation. To control the CNC machine tool, a machining program must be prepared. The machining program for a part includes the relative motion trajectories of the tool and workpiece on the machine, process parameters (such as feed rate, spindle speed, etc.), and auxiliary movements. The machining program is stored on a program carrier in a specific format and code, such as punched paper tape, cassette tape, floppy disk, etc. The program information is then input into the CNC unit through the input device of the CNC machine tool.

CNC Device

The CNC device is the core of a CNC machine tool. Modern CNC devices adopt the form of Computer Numerical Control (CNC), which typically uses multiple microprocessors to implement CNC functions in the form of programmed software, hence also known as Software NC. The CNC system is a position control system that interpolates the ideal motion trajectory based on input data and then outputs it to the executing components to machine the required part. Therefore, the CNC device mainly consists of three basic parts: input, processing, and output. All these tasks are reasonably organized by the computer's system program, allowing the entire system to work in harmony.

1. Input Device: It inputs CNC commands to the CNC device. Depending on the program carrier, there are corresponding input devices. The main ones include keyboard input, disk input, direct communication input from CAD/CAM systems, and DNC (Direct Numerical Control) input connected to a superior computer. Many systems still retain the paper tape input form using optical readers.

• (1) Paper Tape Input Method: The part program can be read in by a paper tape optical reader to directly control the machine tool's movement, or the content of the paper tape can be read into memory, and the part program stored in the memory can be used to control the machine tool's movement.

• (2) MDI (Manual Data Input) Method: The operator can use the keyboard on the control panel to input machining program commands, which is suitable for relatively short programs.

In the EDIT mode of the control device, the machining program is input using software and stored in the memory of the control device. This input method allows the program to be reused. This method is commonly used for manual programming.

On CNC devices with conversational programming functions, by selecting different menus based on the prompts on the display and using a human-machine interaction method to input relevant dimension numbers, the machining program can be automatically generated.

• (3) DNC Input Method: The part program is stored in a superior computer, and the CNC system receives subsequent program segments from the computer while machining. The DNC method is often used for complex workpieces designed using CAD/CAM software, which directly generates the part program.

1. Information Processing: The input device transmits the machining information to the CNC unit, which compiles it into information that the computer can recognize. The information processing part gradually stores and processes the information according to the control program's specifications and then sends position and speed commands to the servo system and main motion control part through the output unit. The input data of the CNC system includes part contour information (start point, end point, straight lines, arcs, etc.), machining speed, and other auxiliary machining information (such as tool changing, speed changing, coolant switching, etc.). The purpose of data processing is to complete the preparations before interpolation. The data processing program also includes tool radius compensation, speed calculation, and auxiliary function processing.

2. Output Device: The output device is connected to the servo mechanism. The output device receives the output pulses from the arithmetic unit according to the controller's commands and sends them to the servo control system of each coordinate. After power amplification, it drives the servo system, thereby controlling the machine tool to move according to the specified requirements.

Servo and Measurement Feedback System

The servo system is an important part of a CNC machine tool, used to realize the feed servo control and spindle servo control of the CNC machine tool. The role of the servo system is to convert the command information received from the CNC device into linear or angular displacement movement of the machine tool's executing components after power amplification and shaping processing. Since the servo system is the final link of the CNC machine tool, its performance directly affects the technical indicators such as the precision and speed of the CNC machine tool. Therefore, the servo drive device of the CNC machine tool is required to have good fast response performance, accurately and sensitively track the digital command signals sent by the CNC device, and faithfully execute the commands from the CNC device to improve the system's dynamic following characteristics and static tracking accuracy.

The servo system consists of two main parts: the drive device and the executing mechanism. The drive device is composed of a spindle drive unit, a feed drive unit, a spindle servo motor, and a feed servo motor. Stepper motors, DC servo motors, and AC servo motors are commonly used drive devices.

The measurement components detect the actual displacement values of each coordinate axis of the CNC machine tool and input them into the CNC device of the machine tool through the feedback system. The CNC device compares the actual displacement values fed back with the command values and outputs the displacement commands required to reach the set values to the servo system.

Machine Tool Body

The machine tool body is the main part of a CNC machine tool. It includes the bed, base, column, beam, slide, worktable, spindle box, feed mechanism, tool holder, and automatic tool changer and other mechanical components. It is the mechanical part that automatically completes various cutting processes on the CNC machine tool. Compared with traditional machine tools, the CNC machine tool body has the following structural characteristics:

1. It adopts a new machine tool structure with high stiffness, high shock resistance, and small thermal deformation. Methods such as improving the static stiffness of the structural system, increasing damping, adjusting the mass and natural frequency of structural components are commonly used to improve the stiffness and shock resistance of the machine tool body, so that it can adapt to the needs of continuous automatic cutting processing on CNC machine tools. Measures such as improving the machine tool's structural layout, reducing heat generation, controlling temperature rise, and adopting thermal displacement compensation are taken to reduce the impact of thermal deformation on the machine tool body.

2. High-performance spindle servo drives and feed servo drives are widely used, shortening the transmission chain of CNC machine tools and simplifying the structure of the machine tool's mechanical transmission system.

3. High-efficiency, high-precision, and backlash-free transmission devices and moving parts are adopted, such as ball screw nut pairs, plastic sliding guides, linear rolling guides, and hydrostatic guides.

CNC Machine Tool Auxiliary Devices

Auxiliary devices are necessary supporting devices to fully leverage the functions of CNC machine tools. Commonly used auxiliary devices include pneumatic and hydraulic devices, chip removal devices, cooling and lubrication devices, rotary worktables and CNC indexing heads, protective devices, lighting, and various other auxiliary devices. Five-axis simultaneous machining center.