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What is a CNC machine tool? Composition of CNC machine tools

Add Date: 2018/4/18    Views: 27

Numerical Control Machine Tools (referred to as Numerical Control Machines) is a control technology that applies digital computing technology to machine tools. It expresses various control information in the machining process with coded numbers and inputs the numerical control device through the information carrier. Through the arithmetic processing, various control signals are sent by the numerical control device to control the movement of the machine tool, and the parts are automatically processed according to the shape and size required by the drawings. CNC machine tools solve the problem of complex, precise, small-batch, multi-species parts processing. It is a kind of flexible, high-efficiency automatic machine tool and represents the development direction of modern machine tool control technology. It is a typical mechanical and electrical integration. Products.

The composition of CNC machine tools:
The basic components of CNC machine tools include the carrier of machining programs, numerical control devices, servo drives, machine tools and other auxiliary devices. The following outlines the basic working principles of each component.

1. Processing program carrier
When a CNC machine tool works, it does not require the worker to directly operate the machine tool. To control the CNC machine tool, a machining program must be programmed. The part machining program includes the relative movement trajectory of the tool and the workpiece on the machine tool, the process parameters (feed rate, spindle speed, etc.) and auxiliary movement. The part machining program is stored on a program carrier with a certain format and code, such as punched tape, cassette tape, floppy disk, etc., and the program information is input to the CNC unit through the input device of the numerical control machine tool.

2. CNC device
The numerical control device is the core of the numerical control machine tool. The modern numerical control device adopts the form of CNC (Computer Numerical Control). This type of CNC device generally uses multiple microprocessors to realize the numerical control function in the form of programmed software. Therefore, it is also called Software NC. The CNC system is a position control system that interpolates the ideal trajectory based on input data and outputs it to the executing component to machine the desired part. Therefore, the numerical control device mainly consists of three basic parts: input, processing and output. All of these tasks are properly organized by the computer's system program, so that the entire system works in a coordinated manner.
(1) Input device The numerical control instruction is input to the numerical control device. According to different program carriers, there are corresponding input devices. At present, there are mainly keyboard input, disk input, direct communication mode input of CAD/CAM system and DNC (direct numerical control) input connected to the upper computer. There are still many systems that still retain the paper tape input format of the optical reader.
1) Tape input method. You can use a paper-opto-electronic reader to read the part program and directly control the movement of the machine. You can also read the contents of the tape into memory and control the movement of the machine with the part program stored in the memory.
2) MDI manual data input method. The operator can use the keyboard on the operation panel to input the instructions of the machining program, which is suitable for short programs.
In the edit state of the control device (EDIT), the processing program is input by software and stored in the memory of the control device. This input method can reuse the program. This method is generally used for manual programming.
On a numerical control device with a session programming function, different menus can be selected according to the questions presented on the display. Using the method of human-machine dialogue, the relevant size numbers can be input, and a processing program can be automatically generated.
3) Direct digital input using DNC. The part program is stored in the upper computer, and the CNC system receives the subsequent program segment from the computer while processing. The DNC method is often used in cases where complex parts designed using CAD/CAM software are used and part programs are directly generated.
(2) The information processing input device transmits the processing information to the CNC unit and compiles it into information that can be recognized by the computer. The information processing part gradually stores and performs processing according to the control program, and then sends the position and speed command to the servo via the output unit. System and main motion control section. The input data of the CNC system includes: contour information of the part (starting point, end point, straight line, arc, etc.), processing speed, and other auxiliary processing information (such as tool change, speed change, coolant switch, etc.). The purpose of data processing is to complete the interpolation. Complementary preparations. The data processing program also includes tool radius compensation, speed calculation, and auxiliary function processing.
(3) Output Device The output device is connected to the servo mechanism. The output device receives the output pulse of the operator according to the command of the controller and sends it to the servo control system of each coordinate. After the power is amplified, the servo system is driven to control the movement of the machine according to the specified requirements.

3. Servo system and measurement feedback system
The servo system is an important part of the CNC machine tool and is used to realize the feed servo control and spindle servo control of the CNC machine tool. The role of the servo system is to receive the command information from the numerical control device, and after power amplification and shaping processing, it is converted into a linear displacement or an angular displacement movement of a machine tool execution component. As the servo system is the last link of the CNC machine tool, its performance will directly affect the technical indicators such as the accuracy and speed of the CNC machine tool. Therefore, the servo drive device of the CNC machine tool is required to have a good rapid response performance and accurately and sensitively track the numerical control device. The digital command signal is issued and the instructions from the numerical control device can be faithfully executed to improve the dynamic follow-up characteristics and static tracking accuracy of the system.
The servo system includes two major parts: drive device and actuator. The drive device consists of a spindle drive unit, a feed drive unit, a spindle servo motor, and a feed servo motor. Stepping motors, DC servo motors and AC servo motors are commonly used drive devices.
The measuring element detects the actual displacement value of each coordinate axis of the CNC machine tool and inputs it to the numerical control device of the machine tool via the feedback system. The numerical control device compares the actual displacement value returned by the feedback with the command value and outputs the set value to the servo system. The required displacement instruction.

4. Machine body
The machine tool host is the main body of the numerical control machine tool. It includes machine parts such as bed, base, column, beam, slide, table, spindle box, feed mechanism, tool holder and automatic tool changer. It is a mechanical part that automatically performs a variety of cutting operations on CNC machine tools. Compared with traditional machine tools, the main body of CNC machine tools has the following structural features:
(1) New machine tool structure with high rigidity, high shock resistance and less thermal deformation. Normally, the stiffness and shock resistance of the main machine are improved by increasing the static stiffness of the structural system, increasing the damping, adjusting the mass of the structural parts and the natural frequency, so that the main body of the machine tool can adapt to the needs of continuous automatic machining of the numerical control machine tool. Adopting measures such as improving the layout of machine tools, reducing heat generation, controlling temperature rise, and using thermal displacement compensation can reduce the impact of thermal deformation on the machine tool.
(2) High-performance spindle servo drive and feed servo drive devices are widely used, which shortens the transmission chain of CNC machine tools and simplifies the structure of the machine's mechanical transmission system.
(3) The use of high transmission efficiency, high precision, no gap between the transmission and moving parts, such as ball screw nut pairs, plastic sliding guides, linear rolling guides, static pressure guides and so on.

5. Auxiliary device of CNC machine tool
Auxiliary devices are necessary devices to ensure the full use of CNC machine tools. Common auxiliary devices include: pneumatic, hydraulic devices, chip removal devices, cooling and lubrication devices, rotary table and numerical control indexing head, protection, lighting and other assisting equipments.

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