Composition and Working Principle of CNC Machine Tools
CNC machine tools consist of several components, including program preparation and program carrier, input device, numerical control device (CNC), servo drive and position detection, auxiliary control device, and machine tool body.
1. Program Preparation and Program Carrier
The CNC program is the working instruction for the automatic machining of parts on CNC machine tools. Based on the process analysis of the machined parts, the relative position of the part coordinate system on the machine tool coordinate system is determined, including the installation position of the part on the machine tool, the dimensional parameters of the relative motion between the tool and the part, the process route of the part machining, the process parameters of the cutting process, and the actions of auxiliary devices. After obtaining all the motion, dimension, and process parameter information of the part, a CNC program is prepared using standard numerical control codes composed of text, numbers, and symbols, following specified methods and formats. The programming can be done manually, or for complex-shaped parts, it can be done automatically using dedicated programming machines or general-purpose computers through automatic programming (APT) or CAD/CAM design.
The prepared CNC program is stored on a storage medium that is convenient for inputting into the CNC device. It can be punched tape, magnetic tape, magnetic disk, etc., depending on the design type of the CNC device.
2. Input Device
The input device is used to transfer and store the numerical control codes from the program carrier (information carrier) into the CNC system. Depending on the type of control storage medium, the input device can be an optical reader, magnetic tape drive, or floppy disk drive. The CNC machine tool program can also be directly input into the CNC system manually using a keyboard. The CNC machining program can also be transmitted to the CNC system by a programming computer via RS232C or through network communication.
There are two different ways of inputting the part machining program: one is reading and processing simultaneously (when the CNC system memory is small), and the other is reading the entire part machining program into the internal storage of the CNC device at once and retrieving it segment by segment for processing.
3. Numerical Control Device (CNC)
The numerical control device is the core of the CNC machine tool. The numerical control device retrieves one or several segments of the CNC machining program from the internal memory or from the input device. After compilation, calculation, and logical processing by the logical circuits or system software of the numerical control device, it outputs various control information and instructions to control the operation of different parts of the machine tool, enabling them to perform specified sequential movements and actions.
The contour of a part is often composed of straight lines, arcs, or other non-circular curves. The tool must move according to the shape and dimensional requirements of the part during the machining process, following the path of the geometry. However, the input part machining program can only contain data such as the start and end coordinates of each line segment, which is not sufficient to meet the requirements. Therefore, trajectory interpolation is performed, which involves "densifying the data points" between the start and end coordinates of each line segment. This process calculates a series of intermediate point coordinates and outputs corresponding pulse signals to control the feed speed, feed direction, and feed displacement of each coordinate axis (i.e., the feed motion's various execution elements).
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4. Drive Device and Position Detection Device
The drive device receives instruction information from the numerical control device and, after power amplification, precisely drives the moving components of the machine tool according to the requirements of the instruction information to produce parts that meet the pattern requirements. Therefore, its servo accuracy and dynamic response performance are among the important factors that affect the machining accuracy, surface quality, and productivity of CNC machine tools. The drive device consists of a controller (including power amplifier) and an actuator. Currently, most systems use DC or AC servo motors as actuators.
The position detection device measures the actual displacement of each coordinate axis of the CNC machine tool. After inputting this information into the numerical control device through a feedback system, the numerical control device compares the feedback actual displacement value with the set value and controls the drive device to move according to the set instruction value.
5. Auxiliary Control Device
The main function of the auxiliary control device is to receive the switch signal instructions output by the numerical control device. After compilation, logical discrimination, and motion processing, the corresponding electrical components are driven through power amplification to actuate the mechanical, hydraulic, pneumatic, and other auxiliary devices of the machine tool to perform the switch actions specified by the instructions. These controls include speed variation, reversing, and start-stop instructions for the spindle drive components, tool selection and exchange instructions, start and stop of cooling and lubrication devices, loosening and clamping of workpieces and machine tool components, indexing of the indexing worktable, and other switch-assisted actions.
Programmable Logic Controllers (PLCs) are widely used as auxiliary control devices for CNC machine tools due to their fast response, reliable performance, ease of use, programming and program modification, and the ability to directly control machine tool switches.
6. Machine Tool Body
The machine tool body of a CNC machine tool is similar to that of a conventional machine tool and consists of a spindle drive device, feed drive device, bed, worktable, auxiliary motion devices, hydraulic and pneumatic systems, lubrication system, cooling device, etc. However, CNC machine tools have undergone significant changes in overall layout, appearance, transmission systems, tool systems, and operating mechanisms. These changes are aimed at meeting the requirements of CNC machine tools and fully leveraging their characteristics.
