Working principle of CNC machine tools

Working principle of CNC machine tools

CNC machine tools are machine tools controlled by digital information. Any machine tool that uses coded digital information to record the tool movement trajectory information on the program medium, and then sends it to the CNC system for decoding and calculation to control the relative movement of the machine tool and the workpiece, and processes the required workpiece is a CNC machine tool.

Before the CNC machine tool processes the workpiece, it is necessary to analyze the part drawing and formulate the part processing process plan. Clarify the processing process parameters, and then compile the CNC processing program according to the programming rules. After the geometric information and process information of the processed parts are converted into digital information, they can be input into the CNC system of the machine tool in different ways. After checking that there are no errors, the machine tool can be started and the CNC processing program can be run. The CNC device will automatically complete various control instructions issued by the CNC processing program. If there is no fault, it will not be processed until the processing program is finished. The control process of CNC processing is very similar to the printing process of a computer-controlled printer, especially the drawing process of a computer-controlled plotter.

Digital control is relative to analog control. Digital control systems or computer digital control systems use word length to represent information of different precisions, can perform complex arithmetic operations, logical operations and information processing, and realize the conversion of information processing methods and processes by changing software (rather than circuits or mechanical mechanisms), with good flexibility.

CNC systems are convenient, reliable and highly accurate, and are widely used in various aspects such as trajectory, detection and auxiliary motion control of mechanical motion. Among them, trajectory control is the main control content of machine tools and industrial robots.

Composition of CNC machine tools

CNC machine tools are generally composed of input and output devices, CNC devices (CNC), servo units, drive devices (or actuators), programmable controllers (PLCs) and electrical control devices, auxiliary devices, machine tool bodies and measuring devices.

1) Input and output devices

It is an interactive device for the CNC system of machine tools and operators to exchange information and realize human-machine dialogue.

The function of the input device is to convert the CNC code on the program carrier into a corresponding electrical pulse signal, which is transmitted and stored in the CNC device. At present, the input devices of CNC machine tools include keyboards, disk drives, photoelectric readers, etc., and their corresponding program carriers are disks and punched paper tapes. The output device is a display, which can be a CRT display or a color LCD display. The function of the output device is: the CNC system provides the operator with necessary information through the display. The displayed information can be the program being edited, coordinate values, and alarm signals, etc.

(2) CNC device

It is the core of the computer numerical control system and consists of hardware and software. It receives the pulse signal sent by the input device. After the signal is compiled, calculated, and logically processed by the system software or logic circuit of the CNC device, it outputs various signals and instructions to control the various parts of the machine tool so that it can perform prescribed and orderly actions. The most basic of these control signals are the feed speed, feed direction, and displacement instructions of each coordinate axis (i.e., each actuator for feed motion) (sent to the servo drive system to drive the actuator for feed motion), as well as the speed change, reversing, and start/stop signals of the spindle, the tool instruction signals for selecting and exchanging tools, the control of the start/stop of coolant and lubricating oil, the control of the release and clamping of workpieces and machine tool parts, and the auxiliary instruction signals for controlling the indexing table rotation, etc.

The CNC device mainly includes a microprocessor (CPU), memory, local bus, peripheral logic circuits, and interfaces that connect with other components of the CNC system.

(3) Programmable Logic Controller (PLC)

The CNC machine tool completes the control function through CNC and PLC. Among them, CNC mainly completes functions related to digital operations and management, such as part program editing, interpolation operations, decoding, and position servo control of tool movement. The PIC mainly completes some actions related to logical operations. It receives CNC control codes M (auxiliary functions), S (spindle speed), T (tool selection, tool change) and other switch action information, decodes the switch action information, converts it into corresponding control signals, and controls the auxiliary devices to complete the corresponding switch actions of the machine tool, such as workpiece clamping, tool replacement, coolant switching, and other auxiliary actions. It also receives instructions from the machine tool operation panel. On the one hand, it directly controls the action of the machine tool (such as manual operation of the machine tool), and on the other hand, it sends part of the instructions to the CNC device for the control of the processing process.

In the FANUC system, the PLC specifically used to control the machine tool is recorded as PMC, which is called a programmable machine tool controller.

(4) Servo unit

The servo unit receives speed and displacement instructions from the CNC device. These instructions are transformed and amplified by the servo unit and then converted into the speed, direction and displacement of the machine tool feed motion through the drive device. Therefore, the servo unit is the link between the CNC device and the machine tool body. It amplifies the weak instruction signal from the CNC device into a high-power signal to control the drive device. The servo unit is divided into spindle unit and feed unit, etc. The servo unit is divided into open-loop system, semi-closed-loop system and closed-loop system in terms of its system.

(5) Drive device

The drive device converts the instruction signal amplified by the servo unit into mechanical motion, drives the machine tool worktable through mechanical connection components, so that the worktable can be accurately positioned or make strict relative motion according to the specified trajectory, and process parts with shape, size and precision that meet the requirements. Currently, the commonly used drive devices are DC servo motors and AC servo motors. AC servo motors are gradually replacing DC servo motors.

The servo unit and the drive device are collectively referred to as the servo drive system. It is the power device for the machine tool. The instructions of the computer numerical control device must be implemented by the servo drive system. The servo drive device includes the spindle drive unit (mainly controls the speed of the spindle) and the feed drive unit (mainly controls the speed and position of the feed system). The servo drive system is an important part of the CNC machine tool. In a sense, the function of the CNC machine tool mainly depends on the CNC device, and the performance of the CNC machine tool mainly depends on the servo drive system.

(6) Machine tool body

That is, the mechanical parts of the CNC machine tool, including the main motion parts, feed motion execution parts (worktable, carriage and its transmission parts) and support parts (bed, column, etc.), and also includes auxiliary devices with cooling, lubrication, indexing and clamping functions. CNC machine tools of the machining center type also have tool magazines for storing tools, manipulators for exchanging tools and other parts. The composition of the mechanical parts of CNC machine tools is similar to that of ordinary machine tools. Due to the requirements of high speed, high precision, large cutting volume and continuous processing of CNC machine tools, their mechanical parts have higher requirements in terms of precision, rigidity, vibration resistance, etc.

In addition, in order to ensure the full play of the functions of CNC machine tools, there are some auxiliary systems, such as cooling systems, lubrication systems, hydraulic (or pneumatic) systems, chip removal systems, protection systems, etc.