The most basic components of a CNC MILLING MACHINE include six parts: I/O device, CNC device, servo drive device, measurement feedback device, auxiliary device, and machine tool body. Below, we will provide a detailed introduction to these six parts.
1. I/O device
I/O devices are used for inputting/outputting data such as numerical control machining or motion control programs, machining and control data, machine parameters, coordinate axis positions, and the status of detection switches. Keyboard and monitor are essential and basic I/O devices for CNC equipment. As peripheral devices of CNC systems, desktop computers and portable computers are currently one of the commonly used I/O devices
2. Numerical Control Device
The numerical control device is the core of the numerical control system, which consists of I/O interface circuits, controllers, arithmetic units, and memory. The function of a numerical control device is to compile, calculate, and process the data input by the input device through internal logic circuits or control software, and output various information and instructions to control the various parts of the machine tool to perform specified actions.
Among these control information and instructions, the most basic ones are the coordinate axis feed rate, feed direction, and feed displacement generated through interpolation operation, which are provided to the servo drive device. After amplification by the driver, the displacement of the coordinate axis is ultimately controlled. These control information and instructions directly determine the movement trajectory of the tool or coordinate axis.
3. Servo drive device
Servo drive devices are usually composed of servo amplifiers (also known as drivers or servo units) and actuators. On CNC machine tools, AC servo motors are generally used as actuators. At present, linear motors have been used on advanced high-speed machining machines. In addition, there were also simple CNC machine tools produced in the 20th century that used DC servo motors, as well as stepper motors as actuators. The servo amplifier must be used in conjunction with the drive motor.
4. Measurement feedback device
The measurement feedback device is the detection link of closed-loop (semi closed loop) CNC machine tools. Its function is to detect the actual speed and displacement of the actuator or worktable through modern measuring components (such as pulse encoders, rotary transformers, induction synchronizers, gratings, magnetic rulers, laser measuring instruments, etc.), and feed it back to the servo drive device or CNC device to compensate for the feed rate or motion error of the actuator, in order to improve the accuracy of the motion mechanism. The position of the signal feedback detected by the measuring device depends on the structural form of the CNC system. Servo built-in pulse encoders, speed measuring machines, and linear gratings are commonly used detection components.
The advanced servo drive device adopts digital servo drive technology (referred to as digital servo), and the servo drive device is connected to the numerical control device through a bus. Feedback signals are mostly connected to the servo drive device and transmitted to the numerical control device through the bus. The feedback device only needs to be directly connected to the numerical control device in a few occasions or when using analog controlled servo drive devices (referred to as analog servos).
5. Auxiliary control mechanism
The auxiliary control mechanism refers to the control components between the numerical control device and the mechanical and hydraulic components of the machine tool. Its main function is to receive spindle speed, steering, and start stop commands output by the CNC device, tool selection and exchange commands, start stop commands for cooling and lubrication devices, release and clamping commands for workpieces and machine parts, auxiliary command signals for worktable indexing, as well as signals for the status of detection switches on the machine tool. After necessary compilation, logical judgment, and power amplification, it directly drives the corresponding execution components to drive the mechanical components, hydraulic and pneumatic auxiliary devices of the machine tool to complete the actions specified in the commands. It is usually composed of PLC and strong current control circuit. PLC can be integrated with CNC in structure (built-in PLC) or relatively independent (external PLC).
6. Machine tool body
The machine tool body is the mechanical structural component of a CNC machine tool, which consists of a main transmission system, a feed transmission system, a bed body, a worktable, as well as auxiliary motion devices, hydraulic/pneumatic systems, lubrication systems, cooling devices, chip removal, protection systems, and other parts. In order to meet the requirements of CNC technology and fully utilize the performance of machine tools, CNC machine tools have undergone significant changes in overall layout, appearance, transmission system structure, tool system, and operational performance compared to ordinary machine tools.
Working principle of CNC milling machine
On traditional metal cutting machine tools, the operator needs to constantly change the parameters such as the tool's motion trajectory and speed according to the requirements of the drawing when machining parts, so that the tool can cut the workpiece and ultimately produce qualified parts.
The machining of CNC milling machines actually applies the principle of "differentiation", and its working principle and process are briefly described as follows.
1. Based on the tool path required by the machining program, the CNC device differentiates the path according to the corresponding coordinate axis of the machine tool, using the minimum movement amount (pulse equivalent) as the unit, and calculates the number of pulses required to move each coordinate.
2. Using the "interpolation" software or "interpolation" operator of the numerical control device, fit the required trajectory with an equivalent line in units of "minimum movement", and find the closest fitting line to the theoretical trajectory.
3. The numerical control device continuously assigns feed pulses to the corresponding coordinate axes based on the trajectory of the fitted line, and drives the machine tool coordinate axes to move according to the assigned pulses through servo drive.
The following conclusion can be drawn from the above:
① As long as the minimum movement amount (pulse equivalent) of the CNC machine tool is small enough, the fitted line used can effectively replace the theoretical curve.
② By changing the pulse allocation method of the coordinate axis, the shape of the fitted polyline can be altered, thereby achieving the goal of changing the machining trajectory.
③ By changing the frequency of the allocated pulses, the speed of the coordinate axis (tool) can be altered
This achieves the fundamental purpose of controlling the tool movement trajectory of CNC machine tools.
The method of calculating and determining intermediate points between known points of an ideal trajectory (contour) through data point densification based on a given mathematical function is called interpolation; The number of coordinate axes that can participate in interpolation simultaneously is called the linkage axis number. Obviously, the more linkage axes a CNC machine tool has, the stronger its performance in machining contours. Therefore, the number of linkage axes is an important technical indicator for measuring the performance of CNC machine tools.
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