A CNC system has many functions, including basic functions—essential functions of the CNC device; and optional functions—functions available for user selection. Some optional functions are designed to address different machining objects, some to improve machining quality, some to facilitate programming, and some to improve operation and maintenance performance. Some optional functions are related, requiring the selection of one function to be replaced by another. Therefore, selection must be based on the machine tool's design requirements. Avoid excessive steps in selecting functions without proper analysis, as this can lead to the omission of related functions, reducing the machine tool's functionality and causing unnecessary losses.
A high-efficiency CNC lathe is short for numerical control machine tool, which is an automated machine tool equipped with a program control system. This control system can logically process programs with control codes or other symbolic instructions, decode them, and thus enable the machine tool to move and process parts.
High-efficiency CNC lathes are classified according to their process applications.
Heavy-duty CNC lathes, including CNC lathes, CNC drilling machines, CNC milling machines, CNC grinding machines, CNC boring machines, and machining centers. These machine tools are suitable for single-piece, small-batch, and multi-variety parts processing, featuring excellent dimensional consistency, high productivity and automation, and high equipment flexibility.
CNC machine tools for metal forming; these machine tools include CNC bending machines, CNC combination punching machines, CNC pipe bending machines, CNC rotary head presses, etc.
CNC special processing machine tools; these machine tools include CNC wire (electrode) cutting machine tools, CNC electrical discharge machining machine tools, CNC flame cutting machine tools, CNC laser cutting machine tools, special combination machine tools, etc.
Other types of CNC equipment; non-machining equipment using CNC technology, such as automatic assembly machines, multi-coordinate measuring machines, automatic drafting machines, and industrial robots.
The basic components of a high-efficiency CNC lathe include a machining program carrier, a CNC device, a servo drive device, the machine tool body, and other auxiliary devices. The basic working principles of each component are briefly explained below.
1. Processing program carrier
When heavy-duty CNC lathes are in operation, workers do not need to directly operate the machine tool. To control the CNC machine tool, a machining program must be created. The part machining program includes the relative motion trajectory of the tool and workpiece on the machine tool, process parameters (feed rate, spindle speed, etc.), and auxiliary motions. The part machining program is stored in a specific format and code on a program carrier, such as punched paper tape, cassette tape, or floppy disk. The program information is then input into the CNC unit through the input device of the CNC machine tool.
2. CNC device
The core of a high-efficiency CNC lathe. Modern CNC devices all adopt the CNC form. These CNC devices generally use multiple microprocessors to implement numerical control functions in the form of programmed software, hence the name Software CNC. A CNC system is a position control system that interpolates the ideal motion trajectory based on input data and then outputs it to the actuators to machine the required parts. Therefore, a CNC device mainly consists of three basic parts: input, processing, and output. All these tasks are rationally organized by the computer's system program, enabling the entire system to work in a coordinated manner.
(1) The input device inputs numerical control instructions to the numerical control device. Depending on the program carrier, there are different input devices. Currently, the main input devices are keyboard input, disk input, direct communication input with CAD/CAM system, and DNC (direct numerical control) input connected to the upper-level computer. Many systems still retain the paper tape input form of photoelectric reader.
1) Paper tape input method. The part program can be read into the paper tape photoelectric reader to directly control the machine tool movement, or the contents of the paper tape can be read into the memory and the part program stored in the memory can be used to control the machine tool movement.
2) MDI Manual Data Input Method. The operator can input machining program instructions using the keyboard on the control panel. It is suitable for relatively short programs.
In high-efficiency CNC lathes, machining programs are input via software in the EDIT (Edit-In-Time) mode of the control device and stored in the control device's memory. This input method allows for program reuse. This method is generally used for manual programming.
On a CNC device with conversational programming capabilities, you can select different menus according to the prompts on the display, input the relevant dimensional numbers through human-machine dialogue, and the machining program can be automatically generated.
3) High-efficiency CNC lathes employ Direct Numerical Control (DNC). The part program is stored in the host computer, and the CNC system receives subsequent program segments from the computer while machining. DNC is often used for complex workpieces designed with CAD/CAM software and where part programs are directly generated.
(2) The information processing input device transmits the machining information to the CNC unit, compiles it into information that the computer can recognize, and then the information processing part stores and processes it step by step according to the control program. Finally, the output unit sends position and speed commands to the servo system and the main motion control part. The input data of the CNC system includes: the contour information of the part (start point, end point, straight line, arc, etc.), machining speed and other auxiliary machining information (such as tool change, speed change, coolant switch, etc.). The purpose of data processing is to complete the preparatory work before interpolation calculation. 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 arithmetic unit according to the command of the controller and sends it 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.
In high-efficiency CNC lathes, tool selection and cutting parameter determination are completed in a human-machine interactive environment, a stark contrast to conventional machine tool processing. This also requires programmers to master the basic principles of tool selection and cutting parameter determination. High-precision CNC lathes fully consider the characteristics of CNC machining during programming, enabling the correct selection of cutting tools and parameters. High-precision CNC lathes must adapt to the high speed, high efficiency, and high degree of automation of CNC machine tools, and generally should include general-purpose cutting tools, general-purpose connecting tool holders, and a small number of special-purpose tool holders. Tool holders connect the cutting tools and are mounted on the machine tool's power head, and therefore have gradually become standardized and serialized.
High-efficiency CNC lathe control technology is a high-tech field integrating computer, electronics, electrical, electric drive, automatic control and measurement, mechanics, hydraulics, and pneumatics. It effectively solves the problems of machining complex, precision, and small-batch parts. With the application of new technologies such as computer-aided design, computer-aided manufacturing, and computer-aided process design, it plays an irreplaceable role, especially in automated and flexible manufacturing systems, driving continuous product upgrades. This, in turn, promotes the continuous improvement and development of CNC technology itself.
In a narrow sense, a high-precision CNC lathe is a machine tool whose movement is controlled by digital characters to complete machining. The CNC we commonly refer to, however, is Computer Numerical Control machine tools, which emerged in the late 1950s and developed rapidly after the 1970s with the advancement of microcomputer technology. They have now become indispensable and irreplaceable processing equipment in the machining industry.
The specific values for high-efficiency CNC lathes should be determined based on the machine tool manual, cutting parameter handbook, and experience. For high-precision CNC lathes, the following factors need to be considered:
1. Depth of cut t. When the rigidity of the machine tool, workpiece, and cutting tool allows, t equals the machining allowance, which is an effective measure to improve productivity. To ensure the machining accuracy and surface roughness of parts, a certain allowance should generally be left for finishing. The finishing allowance for CNC machine tools can be slightly smaller than that for ordinary machine tools.
2. Cutting width L. Generally, L is directly proportional to the tool diameter d and inversely proportional to the cutting depth. In the machining process of economical CNC machine tools, the general range of L is: L = (0.6~0.9)d.
3. Cutting speed v. Increasing v is also a measure to improve productivity, but v is closely related to tool life. As v increases, tool life decreases sharply, so the selection of v mainly depends on tool life. In addition, the cutting speed is also greatly related to the material being machined. For example, when milling 30CrNi2MoVA alloy steel with an end mill, v can be around 8 m/min; while when milling aluminum alloy with the same end mill, v can be selected to be above 200 m/min.
4. Spindle speed n (r/min). The spindle speed is generally selected based on the cutting speed v. The calculation formula is: v = ∏nd/1000. CNC machine tools typically have a spindle speed adjustment (multiplier) switch on the control panel, which allows for integer multiplier adjustments to the spindle speed during machining.
5. Feed rate vF. vF should be selected based on the machining accuracy and surface roughness requirements of the part, as well as the materials of the cutting tool and workpiece. Increasing vF can also improve production efficiency. When the surface roughness requirement is low, a larger vF can be selected. During machining on high-precision CNC lathes, vF can also be manually adjusted via the adjustment switch on the machine tool control panel, but the maximum feed rate is limited by the rigidity of the equipment and the performance of the feed system.
The programmable logic controller (PLC) selection function offers two types: built-in and stand-alone. Built-in is preferable. Different models are available, and the selection should first be based on the number of input/output signal points between the CNC device and the machine tool. The selected number of points should be slightly more than the practical number to accommodate future additions and changes to control performance. Secondly, the storage capacity should be selected based on the estimated program size. As the program size increases with the complexity of the machine tool, the storage capacity also increases; a reasonable selection should be made based on specific circumstances. Other technical specifications, such as processing time, instruction functions, timers, counters, and internal relays, should also be considered, and their quantities should meet design requirements.
Selection of feed drives for high-efficiency CNC lathes
(1) AC servo motors are preferred because compared with DC motors, they have smaller rotor inertia, better dynamic response, larger output power, higher speed, simpler structure, lower cost, and are not restricted by the application environment.
(2) Taiwan CNC lathes select the appropriate servo motor by correctly calculating the load conditions applied to the motor shaft.
(3) Feed drive manufacturers provide a series of complete products for the feed speed control units and servo motors they produce. Therefore, after selecting the servo motor, the corresponding speed control unit should be selected from the product manual.
Selection of high-efficiency CNC lathe spindle drive
(1) The mainstream spindle motor should be given priority because it does not have the limitations of commutation, high speed and large capacity like DC spindle motors. It has a large constant power speed regulation range, low noise and low price. At present, 85% of CNC machine tools in the world use AC spindle drive.
(2) Select the spindle motor according to the following principles:
①Calculate the cutting power according to different machine tools, and the selected motor should meet this requirement;
②Based on the required spindle acceleration and deceleration time, calculate that the motor power should not exceed the maximum output power of the motor;
③ In situations where the spindle is required to start and brake frequently, the average power must be calculated, and its value must not exceed the continuous rated output power of the motor;
④ In cases where constant surface speed control is required, the sum of the cutting power required for constant surface speed control and the power required for acceleration should be within the power range that the motor can provide.
(3) Taiwanese CNC lathe spindle drive manufacturers provide a series of complete products for spindle speed control units and spindle motors. Therefore, after selecting the spindle motor, the corresponding spindle speed control unit should be selected from the product manual.
(4) When a high-efficiency CNC lathe requires spindle orientation control, a position encoder or magnetic sensor shall be selected to achieve spindle orientation control according to the actual situation of the machine tool.
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