I. What is a CNC machine tool?
CNC machine tools, short for numerical control machine tools, are automated machine tools 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.
Compared with conventional machine tools, CNC machine tools have the following characteristics:
1. High machining precision and stable machining quality;
2. It can perform multi-axis linkage and process parts with complex shapes;
3. When the parts being processed change, generally only the CNC program needs to be modified, which can save production preparation time;
4. The machine tool itself has high precision and rigidity, and can select favorable machining parameters, resulting in high productivity (generally 3 to 5 times that of ordinary machine tools).
5. Machine tools have a high degree of automation, which can reduce labor intensity;
6. It requires high qualifications from operators and even higher technical skills from maintenance personnel.
CNC machine tools generally consist of the following parts:
1. Main unit: This is the core of a CNC machine tool, including the machine bed, column, spindle, feed mechanism, and other mechanical components. It is the mechanical component used to perform various cutting operations.
2. The numerical control device is the core of the CNC machine tool, including hardware (printed circuit board, CRT display, key box, paper tape reader, etc.) and corresponding software. It is used to input digitized part programs and complete the storage of input information, data transformation, interpolation calculation and realize various control functions.
3. Drive unit: This is the driving component of the CNC machine tool's actuator, including the spindle drive unit, feed unit, spindle motor, and feed motor. Under the control of the CNC device, it achieves spindle and feed drive through an electrical or electro-hydraulic servo system. When several feeds are linked, positioning, linear, planar curve, and spatial curve machining can be completed.
4. Auxiliary devices refer to essential supporting components for CNC machine tools, used to ensure their operation, such as cooling, chip removal, lubrication, lighting, and monitoring. These include hydraulic and pneumatic devices, chip removal devices, exchange worktables, CNC rotary tables, and CNC indexing heads, as well as cutting tools and monitoring/detection devices.
5. Programming and other auxiliary equipment, which can be used to program and store parts outside the machine.
Since the first CNC machine tool was developed by MIT in 1952, CNC machine tools have been widely used in the manufacturing industry, especially in the automotive, aerospace and military industries. CNC technology has developed rapidly in both hardware and software.
Detailed Explanation of PLC Programming for CNC Machine Tools
The PLC of CNC machine tools provides a complete programming language, which can be used to create different control programs according to different control requirements. The ladder diagram method is currently the most widely used programming method. It is similar in form to a relay control circuit diagram, and is simple, intuitive, easy to read, and easy to understand.
The PLC programming steps in CNC machine tools are as follows:
(1) Determine the object of control;
(2) Create the schematic diagrams, address tables, and PLC data tables for the input and output signal circuits;
(3) Based on the analysis of the working principle or action sequence of CNC machine tools, use flowcharts, timing diagrams, etc. to describe the logical sequence relationship between signals and machine tool movements, and design and produce ladder diagrams;
(4) Convert the ladder diagram into the format of the instruction list, then write the sequential program using the programmer keyboard, and then debug and modify it using a simulation device or simulator.
(5) The sequential program, which has been repeatedly debugged and confirmed to be error-free, is solidified into the EPROM, and the program is stored on a floppy disk or optical disk. At the same time, relevant drawings and maintenance information are compiled.
Table 1 lists the graphic symbols for ladder diagrams in the FANUC series.
The following example, using spindle orientation control of a CNC machine tool, illustrates the application of PLC in CNC machine tools.
When machining on a CNC machine tool, the spindle orientation function is used for automatic tool changing or precision boring. Figure 1 shows the PLC control ladder diagram for the spindle orientation function.
Figure 1. Ladder diagram for spindle orientation control of CNC machine tool
In ladder diagram 1, AUTO is the automatic operation status signal; it is "0" for manual operation and "1" for automatic operation. M06 is the tool change command, and M19 is the spindle orientation command. These two signals are connected in parallel as the control signals for spindle orientation control. RST is the CNC system reset signal. ORCM is the spindle orientation relay. ORAR is the orientation completion signal input from the machine tool. Additionally, a timer TMR function is set here to detect whether spindle orientation is completed within a specified time. A 4.5-second delay is set on the monitor via the manual data input (MDI) panel and stored in data storage unit 203. If orientation control cannot be completed within 4.5 seconds, an alarm signal will be issued. R1 is the alarm relay. The data next to the ladder diagram symbols in the diagram represents the cell address of the PLC's internal memory; for example, 200.7 represents the 7th bit of storage unit 200 in the data memory. These addresses can be specified by the PLC programmer as needed.
