The realization of precision cutting CNC lathe bas

2022-07-26
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Implementation of precision cutting CNC lathe based on PLC Abstract: This paper presents a design scheme of CNC lathe suitable for precision cutting. In this scheme, the mechanical trigger sensor is used to trigger the sensor by controlling the turning tool movement. The PLC transmits the signal obtained by the sensor to the NC system, and then the system carries out the corresponding data processing. The connection line between the mechanical trigger sensor and the input extension end of PLC is simple and easy to maintain, which reduces the cost compared with other sensors

key words: PLC; numerical control machine; precise; NUM; Sensor

1 preface

with the development of science and technology and the progress of manufacturing technology, the demand for product diversification is becoming stronger and stronger, the renewal cycle of products is becoming shorter and shorter, and the proportion of small and medium-sized batch production is significantly increasing, thus putting forward higher requirements for manufacturing equipment. In order to meet the needs of the market, manufacturing equipment is required to have high efficiency, high quality, high flexibility and low cost performance. As an automatic processing equipment, CNC machine tools are widely used. At the same time, with the development of modern mechanical manufacturing industry to a higher level, CNC machine tools will also become the basic equipment of flexible manufacturing cell (FMC), flexible manufacturing system (FMS) and computer integrated manufacturing system (CIMS). As the necessary basic equipment for manufacturing products with complex shape, high quality and high precision, computer numerical control system has become an important part of today's advanced manufacturing technology

plc (programmable logic controller) is a new type of industrial control device based on computer technology, which gradually developed in the late 1960s. As a new product of computer technology applied in the field of industrial control, PLC is also an indispensable part of open CNC system. It plays an important role in the control of switching value. Modern advanced CNC machine tools can generally be divided into three parts: machine tool body (MT), NC and PLC. In CNC machine tools, NC and PLC coordinate and cooperate to jointly complete the control of CNC machine tools. NC mainly completes "digital control" such as management scheduling and trajectory control, and PLC mainly completes some actions related to logic, such as tool replacement, workpiece clamping, and start and stop of coolant and lubricant. PLC technology has been widely used in various industrial process control and automatic production line control, and has become a very important application technology in the field of industrial automation

there are two types of control information on the NC machine tool: one is the position information that controls the coordinate axis of the machine tool feeding movement, such as the front, back, left and right movement of the NC machine tool workbench; The up and down movement of the headstock and the rotary movement displacement around a certain linear axis. These controls are realized by controlling the servo feed motor based on the difference between the theoretical position and the actual feedback position calculated by interpolation, of which US $124million is used for the research and development of graphene technology. The core function of this control is to ensure the realization of the contour track of the machined parts. Except for point processing, the strict proportional relationship must be maintained between the movements of each axis at any time. This kind of digital information is processed by CNC system (special computer), namely "digital control". The other is to control the operation of the CNC machine tool, such as the start, stop and reverse of the spindle, the replacement of the tool, the clamping and loosening of the work pieces, and the operation of the hydraulic, cooling and lubrication systems according to the pre-defined logic sequence, based on the status of the switch signals such as the travel switches, sensors, buttons and relays in the CNC system and on the machine tool. This kind of control information is mainly the sequential control of switching value signals, which is generally completed by PLC

2 functional analysis of precision cutting CNC machine tools

precision cutting CNC machine tools digitally control the movement of the tool through the CNC system to realize the cutting of the workpiece. When compiling the NC turning program, the tool is not considered. Before machining, the user must input four compensation parameters, i.e. X-axis compensation amount, z-axis compensation amount, tool tip arc radius and tool tip form, into the NC system, which will perform compensation calculation according to the program. Among the four parameters, the form of the tool tip is confirmed according to the provisions of the NC system. The arc radius of the tool tip can be measured by the R gauge, while the measurement of the X and Z axis compensation of the tool is relatively difficult. The use of the automatic tool setting instrument can well solve this problem. Therefore, most CNC machine tools and machining centers are equipped with various types of tool setting devices, such as the external tool setting instrument, the internal optical tool setting instrument, and the contact automatic tool setting device. Due to the low degree of standardization of tool clamping for general CNC lathes in turning centers, the tool setting accuracy of the tool setting instrument outside the machine is relatively low, and the special tool setting instrument outside the machine has high cost, complex operation and requires special operation space, so its practicability is poor. It is undoubtedly a simple and fast tool setting method to adopt the contact type automatic tool setting device in the machine. It can conveniently and automatically measure the fixed tool compensation value of the tool, greatly reduce the tool setting time and improve the machining efficiency of the machine tool. Therefore, this paper aims to design an in-machine contact CNC lathe to achieve precise tool setting before CNC turning, improve productivity and reduce processing costs. The main problems to be solved are as follows: the automatic tool setting instrument needs a high-precision electronic probe (sensor), which can trigger accurately at the trigger point and has a fast response time; The measuring head of the tool setting instrument is in rigid contact with the tool tip, and a buffer device shall be added to protect the surface of the measuring head. The pressure shall be controlled at about 1 ~ 10MPa, so that the measuring head of the sensor will not be damaged and pits will be formed; The system can use the position measuring device of the machine tool to measure. By recording the coordinates of different tool tip trigger points (x, Z hydraulic pressure testing machine of Jinan testing machine factory), a group of coordinate values can be easily obtained. After analysis and calculation, each tool compensation value can be determined; The device (connecting arm) for installing and fixing the tool setting instrument shall meet the corresponding accuracy requirements, meet the requirements of parallelism and verticality, and have good rigidity and easy operation

3 overall design of precision cutting CNC machine tool

the function of precision cutting mainly needs to ensure the cutting accuracy of the tool. Therefore, the accuracy of the sensor, the main component of the data machine tool, is required to be guaranteed. The function of the sensor is to sense and detect the information of one form and convert it into the information of another form, Convert the measured physical quantity (tool tip position) into the physical quantity expressed by the output signal (current and voltage) according to a certain law. The sensor of precision cutting CNC machine tool is composed of the following parts:

Figure 1: sensor composition of CNC machine tool

in this paper, the selection of sensor should have considerable accuracy to complete the following functions: 1) realize the sensing of x-axis and z-axis. To obtain the coordinate values of x-axis and z-axis, the tool setting instrument must trigger the sensor at the same point for different tools, Then the coordinate value of the point can be obtained by using the function of CNC system and programming. In fact, the function to be completed by the sensor is a switching value, and different tools can be triggered at the same point. 2) Due to the different tool deflection angles, the sensors cannot be made into two pairs of sensors perpendicular to each other in the x-axis and z-axis, so the tool tip coordinates obtained may not be the real tool tip coordinates in the z-axis coordinates

the mechanical switch sensor is used in this paper. A switch output is obtained by mechanical trigger. When the tool tip and the sensor trigger go into the preset position in parallel, the circuit is connected to get the trigger signal. The accuracy of mechanical sensor is relatively poor, but as long as the design is reasonable, the error can also be controlled within a reasonable range. On the other hand, it can be designed by itself to take into account the difference of tool deflection angle and the size and connection mode of sensor. This kind of sensor is simple and applicable, with low cost, and has great market promotion value

4 programming of PLC and CNC system

num1020/1040 CNC system is a new CNC system developed by num in 1995. It is a compact and fully functional 32-bit CNC system, and is fully compatible with num1060 series systems. It is especially suitable for 1 ~ 6 axis CNC machine tools. Its hardware features are as follows: GSP motherboard using CISC (very large scale integrated circuit) technology; The motherboard is connected with a pluggable (separated) small template. Considering the connection between the CNC system and the outside of the system, Num makes the functional modules connected with the outside world into pluggable small modules to facilitate the future maintenance of users. It is divided into axis module, display module and communication module; Num1020/1040 uses +24vdc as its power input. Since the CNC system is a weak current circuit, using +24vdc as its power input can greatly reduce the impact of its heat source and unstable factors. Users can put the +24vdc regulated power supply in the electrical cabinet, which greatly improves the reliability of the entire CNC system; The internal integration of PLC functions and the internal integration of PLC functions improve the internal communication ability between PLC and CNC, and enhance the logic control of CNC machine tools; 32 input and 24 output modules of PLC and 32 input and 24 output modules of num can be connected with peripheral circuits, and this module is connected with NUM CNC system through cables provided by num, which improves the reliability of the whole machine tool. (if there is a problem, only this module can be damaged, which will not damage the CNC system); The communication of optical fiber technology and the expansion of PLC input and output points are connected through optical fiber, which simplifies the connection of lines; The shaft transfer module, the encoder of the machine tool and the circuit to the servo can be directly connected to this module and connected with the shaft plate of the CNC system through it, which improves the reliability of the CNC system. In addition, the axis connection of num is different from other CNC systems. The axis module of num connects all the information of this axis (such as encoder, speed signal and zero return switch). If there is a problem with the axis of the machine tool, you can directly replace the plug on the axis module, and you can quickly find out the problem (inside or outside the system); Compact and practical operation panel. The CRT of the display and the computer are compatible. There are 47 function keys connected with the NC, 6 user-defined keys and serial communication interfaces, which can be connected to the keyboard of the PC (direct plugging)

according to the design requirements, when the sensor detects the signal, the program of the CNC system is not monitored. At this time, the tool tip coordinate value cannot be recorded for data processing. First stop the feeding motor, wait for the operator to give instructions, and then proceed to the next step. Therefore, this function should be realized by creating an intelligent factory control conforming to industry 4.0 through PLC. Connect q001.0 and q001.1 terminals with two feed motors respectively to realize separate control. Secondly, the sensor has four probes, but the control of the feed motor is the same: any sensor that gets a signal must stop the corresponding motor at the same time, and then carry out the corresponding data processing

the signals from the sensors of the NC machine tool are transmitted to the PLC through the interface circuit. The PLC transmits the obtained signals to the CNC through the exchange area. The CNC processes the information and then transmits it to the PLC. The PLC controls the movement of the x-direction motor and the z-direction motor. The interface circuit between the NC system and the sensor is shown in Figure 2:

as shown in the figure is the wiring diagram of PLC, connecting the four input ports%i001.0,%i001.1,%i001.2,%i001.3 with the four sensors respectively, and then with com

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