With the development of science and technology, mechanical manufacturing technology has undergone profound changes. Due to the strong demand for diversification of products in the society, the proportion of multi-variety, small- and medium-sized batch production has increased significantly. It has been difficult to adapt to high-efficiency, high-quality and diversified processing requirements using traditional ordinary processing equipment. The application of machine tool numerical control technology has greatly shortened the preparation time of machining and the continuous improvement of the whole process automation of machining, and also enhanced the ability of the manufacturing system to adapt to various production conditions.
The basic components of CNC machine tools include machining programs, input devices, CNC systems, servo systems, auxiliary control devices, feedback systems, and machine tools. The machining program can be programmed manually (such as when machining a simple workpiece with a lathe CNC system). Complex machining requirements can be plotted on a computer (such as when milling machines and machining centers machine curved surfaces), and then the machining program is generated. The input of the program can be manually input by the panel of the numerical control system, or can be transmitted by cable through the communication port of the computer, or can be transmitted by the computer USB interface.
When purchasing a CNC machine tool, it can be considered from three aspects. First, whether the machine body can meet its own processing requirements, and the quality of the machine tool. Next is the numerical control system. There are many types of numerical control systems. Choosing the right system is the key to purchasing CNC machine tools. Finally, the drive unit is also the key to machine control. Different drive units can achieve different machining accuracy. When selecting the drive unit, the appropriate drive unit should be selected according to the precision requirements of the machined workpiece.
The following analysis from the numerical control system and the drive unit:
1. Selection of CNC system
The numerical control system is the "brain" of the numerical control machine tool, and the machine control information is calculated and processed. According to the principle of numerical control system, it can be divided into two categories: economical numerical control system and standard numerical control system.
1.1 Economical CNC system
From the perspective of control methods, the economical numerical control system generally refers to the open-loop numerical control system, which has the advantages of simple structure, low cost, convenient maintenance and debugging, low operation and maintenance costs, etc., but is affected by the torque frequency characteristics and accuracy of the stepping motor, the feed rate, The torque is mutually restricted and the performance improvement is limited. Therefore, the economical numerical control system is often used in CNC wire cutting and some economical CNC lathes, milling machines, etc., which are not required for speed and precision, and are widely used in the numerical control transformation of ordinary machine tools.
The open-loop numerical control system is an index control system without position detection device. The numerical control system sends out a certain number and frequency of command pulses, and the drive unit performs machine positioning. In the case of the open-loop system affected by external factors, the machine does not move or the action is not in place, but the system has reached the specified position when the machine tool reaches, and the machining accuracy of the machine tool will be greatly reduced. However, due to its simple structure, rapid response, stable and reliable work, convenient commissioning and maintenance, and very low price, it still has the largest market in China.
1.2 Standard CNC system
The standard CNC system includes a semi-closed loop CNC system and a full closed loop CNC system.
The semi-closed loop numerical control system generally refers to the position signal (photoelectric encoder) of the servo motor of the machine tool fed back to the numerical control system. The system can automatically perform position detection and error comparison, and can compensate part of the error, so the control precision is better than the open loop numerical control system. It should be high, but lower than the full-closed CNC system.
The full-closed CNC system not only includes the position feedback of the servo motor of the machine tool, but also the position signal of the position detecting device of the machine table (usually with a grating scale) is fed back to the system to form all position follow-up control. The system is in the process of machining. All position errors are automatically detected and compensated.
The machining accuracy of the full-closed CNC system is the highest, but the debugging and maintenance of this system is extremely difficult, and the price of the system is very high, which is only suitable for medium and high-grade CNC machine tools.
Because the price of the open-loop control system is much lower than that of the closed-loop control system, when selecting the numerical control system, it is necessary to consider the numerical control system to account for the price of the entire CNC machine tool, and then according to the configuration of the machine tool and the requirements of the machine itself, The low-end machine tool adopts an open-loop control system, and the medium and high-end machine tools adopt a closed-loop control system.
2, the selection of the drive unit
The driving unit includes two parts of the driving device and the motor. The selection of the driving unit mainly lies in the selection of the driving device. Since the motor is a common component, the performance difference exists only in different manufacturers and models.
The drive motor can be mainly divided into three types: reactive step drive motor, hybrid (also called permanent magnet reaction) step drive motor and servo drive motor.
The rotor of the reactive stepping drive motor has no winding, and the reaction torque generated by the excited stator winding realizes stepping operation. The rotor of the hybrid stepping motor uses permanent magnets, and the electromagnetic torque generated by the excitation and permanent magnets is stepped. The stepping motor is controlled by the pulse. The rotation direction of the motor can be changed by changing the order of energization. Changing the frequency of the pulse can change the rotation speed of the motor. Stepper motors have a certain step accuracy and no cumulative error. However, the efficiency of the stepping motor is low, the ability to drag the load is not large, the pulse equivalent cannot be too large, and the speed adjustment range is not large. At present, the stepping motor can be divided into two phases, three phases, and five equals. The commonly used three-phase stepping motor, such as the DY3A of Guangzhou CNC, is a three-phase hybrid stepping driver. In the past, stepper motors have dominated the market for a long time, but they are gradually being replaced by servo motors.
At present, the commonly used servo motor is an AC servo motor. A photoelectric encoder is installed at the shaft end of the motor, and the rotor angle is used for frequency conversion control. From the lowest speed to the highest speed, the servo motor can run smoothly with low torque ripple. The servo motor has a long overload capability, a small moment of inertia and a large stall torque. The servo motor has a small starting frequency and can quickly accelerate from the lowest speed to the rated speed.
The AC servo motor is used as the driving device to form a high-precision, high-performance semi-closed loop or closed-loop control system like the DC servo motor. Because the AC servo motor has a brushless structure, it requires almost no maintenance, and the volume is relatively small, which is beneficial to the improvement of the speed and power. DC servo motors have been replaced in a wide range. After the emergence of the all-digital AC servo system using high-speed microprocessor and dedicated digital signal processor (DSP), the original hardware servo control became software servo control, and some advanced algorithms in modern control theory were realized, which greatly improved The performance of the servo system, so the servo unit can greatly improve the processing efficiency and processing accuracy, but the price of the servo drive unit is also high. With the gradual improvement of servo control technology, the servo drive unit is gradually becoming the main force of the drive unit, and the price of the servo drive unit is gradually decreasing.
There are two types of servo drives. One uses pulse control, which is closed to the motor, but is not fed back to the CNC system. This driver can be called servo control of open loop control to some extent. The other uses the voltage control mode to control the speed of the motor through the voltage level. The feedback signal of the motor is fed back to the CNC system for position control through the driver.
When selecting a drive unit, also consider the proportion of the drive unit price in the entire CNC machine. The whole CNC machine tool has a lower price and generally selects the step drive unit, while the higher price machine selects the servo drive unit. However, when selecting the drive unit, it is also necessary to consider the matching problem between the drive unit and the numerical control system. When selecting the closed-loop control system, the closed-loop servo drive unit must be selected. AC servo systems outperform stepper motors in many performance aspects. However, in some occasions where the requirements are not high, stepper motors are often used to perform the motor. Therefore, in the design process of the control system, it is necessary to comprehensively consider the control requirements, cost and other factors, and select the appropriate control motor.
3, function selection
The above is based on the machining accuracy of the CNC system, in addition to the consideration of the function selection of the CNC system.
3.1 Control axis
The number of axes controlled by the CNC system is also the key to choice. The control axis can be divided into a linear feed axis and a rotary axis. According to the number of control axes, it can be divided into two-axis linkage, three-axis linkage, multi-axis linkage and the like. The more the number of control axes, the more complex the machine can process, but the higher the cost. At present, the lathe is generally linked by two linear moving axes, and sometimes a linear moving shaft or a rotating shaft is attached. Milling machines are generally linked by three linear moving axes, sometimes with a linear moving axis or rotating axis. The high-end system has more axes, and the highest level of machine tool manufacturing is the five-axis CNC machine system. Three axes are linear moving axes, two rotating axes, and five-axis linkage can process complex spatial surfaces. . Of course, this requires high-end CNC systems, servo systems and software support, and the requirements for machine tools are also extremely high.
The more control axes, the more the price of the CNC system grows. Therefore, when selecting the numerical control system, it is necessary to select according to the motion axis of the machine tool. The redundant control axis does not improve the control precision of the machine tool, but increases the cost of the numerical control system.
3.2 graphic display
The system's graphic display function, which is used to simulate the part processing process, showing the cutting path of the real tool on the blank, can choose two different planes in the Cartesian coordinate system, or can choose three-dimensional three-dimensional stereo with different viewing angles, which can be processed. At the same time, real-time display can also be used for quick drawing of the machining process in the mechanical locking mode. It is an effective tool for checking the part processing program, improving programming efficiency and real-time monitoring.
3.3 DNC transmission function
It is well known that the programming of parts consisting of non-circular curves or faces is very difficult. The usual method is to subdivide them into small three-dimensional straight line segments by means of calculations from general-purpose computers. This kind of processing program of several hundred KB is often encountered, and the general CNC system provides a program storage capacity of 64 to 128 KB, which brings great difficulty to mold processing. The DNC communication function has two working modes, one of which is to transfer the program in the general-purpose computer to the storage area of ​​the machining program of the numerical control system once (if its capacity is large enough), and the second is to use the program in the general-purpose computer. It is transferred to the buffer memory of the CNC system one by one and transferred while machining until the machining is finished. The problem of processing large-capacity program parts has been completely solved. Although it is necessary to add a certain cost to this function, it is indeed a practical function. Therefore, it is recommended to select the DNC transmission function as an essential function when selecting the numerical control system.
3.4 rigid tapping
Tapping is a common function of CNC machine tools. What method is a problem worth considering. The rigid tapping function must use the servo motor to drive the spindle. It is not only required to add a position sensor to the spindle, but also has strict requirements on the clearance and inertia of the spindle drive mechanism. The electrical design and adjustment also have a certain amount of work, so this function The cost cannot be ignored. For the user, if the flexible tapping can be used for flexible tapping, or the speed of the machine itself is not high, it is not necessary to use the rigid tapping function.
The above-mentioned problems are often encountered in the functional configuration of CNC machine tools. As a CNC machine tool design and sales personnel must clearly understand the various functional uses of the CNC system, according to the actual situation of the machine tool, the user is economically rational, functional and CNC machine tools with higher price ratios reduce unnecessary waste.
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