With the development of science and technology, mechanical manufacturing technology has undergone profound changes. Due to the stronger demand for product diversification in the society, the proportion of multi-species, medium and small-scale production has increased significantly. It has been difficult to adapt traditional processing equipment to high-efficiency, high-quality, and diversified processing requirements. The application of machine tool numerical control technology has greatly shortened the pre-preparation time of mechanical processing and continuously improved the level of automation in the whole process of machining, and also enhanced the ability of the manufacturing system to adapt to changes in various production conditions.
The basic components of CNC wire-cutting machine tools include machining programs, high-frequency power supplies, drive systems, numerical control systems, and machine tool bodies. Machining programs can be written manually (such as the early 3B instructions), and are now mapped on computers (such as current CAXA, HL, HF, YH, etc. programming software), and then generate machining programs. The input of the program can be manually input by the panel (single board machine) of the CNC system, or can be transmitted through the 232 serial port of the computer, or it can be transmitted by the computer USB interface.
In the selection of CNC wire-cutting machine tools can be considered from three aspects, the first is whether the machine body can meet their own processing requirements, the quality of the machine tool. Followed by the numerical control system, the numerical control system has many kinds, chooses the appropriate system to be the key to purchase the numerical control engine bed. The last is the drive unit, which is also the key to the control of the machine tool. Different machining units can achieve the same machining accuracy. When selecting the drive unit, the drive unit must be selected according to the precision requirements of the machined workpiece.
The following analysis from the machine body, numerical control system and drive unit is carried out in three aspects:
1, the choice of machine body
First of all, the design of the machine tool and the size and weight of the workpiece must be optimally matched. For medium and large load tables, a full-support machining center structure is used. This design can have enough load bearing, stiffness, accuracy, vibration resistance and precision retention. Followed by the mechanical transmission of the feed system to use ball screw, ball screw is superior to the triangular thread screw and trapezoidal thread screw, and the screw diameter is required to be as large as possible, increase the rigidity. Once again, the guide rails and the guide rails of the table are the key to ensure the accuracy of the table. Users should pay great attention to the selection of models. First, observe the size of the cross-section of the guide rail. Under the same conditions, the thicker and more rigid, the more difficult it is to deform during processing, so as to ensure the highest accuracy and durability of the machine tool in long-term work. The common guide rail structures on the market today are as follows:
1 inlaid steel ball type rolling guide; 
2 inlaid steel roller type rolling guide; 
3 linear rolling guides.
The difference between the first and the second is on the roller body of the guide rail, one is the ball and the other is the roller. The ball and the guide surface are in point contact, the roller and the guide surface are in line contact, so its wear resistance and bearing capacity are much better than the ball type. The linear slide rail is a kind of rolling guide, which is infinitely rolling cycled between the slider and the slide rail by the steel ball, so that the load platform can easily move linearly along the slide rail with high precision, and its friction coefficient can be reduced to the traditional Slide guide 1/50, making it easy to achieve μm-level positioning accuracy. The design of the terminal unit between the slider and the slide rail enables the linear slide rail to bear loads in all directions such as up and down, left and right, and the linear slide rail has a more smooth and low noise movement characteristic. The precision and bearing capacity are much better than ball and roller type. At present, the third structure is adopted in the machine tools imported by Sodick Corporation of Japan, Mitsubishi Corporation of Japan, Charmilles Corporation of Switzerland, and Ruiach Company, so the third type of structure should be considered by the user when selecting the type.
2. Optional CNC system
The numerical control system is the "brain" of the numerical control machine tool, and operates and processes the machine tool control information. According to the principle of numerical control system can be divided into economical numerical control system and standard type numerical control system.
2.1 Economical Numerical Control System
From the perspective of control methods, economical CNC systems generally refer to open-loop numerical control systems. The open loop numerical control system is an exponential control system without a position detection device itself. 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 an open-loop system under the influence of external factors, the machine tool does not operate or move in place, but the system has reached the designated position when the machine tool reaches the specified position. At this time, the machining accuracy of the machine tool will be greatly reduced. However, due to its simple structure, rapid response, stable and reliable operation, convenient commissioning and maintenance, combined with very low prices, but by stepper motor torque and frequency characteristics and accuracy, feed speed, torque constraints between the three, performance Increased restrictions. Therefore, the economical CNC system is currently used in CNC wire cutting and some economical wire-cutting wire-cutting machine tools with low speed and accuracy requirements. It has also been widely used in the numerical control transformation of ordinary wire walking machine tools.
2.2 Precision CNC System
Precision CNC system includes semi-closed-loop numerical control system and full-closed-loop numerical control system.
The semi-closed-loop numerical control system generally refers to the position signal (optical 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 and control part of the error, so its control accuracy is better than that of the open-loop numerical control system. To be high, but lower than the closed-loop numerical control system.
In addition to the position feedback of the servo motor of the machine tool, the full-closed-loop numerical control system also feedbacks the position signal of the position detection device of the machine tool table (generally using a grating ruler) to the system, thereby forming all position follow-up control, and the system is in the process of processing. Automatically detect and compensate for all position errors.
The full-closed-loop numerical control system has the highest machining accuracy, but the commissioning and maintenance of such systems is extremely difficult, and the system's price is very high. It is only applicable to medium-to-high-end CNC machine tools.
Because the price of an open-loop control system is much lower than that of a closed-loop control system, when selecting a numerical control system, it is necessary to consider the numerical cost ratio of the numerical control system to the entire numerical control machine tool, and then according to the configuration of the machine tool and the requirements of the machine tool itself, The low-end machine tool adopts an open-loop control system, and medium- and high-grade machine tools use a closed-loop control system.
3, the matching of the drive unit
The drive unit consists of a drive unit and a motor. The purchase of the drive unit mainly depends on the selection of the drive unit. Because the motor is a common component, the performance difference exists only in different manufacturers and models.
Drive motors can be divided into three major categories: reactive stepper drive motors, hybrid (also known as permanent magnet reaction) stepper drive motors and servo drive motors.
Reactive stepper drive motor rotor without windings, by the excitation of the stator windings generate reaction torque to achieve stepping operation. The permanent magnet for the rotor of the hybrid stepper motor is stepped by the electromagnetic torque generated by excitation and permanent magnets. The stepper motor is controlled by pulses. The direction of rotation 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. The stepper motor has a certain step accuracy and there is no cumulative error. However, the efficiency of the stepping motor is low, the ability to drag the load is not large, the pulse equivalent can not be too large, and the speed range is not large. At present, the stepping motor can be divided into two phases, three phases, and five equal ones. The commonly used stepping motor is a five-phase stepping motor. For a long time in the past, stepper motors accounted for a large market, but are currently being replaced by servo motors.
At present, the commonly used servo motor is an AC servo motor, and an optical encoder is installed at the shaft end of the motor, and the angle of the rotor is used for frequency conversion control. From the lowest speed to the highest speed, the servo motor can run smoothly with small torque ripple. The servo motor has a long overload capacity, a small moment of inertia and a large stall torque. The servo motor has a very 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, which can be the same as a DC servo motor to form a high-precision, high-performance semi-closed-loop or closed-loop control system. Because the AC servo motor has a brushless structure, it needs little maintenance, and its volume is relatively small, which is beneficial to the increase of speed and power. At present, DC servo motors have been replaced in a wide range. After the emergence of a fully digital AC servo system using high-speed microprocessors and dedicated digital signal processors (DSPs), the original hardware servo control was changed to software servo control, and some advanced algorithms in modern control theory were realized, which greatly improved the Because of the performance of the servo system, the servo unit can greatly improve machining efficiency and machining 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 also gradually reduced.
There are two servo drives. An impulse control method is adopted. Such a driver and the motor are closed loop, but they are not fed back to the numerical control system. This kind of actuator can be called open-loop servo control to some extent. Another kind adopts the voltage control way, carries on the electrical machinery rotational speed control through the high and low voltage, the electrical machinery feedback signal passes the driver feedback to the numerical control system to carry on the position control.
When selecting the drive unit, the proportion of the drive unit's price in the entire CNC machine tool should also be considered. The lower price of the entire CNC machine tool generally chooses the stepping drive unit, while the higher-priced machine tool selects the servo drive unit. However, when selecting the drive unit, the matching between the drive unit and the numerical control system must also be considered. When selecting the closed-loop control system, the closed-loop servo drive unit must be selected. AC servo systems are superior to stepper motors in many performance aspects. However, stepper motors are often used to perform the motor in some less demanding applications. Therefore, in the design process of the control system, various factors such as control requirements and costs should be comprehensively considered, and appropriate control motors should be selected.
4, function selection
The above considerations are based on the machining accuracy of the CNC system. In addition to this, consideration must be given to the selection of the functions of the CNC system.
4.1 Control axis
The number of axes controlled by the numerical control system is also the key to the choice. According to the number of control axes can be divided into two-axis linkage, four-axis linkage, multi-axis linkage and so on. The greater the number of control axes, the more complex the shape the machine can machine, but the higher the cost. At present, the wire cutting machine generally uses two linear moving axes for linkage, and there are two additional linear moving axes for the taper device. The high-end system has more linkage axes, representing the highest level of the wire cutting machine manufacturing industry is the five-axis linkage numerical control system, in which four axes are XYUV linear movement axis, one axis is the Z axis as the vertical movement axis, five axes Linkage can be processed more complex space parts. Of course, this requires the support of high-end numerical control systems, servo systems, and software. The requirements for machine tools are also extremely high.
The more control axes, the number of CNC systems grows geometrically. Therefore, when choosing the numerical control system, it is necessary to select according to the movement axis of the machine tool itself. The redundant control axes cannot improve the control precision of the machine tool, but increase the cost of the numerical control system.
4.2 Graphical display
The system's graphical display function, which is used to simulate the part machining process, displays the cutting path of the real tool on the workpiece, can select a plane in the rectangular coordinate system, and can also select a three-dimensional three-dimensional view of different perspectives, which can be made at the same time The real-time display can also be used to quickly delineate the machining process in a mechanically locked manner. It is an effective tool for inspecting part machining programs, improving programming efficiency and real-time monitoring.
These kinds of problems are often encountered in the functional configuration of CNC wire-cutting machine tools. As a design and sales person of a CNC machine tool and an investment buyer, it is necessary to clearly understand the various functional uses of the CNC system, according to the actual situation of the machine tool. Users are equipped with CNC machine tools that are economically reasonable, with high functions and prices, and reduce unnecessary waste.
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