The difference between the first type and the second type is on the roller of the guide rail, one is a ball and the other is a roller. The ball is in point contact with the rail surface, and the roller is in line contact with the rail surface, so its wear resistance and bearing capacity are much better than the ball type. The linear slide is a kind of rolling guide, which is made by the steel ball infinite rolling cycle between the slider and the slide rail, so that the load platform can easily move linearly along the slide rail with high precision, and the friction coefficient can be reduced to the conventional one. The 1/50 of the sliding guide makes it easy to achieve the μm-level positioning accuracy. The design of the final unit between the slider and the slide rail enables the linear slide rail to simultaneously withstand loads in all directions, such as up, down, left and right, and the linear slide has smoother and less noisy motion characteristics. It maintains its precision and load carrying capacity much better than ball and roller. At present, the third structure is adopted in the machine tools imported by Sodick Corporation of Japan, Mitsubishi Corporation of Japan, Charmilles of Switzerland, and Reach, so by comparison, the user should consider the third structure when selecting the model.
2. 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.
2.1 Economical CNC system
The economical numerical control system is generally referred to as an open-loop numerical control system in terms of control methods. 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 operation, convenient debugging and maintenance, and the price is very low, it is subject to the mutual limitation of the torque frequency characteristics and accuracy of the stepping motor, the feed speed and the torque. Improvement is limited. Therefore, the economical numerical control system is currently used in the CNC fast-moving wire cutting and some economical medium-cut wire cutting machine tools with low speed and precision requirements, and is also widely used in the numerical control transformation of ordinary fast wire cutting machine tools.
2.2 Precision CNC System
The precision 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 choosing the numerical control system, it is necessary to consider the numerical control system to occupy the price-cost ratio of the whole CNC 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 the medium and high-end machine tools adopt a closed-loop control system.
3, 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 five-phase stepping motor. 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.
4, 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.
4.1 Control axis
The number of axes controlled by the CNC system is also the key to choice. According to the number of control axes, it can be divided into two-axis linkage, four-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 wire cutting machine generally uses two linear moving axes to interlock, and the taper device has two additional linear moving axes. The high-end system has more axes, and the highest level of wire-cutting machine tool manufacturing is the five-axis linkage CNC system. Four axes are XYUV linear moving axes, one axis is Z axis for up and down linear moving axis, five axes. More complex space parts can be machined when linked. 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.
4.2 Graphic display
The system's graphic display function, which is used to simulate the part machining process, showing the cutting path of the real tool on the workpiece. You can select one plane in the Cartesian coordinate system, or you can select three-dimensional ones with different viewing angles, which can be processed at the same time. Real-time display, also can be quickly depicted in the mechanical locking mode, is an effective tool to verify the part processing program, improve 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 staff of CNC machine tools and investment buyers, they must clearly understand the various functional uses of the CNC system, according to the actual situation of the machine tool. For users to configure CNC machine tools that are economical, functional and cost-effective, reduce unnecessary waste.
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