The numerical control system includes numerical control device, feed drive (feed speed control unit and servo motor), spindle drive (spindle speed control unit and spindle motor) and detection element. The selection of numerical control system should include the following contents.
- Selection of numerical control device
(1) Selection of types
Generally speaking, CNC drilling, milling, EDM and other devices should be selected according to their types.
(2) The choice of performance
The performance of different CNC devices varies greatly. The number of input control axes is single axis, 2 axis, 3 axis, 4 axis, 5 axis, or even more than 10 axis, 20 axis; the number of linkage axes is 2 or more than 3 axis, the maximum feed speed is 10m / min, 15m / min, 24m / min, 240m / min; the resolution is 0.01mm, 0.001mm, 0.0001mm. According to the actual needs of machine tools, for example, 2-axis or 4-axis (double turret) control should be selected for general turning, and more than 3-axis linkage should be selected for plane parts processing.
(3) Function selection
A numerical control system has many functions, including the basic function — the essential function of the numerical control device; the selection function — the function for the user to choose. Some of the selection functions are to solve different processing objects, some to improve processing quality, some to facilitate programming, some to improve operation and maintenance performance. Some of the selection functions are related, and you have to select another one to select this one. Therefore, it is necessary to select according to the design requirements of the machine tool, not without analysis, but with too many steps to select the function, and the relevant functions are omitted, which will reduce the function of the CNC machine tool and cause unnecessary losses.
There are two kinds of PLC in the selection function: built-in PLC and independent PLC. It is better to choose the built-in type, which has different models. First of all, it should be selected according to the number of input and output signals between the numerical control device and the machine tool. The selected points should be a little more than the practical points. One cup may meet the needs of adding and changing the control performance. Secondly, the size of sequential program should be estimated and the storage capacity should be selected. With the increase of the complexity of the machine tool, the storage capacity of the program increases, which should be reasonably selected according to the specific situation. There are also processing time, command function, timer, counter, internal relay and other technical specifications, the number should also meet the design requirements.
(4) Xu Ze of price
Different countries, different manufacturers of numerical control devices, produce different specifications of products, the price is very different, should meet the control type, performance, function selection, on the basis of comprehensive analysis of the performance price ratio, choose the numerical control device with high performance price ratio, in order to reduce the cost.
(5) The choice of technical service
When selecting the CNC device that meets the technical requirements, we should also consider the reputation of the manufacturer, whether the product instructions and other documents are complete, and whether the users can be trained in programming, operation and maintenance personnel. Whether there is a special technical service department to provide long-term spare parts and timely maintenance services, so as to give full play to technical and economic benefits.
- Selection of feed drive
(1) AC servo motor is preferred, because it has small inertia, good dynamic response, large output power, high speed, simple structure, low cost and unlimited application environment compared with DC motor.
(2) Through the correct calculation of the load conditions on the motor shaft to select the appropriate specifications of the servo motor.
(3) The feed drive manufacturer provides a series of complete products for the feed speed control unit and servo motor, so after the servo motor is selected, the corresponding speed control unit is selected by the product manual.
- Selection of spindle drive
(1) The mainstream spindle motor is preferred, because it does not have the limitation of commutation, high speed and large capacity as DC spindle motor, has large range of constant power speed regulation, low noise and low price. At present, 85% of CNC machine tools are driven by AC spindle.
(2) Select the spindle motor according to the following principles:
① According to different machine tools to calculate the cutting power, the selected motor should meet this requirement; ② according to the required spindle acceleration and deceleration time, the calculated motor power should not exceed the maximum output power of the motor; ③ when the spindle is required to be frequent The output power of the motor must be within the range of the required constant power and the constant speed of the surface, which should be controlled continuously.
(3) The spindle drive manufacturer provides a series of complete products for the spindle speed control unit and the spindle motor, so after the spindle motor is selected, the corresponding spindle speed control unit is selected by the product manual.
(4) When the spindle orientation control is needed, the position encoder or magnetic sensor is selected to realize the spindle orientation control according to the actual situation of the machine tool.
- Selection of detection elements
(1) According to the position control scheme of the CNC system, the linear displacement of the machine tool is measured directly or indirectly, and the linear or rotary detection elements are selected. At present, the closed-loop pulse rotary encoder and rotary transformer are widely used for angle measurement
(2) According to the accuracy or speed required by CNC machine tools, the position or speed detection elements (test generator, pulse encoder) are selected. Generally speaking, large machine tools mainly meet the speed requirements, while high-precision, medium and small machine tools mainly meet the accuracy requirements. The resolution of detecting element is generally one order of magnitude higher than the machining accuracy.
(3) At present, the photoelectric pulse encoder is the most commonly used detection element of CNC machine tools. It selects the corresponding specifications of pulse encoder according to the ball screw pitch of CNC machine tools, the minimum moving speed of CNC system, command rate and detection rate.
(4) When selecting the detecting element, the corresponding interface circuit of the numerical control device should be considered.
