The CNC system of CNC machine tools
There are many factors that affect the process of CNC machine tools, and when analyzing the process of workpieces, the characteristics of CNC machine tools should be considered. Considering a series of factors such as the arrangement of part process routes, selection of machine tools, selection of cutting tools, and clamping of parts. Different CNC machine tools correspond to different processes and workpieces, and how to choose a reasonable machine tool has become the key to improving efficiency and reducing investment for enterprises. The CNC system of a CNC machine tool includes a CNC device, feed drive (feed rate control unit and servo motor), spindle drive (spindle speed control unit and spindle motor), and detection components. When selecting a CNC system, the above content should be included.
1、 Selection of CNC devices
（1） Type selection
Select the corresponding CNC device according to the type of CNC machine tool. Generally speaking, CNC devices are suitable for machining types such as turning, drilling, boring, milling, grinding, stamping, and electrical discharge cutting, and should be selected accordingly.
（2） Performance selection
The performance of different CNC devices varies greatly, such as the number of control axes including single axis, 2-axis, 3-axis, 4-axis, 5-axis, and even more than 10 or 20 axes; There are 2 or more linkage axes, and the maximum feed speed is 10m/min, 15m/min, 24m/min, 240m/min; The resolution is 0.01mm, 0.001mm, and 0.0001mm. These indicators are different, and the prices are also different. They should be based on the actual needs of the machine tool. For example, for general turning machining, 2 or 4 axes (double tool holder) control should be selected, and for flat parts machining, 3 or more axes linkage should be selected. Don’t pursue the latest and highest level, choose wisely.
（3） Selection of functions
The CNC system of CNC machine tools has many functions, including basic functions – essential functions of CNC devices; Selection function – a function for users to choose from. Some functions are selected to solve different machining objects, some to improve machining quality, some to facilitate programming, and some to improve operational and maintenance performance. Some selection functions are related, and selecting this option requires selecting another option. Therefore, the selection should be based on the design requirements of the machine tool. Do not choose too many functions without analysis, and omit relevant functions, which will reduce the functionality of the CNC machine tool and cause unnecessary losses.
There are two types of programmable controllers in the selection function: built-in and independent. It is best to choose an internal type, which has different models. Firstly, the selection should be based on the number of input and output signal points between the CNC device and the machine tool. The selected number of points should be slightly higher than the actual number of points, and one cup may require additional and modified control performance. Secondly, it is necessary to estimate the size of sequential programs and select storage capacity. The program size increases with the complexity of the machine tool, and the storage capacity also increases. It should be selected reasonably according to the specific situation. There are also technical specifications such as processing time, instruction function, timer, counter, internal relay, etc., and the quantity should also meet the design requirements.
（4） Price selection
Different countries and CNC device manufacturers produce different specifications of products with significant price differences. Based on the selection of control types, performance, and functions, a comprehensive analysis of the performance price ratio should be conducted to select CNC devices with higher performance price ratios in order to reduce costs.
（5） Selection of technical services
When selecting CNC devices that meet technical requirements, consideration should also be given to the reputation of the manufacturer, whether the product usage instructions and other documents are complete, and whether it is possible to provide training to users on programming, operation, and maintenance personnel. Is there a dedicated technical service department that provides long-term spare parts and timely maintenance services to maximize technical and economic benefits.
2、 Selection of feed drive
（1） Priority should be given to using AC servo motors
Because compared with DC motors, it has smaller rotor inertia, better dynamic response, higher output power, higher speed, simpler structure, lower cost, and unrestricted application environment.
（2） Calculate load conditions
Choose a suitable servo motor specification by correctly calculating the load conditions applied to the motor shaft.
（3） Select the corresponding speed control unit
The feed drive manufacturer provides a complete set of products for the feed rate control unit and servo motor produced, so after selecting the servo motor, the corresponding speed control unit is selected according to the product manual.
3、 Selection of spindle drive
（1） Priority should be given to mainstream spindle motors
Because it does not have the limitations of commutation, high speed, and large capacity like DC spindle motors, it has a wide range of constant power speed regulation, low noise, and is cheap. Currently, 85% of CNC machine tools internationally use AC spindle drive.
（2） Select the spindle motor as required
① Calculate the cutting power based on different machine tools, and the selected motor should meet this requirement; ② According to the required spindle acceleration and deceleration time, calculate that the motor power should not exceed the maximum output power of the motor; ③ In situations where frequent starting and braking of the spindle is required, the average power must be calculated, and its value cannot exceed the continuous rated output power of the motor; ④ In situations where constant surface control is required, the sum of the cutting power required for constant surface speed control and the power required for acceleration should be within the power range that the motor can provide.
（3） Select the corresponding spindle speed control unit
The spindle drive manufacturer provides a complete set of products for the spindle speed control unit and spindle motor produced. Therefore, after selecting the spindle motor, the corresponding spindle speed control unit is selected according to the product manual.
（4） Choose directional control method
When directional control of the spindle is required, a position encoder or magnetic sensor can be selected according to the actual situation of the machine tool to achieve spindle directional control.
4、 Selection of detection components
（1） Choose measurement method
According to the position control scheme of the CNC system, the linear displacement of the machine tool is measured directly or indirectly, and linear or rotary detection components are selected. At present, CNC machine tools widely use semi closed loop control, using rotary angle measuring components (rotary transformers, pulse encoders).
（2） Consider detection accuracy and speed
According to the requirements of CNC machine tools, whether to detect accuracy or speed, select position or speed detection components (testing generators, pulse encoders). Generally speaking, large machine tools are mainly designed to meet speed requirements, while high-precision and small and medium-sized machine tools are mainly designed to meet accuracy requirements. The resolution of the selected detection component is generally one order of magnitude higher than the machining accuracy.
（3） Select pulse encoders of corresponding specifications
Select the corresponding specifications of pulse encoders based on the ball screw pitch of the CNC machine tool, the minimum movement speed of the CNC system, the command multiplier, and the detection multiplier.
（4） Consider interface circuits
When selecting detection components, it is important to consider that the CNC device has corresponding interface circuits.