1.3.2 Classification of CNC machine tools
Nowadays, CNC machine tools have developed into mainstream machine tools with complete varieties and various specifications, which can meet modern production. CNC machine tools can be classified and evaluated from different angles, usually according to the following methods
1.Classification by process use
General CNC machine tools
Like traditional general-purpose machine tools, this kind of machine tools include CNC lathes, milling machines, boring machines, drilling machines, grinding machines, etc., and each kind of CNC machine tool also has many varieties. For example, CNC milling machines include CNC end milling machines, CNC horizontal milling machines, CNC tool milling machines, CNC gantry milling machines, etc. This kind of CNC machine tool is similar to the general machine tool in technology, but the difference is that it can process parts with complex shapes
CNC machining center
CNC machining center is developed on the basis of general CNC machine tools. It is a kind of CNC machine tool with automatic tool change device, which is composed of adding a tool magazine (which can accommodate 10~10 tools) and an automatic tool change device on the general CNC machine tool, which makes the CNC machine tool further develop in the direction of automation and efficiency.
The difference between CNC machining center and general CNC machine tools is that after the workpiece is clamped once, the CNC device can control the machine tool to automatically change the tool, and continuously automatically complete the multi process processing of milling, boring, drilling, comparing and tapping on each processing surface of the workpiece. Most of these machine tools are mainly boring and milling, which are mainly used to process box parts. Compared with general CNC machine tools, it has the following advantages:
① The number of machine tools is reduced, which is convenient for management. For parts with multiple processes, only one machine tool can complete all processing, and the warehouse of semi-finished products can be reduced.
② Since the workpiece is only clamped once, the positioning error caused by multiple installations is reduced, and the machining quality can be guaranteed by relying on the accuracy of the machine tool
③ The process is centralized, which shortens the auxiliary time and improves the productivity
④ Because the parts can be processed in multiple processes with one clamping on a machine tool, the number of special jigs is greatly reduced and the production preparation time is further shortened
Because of many advantages of CNC machining center machine tools, it plays a very important role in the production of CNC machine tools
In addition, there is another kind of machining center developed on the basis of lathe, which takes shaft parts as the main processing object. In addition to turning and boring, it can also carry out drilling, milling and tapping of any part on the end face and peripheral surface. This kind of machining center also has a tool magazine, which can install 4~12 cutters. Conventionally, such machine tools are called turning centers.
Multi coordinate CNC machine tool
Some parts with complex shapes cannot be processed by three coordinate numerical control machine tools, such as propeller and aircraft curved surface parts. It requires the synthetic motion of more than three coordinates to process the required shape. Therefore, multi coordinate NC machine tools appear, which are characterized by the large number of axes controlled by the NC device and the complex structure of the machine tool. The number of coordinate axes usually depends on the process requirements of the processing parts. At present, four axis, five axis and six axis CNC machine tools are commonly used. At this time, the X, y and Z coordinates can be linked with the rotation of the turntable and the swing of the cutting tool, and complex surface parts such as wings can be processed.
2.Classification by the characteristics of motion control
According to the control of the relative motion path between the tool and the workpiece, CNC machine tools can be divided into point control CNC machine tools, linear control CNC machine tools, contour control CNC machine tools, etc.
Point control CNC machine tool
This kind of NC machine tool only needs to control the tool to move from one position to the next, regardless of its motion path, and only requires that the tool can finally reach the target position accurately, that is, it only controls the coordinate value of the end of the stroke, and does not carry out any cutting during the movement. As for the moving speed and route between the two critical points, it depends on the productivity. In order to have the highest possible productivity on the basis of accurate positioning, the movement between the two points first moves quickly to the position close to the new positioning point, then slows down and approaches the positioning point slowly to ensure its positioning accuracy
Point control can be used for the motion control of CNC coordinate boring machines, CNC drilling machines, CNC punches, CNC measuring machines and other machine tools. Machine tools controlled by point control form are called point control CNC machine tools
Linear control CNC machine tool
The NC machine tool with linear control refers to the NC machine tool that can control the workbench or tool of the machine tool to move and cut in a straight line parallel to the coordinate axis (or an oblique line of 45 ° with the coordinate axis) at the required feed speed. When this kind of CNC machine tool works, it is necessary to control not only the position between the two related points, but also the moving speed and route (trajectory) between the two related points. Its route is generally composed of straight lines parallel to each axis. The difference between it and point control CNC machine tools is that when the moving parts of CNC machine tools move, they can be cut along the direction of a coordinate axis (generally, they can also be cut along a 45 ° oblique line, but not along a straight line with any slope), and their auxiliary functions are more than those of point control CNC machine tools, such as spindle speed control, circular feed processing, tool selection and other functions.
This kind of CNC machine tools mainly include simple CNC lathes, CNC boring and milling machines, etc. The corresponding numerical control device is called linear control device
Contour control CNC machine tool
The control device of this kind of NC machine tool can continuously control two or more coordinate axes at the same time.
When machining, it is necessary to control not only the starting point and the end point, but also the speed and position of each point in the whole machining process, so that the machine tool can process parts with complex shapes that meet the requirements of the drawing. Most of them have a series of functions, such as two or more coordinates linkage, tool radius compensation, tool length compensation, axial motion error compensation of CNC machine tools, lead screw pitch error compensation, tooth side clearance error compensation, etc. This kind of CNC machine tool can process parts with complex shapes such as curved surfaces and impellers.
This kind of CNC machine tools typically include CNC lathes, CNC milling machines, machining centers, etc., and their corresponding CNC devices are called contour control devices (or continuous control devices).
According to the number of linkage (simultaneous control) axes, contour control CNC machine tools can be divided into two axis linkage control CNC machine tools, two axis half coordinate linkage control CNC machine tools, three axis linkage control CNC machine tools, four axis linkage control CNC machine tools, five axis linkage control CNC machine tools, etc. Multi axis (more than three axes) control and programming technology is a subject of development and research in the field of high technology. With the continuous improvement of product complexity and machining accuracy in the field of modern manufacturing technology, the application of multi axis linkage control technology and its machining programming technology is becoming more and more common.
3.Classified by the control mode of servo system
According to whether there is a detection feedback device for the controlled quantity, CNC machine tools can be divided into open-loop CNC machine tools and closed-loop CNC machine tools. According to the position of the measuring device, closed-loop CNC machine tools can be divided into full closed-loop CNC machine tools and semi closed-loop CNC machine tools. On the basis of the above three control modes, hybrid control CNC machine tools have also been developed.
Open loop control CNC machine tool
The open-loop control CNC machine tool has no detection feedback device. The signal sending process of the numerical control device is one-way, so there is no problem of system stability. Due to the unidirectional flow of the signal, it does not check the actual position of the moving parts of the machine tool, so the machining accuracy of the machine tool is not high, and its accuracy mainly depends on the performance of the servo system. When the system works, the input data is calculated by the numerical control device to distribute the command pulse, and the controlled workbench is moved by the servo mechanism (the servo element is usually a stepping motor)
This kind of CNC machine tool has the advantages of simple debugging, easy and stable system, low precision and low cost. It is often seen in the technical transformation of economic small and medium-sized CNC machine tools and old equipment.
Closed loop control CNC machine tool
The open-loop control accuracy can not meet the machining accuracy requirements of precision machine tools and large machine tools. Therefore, a detection feedback device is added to the NC machine tool to detect the position of the moving parts of the NC machine tool at all times during machining to make it consistent with the position required by the NC device, so as to achieve high machining accuracy。
The servo system receives the actual position feedback signal measured at the end of the workbench at any time, compares it with the command position signal sent by the numerical control device, and controls the movement of the feed shaft by its difference. This kind of system with feedback control is called closed-loop control system in the electrical field. Because this position detection signal is taken from the CNC machine tool workbench (the end actuator of the transmission system), all errors of the whole transmission system can be eliminated, and the system accuracy is high. However, many mechanical transmission links are included in the loop of closed-loop control. The friction characteristics, stiffness, clearance and other nonlinear factors of each component will directly affect the stability of the system. The manufacturing and debugging of the system is difficult and the cost is high. The closed-loop system is mainly used for some high-precision CNC milling machines, ultra precision CNC lathes, ultra precision CNC grinding machines, large CNC machine tools, etc。
Semi closed loop control CNC machine tool
The detection element of this kind of CNC machine tool is not installed at the end of the transmission system, but at the end of the motor shaft or lead screw shaft. The actual position of the workbench is calculated indirectly through the measured angular displacement of the motor shaft, so the control accuracy is not as high as that of the closed-loop CNC machine tool. Because the workbench is not completely included in the control loop, it is called semi closed-loop control. This control method is between open-loop and closed-loop, and its accuracy is not as high as that of closed-loop, but it can obtain stable control characteristics, and debugging is more convenient than that of closed-loop. At present, most medium and small CNC machine tools use this control mode.
Hybrid control CNC machine tool
Combining the characteristics of the above three control modes selectively, a hybrid control scheme can be formed. This scheme is mainly used in large CNC machine tools. Because large CNC machine tools need much higher feed speed and return speed, and also need quite high precision, if only full closed-loop control is adopted, the transmission chain and workbench of the machine tool are all placed in the control link, and the stability is difficult to guarantee, so hybrid control is often used. In the specific scheme, the hybrid control CNC machine tool can be divided into two forms: one is the open-loop compensation type; The second is semi closed loop compensation type.
① Open loop compensation type. Its basic control mode is the open-loop servo mechanism of the stepping motor, and a correction circuit is added to correct the error of the mechanical system with the feedback signal of the linear displacement measuring element installed on the workbench.
② Semi closed loop compensation type. It uses the semi closed-loop control method to achieve high-precision control, and then uses the linear displacement measuring element installed on the workbench to achieve correction, so as to achieve the unity of high speed and high precision.
