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When purchasing CNC machine tools, the same machine tool body can be equipped with multiple CNC systems. Among the available systems, the performance level varies greatly, which directly affects the equipment price composition. Each company has a series of products of various specifications. The basic principles to reduce the risk of CNC system selection are: high performance-to-price ratio, convenient use and maintenance, and long market life of the system. Then the editor will sort out the risk content of CNC machine tools in the selection. as follows:

Therefore, we cannot one-sidedly pursue high-level and new systems. Instead, it should focus on the performance of the host, make a comprehensive analysis of system performance and price, and select a suitable system. At the same time, the traditional closed-architecture CNC system or the PC-embedded CNC system should be gradually reduced, because the function expansion, change and maintenance of this type of system must rely on the system supplier. The open CNC system with NC embedded in PC structure or SOFT structure should be used as much as possible. The CNC software of this type of system is all installed in the computer, and the hardware part is only the standardized general interface between the computer and the servo drive and the external 1/O. Just as various brands of sound cards, graphics cards and corresponding drivers can be installed on a computer, users can use the open CNC kernel on the WINDOWSNT platform to develop required functions to form various types of CNC systems. In addition, there are many optional functions in addition to the basic functions of the CNC system. Users can select additional functions to be included in the order contract attachments according to their own workpiece processing requirements, measurement requirements, and programming requirements, especially the DNC function for real-time transmission, etc. .

The choice of accuracy level of CNC machine tools depends on the machining accuracy of typical parts. Generally, there are 20 to 30 items in the accuracy inspection of CNC machine tools, but the most characteristic items are: single-axis positioning accuracy, single-axis repeat positioning accuracy, and roundness of test pieces processed by linkage with more than two axes. The positioning accuracy and repeated positioning accuracy comprehensively reflect the comprehensive accuracy of the moving parts of the axis. Single-axis positioning accuracy refers to the error range when positioning at any point in the stroke of the axis, which directly reflects the machining accuracy capability of the machine tool, and the repeated positioning accuracy reflects the positioning stability of any positioning point of the axis in the stroke. This is a basic indicator to measure whether the shaft can work stably and reliably. Among the above two indicators, repeat positioning accuracy is particularly important. At present, the software in the numerical control system has a wealth of error compensation functions, which can stably compensate the system errors of each link in the feed transmission chain. For example, the pitch error and accumulated error of the lead screw can be compensated by the pitch compensation function, and the reverse dead zone of the feed drive chain can be eliminated by backlash compensation.

However, the error compensation function of electronic control cannot compensate for random errors (such as errors caused by changes in the gap, elastic deformation and contact stiffness of each link of the transmission chain). They often follow the load of the workbench, the length of the moving distance, and the moving positioning. The speed and so on reflect the different amount of exercise loss. In some open-loop and semi-closed-loop feed servo systems, the mechanical drive components after the measuring component are affected by various accidental factors and also have considerable random errors, such as the actual positioning position of the worktable caused by the thermal elongation of the ball screw Drift etc. Therefore, a reasonable choice of repeated positioning accuracy can greatly reduce the risk of accuracy selection. The accuracy of milling cylindrical surface or the accuracy of milling space spiral groove (thread) is an index for comprehensive evaluation of the CNC axis (two-axis or three-axis) servo following motion characteristics and the interpolation function of the CNC system of the machine tool. The evaluation index is the cylindrical surface processed by measurement. The roundness. In the trial cutting part of the CNC milling machine, there is also milling bevel square four-side processing, which is also a method to judge the accuracy of the two controllable axes during linear interpolation motion. For the CNC milling machine, the roundness index of the test piece processed by the linkage of more than two axes can not be ignored. For machine tools with high positioning accuracy requirements, attention must also be paid to whether its feed servo system adopts a semi-closed loop or a fully closed loop, and pay attention to the accuracy and stability of the detection components. If the machine tool adopts a semi-closed-loop servo drive mode, the accuracy and stability will be affected by some external factors. For example, the ball screw in the transmission chain is elongated due to the change of operating temperature, which affects the actual positioning position of the worktable and causes the workpiece to be moved. The machining accuracy is affected.

The main feature specifications of CNC machine tools should be selected according to the determined typical workpiece family processing size range. The main specifications of CNC machine tools are the stroke range of several CNC axes and the power of the spindle motor. The three basic linear coordinate (X, Y, Z) strokes of the machine tool reflect the processing space allowed by the machine tool, and the two coordinates X, Z in the lathe reflect the size of the allowable revolving body. Under normal circumstances, the contour size of the workpiece should be within the processing space of the machine tool. For example, if the typical workpiece is a box of 450mm×450mm×450mm, then a machining center with a table size of 500mm×500mm should be selected. The larger work piece is due to the space required to install the fixture. There is a certain proportional relationship between the table size of the machine tool and the stroke of the three linear coordinates. For example, the machine tool with a 500mm×500mm table above, the X axis stroke is generally 700~800mm, the Y axis is 500~700mm, and the Z axis is 500~600mm. Therefore, the size of the working table basically determines the size of the processing space.

In individual cases, the size of the workpiece may be larger than the coordinate stroke. At this time, the processing area on the part must be within the stroke range, and the allowable load-bearing capacity of the machine tool table must be considered, and whether the workpiece interferes with the tool change space of the machine tool, and A series of problems such as interference of accessories such as machine tool guards. The main motor power of a CNC machine tool can also have various configurations on a machine tool of the same specification, which generally reflects the cutting rigidity of the machine tool and the high-speed performance of the spindle. The spindle motor power of a light machine tool may be 1 to 2 levels lower than that of a standard machine tool. At present, the spindle speed of general machining center is 4000~8000r/min, the high-speed machine tool can reach 20,000~70,000 r/min, and the horizontal machine tool can reach 10,000~20,000 r/min. The power of the spindle motor is also doubled. .

The power of the spindle motor reflects the cutting efficiency of the machine tool, and from the other side also reflects the cutting rigidity and the overall rigidity of the machine tool. In modern small and medium-sized CNC machine tools, the mechanical speed change of the spindle box has been seldom used, and the direct-connected spindle with a high-power DC or AC adjustable speed motor is often used, and even the electric spindle structure is adopted. Such a structure has a torque in low-speed cutting. Restricted, that is, the output power of the speed-regulating motor decreases at low speed. In order to ensure the low-speed output torque, a high-power motor must be used. Therefore, the spindle motor of the CNC machine tool of the same specification machine tool is several times larger than that of the ordinary machine tool. When there is a large amount of low-speed machining on a typical workpiece, the low-speed output torque of the machine tool must be checked.

Under the premise of meeting the requirements of the processing technology, the simpler the equipment, the lower the risk. Both turning processing centers and CNC lathes can process shaft parts, but a turning center that meets the same processing specifications is several times more expensive than a CNC lathe. If there is no further Process requirements, the risk of choosing a CNC lathe is definitely less. In the same way, economical CNC lathes should be selected as far as possible among economical and ordinary CNC lathes. In processing boxes, cavities, and mold parts, CNC milling machines and machining centers of the same specifications can meet the basic processing requirements, but the price difference between the two machine tools is about half (not including supporting costs such as air source and tool magazine), so the mold In the process, only the process of changing the tool very frequently is used for the machining center, and the CNC milling machine is selected for the long-term milling with a fixed tool. Many machining centers are currently used as CNC milling machines. The parts that can be processed by CNC lathes can often be processed by ordinary lathes, but most of the parts that can be processed by CNC milling machines cannot be processed by ordinary milling machines. Therefore, priority should be given to comprehensive machining companies that have shaft parts, box and cavity parts Choose a CNC milling machine.

The CNC machine tool is an advanced processing equipment, which is widely used due to its high precision, high reliability, high efficiency, and ability to process complex curved workpieces. However, if the model is not properly selected, it will not be able to exert its due benefits, and a large backlog of funds will be generated, thereby generating risks. The broad selection mainly includes model selection, numerical control system selection, machine precision selection, main feature specification selection, etc. Among them, model selection and numerical control system selection have the greatest risk, followed by machine tool accuracy and main feature specifications. Therefore, the selection risk should be reduced.