The three-line machining center is an advanced machining equipment, which is widely used for its high precision, high reliability, high efficiency, and the ability to process complex curved workpieces. However, if the model is improperly selected, it will not be able to exert its due benefits, and will cause a large backlog of funds, resulting in risks. The generalized selection mainly includes model selection, numerical control system selection, machine tool accuracy selection, and main feature specification selection. Among them, the selection of model and CNC system has the greatest risk, followed by the selection of machine tool accuracy and main feature specifications. Therefore, to reduce the risk of selection, we can start from the following aspects.

1. Model selection

On the premise of meeting the requirements of the processing technology, the simpler the equipment, the lower the risk. Both the turning center and the CNC lathe can process shaft parts, but the price of a turning center that meets the same processing specifications is several times more expensive than that of the CNC lathe. Process requirements, certainly choose CNC lathe less risk. In the same way, economical CNC lathes should be selected as far as possible among economical and ordinary CNC lathes. In the processing of 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 (excluding gas source, tool magazine and other supporting costs), so the mold The machining center is only used for the process of changing the tool very frequently during processing, and the CNC milling machine is used for fixing one tool for a long time for milling. At present, many machining centers are 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, so it should be given priority in comprehensive machining enterprises that have both shaft parts and box and cavity parts. Choose a CNC milling machine.

2. Selection of CNC system

When purchasing a CNC machining center, the same machine body can be configured with a variety of CNC systems. Among the available systems, the performance levels vary greatly, which directly affects the price structure of the equipment. At present, the types and specifications of CNC systems are extremely diverse. The imported systems mainly include FANUC from Japan, SINUMERIK from Germany, MITSUBISHI from Japan, NUM from France, FIDIA from Italy, FAGOR from Spain, and A-B from the United States. The domestic systems mainly include Guangzhou System, Aerospace System, Central China System, Liaoning Lantian System, Nanjing Dafang System, North Kaiqi System, Tsinghua System, KND System, etc. Each company has a series of products of various specifications. The basic principles of reducing the risk of CNC system selection are: high performance-price ratio, convenient use and maintenance, and long market life of the system. Therefore, we cannot unilaterally pursue high-level and new systems. Instead, it should be based on the performance of the host, make a comprehensive analysis of system performance and price, and select an appropriate system. At the same time, the traditional closed-architecture numerical control system or the PC-embedded numerical control system of the NC structure should be gradually selected, because the function expansion, change and maintenance of such systems must rely on the system supplier. The open CNC system with NC embedded in PC structure or SOFT structure should be selected 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 external 1/O. Just as various brands of sound cards, graphics cards and corresponding drivers can be installed on the computer, users can use the open CNC kernel to develop the required functions on the WINDOWSNT platform to form various types of CNC systems. In addition, there are many optional functions other than the basic functions in the CNC system. Users can choose some additional functions according to their own workpiece processing requirements, measurement requirements, programming requirements, etc., and include them in the attachments of the order contract, especially the DNC function of real-time transmission, etc. .

3. Precision selection

The choice of the accuracy grade of CNC machining center depends on the machining accuracy of typical parts. Generally, there are 20 to 30 items for the accuracy inspection of CNC machining centers, but the most characteristic items are: single-axis positioning accuracy, single-axis repeat positioning accuracy, and the roundness of the test piece processed by linkage of more than two axes. The positioning accuracy and repeated positioning accuracy comprehensively reflect the comprehensive accuracy of each moving part of the axis. The single-axis positioning accuracy refers to the error range when positioning any point in the axis's stroke, which directly reflects the machining accuracy of the machine tool, while the repeated positioning accuracy reflects the positioning stability of the axis at any positioning point in the stroke. This is a basic indicator to measure whether the shaft can work stably and reliably. Among the above two indicators, repeated positioning accuracy is particularly important. At present, the software in the CNC system has rich 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 transmission 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). The speed of 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 elements after the measuring element are affected by various accidental factors and also have considerable random errors, such as the actual positioning position of the table caused by the thermal elongation of the ball screw. Drift etc. Therefore, the reasonable selection 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 of the machine tool and the interpolation function of the CNC system. The evaluation index adopts the measured cylindrical surface. of roundness. In the test piece of CNC milling machine, there is also milling of four sides of the inclined square, which is also a method to judge the accuracy of the two controllable axes in the linear interpolation movement. For CNC milling machines, the roundness index of the specimens processed by linkage of more than two axes cannot be ignored. Machine tools with high positioning accuracy requirements must also pay attention to whether its feed servo system adopts semi-closed loop mode or fully closed loop mode, and pay attention to the accuracy and stability of the detection components. If the machine tool adopts the 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 working temperature, which will affect the actual positioning position of the worktable and cause the workpiece to be drifted. The machining accuracy is affected.

4. Selection of main features and specifications of CNC machining centers

The main feature specifications of CNC machining center should be selected according to the determined size range of typical workpiece family. The Z main specifications of a CNC machining center are the stroke range of several CNC axes and the power of the spindle motor. The travel of the three basic linear coordinates (X, Y, Z) of the machine tool reflects the allowable processing space of the machine tool, and in the lathe, the two coordinates X, Z reflect the size of the allowable rotating body. Under normal circumstances, the contour size of the workpiece should be within the processing space of the machine tool. If the typical workpiece is a box with a size of 450mm×450mm×450mm, then a machining center with a work surface size of 500mm×500mm should be selected. The workpiece is slightly larger considering the space required to install the fixture. There is a certain proportional relationship between the size of the machine tool table and the three linear coordinate strokes. For example, the above-mentioned machine tool with a worktable of 500mm × 500mm, 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 work surface basically determines the size of the processing space. In some 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 bearing capacity of the machine tool table, and whether the workpiece interferes with the tool change space of the machine tool, and A series of problems such as the interference of accessories such as machine tool protective covers. The main motor power of the CNC machining center can also have various configurations on the same specification machine tool, which generally reflects the cutting rigidity of the machine tool and the high-speed performance of the spindle. The power of the spindle motor of the light-duty machine tool may be 1~2 grades smaller than that of the standard machine tool. At present, the spindle speed of the general machining center is 4000~8000r/min, the vertical machine tool of high-speed machine tool can reach 20,000~70,000 r/min, and the horizontal machine tool is 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 also reflects the cutting rigidity and the overall rigidity of the machine tool from the other side. In modern small and medium-sized CNC machining centers, the mechanical speed change of the spindle box has been rarely used, and the DC or AC adjustable speed motor with higher power is often used to directly connect the spindle, and even the electric spindle structure is used. This structure is used in low-speed cutting. The torque is limited, that is, the output power of the speed-regulated 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 machining center (CNC machine tool) of the same specification machine tool is several times larger than that of the ordinary machine tool. When there is a lot of low-speed machining on a typical workpiece, the low-speed output torque of the machine must be checked.