With the development of modern manufacturing technology, it is a general trend for enterprises to choose CNC equipment. At present, the numerical control equipment on the market can be described as dazzling, and how to economically and reasonably select the numerical control equipment suitable for the enterprise has always been a topic of concern. This article completely discusses the issues that should be paid attention to in the selection from the technical point of view.
For a manufacturing enterprise, the improvement of production capacity often starts with technological transformation from the aspects of production management, manufacturing process, production equipment, etc., and these parts are mutually influential and restrictive. In the technical transformation, the selection of production equipment, CNC machine tools, maintenance, procurement, etc. must be considered in what environment, how to manage, how to achieve the best economic effects.
The choice of manufacturing equipment is to manufacture some products. The equipment selected may be used for part of the processing of the product parts, and may also be used for all processes. The level of manufacturing depends first on the design of the process. It will determine the method and means to process it, which in turn determines the basic requirements for the use of equipment. This is also the basis for the technical organization and management of production. After the basic requirements for equipment selection are determined, it is also selected according to the technical level of equipment available on the market. For most small and medium-sized production enterprises, it is a development trend to select CNC machine tools to replace old machine tools or enhance production capacity.
Compared with the performance of both ordinary and CNC machine tools, CNC machine tools have the ability to process complex shaped parts and adapt to a variety of processing objects (flexible); high processing quality, precision and processing efficiency; adapt to CAD/CAM networking, suitable for manufacturing processing information Integrated management; high utilization of equipment and low operating costs.
The choice of CNC machine tools is a comprehensive technical problem, and now it can produce a wide range of equipment, both domestically and abroad. After decades of development, CNC machine tools have evolved into a large family group that can fulfill a variety of processing and manufacturing requirements. How to choose suitable equipment from a wide variety of expensive equipment, how to make these equipment fully play a role in manufacturing and meet the future development of the enterprise, how to correctly and reasonably purchase accessories, tools and software matching with the host Technology, after-sales technical services, etc., so that the procurement of equipment can achieve a better investment ratio ... These problems are the problems that the majority of purchasers must consider and deal with one by one.
First, determine the "family" of typical processing workpieces
Determining which parts to process is the first step in choosing a device. According to the needs of technological transformation and production development, the enterprise determines which parts are available, and those processes are prepared to be completed with new processing equipment, taking into account the long-term planning of product development. Group these parts into groups using group techniques to determine the typical part families that are to be prepared for the main machining object. In the classification, there are often problems such as a large difference in the size of the parts, a large difference in the shape of the parts, and the processing time of various parts greatly exceeds the full-load working hours of the equipment. Therefore, it is necessary to further select a typical workpiece family that determines the production program and is closer to the requirements. . The typical workpiece family can be divided into diamond type (box type), board type, rotary body type (disk, sleeve, shaft, flange) and special-shaped type according to the appearance; it can be divided into ordinary level and precision level according to the processing precision requirements. Wait. The classification of typical parts is clear, and the basic processing equipment is relatively clear.
Second, the design of the process specification of the typical part family
After the machining of the parts, the process planning must be carried out with a new perspective on the process flow, including the transformation of the original process production process, the feasibility of exploring new process methods, and the exploration of modern production management and logistics management. Feasibility, explore the feasibility of using advanced tooling tools to greatly improve production efficiency, explore the rational preparation process of CNC equipment and other equipment (general, special aircraft) on the production line, in order to obtain the best after using CNC machine tools. Process manufacturing process. Below are the reasonable processing techniques for several typical types of parts.
☆Axis parts: milling end face center hole → CNC lathe (roughing) → CNC grinding machine (finishing);
☆Flange and disc parts: CNC lathe (roughing)→ turning center (finishing);
☆ cavity mold parts: general machine tool processing shape and base surface → CNC milling machine processing surface → high-speed CNC milling finishing → polishing or electro-corrosion profile;
☆Board parts: double-axis milling machine or gantry milling machine processing large plane → vertical machining center processing all kinds of holes;
☆Box parts: The bottom surface of the vertical machining center → the horizontal machining center is processed on all sides of the process surface.
Consider the following factors in arranging the process:
(1) Select the shortest processing process.
(2) CNC machine tools have considerable adaptability, but they are not omnipotent. From the economic point of view, each part of the typical workpiece family has an economic batch, and should use more advanced technology on the basis of economic batch.
(3) Try to make full use of the various process characteristics of the machine tool, and strive to maximize the comprehensive processing capability of the CNC machine tool (the process characteristics of multiple processes), and the minimum number of machine tools, minimum process equipment and fixtures should be configured in the production process. .
(4) Consider the balance of various equipment capabilities in the production line or production workshop. As the choice of a single CNC machine tool or the configuration of a production line, a single device cannot completely cover all the processing steps of one workpiece, and must have process transfer with other equipment. The production capacity between the devices should be balanced to meet the production. The comprehensive requirements of the beat, so the number of processes on each equipment, the order of the processing steps, etc. should not only play the special features of each CNC machine tool, but also meet the accuracy requirements. It is also necessary to further consider the process benchmark of the workpieces on each machine. use.
(5) The problem often encountered in arranging CNC machining processes is the contradiction between the process concentration and the principle of process processing. In the use of CNC machine tools, the principle of concentration of processes in which multiple processes are concentrated on one machine tool is generally adopted, in order to improve productivity, shorten the machining cycle of parts, and even hope that the workpieces are completely processed in one loading. However, in fact, for some complex and high precision workpieces, due to thermal deformation during machining, workpiece deformation caused by internal stress, fixture clamping deformation, heat treatment aging and other process factors and process editors, It is difficult to complete all processing at one time. The gradual refinement of the machined parts in the basic process guidelines restricts the number of process concentrates. Properly handling the contradiction between the two is an important part of the CNC machining process.
(6) In the arrangement of the typical workpiece family process, the manual adjustment and inspection of each machine tool and production line should be properly arranged, that is, the impact of manual intervention. Enterprises should determine the degree of manual intervention in the process according to their technical equipment capabilities, technical level and technical transformation input, which determines the automation level and functional requirements for selecting CNC machine tools. Appropriate consideration should be given to the appropriate use of manual adjustments to supplement the ability of enterprises to achieve full automation, and to accurately locate the technological capabilities and equipment levels of enterprises.
Third, the selection of the main features of CNC machine tools
Machine specifications should include model, machine specifications and machine main motor power. Under the premise of determining the content of the process, the choice of the model is clearer. For example, the processing of rotary parts is mainly for the selection of equipment such as lathes, turning centers, CNC grinding machines, etc.; the processing of the box should be mainly vertical or horizontal machining centers.
CNC machine tools have been developed into a wide variety of products that can be widely selected. In the selection of models, the simpler the better, the conditions should be met. For example, turning centers and CNC lathes can machine shaft parts, but the price of a turning center that meets the same processing specifications is several times higher than that of a CNC lathe. If there is no further process requirement, the CNC lathe should be reasonable. In the machining cavity mold parts, the same size CNC milling machine and machining center can meet the basic processing requirements, but the price difference between the two machine tools is 20% to 50%, so the process of changing the tool in the mold processing can be arranged. Machining center, and an optional CNC milling machine that fixes a tool for long-term milling.
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 machining space allowed by the machine. The two coordinates (X, Z) in the lathe reflect the size of the allowable rotor. In general, the contour size of the machined workpiece should be within the machining space of the machine. For example, if the typical workpiece is a 450 mm × 450 mm × 450 mm case, then a machining center with a work surface size of 500 mm × 500 mm should be selected. The choice of a work surface that is slightly larger than a typical workpiece is considered for mounting fixtures. The machine tool table size and the three linear coordinate strokes have a certain proportional relationship. For the above-mentioned workbench (500 mm × 500 mm), the x-axis stroke is generally (700-800) mm, and the y-axis is (500-700). The mm and z axes are about 500 to 600 mm. Therefore, the size of the work surface basically determines the size of the processing space. In some cases, the workpiece size is allowed to be larger than the coordinate stroke. In this case, the machining area on the part must be within the stroke range, and the allowable bearing capacity of the machine table and the space interference of the workpiece with the machine tool exchange tool must be considered. A series of problems such as interference with accessories such as machine guards.
The main motor power of CNC machine tools can also have various configurations on the same type of machine tool. Under normal circumstances, the cutting rigidity and spindle high-speed performance of the machine tool are reflected. For example, a light machine tool may be 1 to 2 less powerful than a standard machine tool spindle motor. At present, the spindle speed of general machining center is (4000 ~ 8000) r / min, high-speed machine tool vertical machine can reach (20,000 ~ 70,000) r / min, horizontal machine tool (10000 ~ 20000) r / min, its spindle motor power also Doubled. The spindle motor power reflects the cutting efficiency of the machine tool and reflects the cutting rigidity and the overall rigidity of the machine tool from the other side. In modern small and medium-sized CNC machine tools, the mechanical shifting of the headstock has been used less frequently, and often the AC variable speed motor with a large power is directly connected to the spindle, and even the electric spindle structure is adopted. Such a structure is limited in torque at a low speed, that is, the output power of the speed-regulating motor is lowered at a low speed. In order to ensure a low-speed output torque, a high-power motor is required, so that the spindle machine motor of the same-standard machine tool numerical machine machine is several times larger than that of the ordinary machine tool. When there are a large number of low-speed machining on some typical workpieces of the unit, the low-speed output torque of the selected machine must also be checked. The light machine tool is definitely cheaper in price, and the user is required to comprehensively select the machine tool according to the size of his typical workpiece blank, the cutting ability (metal removal amount per unit time), the required machining accuracy, and the actual tool configuration.
In recent years, the trend of high-speed CNC machine tools has developed rapidly. The spindle has been transferred from several thousand to tens of thousands of revolutions per minute. The rapid moving speed of linear coordinates has risen from (10 to 20) m/min to over 80 m/min. As the price rises, the user unit must make a reasonable choice based on its technical capabilities and supporting capabilities. For example, the maximum speed of the spindle on the vertical machining center can reach (50,000 to 80,000) r/min. Except for some special processing examples, the matching tools are generally expensive. Some high-speed lathes can reach (6000 ~ 8000) r / min or more, at this time the configuration requirements of the turning tool is also very high.
For a small number of special workpieces, only three linear coordinate machining can not meet the requirements, and additional rotary coordinates (A, B, C) or additional work coordinates (U, V, W), etc., can be satisfied in the current machine tool market. However, the price of machine tools will increase a lot, especially for some requirements of multi-axis linkage processing, such as four-axis, five-axis linkage processing, must have comprehensive consideration and arrangement of the corresponding supporting programming software, measurement methods.
Fourth, the choice of machine tool accuracy
The machining accuracy requirements of the key parts of a typical part determine the accuracy level of the selected CNC machine. According to the application, CNC machine tools are divided into simple type, full-function type, ultra-precision type, etc., and the precision that can be achieved is also different. The simple type is also used in some lathes and milling machines. The minimum motion resolution is 0.01mm, and the motion accuracy and machining accuracy are all above (0.03~0.05)mm. The ultra-precision type is used for special processing and its precision can be up to 0.001mm. Here we mainly discuss the most widely used full-function CNC machine tools (mainly machining centers).
According to the precision, it can be divided into ordinary type and precision type. Generally, there are 20 to 30 items of precision inspection items for CNC machine tools, but the most characteristic items are: single axis positioning accuracy, single axis repeat positioning accuracy and two-axis or more linkage processing test. The roundness of the piece.
The positioning accuracy and repeat positioning accuracy comprehensively reflect the comprehensive precision of each moving part of the axis. In particular, repeat positioning accuracy, which reflects the positioning stability of the axis at any positioning point within the stroke, which is a basic indicator to measure whether the shaft can work stably and reliably. At present, the software in the numerical control system has a rich error compensation function, which can stably compensate the systematic errors of each link in the feed transmission chain. For example, the variation of the gap, elastic deformation and contact stiffness of the various links of the transmission chain often reflect different instantaneous movements with the load size of the workbench, the length of the moving distance, and the speed of the moving positioning speed. In some open-loop and semi-closed-loop feed servo systems, the mechanical drive components after the measurement component are affected by various accidental factors, and also have considerable random error effects, such as the actual positioning position of the table caused by the thermal elongation of the ball screw. Drift and so on. In short, if you can choose, then choose the equipment with the best positioning accuracy!
Milling cylindrical surface precision or milling space spiral groove (thread) is a comprehensive evaluation of the machine tool related to the CNC axis (two-axis or three-axis) servo following motion characteristics and numerical control system interpolation function. The evaluation method is to measure the circle of the cylindrical surface. degree. In the CNC machine tool test piece, there is also a milling square square four-edge machining method, and it is also possible to judge the accuracy of the two controllable axes in the linear interpolation motion. When doing this test, the end mill for finishing is mounted on the machine tool spindle, and the circular test piece placed on the workbench is milled. For the small and medium-sized machine tool, the round test piece is generally taken at Ф200~Ф300. Then, the cut test piece is placed on a roundness meter to measure the roundness of the machined surface. There is obvious milling cutter on the milled cylindrical surface to reflect the unstable interpolation speed of the machine; the roundness of the milling has obvious elliptic error, and the gain of the two controllable shaft systems reflecting the interpolation motion is not matched; on the circular surface There is a mark of the stop point at each point of the controllable axis movement direction (in the continuous cutting motion, when the feed tool is stopped at a certain position, a small piece of cut metal is formed on the machined surface) , reflecting that the positive and negative gaps of the axis are not adjusted well.
Single-axis positioning accuracy refers to the error range when positioning at any point within the stroke of the shaft. It directly reflects the machining accuracy of the machine tool, so it is the most important technical index of CNC machine tools. At present, countries around the world have different regulations, definitions, measurement methods and data processing for this indicator. In the introduction of various CNC machine tool samples, the commonly used standards are beautiful.
