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Eight selection steps of CNC machine tools
Click rate: Release time:2012-02-13 Information sources:本站

With the development of modern manufacturing technology, it is a general trend for enterprises to choose CNC equipment. At present, there are a lot of CNC equipment on the market. How to choose the CNC equipment suitable for the enterprise economically and reasonably has always been a topic of concern. This article fully discusses the issues that should be paid attention to in the selection of models from a technical perspective.

For a manufacturing enterprise, improving production capacity often starts with technological management from the aspects of production management, manufacturing process, production equipment, etc., and these parts are mutually influenced and restricted. In the technical transformation, the choice of production equipment, CNC machine tool update, maintenance, procurement, etc. must be considered in what environment to use, how to manage, how to achieve the best economic results and other issues.

The choice of manufacturing equipment is to serve certain products. The selected equipment may be used for part of the process of product parts, or it may be used for all processes. The level of manufacturing depends first on the design of the process. It will determine what methods and means to use for processing, and thus 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 necessary to choose according to what technical level of equipment can be provided on the market. For most of the small and medium-sized batch manufacturing companies, it is a development trend to choose CNC machine tools to replace old machine tools or enhance production capacity.

Compared with the performance of ordinary and CNC machine tools, CNC machine tools have the ability to process complex shaped parts, adapt to a variety of processing objects (strong flexibility); processing quality, accuracy and processing efficiency; adapt to CAD / CAM networking, suitable for manufacturing processing information Integrated management; high utilization rate of equipment and low normal operating costs.

Choosing a CNC machine tool is a comprehensive technical issue. Now no matter domestic or foreign, we can produce and provide a variety of equipment. After decades of development, CNC machine tools have evolved into a huge family group that can complete 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 purchase accessories, tools and software matching with the host correctly and reasonably Technology, after-sales technical services, etc., make the purchased equipment achieve a better investment ratio ... These issues are issues that the majority of buyers must consider and deal with one by one.

1. Identify the typical "workgroup" of workpieces

Determine what kind of parts to process is the first step in selecting equipment. According to the needs of technological transformation and production development, enterprises determine which parts and which processes are to be completed with new processing equipment, and the long-term planning of product development must be considered. Group these parts by grouping technology to determine the typical part family to prepare the main processing object. In the classification, we often encounter problems such as large differences in part specifications, large differences in part shapes, and various parts processing hours that greatly exceed equipment full-load hours. Therefore, we must 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), plate type, rotary body type (disk, sleeve, shaft, flange) and special type according to the appearance; according to the requirements of processing accuracy, it can be divided into ordinary level and precision level Wait. The classification of typical parts is clear, and the basic processing equipment is relatively clear.

Second, the design of process specifications for typical parts families

After determining the parts to be processed, a new planning and design of the process flow must be carried out with the scientific viewpoint of CNC machining technology. This includes the transformation of the original process production flow, 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 to greatly improve production efficiency, explore the reasonable preparation process of CNC equipment and other equipment (general and special machines) on the production line, etc. The purpose is to get the best after using CNC machine tools Process manufacturing process. The following is a reasonable processing technology of several typical parts.

☆ Shaft parts: milling the end face to drill a center hole → CNC lathe (rough machining) → CNC grinder (finishing);

☆ Flange and disc parts: CNC lathe (rough machining) → turning center (finish machining);

☆ 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 surface;

☆ Plate parts: double-axis milling machine or gantry milling machine for processing large planes → processing all kinds of holes on vertical machining centers;

☆ Box parts: processing bottom on vertical machining center → processing all sides on horizontal machining center.

Consider the following factors in arranging the technological process:

(1) Select the shortest process flow.

(2) The CNC machine tool has considerable adaptability, but it is not omnipotent. From an economic point of view, each part in a typical workpiece family has an economic batch, and more advanced technological methods should be used on the basis of economic batch.

(3) Try to give full play to the various process characteristics of the machine tool, and strive to maximize the comprehensive processing capabilities of the CNC machine tool (multi-process concentrated process characteristics), the minimum number of machine tools, the least process equipment and fixtures should be configured in the production process .

(4) It is necessary to consider the balance of various equipment capabilities in the production line or workshop. As the choice of a single CNC machine tool or the configuration of a production line, it is impossible for a single device to completely cover all the processing procedures of a workpiece. There must be a process conversion with other devices. The production capacity of each device must be balanced to meet the production. The comprehensive requirements of the beat, so the number of processes and the sequence of processing steps on each equipment must be arranged to not only take advantage of the advantages of each CNC machine tool and meet the accuracy requirements, but also further consider the reasonableness of the process benchmark when the workpieces are transferred on each machine tool use.

(5) The problem often encountered in the arrangement of CNC machining processes is the contradiction between the concentration of procedures and the principle of gradual refinement of technological processing. In the use of CNC machine tools, people generally adopt the process concentration principle of concentrating multiple processes on one machine tool, in order to pursue increased productivity, shorten the processing cycle of parts, and even hope that the workpieces are all processed in one installation. However, in fact, for some complex and high-precision workpieces, due to thermal deformation during processing, workpiece deformation caused by internal stress, clamping deformation of tooling fixtures, heat treatment required aging and other process factors and programmers ’operating factors, etc., It is difficult to install all the cards at once to complete the processing. The requirements for the gradual refinement of processed parts in the basic process guidelines restrict the number of processes concentrated. Properly handling these two contradictions is an important part of the CNC machining process.

(6) In the arrangement of typical workpiece family process flow, the manual adjustment and detection 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 own technical equipment capabilities, technical level and the intensity of technical transformation investment, which determines the level of automation and functional requirements for the selection of CNC machine tools. Objective consideration should be given to the appropriate use of manual adjustments to supplement the enterprise's ability to achieve full automation, and the precise positioning of the enterprise's process capability and equipment level.

Third, the selection of main features and specifications of CNC machine tools

Machine tool features and specifications should include model, machine tool specifications, and machine tool main motor power. Under the premise of determining the process content, the model selection is more clear. For example, the main parts available for the processing of rotary parts are lathes, turning centers, CNC grinders, etc .; the processing of boxes should be based on vertical or horizontal machining centers.

CNC machine tools have developed into a wide variety of products that can be widely selected. In the selection of models, the simpler the better as long as the processing technology requirements are met. For example, both turning 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 are no further process requirements, it should be reasonable to choose a CNC lathe. In the processing of cavity 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 20%-50%, so the process of frequently changing tools can be arranged in mold processing. The machining center is selected, and a CNC milling machine can be used to fix a tool for milling for a long time.

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 allowable machining space of the machine tool, and the two coordinates (X, Z) in the lathe reflect the size of the allowed slewing body. Under normal circumstances, the contour size of the workpiece should be within the processing space of the machine tool. For example, a typical workpiece is a box of 450 mm × 450 mm × 450 mm, then a machining center with a worktable size of 500 mm × 500 mm should be selected. The selection of the work surface is slightly larger than the typical workpiece for the consideration of the installation fixture. The size of the machine table and the three linear coordinate strokes have a certain proportional relationship. For the machine tool of the above table (500 mm × 500 mm), the x-axis stroke is generally (700-800) mm, and the y-axis is (500- —700) mm, the z axis is about (500—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. At this time, the machining area on the part must be within the stroke range, and the allowable carrying capacity of the machine tool table, and whether the workpiece interferes with the machine tool exchange tool space, A series of problems such as interference with machine tool protective covers and other accessories.

The main motor power of the CNC machine tool can also have various configurations on the machine tools of the same specification. In general, it reflects the cutting rigidity of the machine tool and the high-speed performance of the spindle. For example, the power of the spindle motor 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 the general machining center is (4000-8000) r / min, the high-speed machine tool vertical machine tool can reach (20000-70000) r / min, and the horizontal machine tool (10000-20000) r / min. The motor power has also increased exponentially. The spindle motor power 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 less used, and the AC power-adjustable motor with a relatively large power is often used to directly connect the spindle, or even the electric spindle structure. This structure has limited torque at low speed, that is, the output power of the speed-regulated motor decreases at low speeds. In order to ensure low-speed output torque, a high-power motor must be used. Therefore, the spindle motor of the CNC machine tool of the same specification is several times larger than the ordinary machine tool. When there are a lot of low-speed machining on some typical workpieces of the unit, the low-speed output torque of the selected machine tool must also be checked. Light machine tools are definitely cheaper in price, requiring users to choose the machine tool based on their typical workpiece blank size, cutting capacity (metal removal per unit time), required processing accuracy, and what kind of tools can be configured.

In recent years, the high-speed trend of CNC machine tools has developed rapidly. The spindle has changed from thousands to tens of thousands of revolutions per minute. The linear coordinate rapid movement speed has increased from (10-20) m / min to more than 80m / min. Of course, the price of machine tools has also increased. With a corresponding increase, user units must make reasonable choices based on their technical capabilities and supporting capabilities. For example, the maximum speed of the spindle on a vertical machining center can reach (50000-80,000) r / min. Except for some special processing cases, the matching tools are generally very expensive. Some high-speed lathes can reach more than (6000-8000) r / min, and the configuration requirements of turning tools are also very high.

For a small number of special workpieces, machining with only three linear coordinates cannot meet the requirements. It is necessary to add rotary coordinates (A, B, C) or additional work coordinates (U, V, W), etc. These requirements can currently be met on the machine tool market. However, the price of machine tools will increase a lot, especially for some requirements of multi-axis simultaneous processing, such as four-axis and five-axis simultaneous processing, the corresponding supporting programming software and measurement methods must be fully considered and arranged.

Fourth, the choice of machine tool accuracy

The precision requirements of the key parts of typical parts determine the accuracy grade of the CNC machine tool. According to the application, CNC machine tools are divided into simple type, full-function type, ultra-precision type, etc., and the accuracy they can achieve is also different. The simple type is currently used in some lathes and milling machines. Its minimum motion resolution is 0.01mm, and its motion accuracy and machining accuracy are both above (0.03-0.05) mm. The ultra-precision type is used for special processing, and its accuracy can reach 0.001mm or less. Here mainly discusses the most widely used full-function CNC machine tools (mainly machining centers).

According to the accuracy, it can be divided into ordinary type and precision type. The general CNC machine tool accuracy inspection items have 20-30 items, but the most characteristic items are: single-axis positioning accuracy, single-axis repeat positioning accuracy and more than two axis linkage processing The roundness of the test piece.

Positioning accuracy and repeated positioning accuracy comprehensively reflect the comprehensive accuracy of each moving part of the axis. Especially the repeated positioning accuracy, which reflects the positioning stability of the axis at any positioning point in the stroke, which is the basic index to measure whether the axis can work stably and reliably. 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 drive chain. For example, the variable factors such as the gap, elastic deformation and contact stiffness of each link of the transmission chain often reflect different instantaneous amounts of motion with the load of the table, 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 element after the measurement element is affected by various accidental factors and also has a large random error, such as the actual positioning position of the worktable caused by the thermal extension of the ball screw Drift, etc. In short, if you can choose, then choose the equipment with the best repeat positioning accuracy!

The accuracy of milling cylindrical surfaces or the milling space spiral groove (thread) is an index that comprehensively evaluates the servo following motion characteristics of the machine tool's CNC axis (two-axis or three-axis) and the interpolation function of the CNC system. The evaluation method is to measure the circle of the cylindrical surface. degree. There are also milling rhombic four-sided machining methods in the trial cutting parts of CNC machine tools, and the accuracy of the two controllable axes during linear interpolation movement can also be judged. When doing this trial cutting, the end mill used for finishing is installed on the spindle of the machine tool, and the round test piece placed on the worktable is milled. For small and medium-sized machine tools, the round test piece is generally taken from Ф200 to Ф300 Then, put the cut test piece on the roundness meter to measure the roundness of the processed surface. The obvious milling cutter vibration on the milled cylindrical surface reflects the unstable interpolation speed of the machine tool; the roundness of the milling has an obvious ellipse error, which reflects the mismatch of the gains of the two controllable axis systems of the interpolation movement; on the circular surface There are traces of tool stop points on the points of each controllable axis movement reversal direction (in continuous cutting motion, the tool will stop forming feed at a certain position will form a small trace of multi-cut metal on the machining surface) , Reflecting that the forward and reverse clearances of this axis are not adjusted well.

Single axis positioning accuracy refers to the error range when positioning at any point within the stroke of the axis. It directly reflects the processing accuracy of the machine tool, so it is the most critical technical index of CNC machine tools. At present, countries around the world have different regulations, definitions, measurement methods and data processing on this indicator. In the introduction of various CNC machine tool sample materials, the commonly used standards are the United States