How to choose horizontal machining center

11-11-2019

0 Preface


With the advancement of science and technology, the upgrading of products from all walks of life is very fast, which has driven the technological transformation of production lines and the enhancement of production capacity of bottleneck processes. The machining center has also become the main equipment updated by manufacturing enterprises and research institutes. The variety, performance, quality and technical level of the product directly affect the performance, quality, production technology level and economic benefits of other electromechanical products. How to choose a suitable horizontal machining center from a wide variety of expensive equipment, how to correctly and reasonably purchase accessories, tools, software technology, after-sales technical services and other supporting equipment, all of which are reflected in the equipment selection and processing Determination of center parameters and procurement of required processing centers through tendering. This article mainly discusses the selection of milling processing center and the determination of bidding parameters.


1 Selection of machining center


The machining center is an efficient automatic machine tool for machining complex parts, which is composed of mechanical equipment and numerical control system. Because it is equipped with a tool magazine and automatic tool changer, the workpiece can be automatically milled on more than two surfaces after one clamping. Drilling, boring, reaming, tapping and other processes, and automatic tool change can be completed during processing, so the production efficiency and degree of automation are greatly improved. Before selecting a machining center, an enterprise must conduct a full investigation and demonstration. When selecting a machining center, many factors must be considered, including factors such as workpiece processing specifications, machining accuracy, production efficiency, investment conditions, economic benefits, and technical services. The demonstration must be carried out with the joint participation of the technology department, equipment maintenance department, user department, and technical transformation department, so that the equipment's processability, processing accuracy, production efficiency, reliability, maintainability, and economics are in line with the development of the superior department and unit Demand. After the demonstration report is approved, relevant personnel should be organized to select the products of 3 to 5 manufacturers for detailed comparison to form the technical parameters of the bidding documents.


The main components of a general machining center are shown in Table 1. The main technical parameters and configuration are shown in Table 2. The content covers the dimensions and weight parameters, spindle unit parameters, feed parameters, accuracy indicators, tool magazine parameters, and control system related to workpiece processing. And related configurations. The equipment selected and demonstrated shall include the contents in Table 1 and Table 2 and cover the main technical parameters of more than three equipment.




2 machining center parameter selection


2.1 Selection of specifications


The main specifications of CNC machine tools are the travel 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 tool. Therefore, the size of the work surface basically determines the size of the machining space. The work surface should be selected slightly larger than the typical workpiece, in order to take into account the space required to install the fixture. In addition, it is necessary to consider a series of problems such as the allowable bearing capacity of the machine tool table, the space interference between the workpiece and the machine tool exchange, and the interference with the accessories such as the machine tool cover. For a vertical or horizontal machining center with a turntable, the minimum distance between the nose of the spindle to the table or the minimum distance from the center of the spindle to the table and the minimum distance from the end of the spindle to the center of the table should be increased in the specifications to ensure that


The machining requirements of typical workpieces are consistent.


2.2 Selection of accuracy parameters


(1) Positioning accuracy and repeated positioning accuracy Positioning accuracy and repeated positioning accuracy are very important accuracy indexes, which depend on the geometric accuracy of CNC machine tools, the accuracy of the measuring system of the machine tool, the accuracy and stiffness of the feed control system, and their dynamic characteristics. factor. Generally, there are 20 to 30 precision inspection items for CNC machine tools, but the most characteristic items are the single-axis positioning accuracy, single-axis repeated positioning accuracy and roundness of the test piece processed in two or more axes. The positioning accuracy and repeated positioning accuracy comprehensively reflect the comprehensive accuracy of the moving parts of the axis, especially the repeated positioning accuracy. It reflects the positioning stability of the positioning point of the axis within the stroke, which is a measure of whether the axis can work stably and reliably. Basic indicators. At present, the software in the numerical control system has rich error compensation functions, which can stably compensate system errors in each link of the feed transmission chain. The milling accuracy is used to comprehensively evaluate the follow-up motion characteristics of the CNC machine's servo system related to the CNC axis, and it is an indicator of the CNC system's interpolation function. When selecting the accuracy of the machining center, it should be noted that the positioning accuracy is not the processing accuracy of the machine tool, but the accuracy that the machine can achieve can be estimated from the positioning accuracy of the machine tool. The positioning error of this axis is 1 to 2 times (the specific error value is closely related to process factors). Ordinary machining centers can already process 6 to 7-level precision workpieces, and precision machining centers can process 5-level precision workpieces.



(2) Selection of accuracy standards


At present, the regulations, definitions, measurement methods and data processing of the accuracy index differ from country to country. In the introduction of various types of CNC machine tool samples, the commonly used standards are the German VDI / DGQ3441 standard, the US NMT BA standard, the international ISO230 standard, the Japanese JIS B standard, the national standard GB / JB standard, and the manufacturer's enterprise standard, etc. Etc. There is a difference in the accuracy values in evaluation and testing. In terms of position accuracy, some use the statistical method and some use the range method. In principle, the JIS standard specifies the loosest accuracy, and the VDI standard requires the strictest. Therefore, when selecting a model, you should pay attention to knowing what accuracy standards the manufacturer uses, understand whether the control method of system control position accuracy uses direct measurement (closed-loop control) or indirect position measurement (semi-closed-loop control), and understand the CNC system and control. Precision resolution to ensure that the necessary machining accuracy is met.


(3) Geometric accuracy


The geometric accuracy of the machine tool is the basis for ensuring the accuracy of the workpiece processed by the machine tool. Generally, it does not appear in the technical parameters of the tender, but appears in the acceptance clause of the machine tool. However, in order to ensure the accuracy of the machining center, the manufacturer is often required to provide a geometric accuracy inspection report of the delivered user to examine the geometric accuracy and accuracy margin of the bid machining center.


(4) Verification of cutting accuracy.


The verification of the cutting accuracy of the machine tool can be processed according to the inspection items provided by the manufacturer or the test cutting drawings confirmed by both parties. After processing, the dimensional accuracy, shape error, position error and surface roughness of the workpiece are inspected.


2.3 Spindle and drive


The performance index of the spindle and drive determines the cutting performance of the machine. The power and torque of the spindle motor reflects the cutting efficiency of the machining center, and also reflects the cutting rigidity of the machining center and the overall rigidity of the machine tool from one side. Processing is performed by tools. According to the typical workpiece processing scheme, the shank model, spindle unit and drive unit of the tool are selected.


(1) Model of tool holder


The shank model depends on the size of the shank hole of the machine tool spindle. At present, most of the machining center machine tool spindle holes use 7:24 tapered holes specified by ISO. Commonly used are No. 40, No. 45, No. 50, etc., and some are No. 30 and No. 35. The smaller the size of the machine tool, the smaller the size of the shank should be. The small size of the shank is not good for machining large holes and long holes. Therefore, if a machine tool has a large size of shank to choose from, it should be as large as possible. , But the magazine capacity and tool change time are the affected factors to be considered. In recent years, machining centers and CNC milling machines have been moving towards higher speeds. Many experimental data show that when the spindle speed exceeds 10000r / min, the 7:24 cone hole will increase due to the centrifugal force, which will affect the positioning accuracy of the tool holder. Therefore, it is recommended to use the short taper tool holder HSK series recommended by German VDI for high-speed spindles. For the same type of tapered tool holder, there are Japanese BT standard, American CAT standard, German VDI standard, etc. The size of the manipulator caliper they specify is different, and the size of the tightening nail of the tool holder is also different. Must consider all. For users who already have a certain number of CNC machine tools or users who are about to purchase a batch of CNC machine tools, they should choose a single standard tool holder series that is universal to each other.


(2) Spindle unit


The spindle motor power can have various configurations on machining centers of the same specifications. Users should according to their own typical parts blank size, cutting capacity (metal cutting amount per unit time), required machining accuracy, and actual tools that can be configured And other factors. At present, the spindle speed of general machining centers is 4000 ~ 8000r / min, vertical high-speed machining centers can reach 20,000 ~ 70000r / min, and horizontal high-speed machining centers can reach 10,000 ~ 20000r / min. The power of the spindle motor is also doubled. Big. 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.


Focus on the spindle unit speed-power-torque graph, and pay attention to continuous power, torque, and 30-minute power and torque. Pay attention to the cooling requirements of the spindle when selecting a high-speed spindle.


(3) Feeding speed


Increasing the speed of machine tools is the current development trend of the machine tool industry. High speed, small cutting depth and large feed are the development direction of high speed machining. For high-speed cutting, both a high spindle speed and a feed speed that matches the spindle speed are required. At present, the trend of high-speed machining centers is developing rapidly. The spindle speed changes from thousands to tens of thousands of revolutions per minute. The rapid moving speed of linear coordinates has increased from 10 to 20m / min to more than 80m / min. Of course, its functional components such as electric spindles and linear The prices of motors, linear rolling guides, spindle bearings, and related accessories such as encoders, knives, etc. have also risen accordingly, and they are even very expensive. Therefore, users must make reasonable choices based on their own technical and supporting capabilities, and the relevant provisions of the bidding documents must be relevant to their own reality.


2.4 CNC system selection


The CNC system is directly related to the overall benefits of the machine tool. When purchasing a machining center, users can ask the manufacturer to configure their own CNC system. Well-known CNC systems include Japan's FANUC system, Germany's HEIDENHAIN and SIEMENS systems. Each CNC system has many functions, such as random programming, motion graphics display, human-machine dialog editing, and fault management. When selecting system functions, we must comprehensively configure the functions that we need now, and also consider the long-term needs, but we must not blindly pursue the comprehensive functions and cause waste. In addition, there are many selection functions in addition to the basic functions in the CNC system. Users can choose some additional functions according to their own workpiece processing requirements, measurement requirements, and programming requirements.


In the annex to the order contract, especially the DNC function and communication interface transmitted in real time, and the forward-looking function of the high-speed CNC machining system.


2.5 Tool magazine selection


The automatic tool changer is the basic condition to ensure the multi-process centralized processing of the machining center. The tool magazine capacity, tool change time, and failure rate are all issues that users are very concerned about. The investment of the automatic tool changing device accounts for 30% to 50% of the whole machine, and its failure rate also accounts for more than 50% of the machine's failure rate. Therefore, when selecting an automatic tool changer, it is necessary to choose a device with a simple structure and good reliability on the premise of meeting the requirements for use. The tool magazine capacity should be determined based on the number of tools required to complete the entire process in one setup. For a machining center used by a single machine, its tool magazine capacity should not be too large, because in use, each time a new part is replaced, the operator must organize the tools according to the process requirements. The more unrelated tools in the tool magazine, the more workload The bigger. In addition, large-capacity tool magazines have high costs, complicated structures, high failure rates, and correspondingly complicated tool management. Generally, the tool magazine capacity of the vertical machining center is about 20, and the horizontal machining center is about 40.


2.6 Selection of accessories


Accessories for machining centers such as tool presetters, automatic measuring devices, tool monitoring systems, cutting condition monitoring devices, adaptive control devices, various diagnostic devices, etc. These optional functions often appear in large numbers on quotations of imported machine tools. Selecting some of these devices has a great effect on improving the efficiency and quality of the machining center. However, pay attention to its practicality and reliability when selecting. In addition, some accessories must be selected, such as machine tool cooling protection device, chip removal device, spindle oil temperature control device, etc. The use of these devices will result in excellent processing performance and stable processing quality, while also extending the service life of the machine tool.


2.7 Working conditions


Equipment installation and working environment conditions are the prerequisites for the smooth installation and processing of the machining center. They are the practical questions put forward by the user to the manufacturer. The equipment's power supply conditions, gas source conditions, and environmental conditions are clarified to ensure that the equipment can be smoothly installed Installation, commissioning, and acceptance.


2.8 Service and Quality Assurance Terms


General equipment suppliers have better service terms and quality assurance systems, but it is inevitable to differ from the user's requirements. Therefore, the service and quality of purchased products should be clearly stated. The general terms are as follows:


(1) The seller is responsible for free installation and commissioning at the buyer's site, and free training for operation and maintenance personnel;


(2) After the acceptance of the machine tool, the control system provides two years and one year warranty for the machine tool, free spare parts and services are provided, and the service must be timely and effective;


(3) After the warranty period, we can still provide comprehensive preferential technical support and spare parts supply.


3 Conclusion


The selection of a machining center is a highly specialized technical project that involves all aspects of the unit. In order to increase the scientificity of the selection and avoid mistakes, there should be process personnel, product designers, operators, maintenance personnel and Relevant authorities of the unit participated. Choosing the right model and setting up the equipment parameters is not an easy task. It needs to be based on research, comparison, and summarization in order to extract a complete set of technical parameters and achieve the rigorousness of indicators during the bidding process. During the review process, the controllability of the technical quality of the equipment and the strictness of the indicators are reflected in the control of the technical parameters in Table 2. General specifications, accuracy indicators, spindle speed, spindle power, spindle torque, and simultaneous control The number of axes, the scale brand and resolution can be set as key parameters. The controllability of technical quality is reflected in the details of the structure and configuration. The specifications of the equipment can be the same, but the structural details are still different, which reflects the ability of the selection personnel to inspect and compare similar equipment.


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