Master of numerical control slogan

Present situation and development trend of CNC lathes in China

Author: Zhang Caihong, Science and Technology Department of Baoji Machine Tool Factory, translated from: CAD/CAM and manufacturing informatization.

Numerical control machine tool is a mechatronics product integrating mechanical, electrical, hydraulic, pneumatic, microelectronics and information technologies. It is a machine tool with high precision, high efficiency, high automation and high flexibility in mechanical manufacturing equipment. The technical level of CNC machine tools and its percentage in the output and total possession of metal cutting machine tools are one of the important symbols to measure the national economic development and the overall level of industrial manufacturing. CNC lathe is one of the main varieties of CNC machine tools, which occupies a very important position in CNC machine tools. For decades, it has been highly valued by countries all over the world and developed rapidly.

CNC lathes in China entered the market in the early 1970s. Up to now, through the unremitting efforts of major machine tool manufacturers, cooperation with famous foreign machine tool manufacturers, joint venture, technology introduction, prototype digestion and absorption and other measures, the manufacturing level of machine tools in China has been greatly improved, and its output accounts for a large proportion of metal cutting machines. At present, the varieties and specifications of domestic CNC lathes are relatively complete, and the quality is basically stable and reliable, which has entered the stage of practical and all-round development.

According to different market positioning and demand, CNC lathes can be divided into economical CNC lathes and full-function CNC lathes. Based on the experience and practice of developing and producing CNC lathes in Baoji Machine Tool Works for decades, this paper discusses the present situation of CNC lathes in China and the overall development trend of CNC lathes in the future.

First, the status quo of CNC lathe

1. Bed and guide rail

(1) bed

The bed of the machine tool is the basic support of the whole machine tool and the main body of the machine tool. It is generally used to place important parts such as guide rails and headstock. The structure of the lathe bed has a great influence on the layout of the machine tool. According to the relative position between the guide surface of the lathe bed and the horizontal plane, the lathe bed has five layout forms as shown in figure 1. Generally speaking, inclined lathe bed and flat lathe bed inclined slide plate are mostly used in small and medium-sized CNC lathes. Only large CNC lathes or small precision CNC lathes use flat lathe beds, while vertical lathe beds are rarely used. The flat bed has good manufacturability and is easy to process and manufacture. Because the tool rest is placed horizontally, it is beneficial to improve the motion accuracy of the tool rest, but the space under the lathe bed is small and it is difficult to remove chips. The cross slide of the tool rest is longer, which increases the width and size of the machine tool and affects the appearance. The flat bed adopts inclined sliding plate structure and inclined guide rail protective cover, which not only maintains the advantages of good manufacturability of the flat bed, but also keeps the width of the bed not too large.

It's huge. Modern CNC lathes widely use inclined bed and flat bed inclined slide plate structure, because this layout has the following characteristics:

☆ Easy to realize mechatronics;

☆ The machine tool is neat and beautiful, and covers a small area;

☆ It is easy to set a closed protection device;

☆ The chip removal is convenient, and an automatic chip removal device is installed;

☆ Hot chips cut from the workpiece will not accumulate on the guide rail and affect the accuracy of the guide rail;

☆ Good comfort and easy operation;

☆ Easy to install manipulator and realize single machine automation.

For example, CJK6 140H series simple CNC lathes designed and produced by Baoji Machine Tool Factory adopt flat bed and smooth plate structure; CK75 series full-function CNC lathe adopts the structure of inclined slide plate with backward inclined bed. The CK535D fully-functional CNC inverted vertical lathe we just developed adopts the structure of vertical slide plate with vertical bed. The machine tool adopts high-power built-in motorized spindle structure, and the spindle can move along X and Z axes to realize automatic loading and unloading function. The machine tool is equipped with an automatic rotating tool magazine to realize single machine automation, and the machine tool can be easily added to the production line.

A) rear inclined bed-inclined skateboard b) upright bed-upright skateboard

C) flat bed-smooth plate d) front inclined bed-smooth plate

E) flat inclined skateboard

Figure 1 bed layout

② guide rail

The guide rail of lathe can be divided into sliding guide rail and rolling guide rail.

The sliding guide rail has the advantages of simple structure, convenient manufacture and high contact rigidity. However, the traditional sliding guide rail has large friction resistance, fast wear and large difference between static and dynamic friction coefficients, and it is easy to crawl at low speed. At present, CNC lathes do not use traditional slide rails, but use slide rails with wear-resistant tapes and new plastic slide rails. They have good friction performance and long service life.

The rigidity of the guide rail, whether it is simple to manufacture, whether it can be adjusted, whether the friction loss is minimum and whether the initial accuracy of the guide rail can be maintained depend largely on the cross-sectional shape of the guide rail. The cross-section shape of lathe sliding guide rail often adopts mountain-shaped cross-section and rectangular cross-section. Mountain section, as shown in figure 2(a). This kind of cross-section guide rail has high guiding precision, and it will automatically compensate by its own weight when worn. The convex shape of the lower guide rail is beneficial to pollutant discharge, but it is not easy to preserve oil. Rectangular dynamic speed change. The main drive system of full-function CNC lathe mostly adopts stepless speed change. At present, there are two kinds of continuously variable transmission systems: variable frequency spindle system and servo spindle system. Usually, DC or AC spindle motor drives the spindle to rotate through belt drive or through belt drive and reduction gear in the spindle box (to obtain greater torque). Because of the wide speed range and stepless speed regulation of spindle motor, the structure of spindle box is greatly simplified. The spindle motor can output full power and maximum torque at rated speed.

3. Tool rest system

CNC lathe tool rest is an important part of machine tool. The tool rest is used to hold the tool, so its structure directly affects the cutting performance and efficiency of the machine tool. The structure and performance of the tool rest reflect the design and manufacturing technical level of the machine tool to a certain extent. With the continuous development of CNC lathes, the tool structure is constantly being renovated.

The tool rest is the executive part that directly completes cutting, so the tool rest must have good strength and rigidity in structure to bear the cutting resistance during rough machining. Because the cutting accuracy depends on the position of the tool tip to a great extent, it is required that the CNC lathe choose a reliable positioning scheme and a reasonable positioning structure to ensure a high repeated positioning accuracy. In addition, the design of the tool rest should also meet the requirements of short tool changing time, compact structure, safety and reliability.

According to the different tool changing methods, the tool rest system of CNC lathe mainly includes rotary tool rest, row tool rest and automatic tool changing device with tool magazine.

(1) rest of the world tool rest

Tool rest is generally used for small CNC lathes, mainly used for processing strip or disc parts. Its structural form is that a tool rest for clamping various kinds of tools is arranged on a transverse sliding plate along the X-axis direction of the machine tool. The typical arrangement of cutters is shown in Figure 4. This kind of tool rest is very convenient in tool arrangement and machine tool adjustment, and can arbitrarily combine tools with different uses according to the turning process requirements of specific workpieces. After a tool completes the turning task, as long as the transverse sliding plate moves a preset distance along the X axis according to the program, the second tool will reach the machining position, thus completing the tool changing action of the machine tool. This tool changing method is quick and time-saving, which is beneficial to improve the production efficiency of machine tools. CK7620P full-function CNC lathe produced by Baoji Machine Tool Factory is equipped with cutter head.

Fig. 4 Row tool rest

② Rotary tool rest

Rotary tool rest is a typical tool holder commonly used in CNC lathes. The automatic tool change of machine tool is realized by the rotary indexing and positioning of tool rest. According to the machining requirements, it can be designed as a square, hexagonal or disc tool holder, and 4, 6 or more tool holders are installed accordingly. The tool changing action of rotary tool rest can be divided into several steps, such as tool rest lifting, tool rest indexing and tool rest locking. Its action is completed by the instructions issued by the numerical control system. According to the relative position between the rotary shaft of the tool rest and the installation bottom surface, the rotary tool rest can be divided into vertical tool rest and horizontal tool rest. CJK6 140H series simple CNC lathes produced by Baoji Machine Tool Factory are equipped with four-station vertical tool rest or six-station horizontal tool rest, while CK75 series full-function CNC lathes are equipped with eight-station or 12-station horizontal tool rest, as shown in Figure 5.

Fig. 5 Horizontal rotary tool rest

(3) Automatic tool changing device with tool magazine

Too many tools cannot be installed in the above-mentioned row tool rest and rotary tool rest. Even if equipped with two tool holders, there are certain restrictions on the number of tools. When a large number of tools are needed for some reason, an automatic tool changing device with a tool magazine should be adopted. The automatic tool changing device with tool magazine consists of tool magazine and tool changing mechanism.

4. Feed conveying system

The feed servo system is generally used in the feed transmission system of CNC lathe. This is also the special point that CNC lathe is different from ordinary lathe.

The servo system of CNC lathe is generally composed of drive control unit, drive element, mechanical transmission part, actuator and detection feedback link. The drive control unit and the drive element form a servo drive system. Mechanical transmission parts and executive parts constitute a mechanical transmission system. The detection element and the feedback circuit form a detection system.

Feed servo system can be divided into open-loop system and closed-loop system according to different control modes. The closed-loop control mode is usually a servo system with position feedback. According to the position of the position detection device, the closed-loop system is divided into semi-closed-loop system and full-closed-loop system. There are two types of semi-closed loop systems: position detection devices are installed at the ends of lead screws and motor shafts. The former includes the lead screw in the positioning ring, while the latter completely places the mechanical transmission parts outside the positioning ring. The position detection device of the full closed-loop system is installed on the workbench, and the mechanical transmission part is included in the position ring.

The positioning accuracy of open-loop system is lower than that of closed-loop system, but it has simple structure, reliable operation and low cost. Because of the wear, inertia and clearance of the mechanical transmission device that affects the positioning accuracy, the accuracy and rapidity of the open-loop system are poor.

The closed-loop system has high control precision and good rapidity. However, because the mechanical transmission parts are in the control loop, the dynamic performance of the system depends not only on the structure and parameters of the driving device, but also on the stiffness, damping characteristics, inertia, clearance and wear of the mechanical transmission parts. Therefore, the structural parameters of electromechanical components must be considered comprehensively to meet the requirements of the system. Therefore, the fully closed-loop system has higher requirements for machine tools and higher costs. The position detection devices used in the closed-loop system include: pulse encoder, resolver, inductosyn, magnetic ruler, grating ruler and laser interferometer.

Servo motor is the common driving device in the feed servo system of CNC lathe. Servo motors can be divided into DC servo motors and AC servo motors. Because of its high reliability, AC servo motor is basically unnecessary.

It is widely used because of its low maintenance and cost.

Second, the development trend of CNC lathe

The application of numerical control technology has not only brought revolutionary changes to the traditional manufacturing industry, making the manufacturing industry a symbol of industrialization, but also played an increasingly important role in the development of some important industries (IT, automobile, light industry, medical care, etc.). With the continuous development of numerical control technology and the continuous expansion of application fields, the digitalization of equipment needed in these industries has become a major trend of modern development. At present, CNC lathes present the following development trends.

1. High speed and high precision

High speed and precision are the eternal goals of machine tool development. With the rapid development of science and technology, the upgrading speed of mechanical and electrical products is accelerated, and the requirements for machining accuracy and surface quality of parts are getting higher and higher. In order to meet this complicated and changeable market demand, machine tools are developing in the direction of high-speed cutting, dry cutting and quasi-dry cutting, and the machining accuracy is constantly improving. On the other hand, the successful application of motorized spindle and linear motor, the introduction of ceramic ball bearing, low-temperature high-speed ball screw pair with high precision and large lead hollow internal cooling and ball nut strong cooling, and linear guide rail pair with ball cage have also created conditions for the high-speed and precise development of machine tools.

The CNC lathe adopts electric spindle, which eliminates the links of belt, pulley and gear, greatly reduces the moment of inertia of the main drive, improves the dynamic response speed and working accuracy of the spindle, and completely solves the vibration and noise problems of the belt and pulley drive when the spindle runs at high speed. The motorized spindle structure can make the spindle speed reach more than 10000r/min.

Linear motor has high driving speed, good acceleration and deceleration characteristics, excellent response characteristics and following accuracy. The linear motor is used as the servo drive, which saves the intermediate transmission link of the ball screw, eliminates the transmission gap (including the reverse gap), has small inertia, good system rigidity, and can accurately position at high speed, thus greatly improving the servo accuracy.

Because of its zero clearance in all directions, the linear rolling guide pair has very small rolling friction, little wear, negligible heating and excellent thermal stability, which improves the positioning accuracy and repeated positioning accuracy in the whole process.

Through the application of linear motor and linear rolling guide pair, the fast moving speed of machine tool can be increased from the current 10 ~ 20m/mim to 60 ~ 80m/min, even as high as 120m/min.

2. High reliability

The reliability of CNC machine tools is the key index of product quality of CNC machine tools. Whether the CNC machine tool can exert its high performance, high precision and high efficiency and achieve good benefits depends on its reliability.

3. CAD of CNC lathe design and modularization of structure design.

With the popularization of computer application and the development of software technology, CAD technology has been widely developed. CAD can not only replace manual drawing, but also select, analyze, calculate, predict and optimize static and dynamic characteristics of large machinery, and dynamically simulate all working parts of machinery. On the basis of modularization, we can see the three-dimensional geometric model and realistic color of the product in the design stage. Using CAD can also greatly improve the work efficiency and the first success rate of design, thus shortening the trial production cycle, reducing the design cost and improving the market competitiveness.

Modular design of machine tool parts can not only reduce repetitive work, but also quickly respond to the market and shorten the product development and design cycle.

4. Functional compounding

The purpose of functional combination is to further improve the production efficiency of machine tools and minimize the time used for non-machining assistance. Through the combination of functions, the use range of machine tools can be expanded, the efficiency can be improved, and one machine can be used for multiple purposes, that is, a CNC lathe can realize both turning function and milling processing; Or grinding can be realized on a machine tool mainly based on milling. Baoji Machine Tool Factory has successfully developed CX25Y CNC turning and milling compound center, which has X-axis, Z-axis, C-axis and Y-axis. Plane milling and machining of offset holes and grooves can be realized through C axis and Y axis. The machine tool is also equipped with a powerful tool rest and an auxiliary spindle. The auxiliary spindle adopts a built-in motorized spindle structure, and the speed synchronization of the main spindle and the auxiliary spindle can be realized directly through the numerical control system. The workpiece of this machine tool can be processed by one-time clamping, which greatly improves the efficiency.

5. Intelligent, networked, flexible and integrated

2 1 century numerical control equipment will be a certain intelligent system. The content of intelligence includes all aspects of numerical control system: in order to pursue the intelligence of machining efficiency and machining quality, such as adaptive control of machining process and automatic generation of process parameters; In order to improve the driving performance and connection intelligence, such as feedforward control, adaptive operation of motor parameters, automatic identification of load, automatic selection of model, self-correction and so on. Simplify programming and simplify the intelligence of operation, such as intelligent automatic programming and intelligent man-machine interface; There are also intelligent diagnosis and intelligent monitoring to facilitate the diagnosis and maintenance of the system.

Networked numerical control equipment is a hot spot in the development of machine tools in recent years. The networking of CNC equipment will greatly meet the information integration needs of production lines, manufacturing systems and manufacturing enterprises, and it is also the basic unit to realize new manufacturing modes such as agile manufacturing, virtual enterprise and global manufacturing.

The development trend of CNC machine tools to flexible automation system is: from point (CNC single machine, machining center and CNC compound machining machine tools) and line (FMC, FMS, FTL, FML) to surface (workshop independent manufacturing island, FA) and body (CIMS, distributed network integrated manufacturing system), on the other hand, it pays attention to application and economy. Flexible automation technology is the main means for manufacturing industry to adapt to dynamic market demand and update products quickly. It is the mainstream trend of manufacturing development in various countries and the basic technology in advanced manufacturing field. Its key point is to improve the reliability and practicability of the system, aiming at easy networking and integration, and focusing on strengthening the development and improvement of unit technology. CNC single machine is developing towards high precision, high speed and high flexibility. CNC machine tools and their flexible manufacturing systems can be easily connected with CAD, CAM, CAPP and MTS, and will develop towards information integration. Network system is developing towards openness, integration and intelligence.