Linear motor for high speed machine tools and precision high speed ball screw

CNC machine tools, which are the core processing equipment of the equipment manufacturing industry, are developing in the direction of high speed, high efficiency, high precision, intelligence, compounding and environmental protection. In high-speed and ultra-high-speed machining, high dynamic characteristics and control accuracy are required; instantaneous high speed and instantaneous stop in high-speed operation; low vibration, low noise, stable operation; high reliability and long service life. Among various linear drive components, Precicion High-speed Ball Screws (PHS-BS) and AC Linear Motor (AC-LM) are large, precise, and high-speed. The core functional components of the fast servo feed system of CNC equipment that meet the above requirements.

AC linear motor entering the field of numerical control equipment

A linear motor is a driving device that directly converts electric energy into linear motion by means of the principle of electromagnetic action. The world's first linear motor was invented by British physicist Sir Charles Wheatstone and patented in 1845. Originally based on high-speed transportation and traction, after continuous improvement, the scope of application has gradually expanded to computer and office equipment, semiconductor manufacturing equipment, medical equipment, industrial automation, automatic plotter and so on. There are many types of linear motors depending on the application. In recent years, a high-speed linear drive component combined with a linear motor and an aluminum alloy roller guide has emerged. Vmax=600m/min, acceleration 4g, applied in the case of fast grabbing and laying down. Figure 1 shows the LMS and LMC small thrust servo linear motor of HIWIN Corporation of Taiwan, Vmax=210m/min.


Figure 1 LMS, LMC servo linear motor from HIWIN, Taiwan, China
With the development of control technology, electronic technology and material technology, linear motors have experienced a process from rapid industrial load to fast and large thrust, from general industrial fields to large, precise and high-speed CNC equipment. In 1982, Ingersoll developed a high-speed machining center prototype driven by AC-LM. At EMO'93, Ex-cell-O exhibited the first inductive linear motor (Indramat) on each axis. XHC240 horizontal machining center, each axis fast moving speed 80m / min, acceleration 1g, positioning accuracy 0.004mm. The above companies took the first step in the application of AC-LM on CNC machine tools. As a high-speed, high-precision, high-stiffness direct drive system, AC-LM has attracted great attention from the industry. In the subsequent EMO'97, "European linear motor heat" was launched.

Although China began to study industrial linear motors in 1969, Zhejiang University and other units, the application of AC-LM on CNC machine tools has been the last ten years, Tsinghua University, Guangdong Institute of Technology, Shenyang University of Technology, Harbin Institute of Technology and other units. A lot of work has been done on the study of AC-LM for high-speed CNC machine tools. Tsinghua University undertook the national “10th Five-Year” science and technology project, and jointly launched “Linear Motor and Servo System for CNC Machine Tools” with Jiangsu Ruiante Company, which was exhibited at CCMT2004 (Shanghai) Exhibition. China's AC-LM for CNC machine tools is still in the growth stage, and there is still a certain distance from specialized production. In recent years, China's R&D high-speed, high-end CNC machine tools have begun to adopt AC-LM, but most of them are imported from abroad.

At the CIMT International Machine Tool Show, the author conducted a survey of 88 high-speed CNC machine tools (V ≥ 0m / min), 23 of which use AC-LM, accounting for 26% of the number of statistics. Well-known foreign companies such as DMG, Ex-cell-O, Ingersoll, CINCINNATI, GROB, MATEC, MAZAK, FANUC, SODICK, etc. have adopted AC-LM in their high-speed and high-end CNC machine tools, and have AC-LM as innovation and The highlight of market competition. According to senior industry experts, linear motor as an electromechanical system simplifies the mechanical structure and complicates the electrical control, which is in line with the development trend of modern electromechanical technology.

After the AC-LM entered the machine tool field, this new direct drive method revolutionized the traditional machine tool structure. As a core component of high performance and high speed, AC-LM will play an important role in the industrial chain of CNC machine tools, especially high-speed CNC machine tools.

Precision high-speed ball screw pair that challenges AC linear motors

The ball screw pair with mature manufacturing technology and industrial scale has been in existence for more than 100 years since its birth, and has a history of 46 years in China. Its product features continue to expand and improve with the development of the mainframe, from the initial "agile energy-saving transmission" (before the 1960s) to "precise positioning" (after the 1970s), and then from the "large lead" Driven (in the 1980s) to "precision high-speed drive" (in the mid-1990s), the development of "speed" and "precision" can be described as advancing with the times, especially in the 21st century, PHS-BS is rapidly Development, become the two dazzling stars in the CNC equipment servo rapid feed system and AC-LM.

When the ball screw and the ball nut are relatively displaced, the acceleration is ≥1g, the linear velocity is ≥60m/min, or the DN value is ≥120000 (the DN value is defined as: the ball screw pair nominal diameter do(mm) and the ball screw and The product of the relative rotation speed of the ball nut n (r/min), the accuracy reaches P3 or above (national standard GB/T18587), and the performance indicators (torque change, stiffness, temperature rise, noise, etc.) meet the requirements of the host, It can be called a precision high speed ball screw pair (PHS-BS).

Due to the excellent rolling friction characteristics and high transmission efficiency (η=90~95%) and agile and flexible functions of the ball screw pair, it has the advantage of speed increase. There are three ways to speed up: (1) increase the lead Pn, which is the large lead (or super-large lead) ball screw pair we often say. Since the increase of Pn is not conducive to improving the lead accuracy, and the static stiffness of the feed system is rapidly decreased, it is only suitable for fast driving applications where the positioning accuracy is not high. (2) Considering accuracy, speed, and dynamic characteristics, moderately increase Pn and n (rotation speed) and the number of threads. Because of the limitation of the critical speed Nc, the pursuit of high speed is not desirable. (3) Reforming and innovating from the product structure and driving method, for example, the main drive of the variable screw is the nut drive; the single drive is double drive; the screw is hollow, the nut is small, and the rolling element is lighter.

The emergence of PHS-BS originated in the mid-to-late 1990s. NSK Japan first introduced the HMC series dedicated to high-speed CNC machine tools. The diameter is Φ36~55 mm, Pn=16~32mm, Vmax=116m/min, acceleration 1.3g. , DN value = 130,000, accuracy C3 ~ C5 (JIS standard). In recent years, the world's well-known ball screw manufacturers have introduced a number of silent PHS-BS products with DN value ≥ 150,000 (220000), V ≥ 120m / min (200m / min), acceleration ≥ 1.5g, such as: Japan NSK The company's BSS and S3 series, THK's SBK series, Taiwan's HIWIN's Super S series, PMI's FSW series, Germany's Rexroth's FDM-E series, Spain's Korta's NTG series. The earliest Chinese research institute engaged in PHS-BS research was the Beijing Machine Tool Research Institute. In the “Ninth Five-Year Plan”, the development of the GSZ2000 high-speed ball screw integrated measuring instrument was completed. Subsequently, Shandong Jining Bote Company and Shandong University of Technology cooperated successfully. BTJS-03 high speed ball screw counter measuring instrument. At present, the level of domestic PHS-BS is: Vmax ≥ 60m / min (80m / min), acceleration 1.4g, accuracy ≥ P3 level, DN value ≈ 140000.


Figure 2 The μ1000 series vertical machining center of Beijing Machine Tool Research Institute Precision Electromechanical Co., Ltd. adopts PHS-BS to realize fast feed and high precision positioning.
To sum up, the new generation PHS-BS has the following characteristics:

(1) Optimized design of the cyclic return device. The experimental research proves that the cyclic return device is a key link that directly affects the fluency and dynamic characteristics, vibration and noise of the PHS-BS ball, which restricts the improvement of the DN value. The early PHS-BS used a thick-walled, in-line bead tube that allowed the ball nut helix to extend in abutment with the bead tube. In recent years, a new internal circulation structure has been popularized. It is equipped with an "end plug" return device at both ends of the spiral of the ball nut, so that the DN value reaches 200,000, the noise is reduced by 6-7 dB (A), and the nut radial size Reduce it by about 30%.

(2) Optimize the ball chain structure. In order to reduce the centrifugal force of the rolling elements during high-speed rotation, a small diameter ball, a Si3N4 ceramic ball or a DS modified ball is used. In order to block the rolling elements from colliding, squeezing and rubbing at high speed, an isolator made of special engineering plastics and having a lubricating function is added to the ball chain, thereby effectively reducing temperature rise and noise, and increasing the smoothness of the rolling elements.

(3) In order to reduce the influence of the "wedge effect" on the smoothness of the rolling element at high speed, optimize the geometric parameters of the inner and outer thread raceways, and reduce the roughness of the raceway surface, or modify the surface to improve the friction. characteristic.

(4) The ball screw is made hollow, and strong cooling is performed from the inside of the screw. Some companies also arrange damping rods in the inner holes to suppress vibration at high speeds. HIWIN's "Cool Type I" series also applies strong cooling to the ball nut.

(5) In recent years, Germany's Rexroth, INA, Japan THK and other companies have introduced high-speed linear drive devices for AC motors to directly drive ball Nuts (see the 12th issue of this issue for details).

(6) The screw and the nut are respectively driven by two AC motors, and by means of the superposition of the rotation speed and the direction of the rotation, the speed can be doubled and the micro feed can be realized.

(7) Make full use of the ball screw pair to complete the synchronous movement function. Double-motor double-screw drive mode is adopted to improve the stability of the servo feed system at high speed and improve the kinetic energy characteristics. For example, HPC- of CINCINNATI LAMB of the United States 800HP high speed horizontal machining center (see Figure 3). In addition, Germany DMG's DMC63H, Switzerland's DIXI's DHP-80-5x, Japan's Makino's A55E, China's Dalian Machine Tool Group's DHSC500, Ningjiang Machine Tool's NJ-5HMC40, etc., also use dual-motor, double-screw drive.


Figure 3 US CINCINNATI LAMB's HPC-800HP horizontal high-speed machining center X, Y, Z three-axis with dual PHS-BS drive, V = 80m / min, acceleration 1.5g. The double drive of the frame structure greatly improves the stability of rapid feed
According to the author's survey statistics and subsequent tracking at a CIMT international machine tool exhibition, it can be seen that PHS-BS is still the mainstream in mid-range high-speed CNC machine tools, and PHS-BS is also common in some high-end and high-speed CNC machine tools.

Double stars on the same stage, highlights


Germany DMG Company is famous for mass production of various high-performance numerical control equipments. AC-LM is adopted earlier in its servo feed system, and the adoption rate is also very high (both marked “Linear” after the machine model). There are three types of configurations for AC-LM and PHS-BS:

  • All axes are equipped with "quick" CNC equipment driven by AC-LM. For example: DMC85V Linear, DMC75V Linear, DMC105V Linear, DMC60H Linear, DMC80H Linear, and DML80-Fine Cutting laser processing machine.

  • Hybrid drive type. For example: DMF500 Linear moving column type large vertical machining center, AC-LM is arranged on X axis (stroke 5m), V=100m/min; PHS-BS is used on Y and Z axis, V=60m/min. In addition, CTV250, CTX300, CTX420, DMC104V, DMF220F, DMF360F, etc. are all hybrid drive types.

  • All coordinate axes are equipped with a "powerful" machining center driven by PHS-BS. For example: DMC63H high-speed horizontal machining center, X, Y, Z three axes are all driven by PHS-BS (Φ50, Pn=35), V=80m/min, acceleration 1g, positioning accuracy 0.008mm. In addition, there are DMC80H and DMC100H, DMC125H (duo BLOCK) and DMC60T.

    The two stars, AC-LM and PHS-BS, were used by DMG, and they appeared on the same stage in their leading products. The diversity of driving methods is also manifested in the same model and series of high-speed CNC equipment of the same enterprise. Different driving modes are configured according to different use occasions. For example, Germany's Grob's BZ500 is equipped with PHS-BS, while BZ500L is equipped with AC-LM; Japan's Otsuka Iron & Steel's MA-400HA is equipped with PHS-BS, while MAC-Star-400 is equipped with AC-LM; Germany's Hueller- Hille's Specht63 series is equipped with PHS-BS, while the Specht 500L series is equipped with AC-LM and more. DMG and some companies only configure AC-LM on long-stroke X-axis, which is intended to improve the dynamic characteristics and positioning accuracy of the machine while minimizing non-machining time and improving production efficiency. Other axes still use PHS-BS drive. Therefore, the cost performance of the machine tool is more attractive to the user. This “hybrid drive” model is an optimized combination of the “features” of the AC-LM and PHS-BS functions. It aims to maximize the user's individual needs and pursue the best price/performance ratio in innovation. This design concept can really bring tangible benefits to users. The “hybrid drive” model objectively reflects the diversity of market demand and reflects the highlights of the two stars.


    Table 1 Comparison of AC linear motor and precision high speed ball screw
    In fact, both AC-LM and PHS-BS have their own weaknesses, although they have their own advantages. Table 1 compares AC-LM and PHS-BS. It can be seen that both have the best range of application on CNC machine tools.

    The AC-LM drive is uniquely positioned in the following areas of CNC equipment:

    (1) High-speed, ultra-high-speed, high-acceleration and large-volume production, where there are many movements requiring positioning, frequent changes in speed and direction. For example, the production line of the automotive industry and the IT industry, the manufacture of precision and complex molds.

    (2) Large-scale, ultra-long-stroke high-speed machining centers, in the aerospace manufacturing industry, light-alloy, thin-walled, and metal-removed, integral components are “hollowed”. For example, the "Hyper Mach" machining center (46m) of CINCINNATI Company of the United States; the "HYPERSONIC 1400L ultra-high speed machining center of MAZAK Company of Japan (see Figure 4), AC-LM for x and y axes, and fast-forward speed of 120m/min. The entire aluminum block is hollowed out into aircraft parts (the Z axis still uses PHS-BS).

    (3) High dynamic characteristics, follow-up at low speed and high speed, and highly sensitive dynamic precision positioning. For example, a new generation of high-performance CNC electric machining machines represented by Sodick, CNC super-precision machine tools, a new generation of CPC crankshaft grinders, cam grinders, CNC non-garden lathes, etc.

    (4) Light-duty, fast special CNC equipment. For example, DMG80 Fine Cutting laser engraving and punching machine from DMG, Germany, "AXEL3015S" laser cutting machine from LVD of Belgium, and "Hyper Cear510" high-speed laser processing machine from MAZAK.


    Figure 4 Japan's MAZAK Hypersonic 1400L ultra-high speed gantry machining center. X and Y axes are driven by linear motor V=120m/min
    Let us analyze the best application of PHS-BS. Although the DN value of PHS-BS has experienced the speed-up process from 70,000 to 150,000 to 200,000 (220,000), it can be seen from Table 1 that due to the existence of soft ribs of pure mechanical transmission, its linear velocity, acceleration, and stroke The increase in range is always limited. Take the new generation of BSS and S3 series of NSK Japan as an example, the DN value is 220,000 in its product sample. If Φ40×20mm is used, Vmax=110m/min, because Nmax=5500 rpm Very high, the range of travel is obviously not too long due to the critical speed Nc. If a large lead Φ40×40mm product is used, Vmax=220m/min, which obviously cannot meet the occasion of high positioning accuracy. The DN value of 220,000 can reflect the design and manufacturing level of the company from one aspect. If we choose Φ40×20 (double head) mm product, it can be used in the state of n≈4000~5000 rev/min and V=80~100m/min, and its safety, reliability and working life can be higher than expected. In fact, so far, in the high-speed high-end CNC gold cutting machine (except CNC forming machine), the successful example of PHS-BS driving is still not required to require fast-forward speed V≥20m/min.

    According to the author's investigation and analysis: the best application of PHS-BS is: requires V=40~100m/min, acceleration 0.8~1.5(2.0)g, precision P3 level and above, mid-range high-speed CNC equipment and part with stroke range within 3m High-grade CNC equipment.

    Thoughts on revitalization and development

    Recently, in the "Several Opinions of the State Council on Accelerating the Revitalization of Equipment Manufacturing Industry", he proposed: "Developing large-scale, high-precision, high-speed CNC equipment and numerical control systems and functional components, and changing the status quo of large-scale, high-precision CNC machine tools that rely heavily on imports to meet machinery, The needs of industrial development such as aerospace." AC-LM and PHS-BS are important linear drive features in large, precision, especially high-speed CNC equipment. The "Special Plan for Numerical Control Machine Tool Development" (draft), which was developed by the National Development and Reform Commission, listed the development of high-speed machining functional components, high-speed drive technology, and precision machining technology as key technologies. Obviously, the localization and industrialization of major functional components such as CNC systems, electric spindles, AC-LM and PHS-BS as soon as possible are the top priority for revitalizing China's large, sophisticated and high-speed CNC equipment.

    (1) Accelerate revitalization and there is no time to delay. In China, the development of these two types of functional components for high-speed driving and precision positioning lags behind the needs of the market, and there is a clear gap with overseas. Taking PHS-BS as an example, when the second and third generation products have been launched overseas, China is still in the initial stage of PHS-BS first-generation product development, and there is no commercialization. As for the AC-LM used to equip high-speed, high-end CNC machine tools, there is currently no company with strong technical strength and mass production conditions to provide products to the market, and has not yet stepped out of the "college model" research stage. It should be clearly seen that the accelerated development of AC-LM and PHS-BS is an urgent desire for the localization and industrialization of CNC equipment in China, and also an era of strengthening China's overall national strength.

    (2) Develop side by side and complement each other. Although AC-LM and PHS-BS are developing in parallel, different countries have different national conditions, and different companies have different lengths. The recognition and adoption rate of the two driving methods are different. China's national conditions are: the land is not boastful, the population is large, and the labor resources are abundant. It is necessary to proceed from the national conditions of China and adopt the scientific development concept and the guiding ideology of new industrialization to plan the development of AC-LM and PHS-BS. ,Indispensable. Even if the technology of AC-LM matures, the output goes up, and the cost decreases in the future, from the perspective of energy saving and green manufacturing, PHS-BS drive still has a broad market space. While AC-LM will become the mainstream driving method in high-speed (super high-speed) and high-end CNC equipment, PHS-BS will continue to maintain its mainstream position in mid-range high-speed CNC equipment, but if PHS-BS blindly pursues high indicators, In the advantage range of AC-LM, "hard fight" is probably not worth the candle.

    (3) Scientific development focuses on the foundation. Analysis of the reasons for the slow development of China's PHS-BS is mainly due to the long-term neglect of the investment in basic theory and performance testing, and the lack of developmental stamina. In the face of the problems exposed after speed increase, I do not know where to start. Because we lack the in-depth study on the friction theory of rolling spiral pair, the friction characteristics of high speed and micro motion, the motion mechanism of rolling element under different working conditions, vibration and noise, torque characteristics, dynamic and static stiffness characteristics, etc. Speed ​​up the process. And AC-LM has a lot of basic work to do in all aspects of digital control technology, heat dissipation, magnetic isolation, electromagnetic interference, zero position, increased thrust, reduced energy consumption, and modularization of components. The research results of Chinese universities and research institutes should be transformed into productive forces as soon as possible, and advanced scientific and technological achievements and continuous innovation in practice should be used to promote the localization and industrialization of AC-LM and PHS-BS.

    (4) Orderly development and correct orientation. It is necessary to conduct an in-depth investigation of China's CNC equipment market and conduct a calm analysis. In CNC equipment, what is the speed and speed? What is the high-speed gold cutting machine and forming machine? What is the light load and large heavy load? According to the prediction of relevant people, by 2010, about 20% of CNC machines in the world will adopt AC-LM. Experience has shown that the quantity forecast is often not accurate enough. If this 20% refers to all CNC machine tools, then what about high-speed CNC machine tools? Does China also account for 20%? Our analysis of the market should avoid subjective inference and blind optimism.

    Whether developing AC-LM or PHS-BS, we must learn from the lessons of the development of certain electromechanical products in the past, to avoid blind launches, low levels of duplication, and waste of resources. At the same time, the related functional components associated with AC-LM and PHS-BS should be developed simultaneously, such as high-speed, high-rigidity precision rolling linear guide pairs, brakes, and guards.

    • Hex Bolt

    Hexagon Bolt refers to the fastener composed of head and screw. According to the material, the bolt can be divided into iron bolt and stainless steel bolt, namely hexagon head bolt (partial thread) - C grade and hexagon head bolt (full thread) - C grade.

    1. According to the stress mode of the connection, there are common and reamed holes. The bolts used for reaming holes should match the size of the holes and be used when subjected to transverse force

    2. According to the head shape, there are hexagon head, round head, square head, countersunk head and so on. Generally, countersunk head is used in the place where the surface is smooth and no protrusion is required after connection, because countersunk head can be screwed into parts. Round head can also be screwed into parts. The tightening force of square head can be larger, but the size is large. Hexagon head is the most commonly used.

    In addition, in order to meet the needs of locking after installation, there are holes in the head and in the rod, which can prevent the bolts from loosening when they are subject to vibration.

    Some bolts have no thread of smooth rod to do thin, called thin waist bolt. This kind of bolt is conducive to variable force connection.

    There are special high strength bolts on the steel structure. The head will be bigger. The size will also change.

    In addition, there are special uses: T-shaped slot bolts, most of which are used in machine tool fixtures, with special shape, and both sides of the head need to be cut off. Anchor bolts are used to connect the machine and the ground, and there are many shapes, such as U-shaped bolts, and so on.

    There are also special studs for welding, one with thread and the other without thread, which can be welded on the parts, and the other side can directly screw the nut.

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