Spark forming machine driven by the hottest linear

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EDM machine driven by linear servo motor

Abstract: This paper expounds the background of EDM machine driven by linear motor, analyzes the characteristics of linear motor servo system, and discusses the related technical problems

key words: linear motor; Servo system; EDM

1 preface

EDM technology has experienced a development process of half a century. With its unique formability and "cutting" performance of overcoming hardness with softness, it has become an important processing means in the field of mold processing. However, it has also experienced ups and downs, and has been impacted by other processing means again and again, and has been improved and improved again and again, showing its strong vitality

at the turn of the century, EDM technology is once again under unprecedented fierce impact from the cutting technology represented by CNC machining centers. CNC machining center technology has made progress in the high-speed rotation of the spindle, the new type of tool materials, the improvement of the rigidity of the main body of the machine tool, and the development of NC trajectory function. This series of technological progress makes EDM technology face new challenges

taking this opportunity, Sadiq company of Japan took the lead in developing the EDM forming machine with linear servo motor. Based on the traditional and mature EDM technology, this machine tool integrates the latest technology of linear servo system, which greatly improves the performance of EDM

in the specific processing technology of die forming, the main reason to inhibit the practical processing speed of forming processing is the deterioration of the processing state caused by complex shapes. The linear servo system, which is famous for its high-speed motion speed, is used for the high-speed jump function of EDM, which derives the punching free machining process, improves the practical machining speed of EDM in the words of cuilixin, chairman of Shandong innovation group, and shortens the practical machining time of EDM

2 technical characteristics of linear servo system

after adopting the linear servo system, the lead screw transmission link is omitted, which lays the foundation for the high-speed movement of the shaft. In addition, the control system adopts a high-speed 32-bit computer, which greatly improves the motion speed of the shaft. The AQ series linear servo EDM forming machine and LN series CNC power supply of Sadiq company can make the motion speed of the shaft reach 36m/min under the condition of 0.0001mm control equivalent. Such technical indicators make the new generation of EDM forming machine have the comprehensive conditions of high speed and precision machining

(1) the new machining process breaks through the traditional process index

the high-speed movement of the shaft has brought great changes to the traditional EDM forming process. This change is not limited to shortening the time required for clamping, correction and precise positioning due to the high-speed movement, but also realizing the high-speed jump (tool lifting) in machining. Jumping (tool lifting) is an essential measure for EDM. In the machining of complex shapes or deep and narrow cavities, it is almost impossible to machine without jumping (tool lifting) and flushing. However, there is no discharge when jumping (lifting the knife), which is a "slow down" phenomenon in EDM, which affects the utilization of machining time. Fast jumping (knife lifting) shortens the time of "slow down", thereby improving the utilization of time and processing speed. At the same time, high-speed jumping (tool lifting) also creates conditions for increasing the jumping (tool lifting) range, which fully improves the fluid force of the discharge gap during machining, completely eliminates the flushing treatment during rough machining, and shortens the processing time to 1/2 of the original. Figure 1 shows an example of processing a deep and narrow cavity with fast jump and no flushing liquid (cut longitudinally after processing), and the cross-section size of the electrode is 1mm × 38mm, electrode taper 1 °, cavity depth 70mm, electrode material graphite, two electrodes. The total processing time is 5h, of which the rough processing time is 160min, and the surface roughness Ra of the workpiece after processing is 2 μ m。

Figure 1

the advantages of the linear servo system have not only made a breakthrough in the processing parameters of deep and narrow cavities, but also unprecedented expansion of the functional field. Figure 2 is a typical machining example of deep and narrow cavity machined by Y-shaped (three leaf) flame high electrode. The thickness of Y-shaped (trefoil) electrode is only 0.25mm. In people's traditional concept, the processing of such shape is quite difficult. A depth of more than 10mm is regarded as a forbidden zone. The processing depth of this processing example is 40mm, which shows that the practicality of the linear servo system makes it easy for us to enter the traditional forbidden zone

Figure 2

(2) guarantee of stable finish machining, servo system with high response speed and high stable machining

in the EDM machine with DC servo system, the linear motor and sliding plate form an integrated structure. Therefore, the sliding plate displacement is the direct displacement of the linear motor, and there is no transmission link between the two. In this way, the loss of intermediate links and response lag are eliminated in the servo movement. Figure 3 shows the comparison between the commanded position and the actual position of the motion axis of the linear servo system. It can be seen that in the special reversible servo motion of EDM, the dynamic response time is only in the order of milliseconds. Therefore, the high-speed response system of linear motor is an order of magnitude higher than the traditional system of rotating AC/DC servo motor driving ball screw

Figure 3

it is well known that in the finishing process of EDM, the electrode does not contact the workpiece, and the effective discharge gap between them is only more than ten microns. The traditional servo motor and ball screw system have a lag of tens to hundreds of milliseconds between the command pulse and the actual position, so it is very difficult to achieve stable and smooth servo and maintain a high finishing discharge efficiency. The advantages of linear servo system are fully reflected in the process of EDM small gap finishing. The high-speed response of only a few milliseconds and the acceleration performance of more than 1G ensure the continuity of small gaps in the micro process of EDM discharge, greatly improve the utilization rate of finishing pulse, and then improve the machining speed of finishing in the smooth servo movement of linear servo system

(3) comprehensive measures to achieve precision control and long-term accuracy retention

the motion mode of the linear servo system determines that its servo unit must use the linear position feedback element as the position detection link, which is the only way. Therefore, the control and drive of the linear servo system can only be a full closed-loop system, which directly detects the linear displacement of the sliding plate, without the pitch error of the lead screw, the reverse clearance error of the lead screw and the coupling, and also avoids the interference factors such as the wear of the transmission chain parts. The guidance of linear motion also adopts the rolling guide rail which is suitable for the high response speed of linear motor. The comprehensive use of these measures realizes the precision control and long-term accuracy retention of the machine tool

(4) smooth drive of slow micro feed

EDM is characterized by slow average servo speed. During rough machining, the hourly feed distance is only a few millimeters, while during fine machining, the hourly feed distance is only a few tenths of millimeters or even slower. This requires EDM servo to have excellent dynamic characteristics to adapt to frequent braking, reverse and maintain the small gap between the tool and the workpiece. Further, the system is required to have smooth driving characteristics, and the influence caused by the cogging effect of the usual AC motor must be solved. Therefore, the control equivalent and drive equivalent of Sadiq's linear servo system are 0.1 μ m. At the same time, high-density rare earth permanent magnets are used as magnetic field materials when making linear motors. In the distribution and arrangement of magnets, the staggered distribution arrangement method (as shown in Figure 4) is adopted, together with the variable-frequency AC drive system that matches the motor performance and can slightly control the current of each phase. This series of methods ensure the most reasonable magnetic field phase relationship and reasonable current phase relationship, so as to balance the thrust between the armature windings of each phase of the linear motor to the greatest extent, It overcomes the problems of dynamic load and large-scale rate change, minimizes the adverse effects of cogging effect, and makes the servo movement stable

Figure 4

3 related technology supporting the linear servo system

the advent of the linear servo system EDM forming machine and its huge functional potential undoubtedly have formed a strong attraction for thousands of users, and will inevitably stimulate manufacturers to compete for the market by developing and updating products

however, the commercialization process of linear servo system machine tools must not only solve the relevant technical difficulties mentioned above in this paper, but also solve a series of related technologies supporting the linear servo system

(1) the gravity balance and axis locking of the vertical sliding plate (Z-axis)

due to the high-speed motion characteristics and high acceleration characteristics of the linear servo system, the system cannot be used as a gravity balance device in the form of a weight. The reason is that when the motion acceleration of the sliding plate (Z-axis) is greater than 1G, the free fall of the weight cannot track the rising motion of the sliding plate, that is, it loses its balance effect. Therefore, the z-axis balancing device of Sadiq company adopts a cylinder type mechanism (as shown in Figure 5), which relies on the air pressure in the cylinder to achieve gravity balance. This device has the following advantages: ① when the machine tool is in the working state, due to the supporting effect of the air pressure balance device on the gravity of the sliding plate, the motor works in the micro power consumption state. Only when it jumps quickly, the motor enters the high-power operation state; ② The air pressure balance makes the locking function of the vertical axis reliable. After the driver loses power, the gravity of the vertical sliding plate is instantly in a free falling state. At this time, the traditional electromagnetic lock axis is very easy to cause the sliding plate to move by mistake due to the action lag, which has a very disturbing impact on the holding position and processing accuracy. The air pressure balancing device and air pressure clamping will make the locking reliable

figure 5

(2) the self weight of the spindle and its load bearing

the high-speed motion characteristics and high acceleration characteristics of the linear servo system make the self weight of the spindle and the tool electrode suspended on the spindle head "overweight" or "weightless" at the moment of acceleration. If at a certain instant during machining, the spindle makes a rapid jumping motion with the acceleration of a, then the instantaneous gravity of the tool electrode with mass m at this time is w=m (g+a). The change of gravity will inevitably cause great pressure on the drive of the system. Therefore, it is very important to reduce the dead weight of the spindle while ensuring the rigidity of the spindle head. Synthetic ceramic materials with light weight and small thermal deformation are the preferred materials to achieve the above goals. However, it is worth noting that the practical process of the product shows that the high-speed response of the linear guide rail matched with the linear drive system and the strictly symmetrical magnetic field force are more important for smooth driving and preventing the force displacement of the sliding plate. Therefore, the position design, placement and debugging process of a pair of linear motors and a pair of linear guide rails located on both sides of the spindle must be correct, so as to offset the force, which is conducive to obtaining a drive with high straightness and good smoothness

the change of gravity puts forward higher requirements for the output force of the drive system and the clamping force of the spindle. The output force of the linear motor system manufactured by Sadiq company can reach 3000n. The driving principle of the linear motor is the same as that of the traditional AC motor, which can quickly complete the design and development matching with the CNC software. The fixture matched with the linear motor servo system has the same load-bearing as the traditional system,

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