> &At present, in the commercial gantry machine tools, the gantry machine tools whose girders do not move up and down are generally divided into three categories: table mobile, gantry mobile and crossbeam mobile (also known as bridge), as shown in the figure.
Format factory 1.jpg Figure 1 table mobile gantry machine
Format factory 2.jpg Figure 2 gantry mobile gantry machine
Format factory 3.jpg Figure 3 crossbeam mobile gantry machine
In the general traditional understanding, the worktable mobile type meets the conventional cutting of general workpieces, and is also a popular form in the domestic market; the gantry mobile type generally meets the general cutting of longer and heavier workpieces, and is not uncommon in the processing of large workpieces; while the crossbeam mobile gantry machine tool is generally used in the developed countries in Europe and America, but it is still relatively common in China Rarely seen, with the rapid development of high-end machine tools imported from Europe and the United States and high-speed and high-precision machining in recent 10 years, this type of machine tools has also begun to be accepted. This paper is a comparative introduction to the technical characteristics and advantages of the crossbeam mobile gantry machine tool.
&Workpiece bearing
As the worktable of the crossbeam mobile gantry machine tool is fixed, it can be designed to be thick and solid, so it has a very good bearing capacity. Generally speaking, the bearing capacity can be placed as long as the volume of the machine tool allows. In the design of the table mobile machine tool, the moving weight must be taken into account. The thickness of the table is usually only 1 / 2 to 1 / 3 of the fixed table, and the structural strength is weak. Without considering the motion characteristics, the bearing capacity per square meter is only about 1-1.5t, so the application range is obviously small.
&Guide wear and accuracy retention
Under the same cutting conditions, the mass load is an important factor that affects the wear and accuracy retention of guide rail.
In addition to the weight of workpieces, the self weight of worktable itself is also very large. If heavy workpieces are processed for a long time, the wear of guide rail will accelerate, and the accuracy will decline for a period of time. The corresponding gantry machine tool with fixed worktable, because the mass load is fixed, will not change with the change of the workpiece. As long as the appropriate guide rail is selected in the design, the wear of the guide rail will be small and stable, and the machine tool can maintain the accuracy for a long time.
&Fast moving speed and acceleration
The table mobile gantry machine tool can generally reach its marked design speed when it is unloaded, but with the increase of the weight of the workpiece, its moving speed is bound to decline significantly, affecting the efficiency, while the table fixed gantry machine tool can ensure that the design speed can be maintained no matter the load of the workpiece is not related to the movement, which is of great significance in larger machine tools.
At the same time, when users pay attention to the fast-moving speed, they often fall into a misunderstanding, which is to ignore the acceleration. In fact, the impact of the fast-moving speed on the processing efficiency is far less than that of the acceleration, especially in the processing of some complex shaped workpieces, when the acceleration is increased, the processing efficiency can be multiplied.
As the table load of the mobile gantry is a variable, when a large acceleration is set in the machine tool manufacturer, once a heavy workpiece is placed, the machine tool will vibrate, impact and damage the transmission system; although the workpiece of the mobile gantry does not participate in the operation, the center of gravity of the gantry frame is high, and the high acceleration is easy to produce a large overturning force, resulting in If the machine tool is damaged, it is not suitable to set too high acceleration. Because the moving center of gravity of the crossbeam mobile gantry has no large overturning moment on the crossbeam and the motion quality remains unchanged, so long as the appropriate servo motor is selected, it can obtain very high acceleration. If some machine tools are designed reasonably, the acceleration can be close to the vertical machining center, which can not only improve the machining efficiency It can also reduce the surface roughness value.
&Servo motion performance
The motion quality has essential influence on the servo inertia ratio parameter and acceleration parameter. If the motion quality changes, the inertia will change, that is to say, the electrical accuracy parameter will change. The parameters determined when the machine tool leaves the factory are set according to the load during debugging. Once the motion quality changes greatly after leaving the factory, the servo load will also change However, at this time, the user cannot adjust these parameters, which will directly lead to the decrease of electrical accuracy and the inability to process high-precision workpieces. Note that the high-precision refers to not the point motion (two-dimensional) accuracy, but the interpolation accuracy or contour accuracy. Moreover, the parameter that is caused by the load change is not suitable, which will worsen the surface roughness to a certain extent, resulting in The machine tool cant do high speed highlight machining.
This is often when the customer checks and accepts the machine tool, the accuracy of the workpiece is qualified, but once a large workpiece is put on, the accuracy changes, and often can not find the problem. The beam mobile machine tool has a high inherent advantage in the stability and consistency of servo motion performance.
&Machining accuracy and surface roughness
The following figure is the transmission diagram of the gantry machine tool with fixed table and cross beam. It can be seen from the figure that the transmission of the gantry machine tool with movable table has to adopt servo motor and planetary gearbox to provide enough driving force due to the heavy work table. But even the planetary gearbox with high precision still has clearance, of course, this clearance is general People think that it can be compensated by the system. This idea is possible when the worktable is not heavy or the moving speed is not fast. In a word, it is OK when there is no large impact force. But for the gantry machine tool, the load is generally large, and the worktable itself is heavy. Once the high speed is reversed, because of this large inertia, the gap will naturally be brought out, which is the reason for the over cutting. Therefore, only There is a reduction in speed and acceleration to reduce the degree of overcutting.
Format factory 4.jpg Figure 4 worktable Mobile
Format factory 5.jpg Figure 5 beam Mobile
Format factory 6.jpg Figure 6 synchronous motion of the same direction differential clearance
For the crossbeam mobile gantry, because it is driven by two sides, the mechanism with clearance cannot be used on both sides. Therefore, generally, the smaller size gantry adopts the gapless transmission, which is the direct connection between the servo motor and the lead screw (actually, there is still a micron level gap, but it has no impact on the processing, so in order to distinguish it from the previous gap, it is hereinafter referred to as gapless); large gantry Because of the use of rack and pinion drive, there is clearance between the rack and gearbox structure, so the international comparison is to use the same direction differential clearance synchronous motion structure, that is, two servo motors with planetary gearbox drive are used on both sides of the beam, the two servo motors are not moving at the same time, but moving successively, through the successive movement to produce a tension force to eliminate The clearance between the gearbox and the rack and pinion. At the same time, the two groups on both sides must keep synchronous motion. Because of the no gap transmission of the crossbeam mobile gantry, it greatly avoids over cutting, especially can improve the acceleration and still maintain high machining accuracy.
Since there is no gap, there is no over cutting phenomenon. In this way, the acceleration can be increased, and the moving load of the machine tool remains unchanged, so that the inertia ratio can also maintain the higher value of the factory debugging. This is to be able to increase the key electrical parameters such as gain, so that the machine tool can obtain good electrical performance and low surface roughness.
However, it should be noted that the gantry mobile machine tool has a large force arm due to the movement center of gravity and the support point (slider under the column), and there is a large overturning force when doing high acceleration movement, which may lead to over cutting phenomenon. Once the lower acceleration is used, not only the motion efficiency is reduced, but also the good contour accuracy and surface roughness can not be obtained. However, this type of machine tool can be used in the processing of point moving workpieces, and it can process major workpieces with good accuracy and accuracy retention.
Can full closed loop solve the accuracy problem?
As mentioned in the above point, since the worktable mobile machine tool (in this case, the medium-sized and above machine tool) can not get stable and reliable accuracy and surface roughness like the beam mobile machine tool, can full closed-loop be used to solve it? The answer is No. It is true that full closed-loop can improve the accuracy of machine tools, but it needs to be pointed out that the accuracy here refers to the accuracy of position ring, rather than the accuracy of speed ring and current ring. Therefore, in point position processing, full closed-loop can improve the processing accuracy, but in interpolation motion, to get good profile accuracy and surface roughness mainly depends on the speed ring and current ring and other electrical devices Because of the parameter matching and the good circular effect of the full closed loop, the full closed loop can not solve the problem effectively. At the same time, it should be pointed out that if the rigidity and accuracy of the machine tool itself are not good, the use of full closed-loop will also lead to vibration, which will further deteriorate the accuracy and surface roughness of the machine tool.
The crossbeam mobile gantry machine tool not only has the above advantages, but also has the following advantages: ① the range of processing materials is wide, because the guide rail is on the processing range of workpiece, easy to protect, so when processing the wear-resistant guide rail materials, it is not necessary to excessively consider the dust into the guide rail to accelerate wear, especially suitable for ceramics, cast iron, glass fiber, etc. ② the floor area is small, generally 60% of the floor area of the movable gantry of the workbench. ③ it is convenient to process long workpieces. For some workpieces that are longer than the length of the machine tool, the workpieces can be moved for cutting.
In view of the above characteristics, this is why the large-scale high-speed milling and double swing head five axis linkage gantry, which is currently popular in the market, basically adopts the crossbeam mobile gantry structure. Double swing head five axis linkage gantry when the swing angle of the five axis rotation axis, the linear axis of the machine tool needs to follow to reach the predetermined position, which is the RTCP function.
As we all know, as a high-precision CNC machine tool, if we want to get high-precision, we must have the same response of each axis, so each axis must have good servo characteristics and inertia matching. In order to get good servo characteristics and inertia matching, we must control the stable motion quality, and effectively control the quality by the stable servo, so that we can get good characteristics and through the circular circulation To verify. When three linear axes are effectively controlled and can respond to the speed and acceleration of the rotating axis, a good spatial machining accuracy can be obtained.
Therefore, in the three types of gantry structures, because the moving weight of the platform mobile gantry is variable, the inertia and servo characteristics are variable, and generally the high-precision and low surface roughness values are not obtained; while in the gantry mobile structure, because the center of gravity of the gantry moving is high, there will be a overturning moment during the rapid acceleration and deceleration, which leads to the structure can not have a rapid response Once the rapid response will lead to the accuracy change and over cutting, so these two structures are not suitable for the requirements of high-precision five axis machining of large workpieces; while the beam mobile structure is very suitable for high-speed and high-precision machining due to the controllable movement quality and no overturning moment, which is also a common structure mode of large-scale high-speed milling and five axis gantry in the world.
6. Conclusion
At present, the domestic processing is developing towards high speed, high precision and low surface roughness value with the popular trend of developed countries, and the selection of machine tools is also following this trend. The traditional work is changing
Format factory 1.jpg Figure 1 table mobile gantry machine
Format factory 2.jpg Figure 2 gantry mobile gantry machine
Format factory 3.jpg Figure 3 crossbeam mobile gantry machine
In the general traditional understanding, the worktable mobile type meets the conventional cutting of general workpieces, and is also a popular form in the domestic market; the gantry mobile type generally meets the general cutting of longer and heavier workpieces, and is not uncommon in the processing of large workpieces; while the crossbeam mobile gantry machine tool is generally used in the developed countries in Europe and America, but it is still relatively common in China Rarely seen, with the rapid development of high-end machine tools imported from Europe and the United States and high-speed and high-precision machining in recent 10 years, this type of machine tools has also begun to be accepted. This paper is a comparative introduction to the technical characteristics and advantages of the crossbeam mobile gantry machine tool.
&Workpiece bearing
As the worktable of the crossbeam mobile gantry machine tool is fixed, it can be designed to be thick and solid, so it has a very good bearing capacity. Generally speaking, the bearing capacity can be placed as long as the volume of the machine tool allows. In the design of the table mobile machine tool, the moving weight must be taken into account. The thickness of the table is usually only 1 / 2 to 1 / 3 of the fixed table, and the structural strength is weak. Without considering the motion characteristics, the bearing capacity per square meter is only about 1-1.5t, so the application range is obviously small.
&Guide wear and accuracy retention
Under the same cutting conditions, the mass load is an important factor that affects the wear and accuracy retention of guide rail.
In addition to the weight of workpieces, the self weight of worktable itself is also very large. If heavy workpieces are processed for a long time, the wear of guide rail will accelerate, and the accuracy will decline for a period of time. The corresponding gantry machine tool with fixed worktable, because the mass load is fixed, will not change with the change of the workpiece. As long as the appropriate guide rail is selected in the design, the wear of the guide rail will be small and stable, and the machine tool can maintain the accuracy for a long time.
&Fast moving speed and acceleration
The table mobile gantry machine tool can generally reach its marked design speed when it is unloaded, but with the increase of the weight of the workpiece, its moving speed is bound to decline significantly, affecting the efficiency, while the table fixed gantry machine tool can ensure that the design speed can be maintained no matter the load of the workpiece is not related to the movement, which is of great significance in larger machine tools.
At the same time, when users pay attention to the fast-moving speed, they often fall into a misunderstanding, which is to ignore the acceleration. In fact, the impact of the fast-moving speed on the processing efficiency is far less than that of the acceleration, especially in the processing of some complex shaped workpieces, when the acceleration is increased, the processing efficiency can be multiplied.
As the table load of the mobile gantry is a variable, when a large acceleration is set in the machine tool manufacturer, once a heavy workpiece is placed, the machine tool will vibrate, impact and damage the transmission system; although the workpiece of the mobile gantry does not participate in the operation, the center of gravity of the gantry frame is high, and the high acceleration is easy to produce a large overturning force, resulting in If the machine tool is damaged, it is not suitable to set too high acceleration. Because the moving center of gravity of the crossbeam mobile gantry has no large overturning moment on the crossbeam and the motion quality remains unchanged, so long as the appropriate servo motor is selected, it can obtain very high acceleration. If some machine tools are designed reasonably, the acceleration can be close to the vertical machining center, which can not only improve the machining efficiency It can also reduce the surface roughness value.
&Servo motion performance
The motion quality has essential influence on the servo inertia ratio parameter and acceleration parameter. If the motion quality changes, the inertia will change, that is to say, the electrical accuracy parameter will change. The parameters determined when the machine tool leaves the factory are set according to the load during debugging. Once the motion quality changes greatly after leaving the factory, the servo load will also change However, at this time, the user cannot adjust these parameters, which will directly lead to the decrease of electrical accuracy and the inability to process high-precision workpieces. Note that the high-precision refers to not the point motion (two-dimensional) accuracy, but the interpolation accuracy or contour accuracy. Moreover, the parameter that is caused by the load change is not suitable, which will worsen the surface roughness to a certain extent, resulting in The machine tool cant do high speed highlight machining.
This is often when the customer checks and accepts the machine tool, the accuracy of the workpiece is qualified, but once a large workpiece is put on, the accuracy changes, and often can not find the problem. The beam mobile machine tool has a high inherent advantage in the stability and consistency of servo motion performance.
&Machining accuracy and surface roughness
The following figure is the transmission diagram of the gantry machine tool with fixed table and cross beam. It can be seen from the figure that the transmission of the gantry machine tool with movable table has to adopt servo motor and planetary gearbox to provide enough driving force due to the heavy work table. But even the planetary gearbox with high precision still has clearance, of course, this clearance is general People think that it can be compensated by the system. This idea is possible when the worktable is not heavy or the moving speed is not fast. In a word, it is OK when there is no large impact force. But for the gantry machine tool, the load is generally large, and the worktable itself is heavy. Once the high speed is reversed, because of this large inertia, the gap will naturally be brought out, which is the reason for the over cutting. Therefore, only There is a reduction in speed and acceleration to reduce the degree of overcutting.
Format factory 4.jpg Figure 4 worktable Mobile
Format factory 5.jpg Figure 5 beam Mobile
Format factory 6.jpg Figure 6 synchronous motion of the same direction differential clearance
For the crossbeam mobile gantry, because it is driven by two sides, the mechanism with clearance cannot be used on both sides. Therefore, generally, the smaller size gantry adopts the gapless transmission, which is the direct connection between the servo motor and the lead screw (actually, there is still a micron level gap, but it has no impact on the processing, so in order to distinguish it from the previous gap, it is hereinafter referred to as gapless); large gantry Because of the use of rack and pinion drive, there is clearance between the rack and gearbox structure, so the international comparison is to use the same direction differential clearance synchronous motion structure, that is, two servo motors with planetary gearbox drive are used on both sides of the beam, the two servo motors are not moving at the same time, but moving successively, through the successive movement to produce a tension force to eliminate The clearance between the gearbox and the rack and pinion. At the same time, the two groups on both sides must keep synchronous motion. Because of the no gap transmission of the crossbeam mobile gantry, it greatly avoids over cutting, especially can improve the acceleration and still maintain high machining accuracy.
Since there is no gap, there is no over cutting phenomenon. In this way, the acceleration can be increased, and the moving load of the machine tool remains unchanged, so that the inertia ratio can also maintain the higher value of the factory debugging. This is to be able to increase the key electrical parameters such as gain, so that the machine tool can obtain good electrical performance and low surface roughness.
However, it should be noted that the gantry mobile machine tool has a large force arm due to the movement center of gravity and the support point (slider under the column), and there is a large overturning force when doing high acceleration movement, which may lead to over cutting phenomenon. Once the lower acceleration is used, not only the motion efficiency is reduced, but also the good contour accuracy and surface roughness can not be obtained. However, this type of machine tool can be used in the processing of point moving workpieces, and it can process major workpieces with good accuracy and accuracy retention.
Can full closed loop solve the accuracy problem?
As mentioned in the above point, since the worktable mobile machine tool (in this case, the medium-sized and above machine tool) can not get stable and reliable accuracy and surface roughness like the beam mobile machine tool, can full closed-loop be used to solve it? The answer is No. It is true that full closed-loop can improve the accuracy of machine tools, but it needs to be pointed out that the accuracy here refers to the accuracy of position ring, rather than the accuracy of speed ring and current ring. Therefore, in point position processing, full closed-loop can improve the processing accuracy, but in interpolation motion, to get good profile accuracy and surface roughness mainly depends on the speed ring and current ring and other electrical devices Because of the parameter matching and the good circular effect of the full closed loop, the full closed loop can not solve the problem effectively. At the same time, it should be pointed out that if the rigidity and accuracy of the machine tool itself are not good, the use of full closed-loop will also lead to vibration, which will further deteriorate the accuracy and surface roughness of the machine tool.
The crossbeam mobile gantry machine tool not only has the above advantages, but also has the following advantages: ① the range of processing materials is wide, because the guide rail is on the processing range of workpiece, easy to protect, so when processing the wear-resistant guide rail materials, it is not necessary to excessively consider the dust into the guide rail to accelerate wear, especially suitable for ceramics, cast iron, glass fiber, etc. ② the floor area is small, generally 60% of the floor area of the movable gantry of the workbench. ③ it is convenient to process long workpieces. For some workpieces that are longer than the length of the machine tool, the workpieces can be moved for cutting.
In view of the above characteristics, this is why the large-scale high-speed milling and double swing head five axis linkage gantry, which is currently popular in the market, basically adopts the crossbeam mobile gantry structure. Double swing head five axis linkage gantry when the swing angle of the five axis rotation axis, the linear axis of the machine tool needs to follow to reach the predetermined position, which is the RTCP function.
As we all know, as a high-precision CNC machine tool, if we want to get high-precision, we must have the same response of each axis, so each axis must have good servo characteristics and inertia matching. In order to get good servo characteristics and inertia matching, we must control the stable motion quality, and effectively control the quality by the stable servo, so that we can get good characteristics and through the circular circulation To verify. When three linear axes are effectively controlled and can respond to the speed and acceleration of the rotating axis, a good spatial machining accuracy can be obtained.
Therefore, in the three types of gantry structures, because the moving weight of the platform mobile gantry is variable, the inertia and servo characteristics are variable, and generally the high-precision and low surface roughness values are not obtained; while in the gantry mobile structure, because the center of gravity of the gantry moving is high, there will be a overturning moment during the rapid acceleration and deceleration, which leads to the structure can not have a rapid response Once the rapid response will lead to the accuracy change and over cutting, so these two structures are not suitable for the requirements of high-precision five axis machining of large workpieces; while the beam mobile structure is very suitable for high-speed and high-precision machining due to the controllable movement quality and no overturning moment, which is also a common structure mode of large-scale high-speed milling and five axis gantry in the world.
6. Conclusion
At present, the domestic processing is developing towards high speed, high precision and low surface roughness value with the popular trend of developed countries, and the selection of machine tools is also following this trend. The traditional work is changing