The structural characteristics of high-performance machining centers

The difference between high-performance machining centers and high-speed machining centers lies in the fact that they not only have a high-speed rotating spindle, but also design high-precision linear motion guides, high-power spindle motors, precision spindle bearings, ball screws, high-efficiency servo drive motors, and advanced CNC systems. Therefore, the machining center can process high-precision parts with high efficiency, greatly improving market competitiveness.
1. Linear motion guide rail
The speed and accuracy of the axial motion of the machining center are crucial for achieving high-speed cutting. The friction coefficient of a linear motion guide rail is only 1/20 of that of a regular square guide rail. Due to the fact that the contact area between the rollers of the linear motion guide rail and the guide rail is much smaller than that of the square guide rail, the power consumption is also reduced to 1/20 of that of the square guide rail, and it can maintain minimal wear for a long time, greatly improving the service life of the guide rail. The precision linear motion guide rail has a quenched hardness of HRC58-62, which is ground by a precision guide rail grinder. Unlike ordinary square guide rails, it has at least one V-shaped guide rail. Due to the simple structure of the two linear guide rails, it is easy to process, assemble, measure, and select the appropriate roller diameter. The linear motion guide rail has high stiffness and there is no gap between it and the worktable, so it rarely generates vibration and can process parts with low surface roughness, extending the service life of the tool.
2. Precision ball screw
The diameter and pitch of the machine tool ball screw directly affect the accuracy of the machined parts, especially under cutting conditions of feed rate. High performance machining centers using linear motion guides choose single head ball screws with small diameter and fine pitch. Some also use multi head ball screws with coarse pitch. Generally, a transmission scheme using servo motors to drive ball screws is adopted. However, during the operation of the ball screw, the direction of the rotation axis of the rolling element changes when it moves in a spiral motion, resulting in gyroscopic motion. When the gyroscopic moment in motion exceeds the frictional force between the ball body and the raceway, the rolling body will slide, causing severe friction and increasing the temperature of the screw. At the same time, vibration and noise increase, shortening the lifespan of the screw and reducing the transmission quality of the ball screw. A new type of high-performance rolling screw, planetary roller screw, has been developed to effectively solve the above technical problems. With the continuous development of new technologies, under ultra-high feed conditions, the acceleration of the workbench will reach more than 3g, so the inertia force of the moving parts is also quite large. When designing the mechanical part, efforts must be made to reduce the mass of moving parts and the rotational inertia of rotating parts, further improving the stiffness, sensitivity, and accuracy of the feed system.
3. High power machine tool spindle motor
Among the many factors that affect the selection of machine tool spindle motor power, the most important ones are spindle taper, cutting amount (cutting rate) selected during machining, part size, and tool size. Choosing a large taper spindle can perform high-power cutting, but sometimes, in order to accelerate and decelerate quickly, a high-power motor can also be used to drive a small taper spindle. For high cutting rate machining, it is necessary to choose a large taper spindle and a high-power machine tool spindle motor. The material of the parts has little effect on the selection of the spindle motor power of the machine tool. For example, for forgings and castings, high-power cutting is not required. However, when choosing to process at high speeds on the machine tool spindle, a high-power motor must be selected. The reason why high-power drive is also necessary for machining large parts is because it requires the use of large-diameter cutting tools.
4. Spindle bearings
The type and scale of spindle bearings must meet the usage conditions. Large bearings can provide high strength and stiffness. However, large-sized bearings have two drawbacks:
1) Due to the large mass of large bearings and the large contact area between bearings, a large amount of heat is generated at high spindle speeds. Prolonged exposure to a large amount of heat can cause spindle size to increase, affecting machining accuracy.
A high-quality spindle requires a high-power motor to drive it. Although the inner ring of the bearing is cooled with lubricating oil, large bearings increase their load capacity and rotational inertia at high speeds, resulting in an increase in required power. Especially when the spindle speed increases, power consumption increases. But not all power is consumed in cutting. For example, a spindle with 40 horsepower can only have 15-20 horsepower of power acting on the cutting head, while the rest is used to rotate the spindle. For a high-power spindle, it can exert as much power as possible on the cutting workpiece, and can drive the spindle with the highest speed with very little power. The power applied to the spindle can be calculated based on the maximum rotational speed of the spindle under no-load conditions to determine the power consumed by the machine tool. Due to cutting at high speeds, the chuck and tool generate radial deviation and non concentricity under the action of cutting forces, resulting in increased additional forces or unbalanced centrifugal forces.
2) The use of multiple rows of small diameter bearings does not affect the stiffness of the spindle, and it is also quite beneficial for the preloading of the spindle bearings. The preload load on bearings usually refers to the pressure exerted by the spindle on the bearings under static conditions. Generally, preload loads are used to improve spindle stiffness and increase cutting capacity. However, due to the increase in pressure acting on the bearings, the heat generation increases, which also accelerates bearing wear. In order to improve the cutting performance of the cutting tool and extend its service life, small pressure is pre applied to the multi row bearings, which can increase the stiffness of the machine tool spindle and achieve the above goals.
From a long-term perspective, the market demand for magnetic, pneumatic, and hydrostatic bearings will greatly increase. However, currently in high-speed cutting, the most commonly used are the following two types: radial thrust bearings and ball bearings. Under standard machine tool spindle speed conditions, a row of ball bearings and a row of radial thrust bearings are often installed at the front end of the spindle, and two rows of ball bearings are installed at the rear end of the spindle. Because installing a row of ball bearings at the front end of the spindle can greatly improve the spindle stiffness and increase its load-bearing capacity. This is crucial for heavy-duty cutting. However, because ball bearings have a larger contact area and are heavier than radial thrust bearings, they consume more power and generate more heat, which can easily cause an increase in spindle size and lower power utilization. High speed cutting can reduce the radial force acting on the spindle and cutting tools. In this way, the radial thrust bearing installed at the front end of the spindle provides sufficient stiffness and stability, avoiding size expansion caused by heating of the machine tool spindle.
Reasonable selection of bearing materials is equally important as the type of bearing. Although bearings made of bearing steel are still widely used, practice has shown that the use of ceramic bearings in high-speed cutting will exhibit many advantages. Although bearings made of bearing steel are cheap, their weight is much heavier than ceramic bearings of the same specifications. Due to its heavy weight and high heat generation during high-speed cutting, a complex cooling and lubrication system must be installed. At the same time, as the spindle speed increases, the centripetal force acting on the bearing increases, causing the bearing temperature to rise, resulting in an increase in spindle size, affecting the dimensional accuracy of the machined parts, and increasing the power required for the machine tool spindle. Ceramic bearings, due to their lightweight, will effectively solve this technical problem. Cutting tests have shown that the speed at which ceramic bearings increase the spindle size is only 1/40 of that of bearing steel bearings. The reason is that it only has a small centripetal force acting on the bearing during high-speed cutting. At the same time, in order to improve the stiffness and cutting ability of the machine tool spindle, a large preload can also be applied to the ceramic bearings. Due to the above characteristics, ceramic bearings have an increased service life.
Modern machine tool spindle technology allows machine tools to easily adjust the preload applied to the spindle bearings based on the spindle speed. When the spindle speed of the machine tool increases, the load acting on the bearing also increases due to the increase in centripetal force. On the contrary, the load acting on the bearing decreases. Therefore, the heat on the bearing is reduced, and the size expansion of the bearing is reduced. Of course, in high-speed cutting, it is also allowed to pre apply a very small load to the bearing, so that the cutting force acting on the tool is very small, which can reduce the requirement for the stiffness of the machine tool. It is still necessary to pre apply a larger load to the bearings at low spindle speeds, as increasing the cutting force of the tool also increases the force acting on the spindle.
5. Spindle motor and transmission system
At present, there are two connection methods between the machine tool spindle and the motor, one is through a belt or gear; The second is direct transmission, which means directly connecting the spindle motor to the spindle, or installing the spindle motor and spindle simultaneously on a composite device, called a composite spindle.
The advantage of belt or gear transmission is that the spindle motor can achieve high spindle speed even when rotating at a slow speed. This transmission method is inexpensive due to its low motor speed and low input power, but it has the following drawbacks: due to its complex structure, it is prone to malfunctions and inconvenient maintenance. At the same time, vibration will also occur between the belt, gear, and spindle. As the structure is further simplified, the number of moving parts is further reduced, which will also enable the spindle to accelerate and decelerate faster. On the contrary, a belt or gear driven spindle includes a spindle, a shaft seat, an electric motor, a pulley or gear, etc. Each component is composed of materials of different weights, which generate heat due to friction during high-speed rotation. Due to the different weight and force of materials, the heat generated in different places is also different, resulting in different sizes of expansion in various parts of the spindle. In severe cases, it can cause deformation of the spindle, affecting its size, geometric shape, etc. The direct drive spindle, on the other hand, has uniform thermal deformation, and even under ultra-high speed conditions, can be cooled by coolant through the inner hole of the spindle. Therefore, it basically does not affect the spindle accuracy and can more stably ensure machining quality.
6. Cooling and lubrication
In cutting machining, if coolant is not added, it will cause the spindle to expand in size. To ensure the high precision of the machine tool spindle, it is necessary to stably control the spindle and bearings to have a fixed size. At present, ordinary machine tools choose external cooling, internal cooling, or combined internal and external cooling methods to cool the spindle and bearings according to the different spindle structures. However, in general, external cooling should be used as much as possible. By cooling, the heat transferred from the cutting head to the spindle is discharged into the air.
In order to effectively improve the utilization rate of machine tools and reduce power consumption, it is recommended to use mist cooling or spray cooling oil spindle cooling systems for cooling. Especially for high-speed machining machines, it is recommended to choose the spindle cooling system based on the maximum speed achieved by the machine spindle and the material selected for the bearings. Determine the selected cooling system based on the rotation speed of the machine tool spindle and the outer diameter of the bearings. If both types of bearings choose bearing steel bearings, it is recommended to choose a spray oil cooling system. Of course, compared to the two, the latter requires a large amount of cooling and lubricating fluid, which increases the power consumption of the machine tool.
7. CNC system
Modern CNC technology allows machine tools to process at a feed rate of 38.1m/min. The main reason is that high cutting speeds can be selected during machining. At the same time, the machine tool needs to be equipped with high-speed and high-resolution servo motors. In addition, to ensure high accuracy, high-resolution servo motors are also required to have the ability to quickly process signals. Of course, having a high-resolution servo motor without increasing the processor's operating speed is useless. Because high-resolution servo motors have more data to process than their predecessors, they require a high speed for word processing. The high speed of word group processing can quickly process data and send pulses to the servo motor. For this reason, it is generally necessary to configure two 32-bit processors on the servo motor to meet the usage requirements, and to quickly process the large amount of data collected during processing and the need to quickly perform complex calculations on complex part shapes in high-speed machining.
When processing large and complex shaped aircraft parts and plastic compression molds, due to the insufficient memory of the general NC system, it is generally necessary to first store them in the computer, and then input them into the NC system (also known as the CNC system). Of course, for parts with short programs and less complex shapes, a typical NC system can meet the usage requirements because it has a transmission speed of 76800 symbols per second. To improve the rapid feed capacity and machining accuracy of machine tools, it is necessary to develop new computer software. According to actual machining needs, the software developed is required to achieve high precision in machining parts, regardless of the general feed rate or high-speed feed conditions.
To improve the machining accuracy of parts at the selected feed rate, not only can the number of tool passes be appropriately increased, but also the recently developed geometric shape compensation software can ensure excellent machining quality at high production efficiency. Geometric shape compensation software is different from high feed rate software. The latter is used to increase the feed rate of the machine tool or reduce processing time to improve production efficiency, while the former is mainly used to improve the accuracy of processed parts. Its main functions are as follows:
1) Interpolate the acceleration or deceleration in advance. In arc interpolation machining, there is often a significant difference between the assembled program and the actual machining, mainly due to the leading or lagging caused by the acceleration or deceleration of the tool moving along the axis. It can compensate for errors caused by the acceleration or deceleration generated by tool movement.
2) Pre control the forward movement. In addition to the machining system, some errors in the servo control system of machine tools can also reduce the accuracy of machined parts. The newly developed forward feed control system can calculate possible error values based on machining programs, cutting speeds, feed rates, etc., and compensate for actual machining errors before they occur.
3) Perform precise vector compensation. When the machine tool is performing high-speed machining, it is necessary to prioritize selecting a suitable feed rate. Usually obtained by adjusting the amplification factor of the amplification device in the servo motor. When the amplification factor given to the forward direction is adjusted to a large value, the tracking speed of servo control for errors can be improved. Of course, an increase in tracking speed will reduce the stability of the servo control system. The lack of stability in the servo control system will cause the surface roughness of the processed parts to deteriorate. A precise vector compensation system is used to calibrate large machining programs for selectively increasing quantities of parts. The corrected machining program can stabilize the servo control system, especially the stable servo control system of the feed system can eliminate the vibration that causes the surface roughness of the parts to deteriorate.
4) Choose the appropriate deceleration for corner machining. When applying the above three functions, it is usually required that each motion axis decelerates at the corners of the parts to avoid the impact vibration generated by the driving system during rapid tool turning. It requires deceleration to an appropriate feed rate within a given time for processing.
5) For high-performance machining centers, it is not only necessary to design high-speed spindles, but also to design high-performance CNC systems, high-precision linear guides, precision ball screws, bearings, choose appropriate cooling and lubrication methods, and machine tool/tool interfaces. The above technologies have been used to produce many high-performance machining centers and have achieved good economic and social benefits in practical production.