Research on the structure of horizontal turning center

The horizontal turning center is a high-end product developed on ordinary CNC lathes, with compound processing capabilities such as turning, milling, drilling, and tapping. This kind of product can complete all or most of the processing of the workpiece in one clamping, reduce the workpiece transfer between the processes, avoid repeated clamping on different processing equipment of the workpiece, and realize the high-precision and high-efficiency processing of the workpiece. It is widely used in cnc machining of sophisticated and complex parts in the automotive, medical, aerospace and other industries.
In addition to the structural characteristics of general CNC lathes, the turning center should also have the following characteristics:
With C axis function. The C axis is a servo axis around the axis of the lathe’s spindle. This function enables the machine tool to achieve continuous indexing around the axis of the spindle and positioning and locking at any point. Linking with other servo axes and cooperating with the power tool can realize the processing of specific profiles;
With power tool. To realize milling, drilling, tapping and other processing in turning center, in addition to internal and external turning tools, self-driven milling cutters, drills, taps and other tools must be configured to achieve the purpose of process concentration.
This article only discusses the products of the tool holder type turning center, and does not involve the row tool type turning center.
2. Structural analysis of turning center
2.1 C axis
C-axis transmission structure
The rotation drive of the C axis usually has three implementation methods: the spindle servo motor is driven by a belt drive, the feed servo motor is directly driven by a reduction box, and the electric spindle.
The spindle servo motor is driven by a belt drive: the spindle drive and the C-axis drive share a set of transmission devices. Because of the existence of slippage in the V-belt (commonly used in machine tools) transmission, and the necessary tension of the belt transmission has a large additional force on the spindle, the V-belt is rarely used at this time. Synchronous toothed belts are often used in C-axis drive. They are driven by toothed meshing, which has an accurate transmission ratio and a small initial tension, and allows higher speeds and higher transmission accuracy and efficiency.
The feed servo motor is driven by the reduction box: in this way, the C-axis drive and the main shaft drive are two sets of transmission devices. The C-axis drive motor is a feed servo motor, which drives the spindle to rotate at a low speed through a reduction box, while the turning spindle is driven by the spindle servo motor to run at a high speed. Therefore, the spindle component needs a set of devices to realize the switching between the turning spindle and the C-axis drive, so that the transmission system of the C-axis is separated from the spindle. When the C-axis is driven, a large gear ratio is realized through the reduction gearbox, the output speed is low, and the torque is large. The spindle drive can achieve a high speed to meet the speed requirements during turning.
Direct drive of electric spindle: The drive motor rotor is directly fitted on the spindle to realize C-axis drive. The spindle has large inertia moment, short transmission chain and simple structure.
The above three C-axis implementation methods: the belt transmission is limited in its output speed and torque due to the speed and transmission ratio; the servo motor can achieve a large torque through the reduction gear box, but due to the existence of the gear transmission gap, the realization The high-precision C-axis manufacturing cost is very high; because the direct drive mode of the electric spindle has no intermediate transmission link, and the electric spindle itself has a large rotational inertia, its dynamic performance is excellent, and it is currently limited by the low output torque of the motor. In the long run, the direct drive mode of the electric spindle has a bright future.
In practice, the C axis mainly considers its accuracy and stability.
Maintain the accuracy of the C axis mainly through the following measures: ① Select the appropriate angle encoder to achieve position feedback, forming a closed-loop control. The accuracy of the angle encoder is selected according to the design accuracy target. Its mechanical allowable speed and electrical allowable speed are matched with the equipment; and it is installed as required. ②Ensure the accuracy of the C-axis drive structure and reduce the transmission gap in its transmission structure. The transmission clearance not only affects the accuracy of the C axis, but also causes vibration during the cutting process, which adversely affects the machining quality of the parts. For a turning center without a Y-axis, in the machining plane, the C-axis forward and reverse rotation and the X-axis feed are interpolated multiple times to form a machining surface. Vibrations are easily generated during the machining process, and the control of the transmission gap is particularly important. The direct drive of the electric spindle has no intermediate transmission link and almost no transmission gap, which has obvious advantages in this respect.
The stability of the C axis mainly refers to the vibration resistance of the spindle system during cutting. In order to increase its stability, the practices in engineering practice include: increasing the inertia ratio of the spindle system, that is, selecting a main motor with a large rotational inertia or reducing the rotational inertia of the driven part, reducing the influence of the workpiece on the quality characteristics of the spindle system; increasing the damping of the spindle system , To absorb the energy of the vibration source, etc. Due to the complexity of the machine tool vibration, it will not be discussed in detail here.
C axis locking mechanism
There are many structural forms of the locking mechanism, the basic principle is to achieve by applying axial or radial friction. You can choose molded products, or you can design your own. The selection is based on the application and the requirements of the application. Pay attention to the uniform distribution of the clamping points to reduce the generation of additional forces.
The self-designed locking mechanism is generally tightened with the entire circumference of the entire friction plate on both sides, and the force is relatively uniform. It can be used as a spindle system damping by adjusting the clamping force. The molded product is locked by local clamping, and it cannot usually be used as a damper.
2.2 Power tool holder
The power tool holder is a tool holder with a driving device in the tool holder, which can provide power for the rotation of the tool at the tool position, and is the core component of the turning center.
The original power tool holder was composed of an electric tool holder or a hydraulic tool holder with a power drive module added. This type of power tool holder is indexed by a motor built into the tool holder. The power drive module motor is independent, and its indexing speed is relatively slow. Used in low-end turning centers.
With the advent of servo tool holders, there is a power tool holder equipped with a power drive module on the servo tool holder body. The tool holder index and power drive are driven by servo motors respectively, so-called dual servo power tool holders (left in Figure 1) .
With the further development of the tool holder technology, a single-servo powered tool holder has emerged (Figure 1 right), the tool holder index and tool rotation are powered by the same servo motor, and the structure is more compact.
The above three kinds of power tool holders, electric tool holders or hydraulic tool holders with power drive modules are the most economical solutions, which can adapt to the processing requirements of general turning centers. Due to the limitations of the tool holder body performance, as user requirements increase, this kind of The plan will eventually be replaced by other plans. The single servo tool holder is favored by users because of its compact structure and superior performance, and its price is also the most expensive. It is mostly used in some high-end turning center products.
In order to further improve the performance of the tool holder, some machine tool manufacturers apply direct drive technology to the power tool holder. Such as Mori Seiki’s built-in motor turret (see picture). Use the built-in electric spindle to directly drive the rotating tool, eliminating the intermediate transmission links such as gears and belts in the power drive structure of the servo tool holder. The tool holder structure is simplified, reducing the generation of vibration and heat. The performance of the rack is improved.
In order to adapt to the development requirements of turning centers, toolholder manufacturers have introduced functionally integrated toolholder products, such as power toolholders with Y-axis (see Figure 3) and power toolholders with B-axis (see Figure 4). The power tool holder with Y axis is equipped with a single servo power tool holder, equipped with a guide screw, which can realize the movement of a linear axis, which increases the planar processing capacity of the tool holder; the power tool holder with B axis, the rotary table and the power tool The combination of the holders can realize the swing of the tool holder within a certain range, so that the tool holder can realize the processing of more angle planes and holes, and the tool configuration of the machine tool is reduced. These functional integrated tool holder products can be directly installed on the host to achieve the corresponding functions, which objectively simplifies the structure of the host and helps to expand the machining range of the machine tool.
3. Function realization of Y axis
In order to improve the planar machining ability of the turning center, a turning center with a Y axis appeared. The so-called Y axis is the axis of motion in the normal direction of the XOZ plane of the machine tool. With the Y axis, it has the ability to move in the vertical plane of XOZ, the tool can be fed on the Y axis, and the machining range of the turning center is expanded.
The turning center without Y axis can only feed in the X axis direction when processing the plane. When the width L of the processed plane is greater than the tool diameter Dr, the feed in the X axis direction cannot complete the plane processing, only C The axis and X-axis interpolation method is realized by piecewise approximation. The resulting plane is not a real plane, but a curved surface with a large radius of curvature, and there are systematic errors. In order to improve the quality of the processed surface, the use of small-diameter milling cutters and multiple interpolation approximations will inevitably lead to low processing efficiency. Therefore, the machining capacity of the turning center without Y axis is limited.
There are two ways to realize the Y axis function of the turning center: virtual Y axis and direct Y axis.
The principle of the virtual Y axis is shown on the left of Figure 6. The Y axis is formed by X1 axis and X2 axis interpolation, and its coordinate value is converted by X1, X2 and angle α. Direct Y-axis is realized by setting feed axis and single servo motor drive in the normal direction of XOZ plane. Figure 5 shows two different Y-axis turning centers. The left picture shows Harding GS MSY series. The Y-axis is the virtual Y-axis. The right is the Shenyang machine tool HTC3285T2Y2. The Y-axis is the direct Y-axis. Above the X-axis skateboard. The two structures have their own advantages and disadvantages: the virtual Y-axis motion is formed by two-axis interpolation, and the Y-axis stroke is short; the tilt angle of the bed saddle is generally within 75°, and the tool holder is located within the skateboard. The direct Y-axis is driven directly by the motor, and the Y-axis stroke is large; the direct Y-axis tool holder is overhanged from the Y-axis slide. When the machine tool is used for heavy cutting, the rigidity of the Y-axis slide must be fully considered.

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