Turning and milling compound machine tools are the fastest growing and most widely used CNC equipment in compound machine tools. For high-speed precision turning and milling CNC machine tools, the spindle system is an electric spindle structure. The machining accuracy of the machine tool depends to a large extent on the accuracy of the machine tool spindle system, and is affected by the position accuracy of the tool post and the dynamic performance of the machine tool. Therefore, how to improve the accuracy of the spindle system, the positioning accuracy of the tool post and the dynamic performance of the machine tool is the key to the design of the machine tool in the manufacturing and assembly process of the turning-milling compound machine tool. This article analyzes the factors that affect the machining accuracy of the turning and milling machine from the following aspects, and proposes corresponding solutions.
1. Precision of Spindle System of Turning and Milling Machine Tool
1. Indexing accuracy and exact stop accuracy of the electric spindle system
The indexing and exact stop accuracy of the turning and milling machine tool spindle system is an important index to measure the accuracy of the machine tool spindle, and it is also an important factor that affects the machining accuracy of the machine tool. In order to achieve the high indexing accuracy and exact stop accuracy of the electric spindle system, the CNC turning and milling machining centers in the world all adopt a built-in spindle motor (electric spindle for short), and the spindle and the motor rotor are assembled into one body to reduce the impact of the intermediate transmission on the machine tool. Impact. Impact on spindle accuracy. When the motor rotates, the spindle rotates. The spindle speed and position loop adopts high-precision feedback components to achieve full closed-loop control. The position signal collected by the feedback element is processed by the frequency division of the numerical control system, so that the indexing accuracy of the spindle can reach 0.001 degrees (the minimum indexing unit of the spindle is 3.6") or lower, so that the machine tool spindle system has higher indexing accuracy and quasi-stop accuracy .
To achieve the above goals, the following solutions are adopted:
a. The motor rotor and the machine tool spindle adopt a special assembly process to ensure the rigidity of the connection between the machine tool spindle and the inner hole of the motor rotor and transmit a large cutting torque.
b. When the motor spindle is assembled and matched, high-speed dynamic balance is adopted to reduce the residual imbalance of the spindle group and ensure the stability of the spindle during high-speed rotation.
c. The electric spindle system adopts spindle braking and damping devices. When the spindle needs to stop milling or drilling, the braking state of the spindle is to ensure that the spindle will not swing during the milling process. In continuous indexing milling, the spindle is in a damped state to ensure the rotation stability of the spindle during the milling process. The braking state and damping state can be automatically switched and set through programming. Through the above measures, the indexing accuracy of the spindle system is less than 24, and the repeat indexing accuracy of the spindle system is less than 12.
2. Rotation accuracy of electric spindle system
The CNC turning-milling composite machining center is a high-precision machine tool, the spindle group is a key component, and the rotation accuracy of the spindle group directly affects the accuracy of the workpiece.
In order to ensure the high rotation accuracy of the spindle group, the spindle system adopts a series of technological measures such as the connection of the spindle and the motor rotor, the coordination of the stator sleeve and the spindle box, and the coordination of the two ends of the spindle with the bearing group.
a. Before the main shaft is assembled, the accuracy of the main shaft box hole should be ensured. The roundness, dimensional tolerance and coaxiality of the front and rear bearing holes are within the specified range; the radial runout of the main shaft journal, the coaxiality of the front and rear journals, the front and rear journals The tolerance of radial runout and end runout. The spindle end should be controlled within 0.002 mm.
b. According to the bearing outer ring size, the spindle box hole is polished so that the bearing outer ring tolerance is strictly controlled within the specified range; the motor rotor, electric spindle spindle, etc. adopt special assembly processes.
After taking the above measures, the rotation accuracy of the electro-spindle system is guaranteed. The radial and end runout of the spindle is within 0.002 mm, which lays the foundation for ensuring the machining accuracy of the workpiece.
2. Dynamic performance of turning and milling compound machine tools
a. The dynamic performance of the machine tool spindle system. The machining accuracy of the machine tool is not only affected by previous factors, but also by the dynamic characteristics of the spindle bearing system. The vibration of the machine tool depends on the dynamic characteristics of the spindle bearing system. Since the electric spindle bearing system is a complex vibration system, its dynamic characteristics are not only related to the damping and static stiffness of the system, but also related to the structure of the system.
The determined pattern shape is relevant. Therefore, in the design of machine tools, dynamic analysis should be carried out to calculate the first and second natural frequencies of the spindle box, so that the first natural frequency avoids the lowest speed of the spindle, and the second natural frequency avoids the highest speed of the electric spindle. Avoid resonance. And it eliminates the impact of vibration on the machining accuracy of the machine tool.
The overall rigidity of the machine bed has a great influence on the machining accuracy of the turning-milling compound machine tool. In machine tool design, it is necessary to carry out finite element analysis on the machine bed, optimize the structure of the machine bed, increase the rigidity of the machine bed, improve the dynamic performance of the machine tool, and avoid the deformation of the machine tool structure due to insufficient local rigidity of the machine tool, so that the machine tool is in the design Not in force. Thereby affecting the machining accuracy of the machine tool.
three. Tool post indexing positioning accuracy and positioning accuracy
The turning tools, radial and axial power tools can be installed at each workstation of the tool post of the turning-milling composite machining center, and the types and quantities of tools can be configured according to the needs of the workpiece. The position accuracy of the electric tool post is an important factor that affects the machining accuracy of the turning-milling compound machine tool, which should be paid attention to in the design and manufacturing process of the machine tool. In order to improve the indexing and positioning accuracy of the tool post, the following solutions can be used:
a. The consistency of the indexing and positioning of the tool holder has a great influence on the size consistency of the workpiece. For this reason, the electric tool post with high indexing accuracy is selected, and a special assembly process is adopted to ensure that the indexing and positioning accuracy of the tool post is less than 0.005mm.
b. The position accuracy of the X/Z axis of the tool post to realize the X axis tracking servo movement is also an important accuracy index, which is directly related to the size dispersion and consistency of the workpiece. This is a comprehensive accuracy, involving the accuracy of the ball screw and bearing, the servo sensitivity of the tool carrier trailer, the matching of the trailer servo inertia and the load inertia, etc. The high-precision ball screw and bearing matching parts are selected in the design to make the servo inertia and The load inertia is reasonably matched; during assembly, the bearing is pre-tightened and the ball screw is pre-tightened to eliminate the influence of thermal deformation on the accuracy of the bearing and ball screw, ensuring that the tool holder has good servo sensitivity, X/Z direction positioning accuracy and repeatability high. positioning accuracy.