In terms of transmission, it is divided into belt type, gear type, built-in type, direct joint type, etc., the shaft is often classified by transmission form. The overall requirement of the general spindle unit is to ensure that under the same load and speed, it can drive the workpiece or tool to rotate around its axis accurately and stably, and maintain this performance for a long time.
The Basic Requirements of the Mechanical Spindle:
Principle of Spindle Classification:
CNC machine tools mainly include lathes and milling machines, and their applications are different. The spindle of a lathe clamps the workpiece for rotational movement, while the X/Z axis performs cutting movement. Therefore, the chuck and the rotary hydraulic cylinder are important clamping mechanisms. And integrate with the spindle. While the milling machine spindle clamps the handle and the tool rotates, the X/Y/Z axes perform the cutting movement, so the broaching mechanism is an important tool clamping mechanism and is integrated with the spindle
However, the items that must be considered and calculated when designing the spindle:
The Calculation of Spindle Selection for Machine Tool Mechanical Design:
The structural shape of the general CNC machine tool spindle is mainly determined by the type, quantity, position, and installation method of the transmission parts installed on the shaft, bearings, and other parts. For the convenience of assembly, the spindle is often made into a stepped shape. The overhang at the front end of the main shaft should be as short as possible, and the shape of the front end has been standardized. The spindle is usually hollow with a central hole for the passage of a tool drawbar, broach mechanism, or workpiece drawbar. To pass a thicker tie rod and reduce the weight of the shaft, the center hole is often expected to be larger. If the central hole is too large, the rigidity of the spindle will be affected. In engineering applications, machine tools rarely fail due to fatigue fracture of the spindle. However, due to the excessive deformation of the spindle under the action of cutting force, or the self-excited vibration of cutting, the machine tool cannot meet the production requirements. Therefore, when designing the machine tool spindle, rigidity is the design requirement rather than strength. The static rigidity of the spindle reflects the ability of the spindle to resist static external loads. The bending rigidity K of the main shaft is defined as the force required to be applied in the direction of displacement when the front end of the main shaft generates unit radial displacement. The parameters of the spindle include the average spindle diameter D, the spindle inner hole diameter d, the spindle overhang a, and the spindle support span moment L.
The parameters will directly affect the working performance of the spindle, but to simplify the problem, these parameters are determined by static rigidity conditions. That is select values such as D, d, and a to obtain the maximum static rigidity of the spindle. At the same time, the need for high speed and vibration resistance. According to the type of machine tool, the power of the main shaft transmission or the maximum processing diameter, etc., the average diameter D of the main shaft is initially determined. To prevent the temperature, rise of the main shaft from being too high or to ensure the life of the bearing, it is necessary to check the value. Under the principle of not reducing the rigidity of the spindle, the diameter d of the inner hole of the spindle is determined to meet the requirements for the hole diameter of this type of machine tool. Spindle overhang a. Depending on the structure of the spindle end, to increase the rigidity of the spindle system and components, it should be made as small as possible. During cutting, it must bear intermittent impact load. To avoid vibration affecting the surface roughness of machining, in addition to bending rigidity, the torsional rigidity of the spindle must also be paid attention to.
The shear stress is obtained from the torsion angle θ per unit length, and the relationship between the shear stress and the torque T can be obtained according to the principle of the moment. When the bearing is subjected to the torque T. The sum of the shear stress on the cross-section of the bearing and the torque on the centerline of the shaft must be equal to the torque on the cross-section. The twist angle per unit length of the shaft is directly proportional to the torque and inversely proportional to GJ. GJ is called the torsional stiffness of the shaft, and this value shows the resistance of the shaft to torsional deformation. Therefore, the torsional rigidity and torsion angle must be considered when designing the outer diameter of the main shaft, and then the outer diameter of the main shaft should be determined regarding the standard bearing specifications. The main advantages of the gear-type spindle are a high torque and good stability in heavy cutting. The disadvantages are high inertia, unfavorable high speed, high noise at high speed, and high maintenance and assembly technology. The main application of the belt type is that the toothed belt can transmit a large torque and is not easy to produce slippage, but it is easy to produce wind shearing noise at high speed. The V-belt can easily meet the high-speed requirements, but the transmittable torque is small and it is easy to produce due to insufficient belt tension. Slip. The wind shear produced at high speeds is quieter and sharper.
Gear-type spindles and belt-type spindles are suitable for medium-heavy cutting and low-speed applications, while direct-coupled spindles and built-in spindles are used for light cutting and high-feed applications. Direct-coupled spindles are the entry-level form for high-speed cutting applications. The advantages are simple structure and easy direct transmission to high speed. The disadvantages are the high cost of high-speed couplings, the difficulty in the design of the gripping system, and the application technology of couplings. The main advantages of the high-speed built-in spindle are easy to achieve high speed, low vibration, easy modularization, and low noise. The disadvantages are that the spindle needs to solve the problem of thermal temperature rise, high manufacturing and maintenance costs, and high technical requirements for module manufacturing.
Spindle System Relevant Technology - Spindle Moment Design
When cutting, the unit of cutting removal rate is defined as CC. The removal rate is proportional to the cutting width, feed rate, and cutting depth. When the removal rate is fixed, the feed rate can be reduced. The depth of cut can achieve the goal of high-speed cutting, so the built-in and direct-coupled spindles are suitable for high-speed cutting, and high-feed and low-depth cutting.
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