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Correct assembly of ball screw of large CNC machine tool

March 03,2023

With the improvement of the working accuracy requirements of CNC machine tools and machining centers, the high accuracy of the ball screw pair becomes inevitable, and the installation accuracy of the main machine gradually becomes a prominent problem in assembly. In order to meet the requirements of the accuracy of the coordinate position of the machine tool, reduce the deflection of the lead screw, prevent radial and eccentric load, reduce the temperature rise and thermal deformation of each link of the lead screw shaft system, minimize the transmission torque of the motor in service, and improve the reliability of the continuous operation of the machine tool, full attention must be paid to improving the installation accuracy of the ball screw pair on the machine tool.

The most common installation methods of ball screw are: double push - free mode, double push - support mode, and double push - double push mode.

Large horizontal machining center is a high-performance, high-rigidity and high-precision efficient processing equipment, and is an ideal processing equipment for processing various high-precision gearbox parts and other large molds. The three coordinate directions are driven by servo motor and rolling lead screw drive. The three coordinate directions of X, Y and Z have large working stroke. Due to the structural characteristics of the ball screw, the installation of the ball screw in three directions on the main machine becomes a particularly critical problem.
According to the traditional process method, the installation of the ball screw pair is to use the mandrel and positioning sleeve to connect the supporting bearing blocks at both ends and the intermediate screw seat together for correction, and use a dial indicator to align the mandrel axis with the machine tool guide rail, so as to drive the mandrel freely and quickly.

This installation method is convenient for small CNC machine tools and machining centers with small stroke in three coordinate directions. Due to the fit clearance between the mandrel and the locating sleeve, the locating sleeve and the bearing holes supported at both ends, and the middle nut socket hole, the coaxiality error between the installed bearing hole and the nut socket hole is often large, resulting in the increase of the deflection and radial eccentric load of the lead screw shaft, the increase of the temperature and thermal deformation of each link of the lead screw shaft system, and the increase of the transmission torque, resulting in a series of serious consequences such as the overload and overload of the servo motor.

In addition, the actual difference between the bearing holes at both ends and the middle nut seat hole cannot be accurately measured, which affects further accurate adjustment. For CNC machine tools and machining centers with large stroke in the three coordinate direction, because the required mandrel is more than 1500mm, it is difficult to process the mandrel of this length, and the accuracy is not easy to ensure, so it is impossible to install the ball screw pair by using the alignment method of matching the mandrel with the positioning sleeve.

In the production of horizontal machining center, due to the large three-coordinate stroke of the machine tool, in the process of using traditional process methods, due to the out-of-tolerance of the coaxiality of the bearing holes at both ends and the middle nut seat holes, the radial and offset load of the ball screw is increased, which often leads to the overload and overheating of the servo motor, and the servo system alarms, making the machine tool unable to operate continuously. At the same time, it will seriously affect the service life and transmission accuracy of the ball screw pair, and shorten the maintenance cycle of the main machine.

In addition, the axial displacement of the ball screw must be strictly controlled during the installation of the ball screw. This technical index will directly affect the transmission position accuracy of the ball screw feed system. According to the actual verification of:, first of all, the bearing in the bearing seat of the servo motor should be assembled. The bearing plays a major role in the driving process of the ball screw, and the axial displacement of the ball screw should be controlled between 0.015-0.02mm; Then install the bearing in the bearing seat at the other end and control the axial momentum within 0.01 mm, which can effectively ensure the rigidity and accuracy of the ball screw feed system.

The pre-stretching of the ball screw shaft is also very necessary. In order to improve the rigidity and accuracy of the roller ball screw feed system, it is effective to pre-stretch the screw shaft. However, due to the different screw shaft sections, the temperature rise is difficult to set accurately, and the pretension calculated according to the relevant literature can only be used as a reference. In production practice, the screw axis with the target value in the negative direction is often pre-stretched to make the orientation of the positioning accuracy curve of the machine tool workbench close to the horizontal.

In production practice, the no-load torque and no-load current of the ball screw assembled by the above-mentioned new process method in the large machining center in the three coordinate directions are significantly reduced, the servo motor and servo system work normally, and there is no servo alarm in the three coordinate directions. The machine tool can run continuously for more than 72 hours. This fully demonstrates that this new technology can effectively ensure the installation accuracy of the ball screw pair, and this method is not limited by the travel size of the machine tool, and can be effectively used for any travel size of the machine tool. The larger the stroke of the machine tool, the better its incomparable superiority and effect. It provides an effective and reliable method for the installation of ball screw pairs in large CNC machine tools and machining centers.

Generally speaking, products with high manufacturing costs are accompanied by low production efficiency. There are always some important parts and components involving trade secrets and security in enterprises, which are often processed inside the enterprise. The design of parts is very important. It is important to reduce costs, improve production efficiency, and grasp the performance of products. It is important to be flexible and strict in production. In other words, it takes a lot of effort to design a product. You can imagine how big the profit is. Therefore, the manufacturability of product design determines the manufacturing cost and manufacturing cycle of the product. Only by taking the route of matching product design with the technical characteristics of the enterprise can the advantage of the enterprise be brought into play and the production efficiency of the enterprise be improved.

Improving production efficiency is not only the upgrading of technological concepts and equipment of enterprises, but also the improvement of management concepts. At the same time, advanced tools, fixtures, machine tools, etc. are used. To achieve high-speed and efficient cutting, it is also necessary to apply relevant technologies and management tools, especially to optimize the whole processing process with the help of information technology, and continuously improve the processing efficiency through various ways, so as to enhance the core competitiveness of enterprises.


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