New Technology of Ball Screw Assembly Installed on CNC Machine Tools

CNC ball screw is a spiral drive component rolling with the balls between screw and nut, it is a kind of precision, high efficiency, high stiffness, long service life and energy saving advanced transmission components, can transform the motor rotary motion into the table linear motion, thus widely used in machinery manufacturing, especially CNC machine tools and machining centers, provides a good condition for the principal machine high-speed transmission.

With the increasing of precision requirements of CNC machine tools and machining centers, the high precision of ball screws becomes the inevitable trend of development, and its installation accuracy on the main engine has gradually become a prominent problem in assembly. In order to achieve the accuracy requirements of the machine coordinate position, reduce the screw deflection, to prevent radial offset load, reduce the screw shaft heating and thermal deformation, to minimize the transmission torque of the servo motor and improve the reliability of machine tools in continuous work, the installation accuracy of ball screw on the machine toll must be improved.

The common installation methods of ball screw assembly in machine tools are usually the following: double push – free; double push – support; double push – double push.

Large horizontal machining center is an electromechanical integration high-efficiency machining equipment with high performance, high rigidity and high precision. It is an ideal processing equipment for processing various high precision transmission box parts and other large moulds. Its three coordinate directions are used ball screws transmission driven by servo motors, and the three coordinate directions, namely X, Y and Z, with larger operating stroke. Because of the structural characteristics of the ball screw pair, the installation of the ball screws in three directions on the main engine becomes particularly critical.

cnc machine ball screw

First, there are defects for the old installation method of ball screws:

1, According to the traditional method, the installation of the ball screw pair has been followed by the mandrel and the positioning sleeve connect both ends bearing support and the middle screw nut bracket together to correct, with a dial indicator to parallel the mandrel axis and machine tool slide guide and make mandrel drive freely and lightly. This installation method is more convenient for small CNC machine tools and machining centers with less stroke in the three coordinate directions.

As the fit clearance between the mandrel and positioning sleeve, the positioning sleeve and the bearing hole at both support ends and the middle screw nut bracket hole, the coaxial error of the bearing hole and screw nut bracket hole is larger, resulting in the increase of screw deflection and radial offset load, causing a series of serious consequences such as the temperature rise of the screw shaft, the increase of the thermal deformation and transmission torque, resulting in the overloading and overheating of the servo motor, the servo system alarm, affecting the normal operation of the machine tool.

In addition, the actual difference between the bearing holes at the both support ends and the middle nut bracket hole can not be accurately measured, thus affecting further precise adjustment. For the larger CNC machine tools and machining centers with larger stroke, due to the required mandrel mostly above 1500mm, processing is difficult, not easy to ensure the accuracy, therefore cannot use the correct method of mandrel fit with positioning sleeve for ball screw installation.

In the production of horizontal machining center, due to the machine larger stroke of three coordinates, the use of traditional installation method, due to he coaxial error of both support ends bearing hole and middle nut bracket hole, resulting in ball screw radial offset load increases, servo motor overload and overheat, servo system alarm and so on, so that the machine tool can not run continuously, while seriously affect the service life and transmission accuracy of ball screw, shorten the maintenance period of the main machine.

2, The use of other assembly methods, such as mobile saddle, shorten the distance between nut bracket and bearing seat, the nut bracket and bearing seat corrected respectively, due to the two parts corrected separately, and the fit clearance of mandrel and positioning sleeve, the actual application effect is not ideal, the same problems exist.

cnc ball screw installation

Second, the new assembly technology and method:

Through the technical research of this product, after repeated trial exploration and production verification, a more reliable assembly process method has been summarized.

1, First, use the integral special mandrel to correct the nut bracket hole, so that positive and lateral parallelism with the reference rail within 0.01/1000;

2, After fixed the screw nut bracket, use the professional measuring fixture to measure the positive and lateral distance of the nut bracket hole and reference rail. Then, same operation as step 1, and use the professional measuring fixture to measure the positive and lateral distance of the bearing hole from the reference rail. It is required that the nut bracket hole and the reference rail positive and lateral distance is consistent, tolerance is 0.01, and fix the bearing seat. This method uses integral special mandrel, not only shorten the length, and combine the mandrel and positioning sleeve, eliminate the fit clearance between mandrel and positioning sleeve, reliable to ensure parallelism of bearing hole, nut bracket hole and guide rail;

3, By measuring the actual distance, so that both ends of bearing hole and the screw nut bracket hole coaxiality has also been reliable, thus reducing the ball screw deflection and radial offset load, improve the installation accuracy of ball screw.

4, In addition, in the process of installing ball screw, the axial movement of the ball screw must be strictly controlled, and this technical index will directly affect the transmission position accuracy of the ball screw feed system. According to the actual verification result shows that: first, to install the end support bearing at the servo motor end, it played a major role in the transmission process of ball screw, the axial movement of the ball screw is controlled in the range of 0.015~0.02; then, and install the other end of the bearing, the axial movement control in less than 0.01. In this way, the stiffness and accuracy of the ball screw feed system can be effectively guaranteed.

5, The pre-stretching of the ball screw shaft is also very necessary. In order to improve the stiffness and accuracy of the ball screw feed system, pre-stretching of the screw shaft is very effective, but because the different section of screw shaft, and the temperature rise can not be accurately set, so the pre-tension force calculated by relevant literature can only be used as a reference. In production, often pre-stretching the screw shaft with negative value so that the positioning accuracy curve of machine tool table is close to horizontal.

ball screw cnc installation

Third, the conclusion:

In the production, the no-load torque of the ball screw in the three coordinate directions of a large machining center assembled by the above-mentioned new process method is significantly reduced, the no-load current is also significantly reduced, the servo motor and the servo system are working normally, no servo alarm of the three coordinate directions, the machine tool can run continuously for more than 72h. The above result fully shows that the use of new technology, can effectively guarantee the ball screw assembly installation accuracy, moreover, this method is not limited by the machine tool stroke. The larger the stroke of the machine tool, the more it can highlight its advantages. It provides an effective and reliable method for the ball screws installation of large CNC machine tools and machining center.

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