Large CNC machine tool ball screw is: between the screw and nut to ball for the rolling body of the spiral transmission components, it is a precision, high efficiency, high stiffness, high life and energy saving advanced transmission components, It can convert the rotation of the motor into a linear motion of the table, so it is widely used in machinery manufacturing, especially in CNC machine tools and machining centers, providing good conditions for high-speed and high-speed mainframe. With the increasing requirements for the work accuracy of CNC machine tools and machining centers, the high precision of ball screw pairs has become an inevitable trend of development. The mounting accuracy on the host machine has also gradually become a prominent issue in assembly. In order to achieve the requirements of the machine coordinate position accuracy, reduce the lead screw around to prevent radial and bias load, reduce the screw shaft system of all aspects of heating and thermal deformation, to minimize the servo motor drive torque and improve machine work continuously The reliability of the ball screw pair must be increased on the machine tool.
Machine Ball Screws common installation methods:
Ball screw pair usually used in the installation of the following types: Double push - free way; Double push - support; Double push - Double push mode.
Large-scale horizontal machining center, is a high-performance, high rigidity and high-precision mechanical and electrical integration of high-efficiency processing equipment, machining various types of high-precision transmission box parts and other large molds ideal processing equipment. Its three coordinate directions are driven by a servo motor rolling screw drive, the three coordinate directions, that is, X, Y, Z larger working stroke. Due to the structural features of the ball screw pair, the installation of ball screw pairs in three directions on the main unit becomes particularly critical.
First, the old method of installing the ball screw is flawed:
1, according to the traditional process, the installation of ball screw has been along the mandrel and positioning sleeve will be supported at both ends of the bearing seat and the middle nut holder connected together with the dial gauge mandrel axis and the machine to find parallel to the guide rail And make the mandrel drive freely brisk way. This installation method in the three directions smaller stroke smaller CNC machine tools and machining centers more convenient to use. Due to the existence of the mandrel and the positioning sleeve, the bearing sleeve and the bearing holes supported at both ends and the nut sleeve bore at the two ends, there is a large error in the coaxiality between the bearing support hole and the nut seat hole after installation, Increasing the degree of lead screw, the radial bias load increases, causing the temperature of each link of the screw shaft system increases, the thermal deformation becomes larger and the transmission torque increases and a series of serious consequences, leading to servo motor overload, overheating, servo system alarm , Affecting the normal operation of machine tools. In addition, the actual difference between the bearing holes at both ends and the middle nut seat can not be accurately measured, thus affecting the further precise adjustment. For three large CNC machine tools and machining centers in the direction of the three coordinates, due to the required mandrel more than 1500mm, processing difficulties, not easy to ensure accuracy, it can not be used with the mandrel and positioning sleeve alignment method for ball screw installation.
In the production of a horizontal machining center, due to the large three-stroke machine tool, the use of traditional process installation process, due to the coaxiality between the ends of the bearing hole and the middle nut seat hole over tolerance, resulting in ball screw Radial and offset load increase, often appear servo motor overload, overheating, servo system alarm and other phenomena, the machine can not be continuous operation, while seriously affecting the service life of ball screw and transmission accuracy, shorten the maintenance cycle of the host.
2, the use of other assembly methods, such as the use of mobile slide saddle, shorten the distance between the nut seat and the bearing seat, the nut seat and the bearing at both ends are to find the correct method, due to the need to find two positive, plus inspection rod And the sleeve with the gap, the actual application is not satisfactory, the same problem exists.
Second, the new assembly process method:
Through on-site technical research on the product, after repeated exploration and production verification, it is concluded that a more reliable assembly process.
1, First, the use of a single dedicated mandrel nut pin hole correction, with the reference rail to the positive and lateral parallelism within 0.01 / 1000;
2, the nut holder is fixed, the use of professional measuring fixture actually measured the nut seat hole from the reference guide the positive and lateral distance; then, the same overall use of the special rod will be the reference hole and the guide rail positive and lateral Parallelism to find is within 0.01 / 1000, the use of special measuring fixture actually measured the bearing hole from the reference guide the positive and lateral distance, the required hole and the reference guide nut positive and lateral distance consistent with a tolerance of 0.01; the bearing fixed. This method uses a monolithic special rod, not only the length is short, but also the mandrel and positioning sleeve combined into one, eliminating the mandrel and positioning sleeve with the gap between the reliable guarantee bearing hole, nut seat hole and The parallelism of the guide rail;
3, through the actual distance measurement, so that the bearing holes at both ends of the nut seat hole coaxiality has also been a reliable guarantee, so as to reduce the ball screw deputy degree of radial and radial bias load, increased Screw pair installation accuracy.
4, In addition, during the installation of ball screw, the axial movement of ball screw must be strictly controlled. This technical index will directly affect the position accuracy of the ball screw feed system. According to the actual field test shows that: First of all, to install the servo motor end of the bearing within the bearing assembly, which plays a major role in the process of rolling screw drive, the ball screw axial play control between 0.015 ~ 0.02 ; Then, and then the other end of the bearing housing assembly, so that the amount of axial play control less than 0.01. This will effectively guarantee the ball screw feed system stiffness and accuracy.
5, pre-stretch ball screw shaft is also very necessary. In order to improve the rigidity and accuracy of the ball screw feed system, pre-tensioning the screw shaft is very effective, but due to the different cross-section of the screw shaft, and the temperature value is not easy to accurately set, so according to the literature The resulting pre-tension can only be used as a reference. In production, the screw shaft with the target value in the negative direction is often pre-stretched so that the positioning accuracy curve of the machine table approaches the level.
Third, the conclusion:
In production, through the use of the new process of assembly of a large machining center in the three coordinates of the ball screw no-load torque are significantly reduced, no-load current is also significantly reduced, the servo motor and servo system is working properly, not Servo alarm in three directions appears, the machine can run continuously for more than 72h. The above results fully illustrate the use of new technology, ball screw can effectively guarantee the installation of precision, in addition, the method is not subject to the size of the machine stroke. Larger machine tool stroke, the more can highlight its advantages for the large-scale CNC machine tools and machining centers installed ball screw pair provides an effective and reliable method.
