Measures to improve the rotation accuracy of machine tool spindle components

Abstract: The spindle assembly is one of the important parts of the machine tool. When the machine tool is working, the workpiece or the tool is clamped by the spindle to directly participate in the surface forming movement. It is the terminal executive part of the machine tool to realize the cutting process, which directly affects the processing quality and productivity. Therefore, in the production, the rotation accuracy, static stiffness, vibration resistance, thermal deformation and wear resistance of the spindle assembly have high requirements. This article only briefly describes the measures to improve the rotation accuracy of the spindle assembly.

When machining parts, there will always be machining errors, which will affect the accuracy of the parts and cause serious waste. According to a large number of practical proofs, the main factors affecting machining accuracy are theoretical design errors, machine tool manufacturing errors and component wear, fixture errors, tool errors, elastic deformation and thermal deformation of the process system, errors caused by the mechanical properties of the workpiece itself, and operators Measurement errors, etc., as far as the machine tool is concerned, the spindle assembly is a key part that determines the position and movement of the workpiece (tool). Its rotation accuracy error has a great influence on the machining accuracy and surface roughness of the part. The rotation accuracy of the spindle assembly can be improved Machining accuracy of machine tools.

1. Measures to improve spindle rotation accuracy in machine tool manufacturing

The rotation accuracy of the spindle refers to the radial and axial runout values of the workpiece or tool at the front end of the spindle under the condition of no load and low speed after the machine is assembled. To improve the rotation accuracy of the main shaft, in addition to ensuring a certain accuracy of the main shaft and bearings, technological measures can also be taken.

(1) Finishing after assembly. For CNC machine tools, the spindle is relatively precise, requiring high enough rigidity and rotation accuracy. In machine tool manufacturing, the machining accuracy of shafting components is relatively high. It is very difficult to control the machining quality of the parts one by one to make the whole machine reach a certain accuracy index. For example, for the spindle assembly of a coordinate boring machine, the tolerance of the spindle taper axis axis runout is only 0.001~0.002mm. If it is guaranteed by the accuracy of the spindle or bearing, it will require accuracy that cannot be achieved at present. In order to ensure the accuracy of the spindle components, the method of assembly and finishing is adopted in the process. The assembly is completed first. The outer circle of the spindle sleeve is finely ground on a precision cylindrical grinder based on the tapered holes at both ends of the spindle, and then the outer circle of the spindle sleeve is finely ground on the precision cylindrical grinder. The outer circle of the cylinder is the reference, and the taper hole of the spindle is precisely ground. After finishing the fine grinding and passing the inspection, it will be disassembled and cleaned, and then assembled to be the finished product. During maintenance of this type of machine tool, if the bearing needs to be disassembled, mark should be made between the outer ring of the bearing and the box or sleeve hole, and between the inner ring of the bearing and the journal before disassembling. The accuracy before disassembly cannot be guaranteed, and the result is half the effort.

(2) Error matching method. For a spindle with a tapered hole, when measuring the rotation accuracy of the spindle, it is necessary to match the measuring rod with the tapered hole of the spindle. The radial runout of the end of the measuring rod is equal to the error a caused by the front bearing, the error b caused by the rear bearing and Since the machining error c of the spindle taper is twice the vector sum of the three, a, b, and c all have a certain direction. Therefore, if these three errors are assembled in a certain direction, the errors can cancel each other out. First, measure the radial runout Δa, Δb and the direction of the inner ring of the front and rear bearings, and then calculate a and b. The directions of a and b are determined by the directions of Δa and Δb, respectively. Put the main shaft (without bearing) on the V-shaped iron, the size and direction of the machining error c of the tapered hole can be measured. Adjust the position of the inner ring of the front and rear bearings and the main shaft, adjust the three errors into a straight line, make the smaller two (a, b) face one direction, and the largest (c) face the other direction, so that the error can be It is reduced to d, but it cannot be completely offset, as shown in Figure 1. If you choose according to Figure 2, it can be completely offset, but you need to calculate the angles of a, b, and c accurately.

2. Measures to improve spindle rotation accuracy during machine tool use

In mechanical production, facing machine tools with unsatisfactory rotation accuracy of spindle components, qualified products can be processed through certain technical means. Generally, the following methods are used:

(1) Measures to eliminate spindle rotation error. If the rotation accuracy of the main shaft of the machine tool is low, a suitable dial can be designed to separate the transmission and positioning and eliminate the error caused by the radial runout of the main shaft. Figure 3 shows the working diagram of a cylindrical grinder with low spindle precision for grinding parts with higher precision. The dial is used to drive the part to rotate, and the spindle center does not rotate, so that the error of the spindle cannot affect the part.

(2) The specific application of the error matching method. Among the many factors that cause part errors, change the direction of one or several errors so that the error values are in opposite directions and offset each other as much as possible, so as to improve the machining accuracy of the parts and achieve the goal of roughness and precision. As shown in Figure 4, an ordinary precision vertical milling machine mills precision keyways. Since the chuck sleeve and the milling cutter chuck have eccentricity errors, the eccentricity of the milling cutter installation is e1+e2, so the milled keyway width difference is 2 (E1+e2). If the milling cutter chuck is rotated 180º, the installation deviation of the milling cutter is the smallest, which is equal to e1-e2. When e1=e2, the axis of the milling cutter chuck coincides with the axis of the spindle. If the eccentric vibration of the spindle center is too large, you can continue to adjust the eccentricity of the collet sleeve to offset the eccentricity of the spindle, and further improve the processing accuracy of the keyway width. Of course, this also requires careful testing of the accuracy of the spindle and end mill chuck, otherwise it will be counterproductive.

Figure 3 Partial structure diagram of the main shaft of the cylindrical grinder

(3) "Self-drying" processing method. The "self-drying" processing method is widely used in production. It refers to choosing one to install a tool to process the other between two parts that need to ensure the positional relationship. This method is not only used to achieve the final accuracy in machine tool assembly, but also is often used as an effective measure to ensure machining accuracy in the machining of parts. For example, when machining parts with a three-jaw self-centering chuck on a lathe, the center of the inner supporting surface of the three-jaw self-centering chuck is required to be concentric with the axis of the lathe spindle. If it is not concentric, the processed parts will be out of tolerance, as shown in Figure 5. In order to overcome the shortcomings of the low precision of the mutual position of the spindle and the three-jaw self-centering chuck, the "self-drying" machining method can be used. That is, the eccentricity of the center of the three-jaw self-centering chuck to the center of rotation of the spindle is repaired by using the rotation accuracy of the spindle axis of the lathe. As shown in Figure 6, when repairing the three-jaw self-centering chuck, clamp a disc between its jaws, so that the three-jaw self-centering chuck mechanism generates stress similar to that during work, and then performs fine boring or fine grinding. , So that the main shaft of the lathe and the center of the three-jaw self-centering chuck have a higher coaxiality. The coaxiality between the spindle and the three-jaw self-centering chuck obtained by this method is generally lower than that of the lathe spindle, and is at most equal to the accuracy of the lathe spindle. Therefore, the accuracy of the lathe spindle limits the grinding accuracy of the three-jaw self-centering chuck , And if the ground three-jaw self-centering chuck is reused in other machine tools, it needs to be re-grinded to ensure the centering accuracy of the three-jaw self-centering chuck on the machine tool. So the application of this method There are certain limitations.

1. Three-jaw self-centering chuck axis 2. Reference plane 3. Jaws 4. Spindle axis 5. Workpiece

3. Conclusion

The high degree of automation of the machine tool puts forward higher requirements on the machine tool spindle. First, it has a sufficiently high speed and large power to meet the needs of high-efficiency processing; secondly, the change of the spindle speed is rapid and reliable and the spindle speed range is wide, and it can generally be automatic Variable speed; again, the spindle should have high enough rigidity and rotation accuracy to meet the product accuracy requirements; because the accuracy of the spindle assembly has a greater impact on the quality of the parts, the designer is required to choose the transmission mode of the machine tool, the structure and precision design of the gearbox, and the spindle The selection and configuration of the bearing model and the design of the spindle assembly are demonstrated to achieve the effects of high spindle rigidity, high rotation and movement accuracy, good heat dissipation conditions and small thermal deformation. To solve the problem of improving the spindle accuracy, not only pay attention to the design and manufacturing stage of the machine tool, but also pay attention to how to improve the rotation accuracy of the machine tool spindle in the application. Based on this, this article has certain practical significance in guiding the production and maintenance of the machine tool.