The machine tool spindle is a major part of the machine tool. Because the machine tool spindle is used to mount the tool or workpiece, it is the relative position reference and movement reference of the tool or workpiece. The spindle precision of the machine tool is one of the main accuracy indicators of the machine tool, which directly affects the machining accuracy and surface roughness of the machined parts. The rotary error of the spindle of the machine tool is a comprehensive error, which is the drift of the actual rotary axis relative to the theoretical rotary axis during the rotation of the spindle. The rotary error of the machine tool spindle can be divided into three basic forms: the pure radial runout of the spindle, the pure axial tilt of the spindle, and the purely angular swing. Under normal circumstances, the errors of these three basic forms exist at the same time, and the resulting machining error is also a superposition of three types of error effects. Different types of spindle rotation errors have different effects on machining accuracy. The same type of spindle rotation error has different effects on different machining methods. The following section mainly analyzes the effect of pure radial runout on the part machining.
1 The cause of the pure radial runout of the main shaft The pure radial runout of the main shaft means that the actual rotation axis of the main shaft makes a parallel revolution motion about the average axis.
The main cause of the pure radial runout of the spindle is the accuracy error of the spindle journal and the bearing. The bearings used on the machine tool are divided into two types: sliding bearings and rolling bearings. The type of bearing is different, and the influence on pure radial runout is also different.
1) When using a plain bearing to affect the pure radial runout of the spindle with a plain bearing, the spindle rotates with its journal in the bearing bore. For a lathe machine tool, during the machining process, the direction of force of the main shaft is certain, and the main shaft journal is pressed by the cutting force to a fixed place on the surface of the bearing hole. At this time, different parts of the spindle journal contact with a fixed part of the bearing hole, so the roundness error of the journal will cause the spindle to rotate to produce pure radial runout, and the shape error of the bearing hole has a great influence on the accuracy of the spindle rotation. small. For boring machine tools, the cutting force acting on the spindle rotates with the rotation of the boring tool. A fixed part on the journal contacts with different parts of the bearing hole surface, so the roundness error of the bearing hole will cause the boring machine The pure radial runout of the main shaft, and the error of the shaft shape of the main shaft of the trampoline has little effect on the accuracy of the main shaft.
2) Influence of the pure radial runout of the main shaft by using the rolling bearing When the main shaft is supported by the rolling bearing, the cause of the pure radial runout of the main shaft is not only the accuracy of the bearing itself, but also the accuracy of the matching parts of the bearing.
a. Impact of Rolling Bearing Accuracy The raceway shape accuracy and position accuracy of the outer ring and inner ring of the rolling bearing have a similar effect on the pure radial runout of the main shaft as the plain bearing. as shown in picture 2. When turning, the accuracy of the inner ring raceway has a greater influence; when boring, the outer ring raceway has a greater influence on the accuracy of the raceway.
(a) Holes and raceways are different axes (b) Inconsistent shapes and sizes of non-circular rolling elements in the raceways can cause pure radial runout of the spindle. When the larger diameter rolling element is on the left, the inner ring will move to the right, that is, the spindle position will move to the right: Conversely, when the larger diameter rolling element is on the right, the inner ring will move to the left, ie the spindle will move to the left . Since the rolling body cage rotates at a lower speed than the inner ring, the pure radial runout frequency it causes is lower.
The clearance of the rolling bearing also has an influence on the pure radial runout of the main shaft. Bearing bearing area is set on the right, when the inner ring to the right, when a rolling element is in the horizontal position, the inner ring of the right shift than the rolling element is not in the horizontal position is smaller, so that the spindle produces pure radial runout, which The frequency of pure radial runout is much higher than that of the inner ring.
b. Influence of fittings on the rolling bearing The bearing inner ring is a thin-walled part, which is easily deformed after being subjected to stress. The roundness error of the spindle journal will cause the inner ring to deform and cause the pure radial runout of the spindle. Similarly, the outer ring of the bearing is also a thin-walled part. After being installed in the hole of the box body, the roundness error of the box hole will also cause the outer ring raceway to be deformed, causing the pure radial runout of the main shaft.
2 The mathematical expression of the pure radial runout of the main shaft facilitates the study of the influence of the pure radial runout of the main shaft on the machining accuracy of the part. It is assumed that during the processing of the part, the main shaft of the machine tool rotates on the one hand at the angular velocity w, and at the same time the axis of the instantaneous rotation of the main shaft is opposite. Simple harmonic vibration with the average axis in two perpendicular directions (Y, Z coordinates). Establish an inertial Cartesian coordinate system (OXYZ), where O is the ideal axis position of the spindle, and then establish a coordinate system (O1WMN) that is fixedly connected with the spindle. O1 point is the momentary spindle position of the spindle. After coordinate translation and rotation, the coordinate transformation relationship between the two coordinate systems can be obtained.
3 Influence of pure radial runout of the spindle on the part machining 1) Influence on boring processing When boring holes on a boring machine, the boring tool performs rotary motion with the main axis of the boring machine. When the spindle performs pure radial runout, it will make the axis of the shaft move in a simple direction in a certain direction. The punched hole shape is determined by the trajectory of the boring tool in the inertial coordinate system. Let the position of the boring tool tip in the dynamic coordinate system be: M=R, N=0, where R is the radius of the machined hole.
2) Effect on turning machining When machining the outer circle or boring hole on a lathe, the workpiece rotates with the lathe spindle. Therefore, the geometry of the machined surface of the workpiece is determined by the relative trajectory of the tool in the dynamic coordinate system. The coordinate position of the tool nose in the inertial coordinate system is Z=0 and Y=-R. Among them, R is the processing radius.
If Ï1=R-Acosφ and Ï2=R-Acos(p+φ), then D=Ï1+Ï2=2R, it means that the heart line is an equal-diameter curve, that is, the cross-sectional geometry of the workpiece has no diameter error.
When φ = 0, Ï = RA, when φ = Ï€, Ï = R + A. Therefore, O1 is not the center of the actual contour of the workpiece surface after machining. The distance between the center of the workpiece's actual trunk and O1 is A.
Therefore, it can be concluded that the internal and external circular turning on the lathe, spindle radial runout mainly affects the coaxiality error of the workpiece, and the influence on the workpiece roundness error can be ignored.
4 Conclusion Through analysis of the pure radial runout of the spindle, it can be seen that the spindle rotation error has a great influence on the part machining accuracy. Therefore, in machining, effective measures should be taken to reduce the influence of spindle rotation error on the part machining accuracy. The measures taken can be considered from two aspects. The first thing to do is to increase the spindle's turning accuracy. Spindle bearings are the key components that affect the accuracy of spindle rotation. For precision machine tools, precision rolling bearings can be used. Multi-oil wedge dynamic pressure bearings and hydrostatic bearings are also used. At the same time, it is also necessary to improve the accuracy of parts that match the bearing. Secondly, it is necessary to reduce the influence of spindle rotation error on the part machining. The spindle structure with the separation of movement and positioning can be adopted, so that the turning accuracy of the workpiece in the machining process is not affected by the rotation error of the spindle of the machine tool, so that the spindle rotation error is not reflected on the workpiece.
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