In the machining process, there are many shaft parts whose length-to-diameter ratio L/d>25. Under the action of cutting force, gravity and top clamping force, the horizontal slender shaft is easy to bend or even lose stability. Therefore, the stress problem of the slender shaft must be improved when turning the slender shaft.
Processing method: reverse feed turning is adopted, and a series of effective measures such as reasonable tool geometric parameters, cutting amount, tensioning device and bushing tool rest are selected.
01
Analysis of Factors of Bending Deformation in Turning Slender Shaft
There are mainly two traditional clamping methods used for turning slender shafts on lathes: one method is: one clamp and one top installation; the other method is two top installations. Here we mainly analyze the clamping method of one clamp and one top.
Through actual processing analysis, the main reasons for the bending deformation of the slender shaft caused by turning are:
(1) Cutting force causes deformation
In the turning process, the cutting force generated can be decomposed into axial cutting force PX, radial cutting force PY and tangential cutting force PZ. Different cutting forces have different effects on bending deformation when turning slender shafts.
1) Influence of radial cutting force PY
The radial cutting force acts vertically on the horizontal plane passing through the axis of the slender shaft. Due to the poor rigidity of the slender shaft, the radial force will bend the slender shaft to make it bend and deform in the horizontal plane. The effect of cutting force on the bending deformation of the slender shaft is shown in Fig. 1.
2) Influence of axial cutting force PX
The axial cutting force acts parallel to the axis of the slender shaft, forming a bending moment on the workpiece. For general turning, the axial cutting force has little effect on the bending deformation of the workpiece and can be ignored. However, due to the poor rigidity of the slender shaft, its stability is also poor. When the axial cutting force exceeds a certain value, the slender shaft will be bent to cause longitudinal bending deformation. as shown in picture 2.
(2) The influence of cutting heat
The cutting heat generated by processing will cause thermal deformation and elongation of the workpiece. Since the chuck and the top of the tailstock are fixed during the turning process, the distance between the two is also fixed. In this way, the axial elongation of the elongated shaft after being heated is limited, resulting in bending deformation of the elongated shaft due to axial extrusion.
Therefore, it can be seen that the problem of improving the machining accuracy of the slender shaft is essentially the problem of controlling the stress and thermal deformation of the process system.
02
Measures to Improve Machining Precision of Slender Shaft
In the process of machining the slender shaft, in order to improve its machining accuracy, different measures should be taken according to different production conditions to improve the machining accuracy of the slender shaft.
(1) Choose the appropriate clamping method
Among the two traditional clamping methods used for turning slender shafts on the lathe, double-top clamping is used, which can accurately position the workpiece and easily ensure coaxiality. But using this method to clamp the slender shaft, its rigidity is poor, the bending deformation of the slender shaft is large, and it is prone to vibration. Therefore, it is only suitable for installation with small length-to-diameter ratio, small machining allowance, and high coaxiality requirements. tall workpieces.
The machining of slender shafts usually adopts the clamping method of one clamp and one top. However, in this clamping method, if the tip is too tight, in addition to bending the slender shaft, it can also hinder the elongation of the slender shaft when it is turned, causing the slender shaft to be axially squeezed and bent out of shape. In addition, the clamping surface of the jaws may not be in the same axis as the tip hole, which will cause over-positioning after clamping, and may also cause bending deformation of the slender shaft. Therefore, when the clamping method of one clamp and one top is used, the top should use elastic living centers. The slender shaft can be freely elongated after being heated to reduce its bending deformation when heated; at the same time, an open steel traveler can be inserted between the jaws and the slender shaft to reduce the axial contact length between the jaws and the slender shaft and eliminate Over-positioning during installation reduces bending deformation.
(2) Directly reduce the force deformation of the slender shaft
1) Use the heel rest and center frame
The slender shaft is turned by the clamping method of one clamp and one top. In order to reduce the influence of radial cutting force on the bending deformation of the slender shaft, the traditional tool rest and center frame are used, which is equivalent to adding a support to the slender shaft. , which increases the rigidity of the slender shaft, which can effectively reduce the influence of radial cutting force on the slender shaft.
2) The slender shaft is turned by the axial clamping method
The use of the tool rest and the center frame can increase the rigidity of the workpiece, but basically eliminate the influence of the radial cutting force on the workpiece. But it still can't solve the problem that the axial cutting force bends the workpiece, especially for the slender shaft with a relatively large long diameter, this bending deformation is more obvious. Therefore, the slender shaft can be turned by the axial clamping method. Axial clamping turning means that in the process of turning a slender shaft, one end of the slender shaft is clamped by a chuck, and the other end is clamped by a specially designed clamping head. The clamping head applies axial tension to the slender shaft. As shown in Figure 4.
During the turning process, the slender shaft is always subjected to axial tension, which solves the problem that the slender shaft is bent by the axial cutting force. At the same time, under the action of axial tension, the degree of bending deformation of the slender shaft due to radial cutting force is reduced; the axial elongation caused by cutting heat is compensated, and the rigidity and processing of the slender shaft are improved. precision.
3) Turning the slender shaft by reverse cutting method
The reverse cutting method means that during the turning process of the slender shaft, the turning tool is fed from the spindle chuck to the tailstock, as shown in Figure 5.
In this way, the axial cutting force generated during processing makes the slender shaft tensioned, eliminating the bending deformation caused by the axial cutting force. At the same time, the elastic tailstock tip can effectively compensate the compression deformation and thermal elongation of the workpiece from the tool to the tailstock, and avoid the bending deformation of the workpiece.
The middle slide plate of the lathe is modified by turning the slender shaft with double knives, the rear tool holder is added, and the front and rear turning tools are used for turning at the same time, as shown in Figure 6.
picture
Figure 6 Double-knife machining and force analysis
Two turning tools are diametrically opposed, the front turning tool is installed upright, and the rear turning tool is installed reversely. The radial cutting forces produced by the two turning tools during turning cancel each other out. The deformation and vibration of the workpiece are small, and the processing precision is high, which is suitable for mass production.
4) Turning the slender shaft by magnetic cutting method
The principle of magnetic cutting method is basically the same as that of reverse cutting method. During the turning process, the slender shaft is stretched by the magnetic force, which can reduce the bending deformation of the slender shaft during processing and improve the machining accuracy of the slender shaft.
(3) Reasonably control the amount of cutting
Whether the choice of cutting amount is reasonable depends on the magnitude of the cutting force and the amount of cutting heat generated during the cutting process. Therefore, the deformation caused by turning the slender shaft is also different.
1) Depth of cut (t)
On the premise that the rigidity of the process system is determined, as the cutting depth increases, the cutting force and cutting heat generated during turning increase accordingly, causing the stress and thermal deformation of the slender shaft to increase. Therefore, when turning slender shafts, the depth of cut should be minimized.
2) Feed amount (f)
The increase of feed rate will increase the cutting thickness and cutting force. However, the cutting force does not increase proportionally, so the force deformation coefficient of the slender shaft decreases. From the perspective of improving cutting efficiency, increasing the feed rate is more beneficial than increasing the cutting depth.
3) Cutting speed (v)
Increasing the cutting speed is beneficial to reduce the cutting force. This is because, as the cutting speed increases, the cutting temperature increases, the friction between the tool and the workpiece decreases, and the force deformation of the slender shaft decreases. However, if the cutting speed is too high, the slender shaft will easily bend under the action of centrifugal force, which will destroy the stability of the cutting process, so the cutting speed should be controlled within a certain range. For workpieces with relatively large length and diameter, the cutting speed should be appropriately reduced.
(4) Choose a reasonable tool angle
In order to reduce the bending deformation caused by turning the slender shaft, it is required that the cutting force generated during turning should be as small as possible. Among the geometric angles of the tool, the rake angle, leading angle and edge inclination angle have the greatest influence on the cutting force.
1) Front angle (γ)
The size of the rake angle (γ) directly affects the cutting force, cutting temperature and cutting power. Increasing the rake angle can reduce the degree of plastic deformation of the metal layer being cut, and the cutting force can be significantly reduced. Increasing the rake angle can reduce the cutting force, so in the slender shaft turning, on the premise of ensuring that the turning tool has sufficient strength, try to increase the rake angle of the tool, and the rake angle is generally γ=13°～17°.
2) Leading angle (kr)
The size of the main deflection angle (kr) affects the size and proportional relationship of the three cutting force components. With the increase of the entering angle, the radial cutting force decreases obviously, but the tangential cutting force increases at 60°-90°. In the range of 60°～75°, the proportional relationship of the three cutting force components is more reasonable. When turning slender shafts, a leading angle greater than 60° is generally used.
3) Blade inclination (λs)
The inclination angle of the blade (λs) affects the flow direction of chips, the strength of the tool tip and the proportional relationship of the three cutting components during the turning process. As the inclination angle increases, the radial cutting force decreases obviously, but the axial cutting force and tangential cutting force increase. When the blade inclination angle is in the range of -10°～+10°, the proportional relationship of the three cutting force components is reasonable. When turning a slender shaft, a positive edge inclination angle of 0°～+10° is often used to make the chips flow to the surface to be machined.
03
in conclusion
Due to the poor rigidity of the slender shaft, the force and thermal deformation generated during turning are relatively large, and it is difficult to guarantee the processing quality requirements of the slender shaft. By adopting appropriate clamping methods and advanced processing methods, choosing reasonable tool angles and cutting parameters, etc., the processing quality requirements of the slender shaft can be guaranteed.
