Turning of slender shafts has always been difficult. The so-called slender shaft is a shaft part whose length to diameter ratio of the workpiece is greater than 25 (ie L/D>25) is called a slender shaft. Under the action of cutting force, gravity and top tightening force, the horizontal slender shaft is easy to bend or even lose stability. The problem of improving the processing accuracy of the slender shaft is the problem of controlling the force and thermal deformation of the process system. Therefore, reverse feed turning is adopted, with a series of effective measures such as the best tool geometric parameters, cutting parameters, tensioning device and bushing type follower tool holder. In order to improve the rigidity of the slender shaft, obtain good geometric accuracy and ideal surface roughness, and ensure the processing requirements.
Turning of slender shaft.
One, slender shaft is the most common problem in processing
1. Large thermal deformation.
Slim shaft has poor thermal diffusivity and large linear expansion when turning.
2, poor rigidity.
The workpiece is subjected to cutting force during turning, the slender workpiece sags due to its own weight, and it is subject to centrifugal force during high-speed rotation. It is very easy to bend and deform.
3. The surface quality is difficult to guarantee.
The cylindricity and surface roughness of the workpiece are affected by the weight, deformation, and vibration of the workpiece.
2. How to improve the machining accuracy of slender shafts and preventive measures
1. Choose the right clamping method
(1) Double center method clamping method. Double-center clamping is used to ensure accurate positioning of the workpiece and easy to ensure coaxiality. However, if the slender shaft is clamped by this method, the rigidity of the slender shaft is poor, the bending deformation of the slender shaft is relatively large, and vibration is prone to occur. Therefore, it is only suitable for the processing of multi-step shaft parts with low aspect ratio, small machining allowance, high coaxiality requirements.
(2) One clamping and one top clamping method. Adopt a clamping method of one clamp and one top. In this clamping method, if the top is pressed too tightly, in addition to bending the slender shaft, it can also hinder the heat elongation of the slender shaft during turning, resulting in bending deformation of the slender shaft due to axial compression . In addition, the clamping surface of the jaw and the center hole may have different axes, which may cause over-positioning after clamping, which can also cause bending and deformation of the slender shaft. Therefore, when the one-clamp-one-top clamping method is adopted, the center should adopt an elastic movable center, so that the slender shaft can be stretched freely after being heated, reducing its bending deformation when heated; at the same time, an opening can be inserted between the jaws and the slender shaft. Travellers to reduce the axial contact length between the jaws and the elongated shaft, eliminate over-positioning during installation, and reduce bending deformation.
(3) Double knife cutting method. Use double-tool turning slender shaft to refit the slide plate of the lathe, increase the rear tool post, and use two front and rear turning tools for turning at the same time. The two turning tools are diametrically opposed, the front turning tool is installed in front and the rear turning tool is installed in reverse. The radial cutting forces generated by the two turning tools during turning cancel each other out. The workpiece is small in deformation and vibration, and the processing accuracy is high, which is suitable for mass production.
(4) It adopts the tool rest and the center rest. The slender shaft is turned with one-clamp and one-top clamping method. In order to reduce the influence of radial cutting force on the bending deformation of the slender shaft, the tool holder and the center frame are traditionally used, which is equivalent to adding a support on the slender shaft. , Increase the rigidity of the slender shaft, which can effectively reduce the influence of the radial cutting force on the slender shaft.
(5) The slender shaft is turned by the reverse cutting method. The reverse cutting method means that in the turning process of the slender shaft, the turning tool is fed to the tailstock direction from the spindle chuck. In this way, the axial cutting force generated during the machining process causes the slender shaft to be pulled, eliminating the bending deformation caused by the axial cutting force. At the same time, the use of the elastic tailstock center can effectively compensate the compression deformation and thermal elongation of the workpiece from the tool to the tailstock section, and avoid the bending deformation of the workpiece.
2, choose a reasonable tool angle
In order to reduce the bending deformation produced by turning the slender shaft, the cutting force generated during turning is required to be as small as possible. Among the geometric angles of the tool, the rake angle, the entering angle and the blade inclination have the greatest influence on the cutting force. Slender shaft turning tools must ensure the following requirements: small cutting force, reduced radial component force, low cutting temperature, sharp edge, smooth chip removal, and long tool life. It is learned from turning steel that the current angle γ0 increases by 10°, and the radial component force Fr can be reduced by 30%; the entering angle Kr increases by 10°, and the radial component force Fr can be reduced by more than 10%; the blade inclination angle λs is negative When the value, the radial component force Fr is also reduced.
(1) Rake angle (γ0) Its size directly affects cutting force, cutting temperature and cutting power, increasing the rake angle. The degree of plastic deformation of the metal layer to be cut is reduced, and the cutting force is significantly reduced. Increasing the rake angle can reduce the cutting force. Therefore, in slender shaft turning, under the premise of ensuring the strength of the turning tool, try to increase the rake angle of the tool. The rake angle is generally γ0=150. Grinding with chip breaker, chip groove width B=3.5～4mm, with grinding br1=0.1～0.15mm, γ01=-25° negative chamfer, so that the radial force component is reduced, the chip generation is smooth, and the chip curling performance is good. The cutting temperature is low, so it can reduce and prevent the bending deformation and vibration of the slender shaft.
(2) Entering angle (kr) The entering angle of turning tool Kr is the main factor that affects the radial force, and its size affects the size and proportional relationship of the three cutting components. As the entering angle increases, the radial cutting force is significantly reduced. The entering angle should be increased as much as possible without affecting the strength of the tool. The main deflection angle Kr=90° (when the knife is installed is 85°～88°), the deflection angle of the grinding pair is Kr'=8°～100. The radius of the tool tip arc γS=0.15～0.2mm, which is beneficial to reduce the radial distribution. force.
(3) The inclination angle of the blade (λs) affects the flow direction of the chip, the strength of the tool tip and the proportional relationship of the three cutting components during the turning process. With the increase of the blade inclination angle, the radial cutting force is obviously reduced, but the axial cutting force and the tangential cutting force have increased. When the blade inclination is in the range of -10°～+10°, the proportional relationship of the three cutting components is reasonable. When turning a slender shaft, a positive edge angle of +3°～+10° is often used to make the chips flow to the surface to be processed.
(4) The rear angle is small a0=a01=4°～60°, which has the effect of anti-vibration.
3. Reasonably control the cutting amount
Whether the cutting amount is selected reasonably, the cutting force generated during the cutting process and the amount of cutting heat are different. Therefore, the deformation caused by turning a slender shaft is also different. The principle of selecting the cutting amount of the slender shaft for rough turning and semi-rough turning is to reduce the radial cutting component as much as possible and reduce the cutting heat. When turning a slender shaft, generally when the aspect ratio and material toughness are large, a smaller cutting amount is selected, that is, more passes and a small depth of cut are used to reduce vibration and increase rigidity.
(1) Back-cutting amount (ap) Under 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, causing the force and heat of the slender shaft The deformation also increases. Therefore, when turning slender shafts, the amount of back-grabbing should be minimized.
(2) Feed (f) The increase in feed 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 has decreased. From the perspective of improving cutting efficiency, increasing the feed rate is more advantageous than increasing the depth of cut.
(3) Cutting speed (v) Increasing the cutting speed will help 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, the cutting speed is too high to easily cause the slender shaft to bend under the action of centrifugal force, destroying the smoothness 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.
3. Conclusion
The turning of a slender shaft is a relatively common machining method in machining. Due to the poor rigidity of the slender shaft, the force and thermal deformation generated during turning are 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 quantities, etc., the processing quality requirements of slender shafts can be guaranteed.
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