We all know that slender shafts are difficult to process. They have poor rigidity and are subject to large stress and thermal deformation during turning, making it difficult to ensure the processing quality requirements of slender shafts.
Today, let's take a look at how German craftsmen turn slender shafts.
By adopting appropriate clamping methods and advanced processing methods, selecting reasonable tool angles and cutting amounts, etc., the processing quality requirements of slender shafts can be ensured.
The most common problems of slender shafts in processing
1. Large thermal deformation
When turning slender shafts, thermal diffusion is poor and linear expansion is large. When the two ends of the workpiece are pressed tightly, it is easy to bend.
2. Poor rigidity
When turning, the workpiece is subjected to cutting force, the slender workpiece sags due to its own weight, and the centrifugal force during high-speed rotation can easily cause it to bend and deform.
3. Surface quality is difficult to ensure
The workpiece's own weight, deformation, and vibration affect the workpiece's cylindricity and surface roughness.
How to improve the processing accuracy of slender shafts
1. Select an appropriate clamping method
(1) Double center clamping method. The use of double center clamping can accurately position the workpiece and easily ensure coaxiality. However, the rigidity of the slender shaft clamped by this method is poor, the slender shaft is subject to large bending deformation, and it is easy to vibrate. Therefore, it is only suitable for machining multi-step shaft parts with small aspect ratio, small machining allowance, high coaxiality requirements.
(2) One clamp and one push clamping method. In this clamping method, if the center push is too tight, in addition to bending the slender shaft, it can also hinder the heat extension of the slender shaft during turning, causing the slender shaft to be axially squeezed and bent. In addition, the clamping surface of the jaws and the center hole may not be coaxial, which will cause over-positioning after clamping, and can also cause the slender shaft to bend. Deformation. Therefore, when using the one-clamp-one-pushing clamping method, the center should use an elastic live center so that the slender shaft can stretch freely after heating, reducing its bending deformation due to heat; at the same time, an open wire ring can be inserted between the jaws and the slender shaft to reduce the axial contact length between the jaws and the slender shaft, eliminate over-positioning during installation, and reduce bending deformation.
(3) Double-tool cutting method. The double-tool lathe slide is modified to turn the slender shaft, and the rear tool holder is added. The front and rear turning tools are used for turning at the same time. The two turning tools are radially opposite, with the front turning tool installed in the right position and the rear turning tool installed in the wrong position. The radial cutting forces generated by the two turning tools offset each other during turning. The workpiece is subjected to small deformation and vibration, and the processing accuracy is high, which is suitable for mass production.
(4) Use a tool holder and a center frame. The slender shaft is turned by a 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, a tool holder and a center frame are traditionally used, which is equivalent to adding a support to the slender shaft, increasing the rigidity of the slender shaft, and effectively reducing the influence of radial cutting force on the slender shaft.
(5) Use the reverse cutting method to turn the slender shaft. The reverse cutting method means that during the turning process of the slender shaft, the turning tool starts to feed from the spindle chuck to the tailstock. In this way, the axial cutting force generated during the processing causes the slender shaft to be pulled, eliminating the bending deformation caused by the axial cutting force. At the same time, the use of an elastic tailstock tip can effectively compensate for the compressive deformation and thermal elongation of the workpiece from the tool to the tailstock, avoiding the bending deformation of the workpiece.
2. Choose a reasonable tool angle
In order to reduce the bending deformation caused 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, main deflection angle and cutting edge inclination angle have the greatest impact on the cutting force. The slender shaft turning tool must meet the following requirements: small cutting force, reduced radial force, low cutting temperature, sharp blade, smooth chip removal, and long tool life. It is known from turning steel that when the rake angle γ0 increases by 10°, the radial force Fr can be reduced by 30%; when the main deflection angle Kr increases by 10°, the radial force Fr can be reduced by more than 10%; when the cutting edge inclination angle λs takes a negative value, the radial force Fr is also reduced.
(1) Rake angle (γ0) directly affects cutting force, cutting temperature and cutting power. Increasing the rake angle can reduce the plastic deformation of the metal layer being cut and significantly reduce the cutting force. Increasing the rake angle can reduce the cutting force. Therefore, in the turning of slender shafts, the rake angle of the tool should be increased as much as possible while ensuring that the turning tool has sufficient strength. The rake angle is generally set to γ0=150°. The rake face of the turning tool should be ground with a chip breaker groove with a chip groove width of B=3.5~4mm, br1=0.1~0.15mm, and a negative chamfer of γ01=-25° to reduce the radial force component, smooth chip removal, good chip curling performance, and low cutting temperature. Therefore, it can reduce and prevent the bending deformation and vibration of the slender shaft.
(2) Main rake angle (Kr) The main rake angle Kr of the turning tool is the main factor affecting the radial force. Its size affects the size and proportional relationship of the three cutting forces. As the main rake angle increases, the radial cutting force decreases significantly. The main rake angle should be increased as much as possible without affecting the tool strength. The main rake angle Kr = 90° (set to 85°~88° when installing the tool), the secondary rake angle K'r = 8°~100°, and the tool tip arc radius γs = 0.15~0.2mm are conducive to reducing the radial force.
(3) Blade inclination angle (λs) The inclination angle affects the flow direction of chips, the strength of the tool tip, and the proportional relationship of the three cutting forces during turning. As the blade inclination angle increases, the radial cutting force decreases significantly, 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 forces is relatively reasonable. When turning slender shafts, a positive blade inclination angle of +3°~+10° is often used to allow the chips to flow to the surface to be processed.
(4) The back angle is small a0=a01=4°~60°, which plays a vibration-proof role.
3. Reasonable control of cutting parameters
Whether the cutting parameters are selected reasonably or not will have different effects on the size of the cutting force and the amount of cutting heat generated during the cutting process. Therefore, the deformation caused by turning slender shafts is also different. The principle of selecting the cutting parameters for rough turning and semi-rough turning of slender shafts is to reduce the radial cutting force and cutting heat as much as possible. When turning slender shafts, generally when the aspect ratio and material toughness are large, a smaller cutting parameter is selected, that is, more passes and smaller cutting depth to reduce vibration and increase rigidity.
(1) Back cutting depth (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 accordingly, causing the stress and heat deformation of the slender shaft to increase. Therefore, when turning slender shafts, the back cutting depth should be minimized.
(2) Feed rate (f). An increase in feed rate will increase the cutting thickness and the cutting force. However, the cutting force does not increase in direct proportion, so the stress 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 reducing 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, destroying the stability of the cutting process, so the cutting speed should be controlled within a certain range. For workpieces with a larger aspect ratio, the cutting speed should be appropriately reduced.
