Thin and long stainless steel and carbon steel shafts have an aspect ratio greater than 20 and have poor rigidity. Factors such as turning force, turning heat, and vibration generated during processing will directly affect the dimensional accuracy and parallel accuracy of the workpiece. Machining is difficult. When using a higher turning speed to process thin and long shafts with an aspect ratio greater than 100, the processing difficulty will be higher. The conventional processing method for slender shafts is one clamp and one top or two tops.
In the past, we processed slender shafts with diameters greater than 40 and diameter and shape tolerances of level 6 on the first line. It is difficult to meet processing requirements using conventional processing methods, and often results in products being scrapped during finishing, which affects product delivery dates and greatly increases processing costs. After many analyzes and experiments, I have taken certain technical measures in terms of parts heat treatment, clamping, processing methods, cutting tools, etc., and I can process a slender shaft with an aspect ratio greater than 80 and high diameter and geometric tolerances.
Due to the large aspect ratio of the slender shaft, the rigidity is very poor. During turning, the following problems are prone to occur due to the influence of turning force, clamping force, own gravity, cutting heat, vibration and other factors:
1. Cutting is the resultant force of the radial cutting force of production and the radial component of clamping force, which will bend the workpiece and cause vibration when the workpiece rotates, thus affecting the machining accuracy and surface quality.
2. Due to the deformation of the workpiece due to its own weight, the vibration of the workpiece is intensified, affecting the processing accuracy and surface quality.
3. When the workpiece rotates at a high speed, the centrifugal force increases the bending and vibration of the workpiece.
4. During processing, the workpiece will be bent and deformed under the action of cutting heat.
Therefore, when turning slender shafts, there are higher requirements for the selection of cutting tools, machine tools, auxiliary tools, cutting quantities, process arrangements and technical operations. It is required to reasonably select cutting parameters and cutting dosage. When turning, generally when V=30~70m/min, vibration is likely to occur within this speed range. At this time, the corresponding amplitude has a larger value. Above or below this speed range, the vibration shows a weakening trend. When the processing diameter is less than 10mm, take V≤30m/min; when the processing diameter is greater than 10mm, take V≤70m/min, which is the relationship curve between the limit turning width and turning speed. When cutting in the high-speed or low-speed range, natural vibration is less likely to occur. Especially cutting in the high-speed range can not only improve productivity but also avoid chatter, so it is a worthwhile method. With the selection of feed amount f, the vibration intensity decreases as the feed amount f increases. The width increases as the feed rate increases. In order to avoid the occurrence of chatter, if possible, such as: the machine tool has sufficient stiffness, sufficient motor power, the surface roughness parameters of the workpiece are low, etc., a large feed amount should be adopted. Take f=0.15mm when rough turning, f=0.1mm when semi-finishing, and f=0.06mm when finishing. When choosing the cutting depth aP, the cutting amount should not be too large during turning. When the cutting depth and feed amount remain unchanged, the amplitude gradually decreases as the main deflection angle increases. This is because the radial cutting force is reduced and at the same time the actual cutting width will be reduced. When finishing a slender shaft, take Kr=75~80°, and when finishing turning, use dr=85~90° tool for cutting, which can avoid or reduce vibration. The relief angle has little effect on cutting stability, but when the relief angle is reduced to 2~3°, the vibration is significantly weakened. During reproduction, it was also found that after a certain degree of wear on the flank surface, there will be an obvious vibration reduction effect. When the tool tip arc radius rS increases, the radial force increases accordingly. To avoid self-vibration, the smaller rS, the better. However, the subsequent decrease will reduce the tool life and is not conducive to the improvement of surface roughness. Therefore, during processing, the chip breaker width is R1.5~R3, and the tool tip arc is r0.5.

Technical measures that should be taken in CNC machining of thin and long axes:
The traditional clamping method of top or top clamping generally utilizes the positioning principle and uses a tool rest or center frame as an auxiliary and supporting clamp to increase the rigidity of the workpiece. Improve the coaxiality of the workpiece by adjusting the rotation center of the tailstock. During clamping, the outer circumference adopts line contact to achieve a certain directional adjustment effect. This processing method has no problem with slender shafts with low requirements, but it is difficult to process slender shafts with high precision requirements or large aspect ratios to produce qualified products. Due to the top force of the tip, the radial bending force on the shaft increases during processing, thereby increasing the bending deformation of the shaft and reducing the accuracy of the shaft. The cutting heat of reprocessing and the friction heat of the machining center frame cause thermal expansion of the workpiece. Workpiece expansion increases the curvature of the shaft. In addition, the center line of the claws of the tool holder and the center frame may be completely different from the center line of the shaft. Therefore, the traditional method of one clamp and one top is used to process ultra-slender long shafts. Even if the center frame and tool rest are used to increase the rigidity of the parts, bending deformation cannot be eliminated well and the machining accuracy is reduced.
Two-draw processing method for slender shaft:
In view of the shortcomings of the traditional clamping method, the clamping method of two pulls, that is, one clamp and one pull, can be used to solve this problem. When clamping, it is still necessary to place an open wire ring on the clamping layer so that the clamping between the workpiece and the claw becomes a line contact, so as to play a similar role as a directional knuckle. The other end of the workpiece is tightened by a modified tip. The greater the tension, the better the processing effect. According to the previous analysis, it can be seen that in the two-pull processing method, due to the tension at both ends, the radial bending force on the shaft during processing is reduced, and the bending deformation of the shaft is reduced. In addition, cutting heat and friction heat cause the workpiece to undergo thermal expansion growth, and the tensile force can well prevent the workpiece from expanding and bending deformation. Therefore, compared with traditional processing methods, the two-draw processing method can quickly improve the processing accuracy of the workpiece.

For the two-draw turning process of the slender shaft, the slender shaft generally adopts the modified tip internal thread hole to be equipped with the turning thread: rough turning - semi-finishing turning - fine turning and one-draw thread. If you use a tool rest or a center rest during clamping, care should be taken to ensure that 80% to 90% of the parts on each claw surface match the workpiece. The tool should be installed 0.1mm higher than the center line to reduce the cutting force. First, the workpiece is straightened and then rough turned. If there is no problem during cutting, do not stop midway. Since turning tools are constantly subject to normal wear and tear, it is necessary to use an outer diameter micrometer (based on experience) from time to time to measure the change in diameter of the newly cut shaft. At the same time, appropriate micro-infeed should be used to compensate for the wear of the turning tool. The surface roughness after finishing turning can reach Ral2.5. Start with Ral2.5. When the cutting work begins and there are bamboo knots, twists, and vibration marks, the tool must be withdrawn. At this time, you can slow down the speed by one gear or add a large swingable pad in the middle of the shaft for support, which can reduce the centrifugal force and act as a vibration damper. It should be noted that the first knife must cut off the black skin. Since the surface of the workpiece has different hardness and is curved, the shaft must be bent and deformed after rough turning. The workpiece must be straightened based on the size of the deformation. Secondly, for semi-finishing turning, the turning tool is replaced, the tool holder is replaced with a smaller tool holder, a claw with a long claw, and the various procedures of rough turning are used for cutting. The semi-finished rear axle usually does not bend or deform, and the surface roughness can reach about Ra6.3. Finally, during finishing turning, the contact surface between the front blade and the workpiece is usually 1.5 to 2 times the feed amount. If low-speed precision turning is used, the dimensional changes of the workpiece can be measured during en route and random stops, and micro feed can also be made during cutting. This makes it easier to control the dimensional accuracy of the workpiece. But it is not convenient to improve its surface quality. In production, we mostly use blades with red hardness and good grindability for higher speed cutting. The dimensional accuracy can reach level 6 and the surface roughness can reach above Ral.6.
Through experiments, this processing method can process various slender shafts and ensure the size and shape accuracy of the workpiece.