During the machining process, many shaft-like parts have a length-to-diameter (L/d) ratio greater than 25.
Under the combined effects of cutting forces, gravity, and tip clamping forces, a long, thin, horizontally oriented shaft is subject to bending or even instability.
Therefore, when rotating such shafts, it is necessary to improve their stress distribution.
Machining method: Reverse feed turning is employed along with a series of effective measures such as selecting appropriate tool geometry, cutting parameters, clamping devices and using a stable support to support the spindle.
1. Analysis of factors causing bending deformation during turning of long and thin shafts
There are two main traditional clamping methods for turning long, thin shafts on a lathe: one uses a point and a center and the other uses two points.
Here, we mainly analyze the one-end and one-center fixing method, as shown in Figure 1.
Through practical machining analysis, the main reasons for bending deformation of long and thin shafts during turning are:
(1) Deformation caused by cutting forces
During the turning process, the generated cutting forces can be decomposed into axial cutting force PX, radial cutting force PY and tangential cutting force PZ. Different cutting forces have different effects on the bending deformation of long and thin shafts during turning.
1）Effect of radial cutting force PY
The radial cutting force is applied perpendicular to the plane passing through the axis of the long thin shaft. Due to the low rigidity of the long, thin shaft, the radial force will bend the shaft, causing it to deform in the horizontal plane. The effect of radial cutting force on the bending deformation of the long thin shaft is shown in Figure 1.
2）Effect of axial cutting force PX
The axial cutting force is applied parallel to the axis of the long, thin shaft, creating a bending moment in the workpiece. For general turning processes, the effect of axial cutting force on the bending deformation of the workpiece is not significant and can be ignored. However, due to the low rigidity and stability of the long and thin shaft, when the axial cutting force exceeds a certain value, the shaft will bend longitudinally and cause deformation. This is shown in Figure 2.
(2) Effect of cutting heat
The cutting heat generated during machining can cause thermal deformation and elongation of the part. During turning, the center of the chuck and tailstock are fixed and the distance between them remains constant.
As a result, the axial elongation of the long and thin shaft is limited by the fixed distance, leading to axial compression and bending deformation of the shaft when it undergoes thermal expansion.
Therefore, improving the machining accuracy of long, thin shafts is essentially a matter of controlling the forces and thermal deformation in the process.
2. Measures to improve the machining accuracy of long and thin shafts
To improve the machining accuracy of long and thin shafts, different measures should be taken according to different production conditions.
(1) Choosing the appropriate fixing method
Of the two traditional clamping methods used to turn long, thin shafts on a lathe, using a double center point clamping method ensures accurate positioning and coaxiality of the part.
However, this method is not suitable for long and thin shafts with low rigidity, high bending deformation and vibration, and is only suitable for workpieces with low length-to-diameter ratios, small machining tolerances and high coaxiality requirements.
To machine long, thin shafts, a one-end, one-center clamping method is commonly used.
However, if the center of the tailstock is too tight, it may not only bend the long and thin shaft, but also make it difficult to thermally stretch during turning, causing axial compression and bending deformation.
In addition, the clamping surface of the chuck and the center hole of the tailstock may not be coaxial, causing overlap after clamping and resulting in bending deformation of the long, thin shaft.
Therefore, when using the one-end and one-center fixing method, an elastic top should be used to allow the long and thin shaft to elongate freely due to thermal expansion, reducing thermal bending deformation.
At the same time, an open wire ring can be inserted between the mandrel and the long thin shaft to reduce the axial contact length between them, eliminate overlapping during installation, and reduce bending deformation, as shown in Figure 3.
(2) Directly reducing the tension deformation of long and thin shafts
1） Using a steady rest and a center rest
When turning long and thin shafts using a one-end and one-center clamping method, to reduce the influence of radial cutting force on bending deformation, a stable support and a center support are traditionally used.
This adds support to the long, thin shaft, increasing its rigidity and effectively reducing the impact of radial cutting force.
2） Using axial clamping method to rotate long and thin shafts
Although the use of constant support and central support can increase the rigidity of the workpiece and eliminate the impact of radial cutting force, it cannot solve the problem of axial cutting force by bending the workpiece, especially for long shafts and thin ones with large length-to-diameter ratios, where bending deformation is more obvious.
Therefore, an axial clamping method can be used to turn long, thin shafts. Axial clamp turning refers to a process in which one end of the long, thin shaft is clamped by a chuck and the other end is clamped by a specially designed collet chuck that applies axial tension to the shaft, as shown in Figure 4 .
During the turning process, the long and thin shaft is constantly subjected to axial tension, which solves the problem of axial cutting force when bending the workpiece.
Under the action of axial tension, the degree of bending deformation caused by radial cutting force is reduced and the axial elongation caused by cutting heat is compensated, improving the rigidity and machining accuracy of the long and thin shaft.
3） Using reverse cutting method to turn long and thin shafts
The reverse cutting method refers to a process in which the cutting tool advances in the tailstock direction from the spindle chuck during the long and thin shaft turning process, as shown in Figure 5.
In this way, the axial cutting force generated during the machining process leaves the long, thin shaft under tension, eliminating bending deformation caused by the axial cutting force.
At the same time, the use of an elastic center of the tailstock can effectively compensate for the compression deformation and thermal stretching of the workpiece from the cutting tool to the end of the tailstock, preventing bending deformation of the workpiece.
Using a dual-tool approach to turning long, thin shafts on a modified lathe bed with an added rear tool holder, both the front and rear cutting tools can be used simultaneously, as shown in Figure 6.
Two turning tools are positioned radially opposite each other, with the front tool installed in the correct orientation and the rear tool installed in reverse.
The radial cutting forces generated during turning with the two tools cancel each other out, resulting in minimal deformation and vibration of the part and high machining precision, making it suitable for batch production.
4） Magnetic cutting is used for turning slender shafts.
The principle of magnetic cutting is similar to that of reverse cutting. During turning, the slender shaft is stretched by magnetic force, reducing its bending deformation and improving its machining accuracy.
(3) Reasonably control the cutting amount.
The selection of the cutting amount impacts the size of the cutting forces and the amount of cutting heat generated during the cutting process. Therefore, it also affects the deformation caused when rotating slender shafts.
1） Cutting depth
