Shafts are very common mechanical parts, there are a lot of shafts with length to diameter ratio of more than 25 ( L / D > 25 ), we call it a slender (long and thin) shaft.The slender shaft has poor rigidity, due to the impact of cutting force, gravity, and jacking force of tailstock center, resulting in the bending and instability.So, it is more difficult to CNC turning machining it than a regular shaft. How to reduce the deformation of a slender shaft? We will discuss it in this article.
Table of Contents
- Reasons for the Deformation When Machining a Slender Shaft.
- The Measures to Reduce the Deformation of a Slender Shaft.
1. Reasons for the Deformation When Machining a Slender Shaft.
The main reasons are listed as follows.
1). Deformation Caused by the Cutting Forces.
In the turning process, the cutting force generated can be resolved into three component of force, which is axial cutting force Px, radial cutting force Py, and tangential cutting force Py.
The influence of different components of cutting forces on the bending deformation of a slender shaft is different.
a). Effect of the Radial Cutting Force Py.
The radial cutting force is applied vertically to the slender shaft. Due to the poor rigidity, the radial force will cause bending deformation on the slender shaft. As shown in Figure 1.
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Figure 1. Influence of Radial Cutting Force and Force Analysis
b). Effect of the Axial Cutting Force Px.
The axial cutting force acts on the slender shaft in parallel to the axis of the shaft, it forms a bending moment to the workpiece.
For general turning machining, the axial cutting force is not large enough to cause bending deformation on the workpiece, so it can be ignored.
However, due to the poor rigidity and stability of the slender shaft, when the axial cutting force exceeds a certain value, the slender shaft will be bent, leading to longitudinal bending deformation. As shown in Figure 2.
Figure 2. Influence of the Axial Cutting Force and Force Analysis
2). Effects of the Cutting Heat.
In the turning machining, the cutting heat generated will cause elongation of the workpiece, however, the distance between the chuck and the tailstock center is fixed, therefore, the elongation of the axial is under restrictions, resulting in the bending deformation by the axial compression.
So, to improve the machining accuracy of the slender shaft, the key point is to control the force and heat distortion.
2. The Measures to Reduce the Deformation of a Slender Shaft.
1). Adopt the Follower Rest and Steady Rest.
Using the follower rest and steady rest adds support to the slender shaft equivalently, increases the stiffness of the slender shaft, therefore, the effect of radial cutting force on the slender shaft can be reduced effectively.
2).Adopt Tailstock Clamp with Pull-back Force.
The follower rest and steady rest can increase the stiffness of the workpiece, and almost eliminate the influence of radial cutting force on the workpiece, but it can’t solve the problem of bending deformation on the slender shaft by the axial cutting force, especially for the slender shaft with large length to diameter ratio, this kind of bending deflection is more obvious.
We can solve it by adopting a tailstock clamp with pull-back force. By doing so, under the axial tension force, the bending deformation of the slender shaft caused by the radial cutting force will be reduced. As shown in Figure 3 .
Figure 3. Force Analysis for Tailstock Clamp with Pull-back Force.
Also, it compensates for the amount of axial elongation due to cutting heat, thus, improving the rigidity and machining precision of the slender shaft.
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3). Adopt “in reverse” Turning Process.
In the “in reverse” turning method, the cutting tools move towards the tailstock instead of the chuck of the lathe. As shown in Figure 4 .
Figure 4. Force Analysis for “in reverse” Turning Machining Process.
In this method, the axial cutting force generated during the machining process tensions the slender shaft, thereby, eliminating the bending deformation caused by the axial cutting force.
Meanwhile, by using the spring-loaded center, it can effectively compensate for both the workpiece deformation caused by compressing force and the elongation caused by the cutting heat, thus avoiding the bending deformation of the workpiece.
4). Reasonable Control of the Cutting Parameters during the Turning Process.
Different cutting parameters will generate different amounts of cutting force and cutting heat, Therefore, the deformation caused by turning the slender shaft is also different.
a). Depth of the Cut.
With the increase of cutting depth, the cutting force and cutting heat will increase, then the slender shaft deformation caused by cutting force and cutting heat will increase. Therefore, the cutting depth should be reduced to as small as possible when turning the slender shaft.
b). Cutting Feed.
Fine feed is a requirement, to keep cutting pressure low, thus reducing the deformation.
c). Cutting Speed.
Increasing the cutting speed is beneficial to reduce the cutting force.
Because, with the increase of cutting speed, the cutting temperature will increase, however, the friction between the cutting tool and workpiece will decrease, therefore, the deformation of the slender shaft caused by cutting force will decrease.
But if the cutting speed is too high, the slender shaft will bend by centrifugal force, leading to unstable during the cutting process.
So the cutting speed should be controlled in a certain range.
For the workpiece with a large length-to-diameter ratio, the cutting speed should be reduced appropriately.
5). Choosing the Right Cutting Tool Angles.
To reduce the deformation of the slender shaft during the turning machining process, The less cutting force generated, the better.
And in all of the cutting tool angles, the rake angle, cutting edge angle, and cutting edge inclination impact the cutting force mostly.
a). Rake Angle (γ).
It affects the cutting force, cutting temperature, and cutting power directly. By increasing the rake angle, the cutting force can be reduced.
So in the turning machining of the slender shaft, Under the premise of ensuring the tool has enough strength, it is suggested to use a relatively large rake angle, which is generally taken γ=13° ~ 17°.
b). Cutting Edge Angle (Kr) .
It affects the magnitude and proportion of the three cutting components. with the increase of the cutting edge angle, the radial cutting force will decrease, however, the tangential cutting force will increase between the angle of 60° and 90°.
In the angle range from 60° to 75°, the proportional relationship between the three cutting components is more reasonable.
Therefore when turning a slender shaft, the cutting edge angle is generally greater than 60° .
c). Cutting Edge Inclination (λS).
It affects the flow direction of chips in the turning process， the strength of the tip, and the proportional relationship of the three cutting components.
With the increase of cutting edge inclination, the radial cutting force decreases obviously, but the axial cutting force and the tangential cutting force increase.
In the range of -10° ~ +10°, the proportional relationship between the three cutting forces is more reasonable.
So, when turning a slender shaft, we often adopt the positive cutting-edge inclination of 0° ~ +10° to make the chip flow to the surface to be machined.