How To Control The Accuracy of Turning Machining Slim Shaft? How To Improve The Deformation?

How To Control The Accuracy of Turning Machining Slim Shaft? How To Improve The Deformation?

In the machining process, there are a lot of shaft parts of the L/D ratio L / d> 25. In the cutting force, gravity and the top of the center of the tightening force of the role of the transverse slim shaft is easy to bend or even destabilize, therefore, turning slim shaft must improve the slim shaft of the force problem.

Machining method: Reverse feed turning, selection of reasonable tool geometry parameters, cutting dosage, tensioning device and sleeve type tool holder and a series of effective measures.

Analysis of the factors of bending deformation produced by turning slim shafts

There are two main traditional clamping methods used in turning slim shafts on lathes: one is: one clamp and one top mounting; the other is: two top mounting. Here we mainly analyze the clamping method of one clamp and one top.

Analyzed through the actual processing, the main reasons for the bending deformation of the slender shaft caused by turning are:

1. Deformation caused by cutting force

In the turning process, the cutting force can be decomposed into axial cutting force PX, radial cutting force PY and tangential cutting force PZ. different cutting forces on the turning of slender shaft bending deformation is different.

(1) The effect of radial cutting force PY

Radial cutting force is acting vertically through the slender axis axis in the horizontal plane, due to the slender axis of the rigidity of the poor, radial force will be the slender axis of the top of the bend, so that its bending deformation in the horizontal plane. 

(2) The effect of axial cutting force PX

Axial cutting force is acting parallel to the direction of the slender axis axis, it forms a bending moment on the workpiece. For general turning, the effect of axial cutting force on the bending deformation of the workpiece is not large and can be ignored. However, due to the poor rigidity of the slim axis, its stability is poor, when the axial cutting force exceeds a certain value, the slim axis will be bent and longitudinal bending deformation occurs.

2. Effects of cutting heat

The cutting heat generated by machining causes thermal deformation and elongation of the workpiece. Since the chuck and the top of the tailstock are fixed during the turning process, the distance between them is also fixed. In this way, the axial elongation of the slim shaft after being heated is limited, resulting in bending deformation of the slender shaft due to axial extrusion.

Therefore, it can be seen that the problem of improving the machining accuracy of the slender shaft is essentially the problem of controlling the force and thermal deformation of the process system.

measures to improve the machining accuracy of slim shafts

In the process of slim shaft machining, in order to improve its machining accuracy, different measures should be taken according to different production conditions to improve the machining accuracy of slender shaft.

1. Choose suitable clamping method

In the lathe turning slim shaft using the two traditional clamping methods, the use of double top clamping, accurate positioning of the workpiece, easy to ensure coaxiality. But with this method of clamping slim shaft, its rigidity is poor, slim shaft bending deformation is large, and easy to produce vibration. Therefore, it is only suitable for installing workpieces with small L/D ratio, small machining allowance and high coaxiality requirements.

Processing of long and thin shafts usually use a clamp and a top of the clamping method. However, in this clamping method, if the top of the top too tight, in addition to the slender shaft may be bent top, but also hinder the turning of the slender shaft of the thermal elongation, resulting in the slim shaft by the axial compression and bending deformation. In addition, the clamping surface of the jaws and the top of the hole may not be the same axis, the clamping will produce over-positioning, can also lead to the sl shaft to produce bending deformation. Therefore,  the top should be used to elastic live top, so that the elongated shaft can be free to elongate after heat, to reduce its heat bending deformation; at the same time in the jaws and the elongated shaft between the pad into an open steel wire ring, in order to reduce the jaws and the elongated shaft of the axial contact length, to eliminate the installation of the over-positioning, reduce the bending deformation.

Reduction of force deformation of slim shafts

(1) use of a tool holder and center holder

In order to reduce the influence of radial cutting force on the bending deformation of the slim shaft by using the clamping method of one clamp and one top to turn the slim shaft, the traditional use of the heel tool holder and center holder is equivalent to adding a support to the slim shaft, which increases the stiffness of the slim shaft, and can effectively reduce the influence of radial cutting force on the slim shaft.

(2) Turning of slim shafts by axial pulling clamping method

The use of heel tool holder and center frame, although it can increase the rigidity of the workpiece, basically eliminating the impact of radial cutting force on the workpiece. But can not solve the problem of axial cutting force to the workpiece bending, especially for the long diameter of the slim shaft, this bending deformation is more obvious. Therefore, the axial clamping method can be used for turning long and thin shafts. Axial clamp pull turning refers to the process of turning slim shafts, slim shafts at one end by the chuck clamping, the other end by the specially designed clamping head clamping, clamping head to the slender axial tensile force, the axial tensile force.

In the turning process, the slim shaft is always subject to axial tension, which solves the problem of bending the slim shaft by axial cutting force. At the same time, under the action of axial tension, the bending deformation of the slim shaft due to radial cutting force is reduced; the axial elongation due to cutting heat is compensated, and the rigidity and machining accuracy of the slim shaft are improved.

3) Turning of slim shafts by reverse cutting method

The axial cutting force generated in the machining process thus strains the slender shaft, eliminating the bending deformation caused by the axial cutting force. At the same time, the use of elastic tailstock top can effectively compensate for the pressurized deformation and thermal elongation of the workpiece in the section from the tool to the tailstock, avoiding the bending deformation of the workpiece.

Reasonable control of cutting amount

Whether the cutting dosage is selected reasonably or not, the size of the cutting force and the amount of cutting heat generated during the cutting process are different. Therefore, the deformation caused by turning slim shafts is also different.

1）Depth of cut(t)

Under the premise of determining the rigidity of the process system, as the depth of cut increases, the cutting force and cutting heat generated during turning increases, causing the force and heat deformation of the slender shaft to increase. Therefore, when turning slender shafts, the depth of cut should be minimized.

2) Feed (f)

Increasing the feed will increase the cutting thickness and increase the cutting force. However, the cutting force does not increase proportionally, so the force deformation coefficient of slender shaft decreases. If from the point of view of improving cutting efficiency, increasing the feed is more favorable than increasing the depth of cut.

3) Cutting speed (v)

Increasing the cutting speed is favorable to 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 slim shaft decreases. However, the cutting speed is too high and easy to make the slim shaft bend under the action of centrifugal force, destroying the smoothness of the cutting process, so the cutting speed should be controlled in a certain range. For workpieces with larger length and diameter, the cutting speed should be appropriately reduced.

Choose the right tool angle

In order to reduce the bending deformation generated by turning slim shafts, the smaller the cutting force generated during turning is required, and among the geometric angles of the tool, the front angle, main deflection angle and camber angle have the greatest influence on the cutting force.

1) Front angle (γ)

Front angle (γ) its size directly affects the cutting force, cutting temperature and cutting power. Increase the front angle, can make the degree of plastic deformation of the cut metal layer is reduced, cutting force significantly reduced. Increase the front angle can reduce the cutting force, so in the slender shaft turning, under the premise of ensuring that the tool has sufficient strength, try to make the front angle of the tool increases, the front angle is generally taken as γ = 13 ° ~ 17 °.

2) Main deflection angle (kr)

The size of the main deflection angle (kr) affects the size and proportionality of the three cutting forces. As the main deflection angle increases, the radial cutting force decreases significantly, while the tangential cutting force increases at 60° to 90°. In the range of 60° to 75°, the proportionality of the three cutting forces is more reasonable. When turning slim shafts, the main deflection angle of more than 60° is generally used.

3) Camber angle (λs)

The angle of inclination (λs) affects the direction of chip flow, the strength of the tool tip and the proportionality of the three cutting forces during the turning process. As the camber angle increases, the radial cutting force decreases significantly, but the axial cutting force and tangential cutting force increase. The proportionality of the three cutting forces is more reasonable when the camber angle is in the range of -10° to +10°. When turning long and thin shafts, a positive camber angle of 0° to +10° is often used to make the chips flow to the surface to be machined.