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Analysis on turning technology of thin wall parts

Thin-walled sleeve refers to the wall thickness of the part and its radial, axial dimensions, compared to the huge difference, generally dozens or even hundreds of times the metal material parts, with material savings, simple structure and other characteristics. Thin-walled parts have been widely used in various types of petroleum machinery components. But the thin-walled parts of the turning process is more difficult, the specific reason is because the thin-walled parts of their own rigidity, weak strength, in the turning process is very easy to deformation, it is difficult to ensure the quality of parts processing. How to improve the machining accuracy of thin-walled parts is a topic of concern for the machining industry.

Thin-walled parts often appear in the process of turning problems, causes and solutions

We often encounter some thin-walled parts in the process of turning processing. Such as sleeve thin-walled parts (Figure 1), ring thin-walled parts (Figure 2), disc thin-walled parts (Figure 3). This paper analyzes in detail the machining characteristics of thin-walled parts, the clamping method to prevent deformation, the material of turning tool, the selection of cutting parameters and the geometric angle of turning tool. A large number of experiments have been conducted to provide a theoretical basis for better machining of thin-walled parts and to ensure the machining quality in the future.
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Fig.1 Thin-walled parts of shaft sleeve
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Figure 2 Ring thin-walled parts
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Figure 3 thin-walled parts of the disk
Processing characteristics of wall parts
Because of the thin wall parts, in the use of general clamping fixture clamping pressure is very easy to produce deformation, and the clamping force is not enough parts and easy to loosen, thus affecting the dimensional accuracy and shape accuracy of the parts.

As shown in Figure 4, when using the three-jaw chuck clamping parts, under the action of clamping force, the part will slightly become triangular, after turning to get a cylindrical body. However, after releasing the jaws and removing the part, it reverts to an arc triangle due to the elasticity of the part. At this point, assuming that when measured with a micrometer, the diameter in all directions is the same, but the part has been deformed is not a cylinder, this deformation phenomenon we call equal diameter deformation.

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Figure 4 three-jaw chuck clamping
Because the parts are thin, the cutting heat during processing will cause parts deformation, thus making it difficult to control the size of the parts. For the expansion coefficient of metal thin-walled parts, such as in a single installation in a continuous semi-finish turning and finishing, the thermal deformation of the parts caused by cutting heat, will have a great impact on its dimensional accuracy, and sometimes even make the parts stuck in the arbor type of fixture.
Thin-walled class parts in the processing of internal holes, generally using a single-edge boring tool processing, at this time, when the parts are longer, if the tool parameters and cutting amount is not handled properly, will cause difficulties in chip removal, affecting the quality of processing, damage to the tool.
And the impact of clamping force, the part will produce deformation or vibration, dimensional accuracy and appearance roughness is not easy to control. If the tool angle is not correct or wear, resulting in an increase in cutting force, the workpiece appearance will produce chatter to affect the appearance of quality.
Wall parts are poorly rigid, can not use a large cutting amount, low productivity.
Therefore, choose the appropriate clamping method, processing technology, reasonable cutting amount, tool material and angle, reduce the vibration of the parts, adequate cooling and testing are the key factors to ensure the processing of thin-walled parts.

Clamping methods for wall parts

General soft jaw positioning clamping: choose a reasonable clamping force point of action, so that the clamping force in the parts of good rigidity, applicable to the shape and size tolerance requirements of the parts processing.
Advantages: easy loading and unloading length can be positioned, can withstand large cutting forces.
Disadvantages: the positioning point of the part is more concentrated, the part is more serious deformation after tightening.

Fan-shaped soft jaws clamping

The use of fan-shaped soft jaws of the three-jaw chuck (Figure 5), according to the requirements of the dynamic fit with the clamping surface of the processed parts, processing the working surface of the jaws to increase the contact area with the parts.

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Fig. 5 Fan-shaped soft jaws
Advantages: increase the area of clamping force, so that the part support surface increases, the clamping force is evenly distributed on the work surface, can increase the cutting amount, not easy to produce deformation.
Disadvantages: fan-shaped soft jaws are not easy to process.

Rigid arbor clamping

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Figure 6 Rigid arbor
Using cone arbor clamping, the part is directly set in the cone arbor machining.
Clamping with a cylindrical arbor, where the part is mounted on the arbor using axial compression. Reduce the radial deformation of the part.
Advantages: easy loading and unloading of parts, can ensure a high degree of concentricity, technical requirements.
Disadvantage: the inner hole of the part is scratched by the arbor.

Magnetic chucking

the parts are magnetically clamped to the chuck, and the parts are only subjected to axial force, but not radial force.

Advantage: the inner and outer circle of the part can be machined at one time.
Disadvantages: the parts are more troublesome to find the right, the application range is small, not suitable for processing non-ferrous metal parts.

The use of axial clamping fixture: when turning thin-walled parts, do not use radial clamping, but choose the axial clamping method. Parts rely on the end of the axial positioning sleeve to achieve axial clamping, due to the axial distribution of clamping force along the parts, and the parts axial stiffness, will not produce clamping deformation.

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Figure 7 axial clamping fixture
Advantages: small part deformation, good machining quality.
Disadvantages: complex process system, fixture range of application is small.

Increase the process rib

some thin-walled parts can be in its clamping parts specially made several process rib to enhance the rigidity here, so that the clamping force acts on the process rib to reduce the deformation of the parts, after processing, and then remove the process rib.

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Figure 8 Process ribs
Advantages: increases the rigidity of the part and reduces the clamping deformation.
Disadvantages: not suitable for large volume machining.
Clamping with risable arbor: As shown in Figure 9, the workpiece is clamped with the outer circle of the flexible arbor as the positioning reference, and the axial movement of the flexible arbor is achieved by tightening or loosening the clamping screws. As the fit between the rigid mandrel and the elastic mandrel is a tapered fit, the elastic mandrel will expand or contract radially while moving axially to achieve radial clamping or unclamping of the workpiece.
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Figure 9
Advantages: ideal for processing batch parts with poor bore size consistency and low manufacturing cost.
Disadvantages: not suitable for machining parts with high accuracy requirements.

The choice of turning process for wall parts

Rough first, then fine

Turning of thin-walled parts should generally be rough and finish machining separately, rough turning followed by heat treatment. Some parts with complex shape and high precision requirements, need to increase the semi-finishing process between roughing and finishing, so that the roughing deformation is gradually corrected, geometric shape and size accuracy gradually improved. When using the same benchmark, once the workpiece semi-finishing and finishing, the workpiece can be loosened before finishing, and turn it slightly to make it return to the free state, and then clamp the workpiece for finishing, which can also achieve the purpose of correcting the deformation. At the same time, the use of fixtures should reduce the deformation of the workpiece when clamping and turning, so as to ensure the quality of thin-walled parts.

Inner hole first and then outer circle

Because the hole is more difficult to process than the outer circle, easy to produce deformation. Machining the inner hole first, and then machining the outer circle, can use the arbor clamping, within the hole positioning axial clamping, to prevent the parts in the processing of the impact of machining accuracy.

Once completed

In a single clamping to complete the required processing of so size, mainly used in the blank material is bar or with the process of the thin-walled parts processing table.
Example of thin-walled parts machining:
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Figure 10 Thin-walled sleeve
Thin-walled sleeve (shown in Figure 10), small batch production, material 2A12 (hard aluminum), outer circle <440-0.02mm, and hole diameter <40+0.020mm coaxiality requirements of 0.02mm, the two end face parallelism of 0.02mm. if this part is processed in one time, the deformation is very large, so the sub-steps are as follows:

  • 1) The bar is clamped in the three-jaw centering chuck, one end is drilled with a drill <38mm, the inner hole is rough turned to <39mm, the outer circle is rough turned to <45.50-0.03mm, the cut-off length is 60.5mm, and the outside diameter is 0.05mm (for positioning).
  • 2) After the completion of rough turning, turn to the heat treatment aging process, after clamping in the soft three centering chucks, turning a total length of 60.15mm at one end; then do a tooling, boring a bore length of 50mm, the end face of the bore is turned flat, the bore and the outer circle of the part fit. Note that can not be tightened, or the parts of the bore after processing and unloading the inner, hole will be deformed, mainly because the outer circle of the part is not round caused by
  • 3) Tooling (as shown in Figure 11) of the outer circle to pick an external thread [email protected], 50mm long, the inner hole turned into <45.6mm, 50mm long, and then make a pressure cap, the internal thread is [email protected], 50mm long. the outer diameter of the pressure cap is <60mm, knurling, the total length of 53mm, the inner diameter of the pressure cap is <44.5mm.

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Figure 11 Tooling diagram

The selection of cutting dosage when lathe machining wall parts

We all know that in the cutting amount of cutting force is the biggest impact is the back draft (Ap), the biggest impact on cutting heat is the cutting speed (Vc) and tool sharpness. Therefore, turning thin-walled parts should be reduced back draft and appropriate to reduce the cutting speed, at the same time should be appropriate to increase the amount of feed.
In finishing, should use a large cutting speed, small feed (F), when the machine tool accuracy is reduced, to reduce the appropriate cutting speed.
Table 1 thin-walled parts cutting dosage parameters (finish turning)

Part material Blade material Cutting parameters
V(r/min) F(mm/r) Ap(mm)
Carburizing steel YT15 200-300
Quenched and tempered steel YT15 300-400
Stainless steel YD15,YG8 200-300

Thin-walled parts processing turning tool geometry angle selection

Turning tool geometry angle on the cutting force is the biggest impact is the main deviation angle (Kr), the front angle (γ0) and edge inclination angle (λs). Increase the front angle to make the turning tool sharp, smooth chip removal, reduce the friction between the chip and the front tool surface, reduce the cutting force and cutting heat.

  • External fine turning tool Kr=90°-93°, Kr′=15°, α0=14°-16°, α01=15°, γ0 increase appropriately.
  • Inner hole finishing tool Kr=60°, Kr′=30°, γ0=35°, α0=14°-16°, α01=6°-8°, λs=5-6°.

Measures to reduce the vibration generated during turning of thin-walled parts.

  1. Adjust the clearance of lathe spindle, dragging plate, bed saddle, tool holder and sliding parts to make the rotating and sliding parts in the most correct state.
  2. Use vibration-absorbing materials, fill or wrap the parts with soft plastic, rubber band, rubber sheet, soft rubber tube, cotton yarn and other materials. When the workpiece rotates, under the action of centrifugal force, the rubber sheet will be close to the hole wall, which can hinder vibration damping and prevent vibration transmission when turning to reduce vibration and eliminate the role of noise.
  3. Filled with low melting point of the material (such as paraffin) on the low melting point of the material, filled into the thin-walled class parts and mandrel bore only feel the gap, both ends with a plug seal, not only to reduce vibration, but also to reduce deformation.
  4. Wedge-shaped mandrel filling method, the application of aluminum wedge-shaped mandrel, so that the wedge-shaped mandrel and parts bore closely with the purpose of vibration reduction.

Adequate cooling

In the processing of thin-walled parts, thin-walled parts should be cooled, according to the different materials of the parts to choose a reasonable cutting fluid to reduce the cutting temperature, reduce the heat and deformation of the parts to improve machining accuracy.

Conclusion

In this paper, we have analyzed the characteristics of thin-walled parts, the difficulties of machining, the design of turning process by example, from the selection of clamping method, turning tool material, cutting parameters, geometric angle of turning tool, methods to reduce and prevent machining deformation, and the selection of parameters for turning thin-walled parts.

Source: China Thin Wall Parts Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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