Machining of stainless steel thin wall pipe sleeve parts

Stainless steel has the characteristics of high toughness, high thermal strength, low thermal conductivity, serious work hardening, high cutting heat and difficult heat dissipation, which results in high cutting temperature at the tool tip, easy to produce chip accretion, aggravate tool wear and affect the surface quality of thin-walled sleeve parts; the thin-walled pipe sleeve parts have poor rigidity, low strength, easy deformation and difficult machining. Based on the analysis of the cutting performance of thin-walled sleeve materials and the factors affecting the deformation of the parts, the coarse-grained separation is adopted in the process, and the fixture is designed for the structure of the thin-walled sleeve parts; in addition, the reasonable selection of tool material, geometric parameters, cutting parameters and cutting fluid, etc., can significantly improve the precision and appearance of the thin-walled sleeve.

With the improvement of product performance and the development of new products, the requirement of parts accuracy is higher and higher, and the application of stainless steel materials is more and more. Thin walled parts have the characteristics of poor rigidity, low strength and easy deformation. However, due to the high toughness, high thermal strength, low thermal conductivity, severe work hardening, excessive cutting heat and difficult heat dissipation, the cutting temperature at the tool tip is high, the chip adhesion to the edge is serious, and it is easy to produce chip accretion, which not only aggravates the tool wear, but also affects the surface roughness.

Factors affecting the machining accuracy of stainless steel thin-walled pipe sleeve parts

Cutting properties of stainless steel materials

• (1) It has high work hardening property. In the process of cutting, the material to be processed produces plastic deformation, which forms a hardened layer on the machined surface. The appearance of hardened layer aggravates the tool wear, especially the tool boundary wear.
• (2) The thermal conductivity is low. 70% ～ 80% of the heat generated by cutting is carried away by the chips. However, when the thermal conductivity of the material is low, the chip will take less heat. As the temperature of the cutting point increases, the tool life will be shortened.
• (3) High affinity. Due to the high affinity of the processed materials, it is easy to cause chip accumulation at the tool tip and attachments on the back face of the tool, thus reducing the accuracy of the machined surface.

Factors affecting machining accuracy of parts

• (1) It is easy to deform. Due to the thin wall of the workpiece, it is easy to produce deformation under the action of clamping force, which affects the dimensional accuracy and shape accuracy of the workpiece.
• (2) It is easy to be deformed by heating. Because the workpiece is thin, the cutting heat will cause the thermal deformation of the workpiece, which makes the size of the workpiece difficult to control.
• (3) It is easy to vibrate and deform. Under the action of cutting force (especially radial cutting force), vibration and deformation are easy to occur, which affect the dimensional accuracy, shape, position accuracy and surface roughness of the workpiece.

How to improve the machining accuracy of stainless steel thin wall pipe sleeve parts

The pipe sleeve shown in Figure 1 is one of the stainless steel thin-walled sleeve parts in Xuchang Tobacco Machinery Co., Ltd. the material of the workpiece is 3Cr13, and the outer circle dimension tolerance requirements are high. In order to improve the qualified rate of the product, the following aspects are analyzed.

Fig.1 Pipe sleeve

Separation of coarse and fine

Due to the high precision requirements of parts, rough and finish machining are carried out separately when machining inner and outer circles. The separation of rough machining and finish machining can avoid all kinds of deformation caused by rough machining, including elastic deformation caused by clamping force during rough machining and thermal deformation caused by cutting heat.

Fixture design

The fixture design and processing can first process the inner hole and one end face, at this time, a large margin is reserved, and the outer circle is clamped with a long soft grip; in the later process, the elastic chuck (spring expansion device) with centering on the inner surface of the workpiece is selected, and the spring expanding device is divided into the following categories:

• (1) The front push spring expanding device can not be used for axial positioning of the workpiece with inner hole centering on the lathe;
• (2) The fixed spring expanding device with forced exit is used to process workpiece with no hole positioning datum on lathe;
• (3) A fixed spring expanding device operated by ring groove nut;
• (4) A separate spring expanding device for machining thin-walled workpieces.

Here, separate expanding spring device (Fig.2) is used to clamp and process the outer circle. The core element of the fixture is the elastic sleeve. A pair of taper sleeves 2 and 6 are installed on the spindle. When the nut is turned to move to the right, the cone sleeve gives the elastic sleeve a radial force to expand and tighten the workpiece. When the nut is turned in the opposite direction, the workpiece is loosened. The positioning pins 3 and 7 prevent the relative rotation between the elastic sleeve and the conical sleeve and between the conical sleeve and the spindle. The fixture makes the clamping force act on the inner surface of the workpiece evenly, which not only reduces the machining error caused by the deformation of the workpiece, but also improves the positioning accuracy by eliminating the radial clearance, which can ensure the coaxiality requirements of the inner and outer circles.

1. Spindle 2. 6. Taper sleeve 3. 7. Locating pin 4. Workpiece 5. Elastic sleeve 8. Nut
Figure.2 separate spring expanding device (elastic chuck)

Tool selection

(1) Material selection
According to the cutting characteristics of stainless steel, the tool material should have enough strength, toughness, high hardness and high wear resistance, and the adhesion with stainless steel should be small. For turning tools, the tool material should be cemented carbide with high strength and good thermal conductivity, because its hardness and wear resistance are better than high speed steel. Common cemented carbide materials are: tungsten cobalt (YG3, YG6, YG8, YG3X, YG6X), tungsten cobalt titanium (yt30, YT15, YT14, YT5), and general-purpose (yw1, yw2). YW cemented carbide is suitable for machining refractory materials such as high temperature alloy, high manganese steel and stainless steel. The hardness, wear resistance, heat resistance and cutting speed of yw2 are lower than those of yw1, but the toughness and bending strength are better than yw1. Therefore, yw1 cemented carbide tool is selected for finish turning and yw2 cemented carbide for rough machining.
(2) The geometric parameter selection of cylindrical finish turning is shown in Fig.3.

Fig.3 geometric parameters of tool

Selection of cutting parameters for finish turning

The selection of cutting parameters is different according to the workpiece material and blade material, as shown in Table.1.

Selection of cutting fluid

Sulfur based chlorinated oil is widely used in cutting stainless steel. This oil contains active sulfur and can be diluted and mixed with kerosene. This kind of oil has good lubricity and anti welding property. Sulfur based cutting oil should not be too thin for heavy cutting.

Conclusion

The above shows that the machining accuracy can be guaranteed by using the actual cutting method. The geometric parameters of cutting tools, cutting parameters and reasonable selection of cutting fluid can significantly improve the accuracy and appearance of parts.

Table.1 Selection of cutting parameters

Material of workpiece	Material of blade		Cutting amount
		v/m·min-1	f/mm·r-1	ap/mm
45，Q235－A	YT15	100～130	0.08～0.16	0.05～0.5
1Cr13，3Cr13	YW1	110～150	0.076～0.1	0.02～0.3

Source: China Pipe Sleeve 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.)

If you want to have more information about the article or you want to share your opinion with us, contact us at [email protected]

Please notice that you might be interested in the other technical articles we’ve published:

• What is pipe fitting
• Manufacturing and quality control of 800H alloy seamless butt welded pipe fittings
• Improving the processing technology of stainless steel thin-wall pipe sleeves
• Fixture design for turning thin walled pipe sleeve parts
• Machining process analysis of thin wall pipe sleeve
• The method of solving the deformation in the process of machining thin wall sleeve