The method of solving the deformation in the process of machining thin wall sleeve
Small thin-walled sleeve is widely used in instrument, office and other small equipment. The main reason is that it is easy to produce a variety of deformation in the process of clamping and machining. In this paper, the main reasons of its deformation are analyzed and the solutions are put forward. The pneumatic and elastic rubber are used in the design of the fixture, which effectively solves the problem of clamping deformation. It has a certain demonstration significance for the design of other small thin-walled pipe sleeve fixture.
Table of Contents
Fig.1 thin walled pipe sleeve
As shown in Figure 1, a typical small thin-walled sleeve. The material is 316L stainless steel, and the bar is rough. volume production. It is very difficult to process this part according to the drawing requirements and materials. Wechat of metal processing has good content and deserves attention. The thinnest wall thickness is 1 mm. The processing material is 316L stainless steel, which is difficult to cut. The inner diameter is 10-12mm, and the tolerance is only 0.01-0.02MM. The outer diameter is 14-13mm, and the tolerance is only 0.02mm. The inner holes at both ends are symmetrical, and each has an inner groove of 0.5 × 0.1 mm. The chamfering at several places is very small, 0.15-0.25 mm. It has the advantages of thin wall, easy deformation, high precision, small structure, difficult machining and material cutting.
Thin wall is the outstanding feature of this part. In order to better analyze the processing process, we should first analyze the factors that affect the machining accuracy of thin-walled sleeve. Under the action of clamping force and cutting force, thin-walled pipe sleeve is prone to deformation and vibration, which affects the turning accuracy of workpiece. Because of the small heat capacity of the thin wall workpiece, it is easy to cause thermal deformation and the size of the workpiece is difficult to master.
(1) Thermal deformation: due to the thin workpiece, small heat capacity and slow conduction, under the action of cutting heat, the temperature of the workpiece is higher, which will cause the thermal deformation of the workpiece and make the size of the workpiece difficult to control.
Measures to reduce cutting heat: reduce cutting force, pay attention to control the rise of cutting temperature during turning. Firstly, reduce cutting deformation (cutting force) to reduce cutting heat generation, at the same time, increase the heat dissipation area of tool tip and use sufficient cooling and lubricating fluid to disperse cutting heat in time. Cutting fluid with good anti sticking and cooling property should be selected, such as emulsion of extreme pressure additives such as sulfur and chlorine, and the supply of cutting fluid must be sufficient.
(2) Vibration deformation: under the action of cutting force (especially radial cutting force), vibration and deformation are easy to occur, affecting the dimensional accuracy, shape, position accuracy and surface roughness of the workpiece.
The radial cutting force is located in the horizontal plane and perpendicular to the longitudinal cutting direction. The resistance between the tool and the workpiece causes the workpiece to bend. Due to the small cutting amount of the thin wall back, the resistance between the tool tip and the workpiece changes intermittently when the chip breaks, which makes the workpiece and even the cutter vibrate.
Measures to reduce vibration deformation: select machine tools with good rigidity and enough power. At the same time, the rigidity of the tool and workpiece should be improved as much as possible, such as increasing the cross-sectional area of the cutter bar and reducing the overhanging length of the cutter.
(3) Stress deformation: due to the thin wall of the workpiece, it is easy to deform under the action of clamping force, thus affecting the dimensional accuracy and shape accuracy of the workpiece. If the thin-walled parts are clamped with the automatic centering chuck, the parts will deform under the action of the clamping force of three claws, resulting in the roundness error of the parts. If the clamping force is reduced, parts may be loose and scrapped during turning. The clamping force is usually clamped when rough turning and loose when finishing turning to control the deformation of parts.
In order to reduce the deformation of thin wall workpieces, the methods of opening transition ring or increasing the contact area between claw and workpiece are often used. This method is suitable for the workpiece whose wall is not too thin and is easy to process inside and outside structure. It is one of the common methods for thin-walled pipe sleeve workpiece to change radial clamping to axial clamping. Wechat of metal processing has good content and deserves attention. Although the deformation of the workpiece is very small by this method, each part must be locked with a nut, which increases the clamping auxiliary time and the labor intensity of the operator, reduces the production efficiency, and can not meet the requirements of mass production, especially for the mass production of thin-walled pipe sleeves similar to those shown in figure 1 on the CNC lathe.
In order to solve the problem of machining deformation and production efficiency of the part, special fixture must be designed.
The processing process is as follows:
- Process 1: clamping bar material, drilling with 95mm drill bit, rough turning of inner hole → rough and fine turning of end face to requirements → rough turning of outer circle → semi finishing turning of shape → fine turning of shape to dimension and cutting.
- Process 2: clamp with special fixture, semi finish turning the inner hole and end face of cut-off end to dimension → turning inner groove. Turn around, semi finish turning the inner hole at the other end to the dimension → turning the inner groove.
Special fixture design
The design is shown in Fig. 2. The special fixture is used to process the cavity of thin wall parts.
Through the above process analysis, we know that the special fixture is needed for the processing of thin wall parts.
Design of special fixture.
Selection of positioning reference: the inner hole of the workpiece has been drilled in the previous process, and the shape of the workpiece has been processed to the requirements except one chamfer at the cutting end. The outer circle of (143 ± 0009) mm was selected as the radial reference and the left end surface as the axial reference.
Selection of clamping mode: the wall thickness of the part is only about 1 mm, the cutting amount cannot be too large, and the cutting force and clamping force can not be too large. The special clamp for pneumatic elastic rubber can be used.
Figure 2 shows the assembly drawing of the fixture. The connecting plate is connected with the spindle of the machine tool, and the structural form depends on the actual situation. In addition to sealing the compressed air, a hole of 20 mm in the middle is connected with the compressed air pipe in the rotary pneumatic device. The compressed air acts on the elastic rubber sleeve through three symmetrically distributed air holes, and the workpiece is clamped by the deformation of the elastic rubber sleeve. The positioning hole of clamping claw is made after the fixture and machine tool are installed.
Fig.2 assembly drawing of fixture
The main parts of clamping jaw are shown in Fig.3.
Fig.3 assembly drawing of clamping jaw
Selection of cutting tools and cutting parameters
The special pneumatic elastic fixture is used to process the workpiece on a micro precision CNC lathe, which effectively solves the problems of clamping deformation and production efficiency. But if we want to process this kind of workpiece normally, we must choose the tool material, parameters and cutting parameters reasonably.
(1) Tool material and parameter selection: as shown in Fig. 1, the typical thin wall workpiece material grade is y1cr17, which belongs to ferrite type easy cutting stainless steel. The stainless steel has the cutting performance of easy cutting structural steel and the mechanical properties of high-quality structural steel, so it is widely used in production. In general, new high-speed steel materials are often used in the processing of such complex shaped cutting tools as drill bits and taps, such as w12cr4v4mo, w12cr4v5co5 (T15) and w9cr4v5co3. The carbide of YG, yw1, yw2, yh1 and yh2 can be selected for the internal and external turning tools. Wechat of metal processing has good content and deserves attention. In order to make the tool sharp and reduce the cutting force, the tool rake angle of stainless steel should be taken as large as possible, especially for small thin wall workpieces. Generally, the anterior angle is 12 ° to 30 °. In order to ensure sufficient tool strength, the back angle should not be too large. Rough turning should be 6 ° to 10 ° and finish turning should be 10 ° to 20 °.
In order to ensure the machining quality and production efficiency in mass production, tools with higher performance are often selected, such as ultrafine cemented carbide PVD coated tools and cermet PVD coated tools.
(2) Selection of cutting parameters: in order to ensure the machining accuracy and surface quality, smaller back feed, larger cutting speed are generally selected. The cutting rate of rough turning outer round back is 0.3-0.5mm, and that of inner hole is 0.2-0.3mm; that of finishing turning is 0.1-0.15mm; the feed rate of rough turning and finishing turning is 0.02-0.05mm/r, that of grooving and cutting is 0.02mm/r, and that of drilling is 0.07mm/r. The cutting speed of y1cr17 stainless steel can reach over 295 M / min when ultra-fine cemented carbide and cermet PVD coated tools are used. When machining small-sized workpieces, the rotation speed should be selected as high as possible under the premise of the rotary performance of the machine tool. In the example, 1200 R / min is selected for drilling, 3000 R / min is selected for rough and fine turning, and 2000 R / min is selected for cutting.
In addition, when machining thin wall workpieces, the cutting fluid with cooling and lubrication is generally selected. As follows: sulfurized oil, sulfurized soybean oil, kerosene, acid or vegetable oil, carbon tetrachloride plus mineral oil and emulsion, etc.
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.)
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