It’s incredible that one fixture can solve the machining of all thin-walled parts
The processing and clamping of thin-walled parts has always been one of the difficulties in the industry. There are many on-site processing methods, but few are suitable for mass production. We adopt a floating three jaw fixture, which can realize the positioning and clamping of thin-walled parts without excessive clamping force, and greatly improve the machining effect.
As we all know, when the thin-walled parts clamp the outer circle for turning, the parts are easy to deform under the action of clamping force, which affects the machining accuracy, and the dimensional tolerance of the workpiece is not easy to ensure.
For example, the processing of an export part 8T-4556, whose outer circle diameter is 412mm and wall thickness is 25mm, is a thin-walled part, and the blank is produced by casting, which has the characteristics of irregular shape. The process requires turning the end face and inner hole, and the dimensional tolerance zone of inner hole is only 0.08mm under the condition that the outer circle is the clamping and positioning benchmark. During the trial production of the part, we used the traditional soft three claw clamping method to clamp the outer circle. During clamping, the workpiece is easy to deform and affect the machining accuracy. At the same time, due to the irregular outer circle of the blank and unstable clamping positioning, in order to reduce the deformation, it is necessary to reduce the clamping force, but in this way, the clamping may not be tight.
In this regard, we break through the traditional way of increasing the clamping contact area (adding slotted sleeve), and adopt a floating three claw with six point positioning and clamping (as shown in Figure 1), which has achieved good results in practical application.
Fig.1 floating three jaw clamping
Composition and working principle of floating three jaw fixture
The floating three jaw fixture we use includes three jaw body, connecting block, clamping nail and special insert sleeve. The three claw body is connected to the connecting block through the insert sleeve. The connecting block is connected with the lathe chuck like an ordinary three claw. The six clamping nails are respectively fixed on the three claw body in pairs. With the insert sleeve as the fulcrum, together with the three claw body fluctuates with the height of the blank outer circle, there is a small amount of floating (the floating amount is strictly required and can not be given arbitrarily), which plays the role of floating clamping. The tip of the clamping nail belt cone is pierced into the blank surface to a certain depth with the action of the clamping force. Without excessive clamping force, the positioning and clamping of thin-walled parts can be realized to prevent workpiece deformation.
As shown in Fig.2a, because the blank is manufactured by casting process, there are surface depressions at many places (B’) on the blank, which is easy to cause excessive deformation or loose clamping of the workpiece when clamping with ordinary three claws. The floating three claws with six point positioning and clamping can overcome this disadvantage. When the chuck moves to the center, when the clamping nail does not contact the B ‘point of the workpiece, the three claws will take the insert as the center of the circle, and the part will give the clamping nail a force F to make the three claws produce a certain angle, that is, the clamping nail tip a moves to point A1 and the clamping nail tip B moves to point B1. As shown in Fig. 2B, as the clamping action of the three claws continues, Make the two locating pins contact with the workpiece surface at the same time, as shown in Fig. 2c, so that the floating amount of the three claw body overcomes the surface defects of the workpiece. Similarly, the other two claws also move in this way. The tip points of the six clamping nails finally contact the outer circle of the blank at the same time, ensuring the positioning and clamping of the workpiece.
Fig.2 floating clamping principle
From the mechanical analysis, the traditional three claw contact surface is the contact area between the three claws and the blank, which is set as S1, while the contact surface between the floating three claw clamp with six point positioning clamping and the outer circle of the blank is set as S2, so the pressure on the part surface during the two clamping methods is P1 = F1/S1 and P2 = F2/S2 respectively. Because S2 is far less than S1, F1 is far greater than F2 under the same pressure.
In the processing of thin-walled parts, the smaller the clamping pressure, the better. The floating three jaw clamp with six point positioning clamping reduces the stress surface to two points, and S2 is very small. At this time, the pressure at the contact between the clamping nail and the workpiece is large, so that the clamping nail can be easily embedded into the workpiece surface to a certain depth. The clamping does not only depend on the friction force, which greatly increases the stability of clamping. When the clamping force is small, The clamping and positioning of the workpiece is realized.
When processing 8T-4556 with traditional three claws, the workpiece accuracy is difficult to ensure, and the rate of different qualified products is high. In recent years, through the use of the floating three jaw fixture with six point positioning and clamping, the machining effect has been significantly improved, which not only meets the process size requirements of the inner hole, but also has a very good cylindricity of the inner hole.
Source: China Pipe Sleeve Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.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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