What factors can cause workpiece deformation in machining?

The problem of workpiece deformation in machining is a difficult problem to solve. First, we must analyze the causes of the deformation and then take measures to deal with it.

01 the material and structure of the workpiece will affect the deformation of the workpiece

The amount of deformation is directly proportional to the shape complexity, length width ratio and wall thickness, and is directly proportional to the rigidity and stability of the material. Therefore, when designing parts, minimize the influence of these factors on workpiece deformation.

Especially in large parts, the structure should be reasonable. Before processing, we must strictly control the defects such as hardness, porosity and so on, so as to ensure the quality of the blank and reduce the deformation of the workpiece.

02 the deformation of the workpiece during clamping

When the workpiece is clamped, the correct clamping point must first be selected, and then the appropriate clamping force is selected according to the position of the clamping point. Therefore, the clamping point and the supporting point are consistent as much as possible, and the clamping force is acted on the support. The clamping point should be as close as possible to the machining surface, and the position that the force is not easily caused by the clamping deformation is selected.

When there are several directions of clamping force on the workpiece, the order of clamping force should be taken into consideration, and the clamping force of the workpiece and the support should be acted first and not too large. The main clamping force of the balance cutting force should be at the end.            Secondly, the contact area between workpiece and fixture should be increased or axial clamping force should be adopted. Increasing the rigidity of the parts is an effective way to solve the clamping deformation, but because of the characteristics of the shape and structure of the thin-walled parts, it has a lower rigidity. In this way, under the action of clamping force, there will be deformation.

Increasing the contact area between workpiece and fixture can effectively reduce the deformation of workpiece during clamping. For example, when milling the thin-walled parts, a large amount of elastic plate is used to increase the force area of the contact parts. When the inner and outer circle of the thin wall sleeve is cut, the contact area of the workpiece is increased by using the simple opening ring, the use of the elastic core and the whole arc claw. This method is conducive to carrying clamping force and avoiding deformation of parts. Using the axial clamping force, it is also widely used in production. The design and manufacture of special fixture can make the clamping force on the end face, and can solve the bending deformation of the workpiece due to the thin wall of the workpiece and the poor rigidity.

03 the deformation of the workpiece during processing

Workpiece in the cutting process due to the role of cutting force, resulting in elastic deformation toward the direction of force, which we often talk about the knife phenomenon. The corresponding measures should be taken to deal with this kind of deformation, and the cutting tool is sharp when finishing. On the one hand, it can reduce the resistance formed by the friction between the tool and the workpiece. On the other hand, it can improve the heat dissipation capacity of the cutting tool and reduce the residual internal stress on the workpiece.

For example, when milling the large plane of thin wall parts, the single edge milling method is used. The tool parameters select the larger main angle and the larger front angle, in order to reduce the cutting resistance. Because the cutting tool is light and fast, it reduces the deformation of thin-walled parts, and is widely applied in production.

In the turning of thin-walled parts, the reasonable cutting angle is very important to the cutting force, the thermal deformation and the micro quality of the workpiece surface. The tool front angle determines the sharpness of cutting deformation and tool rake angle. The front angle is large, the cutting deformation and the friction force decrease, but the front angle is too big, which will reduce the wedge angle of the cutter, the cutting tool strength is weakened, the tool heat dissipation is poor, and the wear speed is quickened. Therefore, when turning thin steel parts with thin steel parts, they use high speed cutters, the front corners take 6 degrees – 30 degrees, with carbide tools, and the front corners take 5 degrees – 20 degrees.

The tool’s rear angle is large, the friction force is small, and the cutting force decreases correspondingly. In turning thin wall parts with high speed steel tool, the rear angle of the tool is taken 6 degrees – 12 degrees, with carbide cutters, the rear angle is 4 degree – 12 degrees, the larger rear angle is taken when the car is refined, and the smaller rear angle is taken when the car is rough. When the inner and outer parts of the thin-walled parts are taken, a large main angle is taken. Correct selection of cutting tools is a necessary condition for workpiece deformation.

The heat generated by friction between cutting tools and workpiece will also deform the workpiece, so in many cases, high speed machining is chosen. In high speed machining, because the chip is removed in a short time, most of the cutting heat is taken away by the chip, which reduces the thermal deformation of the workpiece. Secondly, in the high speed machining, the deformation of the parts processing can be reduced because of the reduction of the softening part of the cutting layer, and it is favorable to ensure the size and shape precision of the parts. In addition, cutting fluid is mainly used to reduce friction in cutting process and reduce cutting temperature. Proper use of cutting fluid plays an important role in improving tool life and surface quality and machining accuracy. Therefore, in order to prevent deformation of parts, reasonable cutting fluid must be used reasonably.

Reasonable cutting parameters in machining are the key factors to ensure the accuracy of parts. Symmetrical machining is generally adopted to make the stress balanced on the opposite sides to achieve a stable state and the workpiece is smooth after processing. However, when a certain process takes a larger amount of cutting, the workpiece will be deformed due to the imbalance of tensile stress and compressive stress.

Therefore, when we are in the process of rough processing, the amount of back knife and feed can be bigger. When finishing, the tool is generally 0.2 – 0.5mm, the feed is generally 0.1 – 0.2mm/r, even smaller, the cutting speed is 6 – 120m/min, and the cutting speed is as high as possible, but it is not easy to pass the high. Reasonable selection of cutting parameters, so as to reduce the deformation of parts.

To sum up, for the easily deformable workpiece, in the blank and processing technology, the corresponding countermeasures should be adopted. It needs to be analyzed according to different conditions, and a suitable process will be found. Of course, the above method is only to further reduce the workpiece deformation, if you want to get a higher quality workpiece, you need to continue to learn, explore and research.

Post time: May-30-2018
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