Workpiece refers to the processing object in the process of mechanical processing. It can be a single part or a combination of several parts fixed together. The processing methods of workpieces are various, such as turning, milling, planer, grinding, casting, forging and so on. The working procedure of the workpiece varies with the change of the processing mode.
The causes of deformation in workpiece processing - deep hole processing manufacturers come to tell you:
First aspect: deformation caused by workpiece clamping
When clamping a workpiece, the correct clamping point should be selected first, and then the appropriate clamping force should be selected according to the position of the clamping point. Therefore, the clamping point should be as close as possible to the processing surface, and the position where the force is not easy to cause the clamping deformation should be selected so that the clamping force acts on the support.
When there are clamping forces acting in several directions on the workpiece, the sequence of clamping forces should be considered. For the clamping force in the contact between the workpiece and the support, it should first act and not be too large. For the main clamping force in balancing the cutting force, it should act at the back.
Secondly, the contact area between workpiece and fixture should be enlarged or the axial clamping force should be adopted. Increasing the rigidity of parts is an effective way to solve the clamping deformation, but due to the shape and structure characteristics of thin-walled parts, it has lower rigidity. In this way, under the action of clamping force, deformation will occur.
Increasing the contact area between workpiece and fixture can effectively reduce the deformation of workpiece during clamping. For example, when Milling Thin-walled parts, a large number of elastic pressing plates are used to increase the force area of the contact parts; when turning the inner diameter and outer circle of the thin-walled sleeve, whether using simple open transition rings, or using elastic mandrels, arc clamps, etc., the contact area is increased when the workpiece is clamped. This method is conducive to bearing clamping force, thus avoiding the deformation of parts. Axial clamping force is also widely used in production. The clamping force can be applied on the end surface by designing and manufacturing special clamps, which can solve the bending deformation of the workpiece caused by thin wall and poor rigidity of the workpiece.
Second aspect: deformation caused by workpiece processing
In the process of cutting, the workpiece is subjected to the action of cutting force, resulting in elastic deformation in the direction of force, which is what we often call the knife-let phenomenon. Corresponding measures should be taken to deal with this kind of deformation on the cutter. The cutter should be sharp when finishing. On the one hand, it can reduce the resistance caused by friction between the cutter and the workpiece, on the other hand, it can improve the heat dissipation ability of the cutter when cutting the workpiece, so as to reduce the residual internal stress on the workpiece.
For example, when milling the large plane of thin-walled parts, using single-edge milling method, the tool parameters are selected with larger principal deviation angle and larger rake angle, in order to reduce cutting resistance. Because of its light cutting speed, the tool reduces the deformation of thin-walled parts and is widely used in production.
In the turning of thin-walled parts, the reasonable tool angle is very important to the cutting force, the thermal deformation and the micro-quality of the workpiece surface. The cutting deformation and the sharpness of tool rake angle are determined by the size of tool rake angle. Large rake angle reduces cutting deformation and friction, but too large rake angle reduces the wedge angle of the tool, reduces the strength of the tool, reduces the heat dissipation of the tool and accelerates wear. Therefore, when turning thin-walled steel parts, high-speed cutters are usually used, with a rake angle of 6 ~30 and carbide cutters, with a rake angle of 5 ~20.
The cutting force decreases when the tool's back angle is large and friction is small, but too large back angle will also weaken the strength of the tool. When turning thin-walled parts, high-speed steel turning tool is used, the tool's rear angle is 6 12 and carbide tool is used. The rear angle is 4 12 while finishing, the larger rear angle is taken, while roughing, the smaller rear angle is taken. When the inner and outer circles of the thin-walled parts of the car are round, the main deflection angle should be large. Correct tool selection is a necessary condition to deal with workpiece deformation.
The heat generated by friction between tool and workpiece will also cause workpiece deformation, so high-speed cutting is often chosen. In high-speed cutting, because the chips are removed in a relatively short time, most of the cutting heat is taken away by the chips, which reduces the thermal deformation of the workpiece. Secondly, in high-speed machining, due to the reduction of the softening part of the cutting layer, the deformation of the parts can also be reduced, which is conducive to ensuring the size and shape accuracy of the parts. In addition, cutting fluid is mainly used to reduce friction and cutting temperature in the cutting process. Reasonable use of cutting fluid plays an important role in improving tool durability, surface quality and machining accuracy. Therefore, in order to prevent parts from deforming, adequate cutting fluid must be reasonably used.
Reasonable cutting parameters are the key factors to ensure the accuracy of parts. When processing thin-walled parts with high precision, symmetrical processing is usually adopted to balance the stress on the relative two sides and achieve a stable state. After processing, the workpiece is flat. However, when a larger cutting tool amount is adopted in a certain process, the workpiece will be deformed due to the unbalance of tension stress and compression stress.
The deformation of thin-walled parts in turning is multifaceted. The clamping force when clamping the workpiece, the cutting force when cutting the workpiece, the elastic and plastic deformation when the workpiece obstructs the cutting tool, and the thermal deformation occurs when the temperature of the cutting area increases. Therefore, we need to take a larger amount of back feed and knife feed in rough machining; in finish machining, the knife feed is generally 0.2-0.5 mm, and the feed is generally 0.1-0.2 mm/r, or even smaller, the cutting speed is 6-120 m/min, and the cutting speed is as high as possible in finish turning, but it is not easy to be too high. Reasonable selection of cutting parameters can reduce the deformation of parts.
The third aspect is:Stress and Deformation after Machining
After processing, there are internal stresses in the part itself. The distribution of these internal stresses is a relatively balanced state, and the shape of the part is relatively stable. But after removing some materials and heat treatment, the internal stresses change. At this time, the workpiece needs to reaching the balance of force, so the shape changes. This kind of deformation can be solved by heat treatment. The workpiece that needs to be straightened can be stacked to a certain height, and the workpiece can be pressed to a flat state. Then the workpiece and the workpiece can be put into the heating furnace together. Different heating temperature and heating time can be selected according to the different materials of the parts. After thermal straightening, the internal structure of the workpiece is stable. At this time, the workpiece not only gets a higher straightness, but also eliminates the hardening phenomenon, which is more convenient for the further finishing of parts. Castings should be aged to eliminate internal residual stress as far as possible, and the way of remanufacturing after deformation, namely roughing-aging-remanufacturing, should be adopted.
For large parts, profiling processing should be adopted, that is, to predict the deformation of the parts after assembly, and to reserve the deformation in the opposite direction during processing, which can effectively prevent the deformation of the parts after assembly.