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how can the deformation of the workpiece occur during mechanical processing be eliminated?

During the cutting process, due to cutting heat, frictional heat between machine parts, internal stress of the workpiece, and clamping force, the workpiece may deform, resulting in waste products.

1. Thermal deformation

1.1 Thermal deformation of cutting tools

Due to cutting heat, the cutting edge and body become hot, causing deformation and elongation of the cutting head and resulting in changes in the size of the workpiece.

The elongation of the blade is related to the length of the blade extension, cross-sectional size, blade thickness, and sharpness of the cutting edge. The longer the extended length of the blade, the greater its elongation; The cross-sectional area of the blade is inversely proportional to the elongation; The thicker the blade, the smaller the elongation.

1.2. Thermal deformation of machine tools

Due to the heat generated by cutting and friction between machine parts, certain components of the machine tool may become deformed due to heating, such as the deformation of the lathe spindle box, which can increase the center height of the spindle and cause horizontal displacement. The center of the spindle will rise and shift horizontally by a certain size.

1.3. Thermal deformation of workpieces

Due to cutting heat, the workpiece will become hot and the temperature will rise. There are two types of workpiece heating: balanced heating and unbalanced heating; Balanced heating will cause changes in the size of the workpiece while maintaining its shape; When uneven heating occurs, not only does the size of the workpiece change, but the shape also changes.

The temperature at equilibrium heating determines the temperature, mass, and specific heat capacity of the workpiece, and is also related to the cutting parameters.

As the cutting speed increases, the temperature of the workpiece gradually decreases; When the feed rate increases, the heat transferred to the workpiece decreases; When the feed rate increases, the temperature of the workpiece increases.

However, it must be noted that when other conditions are the same, the heavier the workpiece, the smaller the thermal deformation. When processing thin-walled workpieces with large margins, the deformation becomes greater. The temperature rise during the processing of solid materials is about half that of pipes with the same length and diameter.

When turning thin sheet workpieces, the shape of the workpiece changes due to single-sided heating, which is an example: when a large amount of metal is cut off from one side of the workpiece, this side will become hot and the workpiece will deform. If the workpiece is machined on the other side before it cools down, it will deform the workpiece.

When the workpiece is exposed to sunlight or heating, the outside becomes hot while the inside remains cold, which can also cause the workpiece to twist.

2. Deformation caused by internal stress

When there is stress inside the surface of a component without any external load, this stress is called internal stress. The internal stress is balanced with each other, so there is no significant change on the outer surface. Internal stress sometimes reaches almost the limit of failure, but in appearance it is no different from parts without internal stress.

The reasons for generating internal stress are as follows:

2.1. Internal stress of castings

After pouring the metal liquid into the mold, its volume will shrink during solidification and cooling. During shrinkage, it may be obstructed by the mold or due to temperature differences between different parts of the casting during cooling, causing each part to either elongate or compress to generate internal stress.

2.2 Internal stress of forgings and heat-treated parts

The internal stress of forgings and heat-treated parts is mainly caused by uneven cooling during the hot working process. In general, the root cause of internal stress generated during hot processing is due to temperature differences at different locations when the material transitions from a plastic state to an elastic state, or due to changes in the internal structure of the metal.

2.3. Internal stress in cold processing (cold rolling, cold drawing, cold rolling, cold extrusion, punching, rolling, etc.)

During cold working, the surface of the workpiece is hardened and internal stresses are present in the metal layer on the surface. After the removal of the stress layer, the stress is immediately redistributed, causing deformation such as twisting of bars, sheets, discs, etc.

If the material has no internal stress, internal stress will also occur due to cold correction. For thermal correction, it is a common forging operation and is carried out in a heated state. Generally speaking, it does not generate any internal stress.

2.4. Internal stress generated by cutting processing

Internal stress can also be generated during cutting, and its properties are the same as those mentioned above; For example, as a result of rough machining, stress is generated in the surface layer of the workpiece, which causes deformation of the workpiece. In future processing, the stress layer is removed, so the shape of the workpiece needs to be changed again. Internal stress still occurs during precision machining, but due to the small cutting amount at this time, the impact of the generated internal stress is minimal. Of course, special attention should be paid to precision parts.

So how to eliminate internal stress? Generally, the following methods can be used:

2.4.1. Methods for eliminating internal stress in castings

For castings, they can be placed outdoors for 6-18 months before mechanical processing (or after rough machining). Artificial aging treatment can also be used, which involves transferring the solidified casting into a furnace at 100-200 ℃, then slowly raising it to 500-600 ℃, holding it for a long time, and then cooling it very slowly.

2.4.2. Methods for eliminating internal stress in forgings (including hot-rolled parts)

1) Hang the workpiece for 1-2 weeks and strike it for 10-20 minutes every day for artificial aging treatment.

2) After high-temperature aging treatment for rough machining and before semi precision machining, the workpiece is heated to 550 ℃, kept at a temperature of 6-8 hours, and then cooled to 300-200 ℃ before being taken out of the furnace for cooling.

3) Low temperature aging treatment: After semi precision machining and before precision machining, the workpiece is heated in oil to around 180 ℃, held for 24 hours, and then cooled.

2.4.3. Methods for eliminating internal stress in machined parts

For machined parts, the principle of process dispersion should be adopted, which means that only rough machining of all parts of the part is carried out before precision machining of each part. Simultaneously adopting the method of small cutting depth and multiple cutting paths to eliminate or reduce internal stress.

3. Deformation caused by clamping and cutting forces

During cutting, due to different clamping methods of the workpiece and the action of cutting forces during the cutting process, deformation of the workpiece can be caused. There are several common situations:

3.1. Turning long axis

When turning long shafts, two rigid tips are generally used to clamp the workpiece, but this method can easily cause the workpiece to bend. Because at this point, if the workpiece becomes hot, there will be no room for extension and bending will occur. Generally, the following measures can be taken:

① One end is clamped with a chuck, and the other end is supported with a rigid tip.

② One end is clamped with a chuck, and the other end is supported with an elastic tip.

③ One end is clamped with a chuck, and the other end is equipped with a rolling bearing installed in the sleeve at the end of the bed

A small three jaw self centering chuck that can rotate and clamp.

3.2. Turning thin-walled sleeves

When turning thin-walled sleeves, if a three jaw self centering chuck is used to clamp the workpiece, it is easy to deform the workpiece. In this case, the following measures can be taken:

a. Use a slotted sleeve to cover the workpiece and clamp it together with a three jaw self centering chuck for turning.

b. The longer inner hole is plugged with conical plugs (stoppers) at both ends, and the two pointed tips are located in the center hole of the conical plug. A small hole is drilled on the cone plug to facilitate the discharge of hot air from the workpiece hole during processing.

c. Increase the contact surface between the claw and the workpiece. Generally, un quenched steel is welded onto the three claws of a three jaw self centering chuck, and a circular arc surface is machined according to the diameter of the clamped workpiece, and then the workpiece is clamped for processing. Of course, a claw should be identified when loading and unloading workpieces.

d. Repeatedly rotate the outer circle and inner hole. Use a smaller cutting amount to turn the outer circle and inner hole in sequence, then turn the outer circle and inner hole.

e. Simultaneously turning the outer circle and inner hole.

f. Adopt axial clamping of the workpiece.

g. If the number of workpieces is small and short, the sleeve can be machined in one installation, and then the workpieces can be cut off from the chuck.

3.3 Turning bearing seat parts

When turning these types of parts, they are usually installed and clamped on a flower plate or triangular iron with the bottom plane as the reference. In addition to paying attention to the above points, it is best to scrape and grind the reference plane.

3.4. Turning the two end faces of thin sheet parts

When turning such parts, it is not easy to meet the flatness requirements of the two end surfaces. It is best to use two knives at the same time for turning, with the same cutting speed, back cutting amount, and feed rate, from the outer circle to the center. This has a certain effect.