How to solve the problem of processing elastic she

  • Detail

How to solve the problem of processing elastic sheet parts

1 introduction

many deformable elastic sheet parts are encountered in production, such as common washers, friction plates, disc springs, thin plates, flyers, etc. they have poor rigidity, difficult heat dissipation, easy to cause clamping deformation during clamping, warping during processing, which seriously affects the machining accuracy of parts. It is of great significance to study the heat treatment technology and precision clamping technology of this kind of parts, that is, to reduce the heat treatment deformation and clamping deformation of the workpiece, and to reduce the influence of the deformed workpiece on the final accuracy of the parts, so as to ensure the machining accuracy of elastic sheet parts

during the in-situ inspection of most deformable elastic sheet parts in production, it is found that the machining accuracy can fully meet the design requirements, but after removal, it is found that the size and geometric tolerance have changed greatly, and some even greatly exceed the design permission. The reason is: the stiffness of the deformable elastic sheet parts is weak, coupled with the low accuracy of the workpiece datum and poor flatness. The workpiece is deformed due to being sucked by the atmospheric pressure or the clamping force such as the compressed tightening force. It is processed in the deformed state, and the size and geometric tolerance of the workpiece are found to meet the design requirements through in-situ inspection, but the deformation of the workpiece is restored after removal, Its size and geometric tolerance have changed greatly, so that it is beyond the scope of design requirements

when detecting another part of elastic sheet parts that are easy to deform in place, it is found that the size and geometric tolerance of the workpiece are far from the normal machining accuracy of the machine tool, which is due to the deformation of the workpiece caused by the cutting force when cutting the workpiece, and the recovery of the deformed part after the cutting edge leaves

therefore, improving the precision of parts before finishing, reducing the clamping deformation of parts and reducing the cutting force are important measures to improve the machining precision of deformable elastic sheet parts

2 technical ways to improve the machining accuracy of elastic sheet parts

2.1 reduce heat treatment deformation and improve the dimensional position accuracy of the workpiece before finishing

for the workpiece with high mechanical property requirements, heat treatment is generally required to improve the comprehensive mechanical properties of the workpiece. However, due to the volume expansion when austenite transforms into martensite during quenching, great structural stress is generated, and the thermal stress caused by rapid cooling forms a great internal stress, resulting in deformation and even cracking of the workpiece during quenching. For flexible sheet parts that are easy to deform, their deformation is greater due to their weak stiffness. In order to improve the machining accuracy of parts, the deformation of heat treatment must be reduced

design to reduce heat treatment deformation

when designing parts, it should be closely combined with the process, pay attention to the limitations of process conditions, and try to reduce heat treatment deformation from the structure of parts

when designing parts, steel with high carbon content or high alloy elements should be selected as much as possible, because generally, the higher the carbon content and the higher the alloy elements, the lower the end temperature of austenite transformation into martensite, the more residual austenite after quenching, the smaller the structural stress during quenching, and therefore the smaller the deformation. And the hardenability of alloy steel is good. Generally, oil quenching is used. The workpiece is cooled in oil, and the cooling speed is slow. The thermal stress is relatively small, so the deformation and cracking tendency is small

when designing the workpiece, if there is no other influence, the end flange should be increased as much as possible to increase the stiffness of the part

the workpiece of the design heat treatment sizing tooling must be tempered immediately after quenching, that is, the quenched part must be reheated to a temperature lower than the critical temperature of austenite transformation, and cooled to room temperature after holding for a period of time. Design the sizing tooling, put the newly quenched workpiece into the sizing tooling to fix the sizing, and then put it into the furnace to temper and finalize the shape with the workpiece, which can well correct the deformation of the workpiece after quenching. It is proved by experiments that the elastic surface parts with an outer diameter of 350mm and a thickness of 3mm adopt the sizing tooling during heat treatment, and the deformation is 0.20mm. If the sizing tooling is not used, the deformation will be greater than 1mm

2.2 selection of clamping method

when finishing parts, we should focus on the clamping method, try to make the parts basically consistent with their free state in the state to be processed, that is, we should avoid or reduce clamping deformation, and try not to use clamping forms such as suction or clamping, jacking, pressing, etc

clamping method of cushion elastic block

the elastic clamping mechanism is adopted, so that the parts can be positioned and clamped in a free state. A layer of 0.5mm rubber with uniform thickness is placed between the workpiece and the tooling positioning surface. When the workpiece is subjected to the clamping force, the rubber is compressed first, and the elastic deformation of the workpiece is small, so that the shape surface with high accuracy can be processed, and the deformation is small after loosening. This method is suitable for datum machining with high accuracy requirements

three point clamping mode

the workpiece is positioned and clamped with three points. When the workpiece is clamped by the clamping force, because the surface of the workpiece is not under the force, there will be no clamping deformation, and the deformation of the workpiece after being processed and removed is also small. If there is a gap between the support block and the workpiece, it should be filled, and rubber can be padded if necessary. In production practice, if the workpiece size is large and the wall is thin, and the workpiece deforms under the action of cutting force during three-point positioning, auxiliary support can be added to solve it

workpiece bonding and positioning method

the workpiece is bonded to the positioning disc with adhesive in the free state. Because the adhesive has fluidity before curing, it can fill the gap between the workpiece and the positioning disc. When the adhesive is cured, the workpiece and the positioning disc are bonded together to form a whole, which not only plays a positioning role for the workpiece, but also greatly enhances the rigidity of the workpiece

for elastic sheet parts, the surface opposite to the processing surface cannot be selected as the bonding surface, because most adhesives will shrink in volume during curing, and uneven shrinkage will cause workpiece deformation. Therefore, for elastic sheet parts, the outer circle or inner hole of the workpiece should be selected as the bonding surface

when selecting adhesives, it is appropriate to select adhesives with strong adhesion, fast curing time and easy degumming. Cyanoacrylate adhesive is a kind of adhesive with fast curing time (only 3 ~ 5 minutes to process), easy degumming (soaking in hot water or acetone), and strong adhesion. Using this kind of glue for processing test, the result is satisfactory, but the selection of C load sensor when removing the glue: whether the load measurement accuracy meets or exceeds the standards of astm4 and iso7500/1 The load sensor should also consider: range, accuracy, repeatability, offset loading fault, nonlinearity, stable compensation range, temperature zero drift and sensitivity The automatic identification, calibration and zeroing of the load sensor greatly speeds up the experimental process and ensures the consistency and accuracy of the data In addition, the ketone soaking time is long, which affects the processing efficiency

the adhesive used in the bonding and positioning method should be as few as possible, so the bonding tooling must be designed with a limit stop to reduce the positioning burden of the adhesive. At the same time, considering the long soaking time in acetone when removing the adhesive, several holes can be designed at the relative position of the adhesive on the tooling, and the workpiece can be removed by mechanical knocking and vibration through the holes. Experiments have proved that as long as the method is correct, taking the workpiece by mechanical beating and vibration will hardly cause deformation of the workpiece, but the exposed adhesive must be removed with acetone before taking the workpiece

the processing test is carried out by bonding and positioning method, and the workpiece is 350 × 3. The elastic sheet is made of 60Si2MnA. Its positioning, the flatness of both sides after processing can reach 0.02 ~ 0.03mm, and the parallelism can reach 0.03 ~ 0.05mm. If the large plane bonding positioning is adopted, the flatness after processing will be greater than 0.20mm

2.3 tool selection

judge according to the required failure mode

reducing the external force directly acting on the workpiece is an effective measure to reduce workpiece deformation and improve machining accuracy. Increasing the main deflection angle and auxiliary deflection angle of the tool can reduce the effect of cutting force on the normal direction of the workpiece surface, which can effectively reduce the deformation of the workpiece

the experiment shows that the cutting force is significantly different with different blade sharpness. When the cutting depth is small, the difference is more obvious. When the cutting depth is small to a certain value, the unit cutting force increases sharply. This is because in ultra precision cutting, the cutting depth and feed rate are very small, and the difference of cutting edge radius r will obviously affect the deformation. With the increase of R value and 100 mesh wood flour, the cutting deformation will increase significantly. When the cutting depth is very small, the cutting deformation caused by the edge radius accounts for a large proportion of the total deformation, and a small change in R value will make a great change in the cutting deformation. Therefore, in the fine cutting with small cutting depth, the cutting tool with small R value should be used

3 conclusion

the main reason why it is difficult to ensure the machining accuracy of elastic sheet parts is that the workpiece has large deformation after heat treatment, the workpiece has deformation under the action of clamping force during machining, and the deformation recovery of the workpiece after machining has caused great changes in its size and geometric tolerance. The direct action of cutting force is also an important reason for workpiece deformation. To improve the machining accuracy, the technical way is to reduce the heat treatment deformation of the workpiece. When finishing the workpiece, try to fix and clamp the workpiece in a free state to reduce clamping deformation, and try to choose sharp cutting tools. (Article 5) the uncertainty U5 introduced by the resolution of the calibration tensile testing machine Source: Tool Technology)

Copyright © 2011 JIN SHI