How to solve the problem of “wire cutting deformation”

If you split a bamboo from the middle, both halves of the bamboo will bend, most of them will bend less, and the other half will bend much. This is that the material itself will have stress, and the splitting process breaks the original stress balance of the material and restores the balance through deformation. The same is true of workpiece deformation in WEDM, and its cutting process breaks the original stress balance state of workpiece.

1. The causes of workpiece deformation in WEDM.

The deformation of WEDM is related to the structure of the workpiece. The narrow and long female die and male die are easy to deform, and the deformation is related to the complexity of the shape and the ratio of length to width. Workpieces with thin wall thickness are prone to deformation.

If the deformation is very small, it is almost negligible within the precision range required for machining. However, if the deformation exceeds the requirement of machining accuracy, the dimension will be obviously deviated, which will affect the machining shape of the workpiece.
There are many reasons for the deformation, such as material problems, heat treatment problems, structural design problems, process arrangement problems, workpiece clamping and cutting path selection problems during wire cutting. These various reasons will lead to the deformation of the internal stress of the material.

2. Precautionary measures for machining deformation of workpieces

Some measures can be taken to control the deformation of WEDM and prevent the deformation from happening.
1) Rough machining or stress release cutting before cutting
If it is necessary to cut a large area on a piece of material, the relative balance of residual stress in the material will be destroyed and the material will be greatly deformed. We can eliminate most of the stress of the material first by rough machining, removing most of the surplus first, or cutting the path to release the stress.
For WEDM of large die, two main cuts can be made. First, the offset of the main cut is increased by 0.1~0.2mm on one side for the first main cut to release its stress, and then the standard offset is used for the second main cut.For a long narrow shape, the stress-relieving path is cut inside the hole first, and then the shape of the hole is processed.

2) Processing a wire threading hole
When cutting the punch, if the thread hole is not processed, it will be directly cut from the outside of the material, as shown in the following figure (a), because of the stress imbalance of the material, the deformation will be open or closed. It is possible to process wire holes in the material and perform closed contour processing, as shown in the following figure (b), which can obviously reduce the deformation caused by WEDM.

3) Optimize the machining path
Generally speaking, it is best to arrange the starting point of machining near the clamping end, the cutting section separating the workpiece from its clamping part at the end of the machining path, and the stopping point near the clamping end of the blank. In some machining, the unreasonable arrangement of machining paths is also the cause of wire cutting deformation. As shown in the following figure, the more reasonable machining path is: A → B → C → D …→ A. If we follow the clockwise direction: A → L → K → J …→ A, because the workpiece and the clamping part are cut off at the beginning of cutting, the cutting accuracy of the punch is directly affected by the unreliable clamping factors when it is processed to the end of the program.

4) Multiple cutting
Some workpieces still have some deformation after taking some measures. In order to meet the accuracy requirements of workpieces, we can change the traditional habit of cutting to size once and adopt the method of multiple cutting. Multi-cutting method is used in high-speed wire cutting, mainly to achieve better surface roughness, but it is of great practical significance to reduce the deformation of die parts caused by stress problems.

5) Optimization of processing technology of multi-hole concave template.

When the template is cut online, with the effect of the original internal stress and the influence of the thermal stress generated by wire cutting, it will produce non-directional and irregular deformation, which will make the following cutting tools uneven in thickness and affect the machining quality and precision.

In view of this situation, the template with high precision is required in multiple cutting processes. The first cutting cuts off all the waste materials of the die holes. After the waste materials are taken out, the die holes are trimmed in turn by the automatic shifting function of the machine tool: main cutting of cavity A, taking waste materials → main cutting of cavity B, taking waste materials → main cutting of cavity C, taking waste materials → … → main cutting of cavity N, taking waste materials → trimming of cavity A → … → trimming of cavity N, and processing is finished.
This cutting method can make each die hole have enough time to release internal stress after machining, and can minimize the mutual influence and slight deformation of each die hole due to different machining sequences, thus ensuring the machining dimensional accuracy of the template.
However, this method has many threading times and heavy workload, and is more suitable for wire cut WEDM with automatic threading mechanism. After this cutting, the shape and position dimensions meet the requirements of high precision through measurement.

6) Set the amount of multi-segment retention.
For the machining of large and complex workpieces, two or more temporary allowances should be set, and multiple cutting points should be set, as shown in the following figure. When programming, it is processed in an open shape. Before programming, the figure is decomposed into multiple segments and connected in series respectively. When processing, the outline is processed first, and then the remaining part is processed.




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