If you split a piece of bamboo down the middle, both halves of the piece will bend, the larger half will bend less and the smaller half will bend more. This is the material itself will have stress, the splitting process breaks the original stress balance state of the material, through deformation to restore the balance. Wire cutting workpiece deformation is the same reason, the cutting process breaks the original stress balance state of the workpiece.
The deformation of wire cutting is related to the structure of the workpiece. The concave die and punch die with narrow and long shape are easy to deform, and the size of the deformation is related to the complexity of the shape and the aspect ratio. The workpiece with thin wall thickness is prone to deformation.
If the deformation is very small, the deformation is almost negligible within the precision required for machining. But if the deformation exceeds the machining accuracy requirements, the size will appear obvious deviation, affecting the workpiece shape processing.
The causes of deformation are many, such as material problems, heat treatment problems, structural design problems, process arrangement problems and wire cutting workpiece clamping and cutting path selection problems and so on. These various reasons will lead to the deformation of the material under the action of internal stress.
Preventive measures for workpiece deformation during processing
Some measures can be taken to control the deformation of wire cutting processing and prevent the deformation phenomenon.
1) Roughing or stress release cutting before cutting
If a large area of cutting is needed in a piece of material, the relative equilibrium state of residual stress inside the material will be damaged, and the material will produce great deformation. We can first remove most of the stress from the material by roughing (roughing), removing most of the surplus, or by path cutting to release the stress.
For the EDM machining of large concave 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, so that the stress is released, and then the standard offset is used for the second main cut
2) Processing thread holes
When cutting punch, if the thread hole is not processed, it is cut directly from the outside of the material. Due to the stress imbalance of the material, it will produce open-mouth deformation or closed-mouth deformation. Can be processed in the material through the wire hole, closed contour processing, can obviously reduce the deformation of electric discharge wire cutting processing.
3) Optimize the processing path
Under normal circumstances, it is best to arrange the machining cutting point near the clamping end, the workpiece and the clamping part of the cutting section is arranged at the end of the processing path, the suspension point is located near the billet clamping end. In some machining, the unreasonable processing path arrangement is also the cause of machining deformation. As shown in the following figure, the more reasonable processing 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 will be cut off at the beginning of cutting, processing to the end of the program, the cutting accuracy of the punch is directly affected by the clamping uncertainty factor.
4) Multiple cuts
Some workpiece after taking some measures, there are still some deformation, in order to meet the accuracy requirements of the workpiece, can change the traditional habit of cutting to the size, the method of multiple cutting. The multi-cutting method is used in the cutting processing of fast walking silk wire, mainly in order to achieve better surface roughness, but the multi-cutting method has very important practical significance to reduce the deformation of die parts caused by the stress problem.
5) Optimization of machining process of multi-hole and concave formwork
In the process of linear cutting, with the effect of the original internal stress and the influence of the thermal stress generated by linear cutting, the template will produce non-directional and irregular deformation, so that the thickness of the cutting tool behind is uneven, affecting the processing quality and precision.
In view of this situation, the precision requirements of the template is relatively high, in multiple cutting processing. The first cutting will all the type of hole waste cut off, take out the waste, and then by the automatic shift function of the machine tool, in turn to complete the type of hole repair: the main cutting a cavity, take the waste → the main cutting b cavity, take the waste → the main cutting c cavity, take the waste →...... → main cut n cavity, take waste →a cavity repair →b cavity repair →...... →n cavity trimming, processing completed.
This cutting method can make each type of hole after machining enough time to release the internal stress, can reduce the mutual influence of each type of hole due to different processing sequence, the micro deformation to a minimum degree, better ensure the machining precision of the template.
But this processing thread more times, the workload is larger, more suitable for the automatic thread threading mechanism of slow wire cutting machine. After the cutting, the shape and size meet the requirements of high precision.
6) Set the temporary amount of multiple segments
Large, complex shape of the workpiece processing, should set two or more temporary retention, set a number of cutting points, as shown in the figure below. During programming, the pattern is processed in the way of open shape. Before programming, the graph is decomposed into multiple segments and connected in series respectively. During processing, the outline is processed first, and the temporary quantity part is processed finally.