Translation is the most important function of CNC EDM, which directly affects processing efficiency and surface quality. However, not every factory can take full advantage of the translation function. The main reason is that designers do not have enough understanding of electrode size reduction and translation processing. This article will provide a detailed analysis of translational machining in order to provide a useful reference for factory-related personnel.
Electrode size reduction (spark position)
1) The concept of electrode size reduction
There is a spark gap during electrical discharge machining, and for this reason the electrode must be made smaller than the shape to be machined. The reduced value is called electrode size reduction.
Electrode size reduction R=(cavity size-electrode size)/2
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Schematic diagram of electrode size reduction
2) The amount of electrode size reduction determines the processing speed
The energy of electric discharge machining is large, the processing speed will be fast, and the discharge gap will be large. If the electrode size reduction is increased, the processing speed (removal rate) can be increased several times. Another important point is that the roughing conditions are not only fast but also low in loss. That means that if the electrode size is reduced enough, efficient and low-loss conditions can be used.
The amount of picture electrode size reduction determines the speed
How to get good surface quality
The surface obtained by rough machining is relatively rough, but we hope to obtain good surface quality in a short time. The best way to achieve this is to use roughing conditions to machine away the bulk, and then use finishing conditions to machine the surface.
In addition, to reduce processing time, processing conditions must be changed at appropriate times. For example, if you start roughing with a maximum roughness of Ra5.0μm, and you end up with a roughness of Ra0.8μm, you must have multiple processing conditions to transition between roughing and finishing.
1) Bottom surface
The bottom surface can be achieved by changing the conditions and setting the height. But the side surface cannot be realized because the discharge gap of rough machining is larger than that of fine machining.
Picture bottom processing
2) Translational movement to achieve side processing
To machine the side, the electrode must be close to the side.
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Bottom and side processing
Movement in a plane perpendicular to the machining direction is called translation (rocking), and the purpose of translation is to complete side processing.
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Translation and machining direction
Effect of two-dimensional translation on accuracy
1) Shape after translation
First we need to understand the shape after translational processing. If the electrode translates in a certain shape, each part of the electrode must translate in the same shape, and then draw the outer shape of the electrode. The outer shape of the figure is the shape after finishing. This method can be used on any kind of shaking shape, which is an effective method to determine the processing shape.
Some translation will result in inaccurate shapes, but from general considerations the error is not very large. To have a sufficient understanding of these, start with the translational analysis of two-dimensional shapes.
When the picture is translated, each part of the electrode follows the same shape.
2) Circular shake
The electrode will be a little smaller than the actual desired shape in each dimension, so to get the desired shape and size it is necessary to expand the size by an R in each direction. Expanding an R in all directions is equivalent to making a circular motion of R at each point. The picture below shows that the straight parts are correct, but the sharp corners are not enough.
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For a general shape, as shown in the figure below, the electrode size reduction makes the outer corner radius smaller and the inner corner radius larger. This deformation is like a graphic offset. After using circular shaking, the processed shape will be correct. If you use CNC or wire cutting to make electrodes and use offset to determine the amount of electrode reduction, circular translation creates the correct shape without sharp corners.
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Another important point is: circular translation is a standard translation method, without overcutting. If you don't know much about translation, it is recommended to choose this translation method.
3) Square translation
For EDM, corner processing is one of the most important processing. If the cavity itself is square or rectangular, as shown in the figure below, square shaking is better than circular shaking. At this time, the processing efficiency of square translation is higher than that of circular translation.
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But there's a problem if you use square panning for general shapes as well. For example, in the picture below, if you use square translation, the diagonal area will be overcut. The most obvious error is at a 45-degree angle.
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Part of the diagonal line is overcut using square translation
Effect of three-dimensional rocking and translation on accuracy (spherical translation)
The influence of three-dimensional translation on size can be referred to the two-dimensional effect on X-Y plane Y-Z or Z-X plane.
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3D electrode processing
1) Simple shape at the bottom
For general CNC EDM machines, the translation value is constant from top to bottom (this method is called "bottom simple shape"). If the X-Y plane is a circular translation, the X-Z or Y-Z plane is the same as a square shake. This means that the bottom radius and bottom slope are the same. Usually, due to the processing offset of R, the bottom radius and slope will become smaller. If you use an electrode with a simple bottom shape, the sharp corners on the bottom will overcut. The value of overcut is determined according to the ratio of electrode R. For this reason, rough machining is prone to overcutting.
For 3D electrodes, if you want to use a simple bottom shape pattern, then the bottom corner radius and slope of your electrode must be consistent with the final shape.
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2) Complex shape at the bottom
As shown in the picture above, it is difficult to determine the bottom radius of some electrodes, or sometimes the bottom of the electrode is not flat. It is impossible for these electrodes to do as mentioned above. The 3D mode of "bottom complex shape" (spherical translation) solves this problem.
The typical way is: complex shape at the bottom. This appears to be the same as translation of a circle from the side (Z-X or Y-Z plane). There are no overcut areas. This method is also suitable for rough machining if large electrodes are used.
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Simple bottom shape and complex bottom shape
Conclusion about translational function
1) The appropriate amount of translation should be as large as possible, which can greatly reduce the processing time.
2) Basically, circular translation should be used because it has the same R value in all directions. Circular translation is the safest way.
3) For complex cavities, selecting square translation will cause overcutting in sharp corners and hypotenuses; square translation is only suitable for rectangular shapes.
4) For two-dimensional translation of simple shapes, circular translation is used. Its X-Y plane is circular, but X-Z and Y-Z are square translations, so overcutting will also occur for complex bottom shapes.
5) Based on the principle that circular translation is the safest, using three-dimensional spherical shaking, circular translation occurs in all directions, so it is safe in three dimensions.
6) For complex cavities with high precision requirements, three-dimensional spherical vibration must be selected; for most electrical discharge machining, two-dimensional circular translation can generally meet the requirements, and it is easier to obtain better finish and higher efficiency than three-dimensional spherical translation. .
