Nowadays CNC spark machines have been widely used. Shaking function is the most important function of CNC spark machines. Shaking is very effective in reducing processing time and obtaining a good surface. However, not every factory makes good use of shaking function. Main reason is that designers do not want to use enough electrodes to reduce size, they worry that too large will reduce accuracy. If we have enough knowledge of electrode reduction and shaking, we can use appropriate electrode reduction and greatly improve processing efficiency.

Spark gaps exist during electrical discharge machining. For this reason, electrodes must be made smaller than shape to be machined. Reduced value is called electrode reduction.
Electrode size reduction R = (cavity size－electrode size) ÷ 2

 

Schematic of electrode reduction

Energy of EDM is large, processing speed will be fast, and discharge gap will be large. If electrode is reduced in size, processing speed (removal rate) can be increased several times. Another important point is that roughing conditions are not only fast but also low loss. That also means that if electrode reduction is sufficient, conditions of high efficiency and low loss can be used.

 

Electrode reduction determines speed

Surface quality of rough machining is very rough, but we hope to get good surface quality in a short time. The best way to achieve this is to machine most of it under roughing conditions and then finish surface with finishing conditions.
In addition, to reduce processing time, processing conditions must be changed at appropriate time. For example, if you start roughing with a maximum roughness of 40μm, and finally you get a roughness of 5μm, then you must have multiple sections between roughing and finishing to change processing conditions.

Bottom surface can be achieved by changing conditions and setting height. However, because discharge gap of rough machining is larger than that of finishing machining, it cannot be achieved.

 

Bottom processing

To machine side, electrode must be close to side.

 

Underside and side processing

Movement in plane perpendicular to processing direction is called shaking, and purpose of shaking is to complete processing of side.

 

Shaking and processing direction

First we need to understand shape after shaking. If electrode is shaken in a certain shape, each part of electrode must be shaken in same shape, and then outer shape of electrode is drawn. Outer shape of figure is finished shape. This method can be used on any kind of shaking shape, which is an effective method for determining processing shape.
Some shaking can lead to inaccurate shapes, but from general considerations, errors are not that great. We need to know enough about them. Let's start with shaking analysis of two-dimensional shapes.

 

Every part of electrode follows same shape when shaking

Electrode will be a little smaller than actual shape in each size, so to get desired shape size, an R size is enlarged in each direction. Enlarging an R in each direction is equivalent to doing a circular motion of R at each point. Figure below shows that straight part is correct, but sharp corner part is not enough.

 

For general shape, as shown in figure below, reduction of electrode makes outer corner radius small and inner corner radius large. This deformation is like a graphic offset. After using circular shaking, machining shape is correct. If CNC or wire cutting is used as electrode and offset is used to determine electrode reduction, circular shape will make correct shape without sharp corners.

 

Another important point is that circular shaking is standard way of shaking without overcutting. If you don't know much about shaking, it is recommended to choose this shaking method.

For EDM, corner machining is one of the most important machining. If cavity itself is square or rectangular, as shown below, square shaking is better than circular shaking. At this time, square translation is more efficient than circular translation.

 

But if you also use square shaking for general shapes, problems will arise. For example, in figure below, if you use square shaking, oblique area will be overcut. The most obvious error is when angle is 45 degrees.

 

Use a square translation slanted part to be overcut

Effect of three-dimensional shaking on size can be referred to two-dimensional for X-Y plane Y-Z or Z-X plane.

 

3D electrodes

For general CNC EDM machines, shaking value is constant from top to bottom (this method is called "simple shape at the bottom"). If X-Y plane is a circular translation, X-Z or Y-Z plane is same as a square pan. This means that bottom radius and bottom slope are same. Generally, radius and slope of bottom become smaller due to processing offset of R. If you use an electrode with a simple shape at the bottom, sharp corners at the bottom will overcut. Value of overcut should be determined according to ratio of electrode R. For this reason, roughing is prone to overcutting.
For 3D electrodes, if you want to use simple shape of bottom, then bottom corner radius and slope of your electrode must be consistent with final shape.

 

As shown above, for some electrodes, it is difficult to determine bottom radius of electrode, or sometimes bottom of electrode is not flat. These electrodes cannot be done as mentioned above. 3D mode of "Bottom Complex Shape" (spherical translation) solves this problem.
Typical way is: complex shape at the bottom. This appears to side (Z-X or Y-Z plane) same as a circular translation. No overcut areas. This method is also suitable for rough machining if large electrodes are used.

 

Simple bottom shape and complex bottom shape

① Appropriate R, as large as possible, can greatly reduce processing time.
② Basically, circular should be selected because it has same R value in all directions. Circular shaking is the safest way.
③ Choosing a square shake will cause overcutting at sharp corners and hypotenuses, which is only suitable for rectangular shapes.
④ Two-dimensional shaking of simple shapes, using circular shaking, its X-Y plane is circular, but X-Z, Y-Z are square shaking, so it will also overcut complex shape at the bottom.
⑤ Based on principle that circular shaking is the safest, three-dimensional spherical shaking uses circular shaking in all directions, so it is safe in all three sizes.
⑥ For high-precision complex cavities, three-dimensional spherical shaking must be selected; for most electrical discharge machining, generally choose a two-dimensional circular translation to meet requirements, it is easier to obtain a good finish and high efficiency than three-dimensional spherical shaking.