Are You Really Using Traverse Machining on Your CNC EDM Machine Correctly?

Traverse machining is one of the most critical functions in CNC Electrical Discharge Machining (EDM), directly impacting processing efficiency and surface quality. However, not every shop can fully leverage the advantages of traverse machining. The primary reason lies in designers' inadequate understanding of electrode reduction and traverse machining principles.This article provides a detailed analysis of traverse machining to offer valuable insights for personnel involved in these processes. By better understanding these concepts, shops can optimize their EDM operations for improved efficiency and quality.

Electrode Reduction (Spark Gap)

1) Concept of Electrode Reduction

In electrical discharge machining (EDM), a spark gap naturally forms between the electrode and the workpiece. This spark gap necessitates that the electrode be manufactured slightly smaller than the desired final shape to account for the material removal during the EDM process. The difference between the final cavity dimension and the electrode dimension is known as the electrode reduction or electrode offset.

The formula to calculate the electrode reduction ( R ) is:

This ensures that the electrode accurately machines the intended shape, taking into account the necessary clearance provided by the spark gap.

2) Electrode Reduction Determines Machining Speed

The energy level during Electrical Discharge Machining (EDM) directly influences the machining speed. Higher energy levels result in faster processing speeds and larger spark gaps. Increasing the electrode reduction can significantly enhance the material removal rate, often by several times. An important point to note is that roughing conditions not only achieve higher speeds but also result in lower tool wear.This means that with sufficient electrode reduction, you can utilize high-efficiency and low-wear conditions. Essentially, a properly adjusted electrode reduction allows for faster machining while minimizing electrode wear, leading to more efficient and cost-effective operations.

How to Achieve Good Surface Quality?

While rough machining can quickly remove material, it often leaves a relatively rough surface finish. However, achieving good surface quality within a short timeframe is possible by combining rough machining with finishing techniques.

The optimal strategy involves using rough machining conditions to remove the bulk of the material efficiently, followed by finishing conditions to refine the surface. This two-step process ensures that the majority of material is removed quickly while still achieving the desired surface finish.

To minimize overall machining time, it's crucial to adjust machining parameters at appropriate intervals. For instance, if the initial rough machining yields a maximum surface roughness of Ra 5.0 μm and the final target roughness is Ra 0.8 μm, several intermediate machining steps are required to gradually improve the surface finish. These steps help bridge the gap between roughing and finishing, ensuring a smooth transition and maintaining efficiency throughout the process.

1) Bottom Surface

The quality of the bottom surface can be controlled by adjusting the machining conditions and setting the appropriate height. However, this approach cannot be applied to the side surfaces because the discharge gap during rough machining is significantly larger than during finish machining, making it challenging to achieve the same level of precision on the sides.

2) Traverse Machining for Side Surfaces

To effectively machine the side surfaces, the electrode must be positioned close to these areas. This is typically achieved through traverse machining, where the electrode moves in a controlled manner along the side surfaces to ensure precise material removal and achieve the desired finish.

Traverse (Oscillation) Movement in a Plane Perpendicular to the Machining Direction

Movement within a plane that is perpendicular to the machining direction is referred to as traverse or oscillation. The primary purpose of this traverse movement is to enable the precise machining of side surfaces. By moving the electrode in this manner, the process ensures thorough and accurate material removal along the sides, achieving the desired surface finish and dimensions.

Impact of Two-Dimensional Traverse Movement on Precision

1) Shape After Traverse Machining

First, we need to understand the shape resulting from traverse machining. If the electrode moves in a specific pattern during traverse machining, each part of the electrode must follow the same pattern. By tracing the outer boundary of the electrode's movement, the external shape outlined represents the final finish-machined shape.This method can be applied to any type of oscillation pattern and is an effective way to determine the final machined shape. While some traverse movements may lead to slight inaccuracies in shape, generally, these errors are minimal. A thorough understanding of this process is essential, starting with the analysis of two-dimensional shapes undergoing traverse movement.

2) Circular Oscillation

The electrode is slightly smaller than the desired shape in every dimension. To achieve the intended shape size, you need to expand the dimensions by a value ( R ) in all directions. Expanding by ( R ) in all directions is equivalent to each point on the electrode undergoing a circular motion with a radius of ( R ).The following diagram illustrates that while the straight sections are correctly machined, the sharp corners are insufficiently formed.

For general shapes, as shown in the following diagram, the electrode reduction results in smaller external corner radii and larger internal corner radii. This deformation resembles a shape offset. After using circular oscillation, the machined shape becomes correct. If the electrode is made using CNC or wire EDM with offsets to determine the electrode reduction, the circular traverse movement will produce the correct shape without sharp corners.

An Important Point:Circular traverse is the standard traversal method and ensures there is no overcut. If you are not very familiar with traverse techniques, it is recommended to choose this method.

3) Square Traverse

For Electrical Discharge Machining (EDM), corner machining is one of the most critical processes. If the cavity itself is square or rectangular, as shown in the following diagram, square oscillation is preferable to circular oscillation. In this case, square traverse offers higher machining efficiency compared to circular traverse.

However, using square oscillation for general shapes can lead to problems. For example, as shown in the following diagram, using square traverse can result in overcutting in the diagonal areas, with the most noticeable errors occurring at 45-degree angles.

Impact of 3D Traverse Movement on Precision (Spherical Traverse)

The impact of three-dimensional (3D) traverse movement on dimensions can be understood by considering its effects on the X-Y, Y-Z, or Z-X planes, similar to two-dimensional traverse movements.

1) Simple Bottom Shape

For standard CNC EDM machines, the traverse value remains constant from top to bottom, a method referred to as "simple bottom shape." When using circular traverse in the X-Y plane, the behavior in the X-Z or Y-Z planes will resemble that of square oscillation. This consistency ensures that the bottom radius and taper angle are uniform throughout.However, due to the machining offset ( R ), the bottom radius and taper angle tend to be slightly smaller than intended. Using an electrode with a simple bottom shape can lead to overcutting at the bottom corners. The degree of overcut depends on the ratio of the electrode's ( R ), making overcutting more common during roughing operations.For 3D electrodes, if you intend to use the simple bottom shape approach, it's crucial that the bottom corner radii and taper angles of the electrode exactly match the final desired shape to avoid these issues.

2) Complex Bottom Shape

As illustrated in the diagram, some electrodes have irregularities that make it challenging to define a consistent bottom radius, or the bottom surface may not be flat. In such cases, the methods discussed for simple bottom shapes are not suitable. Instead, the "complex bottom shape" (spherical traverse) three-dimensional mode offers a solution.A typical approach for handling complex bottom shapes ensures that the movement in the side views (Z-X or Y-Z planes) mimics circular traverse. This technique effectively prevents overcutting and maintains the integrity of the machined shape. Even when using larger electrodes, this method remains effective for rough machining, ensuring precise and reliable results.

Conclusions Regarding Traverse Function

1. Optimal Traverse Amount: Using an appropriately large traverse amount can drastically reduce machining time without compromising accuracy.

2. Preferred Traverse Shape: In general, circular traverse is recommended because it maintains a uniform ( R ) value in all directions. This makes circular traverse the safest and most reliable option.

3. Complex Cavities and Square Traverse: For cavities with complex geometries, using square traverse can result in overcutting at sharp corners and inclined edges. Therefore, square traverse is only appropriate for rectangular shapes where such issues are minimized.

4. Simple Shapes with 2D Traverse: When applying 2D traverse to simple shapes, circular traverse in the X-Y plane effectively becomes square traverse in the X-Z and Y-Z planes. This can lead to overcutting in cavities with complex bottom shapes, highlighting the limitations of this approach.

5. Safety Principle with Spherical Traverse: Adhering to the principle that circular traverse is the safest, 3D spherical oscillation ensures that circular traverse is maintained in all directions, providing comprehensive safety and precision across all three dimensions.

6. High-Precision Complex Cavities: For cavities requiring high precision, especially those with complex geometries, 3D spherical oscillation is essential. However, for most EDM applications, 2D circular traverse typically suffices and offers advantages in terms of achieving good surface finish and maintaining high efficiency compared to 3D spherical traverse.