**1 Two Main Methods for Precision EDM Grinding of Shaft Parts**
There are two primary techniques used in precision EDM grinding for shaft parts: the block electrode radial feed method and the wire electrode method. The block electrode radial feed method, also known as forming block reverse copying or inverse copying, uses a block electrode that has the same surface shape as the micro-axis busbar. This allows the forming surface to be radially servo-fed against the axis of the rotating fine shaft.
The wire electrode method was developed in 1984 by Professor Masawachi Takahisa from Tokyo University. It is particularly effective for precision EDM grinding of fine shafts. In this method, a wire electrode moves along the guide surface of a fixed guide (not a turning guide) on a numerical control table. The NC system controls the relative motion path of the tangent point of the fixed guide. The wire electrode is servo-fed radially and axially around the rotating fine shaft.
A comparison of these two methods is shown in the figures below:
*Figure 1: Block electrode radial feed method*
1. Micro shaft
2. Block electrode
*Figure 2: Wire electrode grinding method*
1. Micro shaft
2. Fixed guide
3. Wire electrode
*Figure 3: Block electrode tangential feeding method*
1. Micro shaft
2. Block electrode
The block electrode method has a significantly higher material removal rate compared to the wire electrode method—about five times or more. However, because the tool electrode's wear cannot be compensated quickly, processing often gets interrupted. This requires frequent size measurements, monitoring of electrode loss, and updating the discharge working surface. Due to these interruptions, only limited manual interventions can be made, making it difficult to control electrode wear at the end of machining, which affects the workpiece’s shape and dimensional accuracy. Especially in multi-piece production, the consistency of the workpieces is poor.
In contrast, the wire electrode method continuously compensates for energy consumption through its movement, allowing the tool loss to be negligible. The workpiece’s shape and dimensional accuracy are ensured by the NC motion accuracy. The block electrode method is harder to automate due to the need for manual intervention, while the wire electrode method enables continuous roughing and finishing.
Another disadvantage of the block electrode method is that it requires high manufacturing, positioning, and adjustment standards. The wire electrode method also demands precise wire movement mechanisms, leading to higher processing costs.
**2 Block Electrode Tangential Feeding EDM Grinding Method**
To overcome the limitations of the previous methods, a new block electrode EDM method was proposed—tangential feeding. This method allows automatic compensation for electrode wear. The basic principle is shown in Figure 3. In this method, the upper surface of the horizontally placed tool electrode serves as the main working surface, with a pre-set distance 'e' between the upper surface and the workpiece axis. The upper surface of the tool is then servo-fed horizontally during machining. When the electrode surface wears, the loss is automatically compensated by the tangential feed until the desired machining radius is achieved.
The tool electrode must be long enough to allow the workpiece to be withdrawn from an area that hasn’t been worn, ensuring the required shape and dimensional accuracy. This method works like a "mold" where the shape depends mainly on the tool electrode’s surface, and the size depends on the offset position of the electrode relative to the workpiece axis and the discharge gap.
**3 Characteristics of the Block Electrode Tangential Feeding Precision EDM Method**
This method offers automatic compensation for electrode wear, eliminating the need for special mechanisms like those in the wire electrode method. It is especially suitable for batch processing, as one electrode can process multiple workpieces simultaneously, ensuring good consistency in shape and size.
For example, using a block-like copper electrode with medium electrical parameters, the measured electrode wear rate was 1.75%. After machining eight workpieces, the significant loss zone on the electrode front end was about 4 mm long and 2 mm deep, forming a right-angled triangle. With an 80 mm electrode length, up to 160 workpieces could be processed. In contrast, using the radial feed method, only 10 workpieces could be machined with the same electrode surface.
The tangential feed method enhances controllability, enabling high shape and size consistency. Dimensional accuracy depends mainly on the electrode’s manufacturing precision and adjustment level. Trial grinding can help verify the adjustment condition before starting mass production.
Material utilization is higher in the tangential feed method, as the electrode can be reused after being reprocessed. This reduces waste and improves efficiency. Automation is also increased, as once the electrode is adjusted, the workpiece is automatically machined to the desired size, reducing manual intervention.
The pulse equivalent in this method is small, contributing to stable servo feed, especially for fine EDM. While this may slow down the retreat speed, it helps prevent arcing, which is important for maintaining stability.
Overall, the tangential feed method improves productivity, reduces manual effort, and ensures better quality and consistency in precision EDM grinding of shaft parts.
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