(Part-2) Are you selecting a good Graphite grade for your EDM needs?
In our previous blog, the graphite was selected mainly based on the grain size. Indeed, the grain size plays a vital role in EDM though several other essential factors measure the worth of an EDM Graphite.
Some more factors that play an essential role in choosing the grade of the graphite are given below-
1. Wear Ratio:
Wear ratio is calculated based on the percentage of erosion on the electrode that happened with the time or the depth or with the amount of steel sparked out. Though there are no concrete methods to find out the exact wear ratio before the starting of the EDM process, in our experience, the wear ratio can be anticipated on the factors like a profile of the electrode, EDM Sinking machine quality and make, amount of material to be removed from the Job and last but not the least the grade of the Graphite Material to be used in EDM electrode.
2. Metal Removal and Erosion Rate:
The metal removal rate is dependent on the particle size of the graphite used to make the electrode. Our previous blog has understood the importance of particle size in selecting the optimum graphite grade. We must know that the larger the particle size in positive polarity, the faster the metal removal rate can be achieved. Thus, an EDM operator needs to consider whether an ultra-premium (<5µ particle size), premium (5-7µ particle size) or superfine (8-15µ particle size) grade is the right one to use. Fortunately, many graphite grades from many manufacturers are offered today with particle sizes in these ranges.
When considering graphite grades, an EDM operator or Toolroom in charge must remember that an incredibly crucial feature of the newer bundled technology graphite is improving current flow within the electrode. Conventionally produced graphites with their high degree of open porosity and separate particle-to-particle bonding create extended current paths that rob the spark of energy as it passes through the electrode. This can be observed in the form of energy loss by developing the electrode's excessive heating. The bundled technology graphites have less heat generation and pass more of the energy through to the area needed—in the gap—to provide the most efficient EDM techniques available.
3. Surface Finish of the Job:
The surface finish is directly related to the significant particle size of the graphite. In low-quality graphites, the particles' measure and the particles' actual size are essentially the same; however, in graphites using the newer technology and higher quality, there is an optimum particle size smaller than the actual particle size. Thus, a grade with an actual particle size of four microns, for example, will develop a sufficient particle size more like a two- or three-micron graphite. The result of this phenomenon is that a surface finish can be achieved that is fairer and more consistent than would be expected from the four-micron graphite.
4. Machining of Graphite:
The most relevant factors in electrode fabrication are the strength and hardness of the material being cut. High power is often achieved by slightly changing the manufacturing process. An undesirable effect that usually occurs with the increased strength is an increase in the graphite's hardness.
In the newer generation of graphites using the bundled technology, high graphite strength is achieved at surprisingly low hardness values. Together, the properties will provide a very machinable material yet produces thin ribs or other highly detailed features without fear of chipping or cracking the graphite.
5. Cost of the Graphite:
In today's world, the cost is considered an essential factor in evaluating EDM machining needs. The cost of graphites can be too confusing to the purchaser. Sometimes, the customer ends up purchasing the wrong graphite grade because of the expensive initial investment and ends up spending more later on achieving the desired results. Most graphites are sold through various distribution channels, and the end-user prices can vary widely. Generally, the end-user makes his purchasing decisions based upon personal experience with the graphite and the service level that he receives.
However, it must be noted that the price of the graphite itself should generally be a small consideration in the purchasing decision. Of far greater importance are the results observed from the other four of the five key performance factors noted above. Pricing of graphite should only come into play after the particular grade requirements have been well determined through a thorough evaluation of the wear resistance, surface finish, metal removal rate, and the graphite machinability.
The EDM shop owner today is forced to make the right decisions when selecting his electrode material consistently. Luckily he has a tool to guide him in his evaluations. The five key performance factors have proven to be a time tested way to do his evaluations. The EDM operator's good news is that the performance factors have repeatedly shown that new technology, one featuring the bundling of discrete graphite particles, provides the end-user with superior results compared to conventionally manufactured graphites unique particle-to-particle bonding techniques.