From Entegris Poco  Author: Rick Slimp

If you are looking for more details, kindly visit SINOMETAL.

INTRODUCTION

'

Unless you have money to burn, time to spare, and a great mold repair team, never use words like 'equivalent to' or 'as good as' when purchasing your EDM graphite. These phrases or similar terms open the door for substitution of a lower quality inferior grade that will increase your frustrations in the electrode fabrication and EDM areas. If you do not specify exactly the grade you want delivered, you might as well open your wallet and prepare for a robbery. Blunt statements, but all too true.

Through the past three decades, I have been asked many questions concerning carbon and graphite. A question I used to get on a regular basis was, 'Can you make white graphite?' I don't get this one very often anymore, either dark grey/black is in style or they just got tired of hearing my attempt at a technical explanation on why white graphite is not a possibility. Another common question is, 'Do you make pencil lead?' The answer is yes, if you don't mind the pencil cutting through your paper and scratching your desk. On more than one occasion, I have been asked, 'Where is your graphite mined?' A long explanation of natural graphite vs. man- made graphite is required.

More often I receive questions like these: 'How do I know I am getting the grade ordered?', 'Is there a simple test I can do in my shop to determine the grade of graphite being supplied?', and 'With so many grades available, how do I know which grade is best for my EDM application?'

Too often our EDM applications engineers receive calls requesting technical assistance on a job where things are not going well in the electrode manufacturing area of the EDM machine, only to learn that the shop is using the incorrect grade of graphite for the application or they are not getting the grade they thought they had ordered. Often getting the correct electrode material gets the job back on track.

TESTING LAB

'

In the past 40 plus years, our lab has tested hundreds of manufacturers' grades and possibly thousands more being sold as reseller house brands. These grades are each tested for a wide range of physical properties, microstructure, and EDM performance (end wear, corner wear, surface finish, and metal removal rate). There have been approximately 78,000 graphite samples tested to date and the records and photo chip for every sample are maintained. The photo mount for sample number one, taken in , is safely stored for reference if needed.

This allows us to monitor changes in a grade over long periods of time and determine the manufacturer's grade for materials being sold under house brand names. Want to know the physical properties of one of our grades produced in ? No problem; we have the testing results and the sample from that period in our lab. Need to know if another manufacturer's grade or house brand grade has changed in the past few decades? We have the test results available and samples available to answer that question too.

GRAPHITE GRADES

'

Almost every manufacturer produces a series of grades for EDM applications. Some of the manufacturer grades are sold to EDM end users as the manufacturer's grade. When buying a manufacturer's grade, you often have more than one supplier offering the exact grade under the manufacturer's grade name and you also have a known manufacturer in case a concern develops with the material. Knowing the manufacturer and the distributor is a benefit to the customer. A common practice of some manufacturers is to allow resellers/distributors to rename or house brand their grades. House branding removes the tie to a specific manufacturer and specific manufacturer grade. House branding makes the reseller/distributor a sole source for their house brand. The exact manufacturer's grade may be sold as several other house brand grades, but the end users will not be aware that house brand A from company X is the same as house brand B from company Y, etc.

We do not allow our distributors to house brand our materials. When you order a specific graphite grade of ours it must be the same product regardless of the distributor chosen, the region of the world where you buy the graphite, or the length of time between your orders. Electrodes produced from our EDM-3® graphite ordered last week will perform just like EDM-3 graphite electrodes that have been in storage at a customer location for 20 years. The manufacturing process must be highly controlled and repeatable to generate consistency within a block, block to block, batch to batch, or decade to decade.

Almost a half century of highly controlled lab analysis indicates there are no grades that are exactly the same. Each manufacturing process yields a product with different physical properties, microstructure, and performance. In some cases the physical properties are similar, but the performance as an electrode material varies greatly. In other cases, there may be good EDM performance, but the material is too hard to machine into electrodes. To be a good EDM grade, the material must be very consistent, be capable of being machined to fine detail without chipping or wearing tools, and perform well in the EDM process. Excelling in all these areas defines a good EDM material.

In many cases the testing indicates the published specifications are highly exaggerated. For the most part, comparing published specifications is of little to no value since there is no policing of the specifications being published. Making buying decisions based solely on published specifications is not recommended.

GUIDELINES

If you are looking for more details, kindly visit Graphite Electrodes supplier.

'

I offer these simple guidelines when purchasing your EDM graphite requirements.

1. Determine the material grade best suited for your application and know why you are selecting that grade. Help in selecting the correct grade for your application is available if you need
2. Specify the exact grade you want and note 'no substitution' on the quotation request, purchase order, correspondence,
3. Be on the lookout for words such as 'like', 'equiva- lent', or 'type' on the quotation, acknowledgement, packing list, and invoice. Making it clear that 'no substitution is allowed' will help assure that you are getting the exact grade you are ordering and for which you are paying.
4. Know the company supplying your EDM graphite very If you are new to EDM, ask other shops what graphite supplier they are using and their experience with the company. If you get a call out of the blue, offering prices too good to be true, beware. There are many companies in the EDM supply business that have developed a very good reputation for honesty, integrity, and ability to service our industry. Select and buy from one of these companies. They will supply you the exact grade you order, since their good reputation and livelihood are at stake. These companies will gladly provide a 'certification of grade' if requested.

If you have specified the exact grade being ordered, stated 'no substitution' on the documents, ordered from a company with a good reputation in the industry, and you still would like more assurance that the grade you received is exactly what you ordered, we have a solution. We will test a sample of the material at no cost to you. It is not necessary to disclose the grade or the supplier when sending the sample.

We all know that graphite, for the most part, looks the same, feels the same, smells the same, and probably tastes the same. However, no two graphite grades are the same in terms of consistency, quality, and performance. Being an educated buyer will ensure that you are always getting the exact material needed and ordered.

FOR MORE INFORMATION

Select a suitable electrode material for EDM

There has been a long-standing, unresolved debate over whether graphite or copper is a better EDM electrode material. The preference is most always the same in any one geographical region. In North America , the preferred electrode material has shifted from copper to graphite. In Europe and Asia, many argue that copper is the preferable material. Perhaps it's time to take a closer look at both materials to identify the differences and help you select the one that is most appropriate for your EDM applications.

Electrode Material Considerations
Material variety. Graphite is produced with a wide range of material characteristics, enabling you to match the electrode material properties to the EDM application. Less-critical applications with electrode features such as a large radius, an open tolerance or minimal EDM requirements would use an economically-priced graphite material with large particles and lower strengths. However, a highly detailed electrode with critical features, extreme tolerances and stringent EDM requirements would entail a more premium graphite to fit the needs of this application.

On the other hand, due to the high purity value required for efficient EDMing, the types of copper available on the market and used in EDM applications are limited. This minimizes the ability to match material characteristics to the EDM application. The most commonly used types are electrolytic copper and tellurium copper, which vary slightly in elemental composition and are both cast as a solid.

Cost. Copper is often considered a commodity material and can be less costly than graphite in the bulk or blank stage. Many graphite materials are considered specialty materials and, as such, can be more costly. In addition, copper is recyclable whereas graphite is limited in its ability to be reclaimed. However, with the wide range of graphite materials available on the market today, it is possible to find some low-quality EDM grades that are more economical than copper. To establish the true cost of copper versus graphite, the value of machining the electrode must also be considered. Even with the more expensive graphite materials, the machining costs often offset any savings that are realized with the copper.

For example, a quote was recently solicited for a simple electrode blank with a ground finish on the top and bottom. The quote was received with copper at $4.68 per cubic inch, while a premium grade of graphite was quoted at $6.80 per cubic inch, or 45 percent more costly. However, when the cost of machining a simple, finished electrode was included, the story changed. In this case, the finished graphite electrode was quoted at $15.50 each, whereas the copper electrode ('oxygen free') was quoted as $95 each. Due to the soft, ductile characteristic of copper, it is often gummy, and conventional machining practices (such as feeds and speeds) must be altered to accommodate. This can be mitigated with tellurium copper, but EDM performance may be jeopardized. On the other hand, graphite is not gummy and can be conventionally machined very easily. However, machining graphite generates significant dust, which must be collected through an effective vacuum system with HEPA-type filtration.

Detail. In EDM applications, the electrical current tends to concentrate at sharp edges and corners. Copper's low melting point does not allow it to handle current density as effectively as graphite, so features on a single copper electrode must be similar in detail. It is not wise to EDM with varying details on the same copper electrode. This is because the higher current required to efficiently EDM larger electrode details exceeds the current allowable for the smaller detail.

With copper electrodes, high current density on smaller, more intricate detail increases the electrode wear. Also, when machining intricate detail or EDMing at high current densities, a copper electrode may move or flex as material stress is relieved. This limits the ability to maintain the very tight tolerances required on many of today's molds. Graphite performs very well at a high current density even with complex geometry. Graphite electrodes allow for efficient EDMing, even with varying details on the same electrode. Therefore, the number of graphite electrodes required to perform a job can be significantly reduced.

  EDM Performance
Metal removal rates. An electrode material's thermophysical properties determine its ability to process the energy of the EDM cut and remove metal. In generating a spark, peak current is discharged only after the gap between the electrode and workpiece has been ionized. At this point, the electrode emits electrons that collide with the molecules of the dielectric fluid. As a result, the fluid is vaporized and an energy channel is formed that allows the spark to take place. For this to happen, the electrode material must be hot enough for electrons to absorb enough energy to escape and work to create the energy channel.

For copper electrodes to release electrons in the gap, the temperature must be high enough for these electrons to absorb sufficient energy.1  As a result, these high temperatures tend to burn some of the copper electrode away. In order to generate this heat, the on times for copper electrodes are generally much higher than for graphite electrodes. However, due to its carbon base, a graphite electrode is able to emit these electrons at much lower temperatures and does not require the extended on times for electrons to release and create the energy channel. Therefore, the time required to form the energy channel is considerably less. Since the graphite initializes the spark faster, significantly higher metal removal rates are the result.

Wear. Electrode wear is a constant concern because excessive wear results in adding electrodes or redressing electrodes more often. Graphite is able to achieve electrode wear of less than 1 percent in relation to the depth of cut at machine parameters much more aggressive than copper electrodes. This is achieved through a 'replating' process during which molten particles from the workpiece may penetrate into the structure of the graphite electrode and reduce the amount of wear caused by the EDM process. The melting temperature of most standard work metals being EDMed is around 1,500°C. This exceeds that of copper, which is around 1,100°C. Any molten particles being ejected out of the EDM cut are likely to carry a thermoelectric charge and have temperatures that do not permit them to affix to the copper electrode. In this case, the molten particles may actually create secondary discharging and erode the copper electrode away.

This means that the high amperage and long on times of a roughing condition (and the process of replating) actually preserve the graphite electrode but is detrimental to the copper electrode which erodes away at these settings. On the contrary, in the finishing stages, with low amperage and short on-times, the graphite electrode has a tendency to wear at a faster rate than copper. However, since electrode wear is a ratio of the amount of material removed in the EDM cut, the actual wear on either a graphite or copper electrode is minimal in the finishing stage and sometimes immeasurable.

Surface finish. Because it is cast as a solid with no porosity, it goes without saying that copper electrodes provide very fine surface finishes. However, with the sophistication of today's EDM sinker technology, the surface finish gap between graphite and copper has narrowed significantly. Fine-grain graphite electrodes are now able to deliver surface finishes similar to copper with comparable electrode wear. With the proper electrode material selection and machine parameters, graphite is able to achieve near mirror finishes without the use of a powder additive and mirror-like finishes with the additive.

Conducting test burns and tracking results will help determine the monetary impact that the electrode material has on an EDM application. Part 2 of this article will review the results of EDM testing conducted with both graphite and copper, and reveal the bottom-line cost for each. With all of this information, you will then be able make an informed decision on the electrode material type best suited for a specific EDM application.

By |July 29th, |Blog|