Graphite Milling 101

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Graphite electrodes are used in the RAM EDM Process, serving as the negative form of what is being formed in the EDM. The technological advances in CNC machining have helped the graphite milling process just as much as any other sector. Even though some of the advances in CNC machining have reduced the need for the EDM process, it still has its place in mold building and possibly even more in other industries. Check out the video below of one of our graphite mills rough-cutting an electrode. 

The Raw Material

The following information is from www.poco.com. They are a leading manufacturer of graphite and do a great job of describing the various characteristics of graphite. 


“EDM has grown up. EDM has taken its place as a proven, precision technology, chosen for what it can do, rather than what conventional machining can’t do. EDM machine technology has spawned a world of new applications wherein increased importance is placed on the graphite electrode material utilized. While there are many methods used to determine the right material for a job, we believe there are five factors that mean the difference between success and failure, profit and loss.

Metal Removal Rate (MRR)

Metal removal rate is usually expressed as cubic millimeters per hour (mm3/hr) or cubic inches per hour (in3/hr), but in fact could just as realistically be expressed as $/hr. Achieving an efficient MRR is not simply a matter of the right machine settings. It also involves direct energy dissipated in the EDM process. Graphite is generally much more efficient than metallic electrodes, however metal removal rates vary widely between graphite types. With the proper electrode material/work metal/application combination MRR can be maximized.

Wear Resistance (WR)

There are four types of wear: volumetric, corner, end, and side. Of the four, we believe that corner wear is the most important since the contours of the final cut are determined by the electrode’s ability to resist the erosion of its corners and edges. It follows that if an electrode can successfully resist erosion at its most vulnerable points, then overall wear will be minimized, and maximum electrode life achieved. Electrode erosion cannot be prevented, but it can be minimized by choosing the proper electrode material/work metal combination and machining at the optimum settings. The ability of an electrode to produce and maintain detail is directly related to its resistance to wear and its machinability. Minimizing corner wear requires choosing an electrode material that combines high strength with high temperature resistance.

Surface Finish (SF)

Fine surface finish is obtained by a combination of the proper electrode material, good flushing conditions, and the proper power supply settings. High frequency, low power, and orbiting produce the best finish, as these conditions produce smaller, less defined craters in the work metal. The final surface finish will be a mirror image of the electrode’s surface, so Angstrofine and Ultrafine particle, high strength graphites are the best choices for finishing electrodes.

Machinability

Any machinist who has ever machined graphite is aware that graphite cuts very easily. Simply being easy to machine doesn’t necessarily make a material the best choice for an electrode. It must also be strong to resist damage from handling and from the EDM process itself. Strength and small particle size are important so that minimum radii and close tolerances may be achieved. Material hardness is also a factor in graphite machinability, as the harder electrode
materials will be more prone to chipping during the machining process.

Material Cost

Electrode material cost generally represents only a small part of the total EDM job cost. What is too often overlooked, however, is that electrode material cost considered outside the total job cost is completely meaningless. Fabrication time, cutting time, labor, electrode wear— all these factors depend on the electrode material more than on any other factor. Thus it is critical that
you know the properties and performance characteristics of the available electrode materials as they affect the work metals you are machining. Only with this data is it possible to make a cost/performance analysis to determine the true cost of an EDM job.”


 

Electrode Design

Designing electrodes is more like an art than a science. During the EDM process, the electrode orbits around as the electricity cuts the material away. Therefore the electrodes are cut slightly smaller than the workpiece to allow for this movement as well as room for the spark. The amount that the electrode is cut undersize varies by application. Because graphite wears away during the EDM process, it’s often necessary to make more than one electrode for a particular feature. You may start with a larger undercut to allow for more aggressive EDM settings (causing more wear) and then have a second or even third electrode with less undercut for finishing surfaces to size. 

Electrodes are usually not just designed as one big chunk of graphite that makes a bunch of detail. Features like ribs, lettering, and other details are broken up into several electrodes. Breaking electrodes up into several different pieces can save on material but also avoids creating unintended radii during the EDM process.  It’s also important to have graphite milling and EDM machinist working closely to communicate and plan the more complicated projects. 

 

Machining Process

When compared to metals, graphite is easy to machine. However, the challenge is that the graphite is abrasive and wears out cutting equipment quickly. It’s important to use diamond coated tooling (or some other coatings). This helps avoid excessive tool wear, which will inevitably show through in the EDM process.

We can produce very small details using the EDM process with graphite electrodes. The challenge here is that making small details can often require extremely small tooling. The general rule of machining applies where the smaller the tooling, the higher the rpm.

Equipment

Using CNC equipment that can reach over 20,000 rpm is important for efficiency. Ultimately this is related to the amount of material that’s removed with each rotation of the tooling. You don’t necessarily want to move the machine faster at the same rpm. Instead, you can raise the rpm, and in keeping the feed per revolution the same, you raise the feed rate of the machine. All tools have optimum feeds and speeds, but this is an important concept for graphite milling. 

The CNC equipment should also be fitted with an evacuation system to protect moving surfaces inside the machine. When graphite is cut, it turns into an abrasive powder. Using an evacuation system keeps the inside of the machine and the area around the machine as clean as possible. Allowing the graphite dust to get into bearings or bearing surfaces on a CNC machine is asking for a disaster. 

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