Graphite processing can be a tricky business, so putting certain issues first is critical to productivity and profitability.
Facts have proved that graphite is difficult to machine, especially for EDM electrodes that require excellent precision and structural consistency. Here are five key points to remember when using graphite:
Graphite grades are visually difficult to distinguish, but each has unique physical properties and performance. Graphite grades are divided into six categories according to the average particle size, but only three smaller categories (particle size of 10 microns or less) are often used in modern EDM. The rank in the classification is an indicator of potential applications and performance.
According to an article by Doug Garda (Toyo Tanso, who wrote for our sister publication “MoldMaking Technology” at the time, but now it is SGL Carbon), grades with a particle size range of 8 to 10 microns are used for roughing. Less precise finishing and detail applications use grades of 5 to 8 micron particle size. Electrodes made from these grades are often used to make forging molds and die-casting molds, or for less complex powder and sintered metal applications.
Fine detail design and smaller, more complex features are more suitable for particle sizes ranging from 3 to 5 microns. Electrode applications in this range include wire cutting and aerospace.
Ultra-fine precision electrodes using graphite grades with a particle size of 1 to 3 microns are often required for special aerospace metal and carbide applications.
When writing an article for MMT, Jerry Mercer of Poco Materials identified particle size, bending strength, and Shore hardness as the three key determinants of performance during electrode processing. However, the microstructure of graphite is usually the limiting factor in the performance of the electrode during the final EDM operation.
In another MMT article, Mercer stated that the bending strength should be higher than 13,000 psi to ensure that graphite can be processed into deep and thin ribs without breaking. The manufacturing process of graphite electrodes is long and may require detailed, difficult-to-machine features, so ensuring durability like this helps reduce costs.
Shore hardness measures the workability of graphite grades. Mercer warns that graphite grades that are too soft can clog the tool slots, slow down the machining process or fill the holes with dust, thereby putting pressure on the hole walls. In these cases, reducing the feed and speed can prevent errors, but it will increase the processing time. During processing, the hard, small-grained graphite can also cause the material at the edge of the hole to break. These materials may also be very abrasive to the tool, leading to wear, which affects the integrity of the hole diameter and increases work costs. Generally, to avoid deflection at high hardness values, it is necessary to reduce the processing feed and speed of each point with a Shore hardness higher than 80 by 1%.
Because of the way that EDM creates a mirror image of the electrode in the processed part, Mercer also said that a tightly packed, uniform microstructure is essential for graphite electrodes. Uneven particle boundaries increase porosity, thereby increasing particle erosion and accelerating electrode failure. During the initial electrode machining process, the uneven microstructure can also lead to uneven surface finish-this problem is even more serious on high-speed machining centers. Hard spots in the graphite can also cause the tool to deflect, causing the final electrode to be out of specification. This deflection may be slight enough that the oblique hole appears straight at the entry point.
There are specialized graphite processing machines. Although these machines will greatly speed up production, they are not the only machines that manufacturers can use. In addition to dust control (described later in the article), past MMS articles also reported the benefits of machines with fast spindles and control with high processing speeds for graphite manufacturing. Ideally, rapid control should also have forward-looking features, and users should use tool path optimization software.
When impregnating graphite electrodes—that is, filling the pores of the graphite microstructure with micron-sized particles—Garda recommends the use of copper because it can stably process special copper and nickel alloys, such as those used in aerospace applications. Copper impregnated graphite grades produce finer finishes than non-impregnated grades of the same classification. They can also achieve stable processing when working under adverse conditions such as poor flushing or inexperienced operators.
According to Mercer’s third article, although synthetic graphite-the kind used to make EDM electrodes-is biologically inert and therefore initially less harmful to humans than some other materials, improper ventilation can still cause problems. Synthetic graphite is conductive, which can cause some problems to the device, which may short-circuit when it comes in contact with foreign conductive materials. In addition, graphite impregnated with materials such as copper and tungsten requires extra care.
Mercer explained that the human eye cannot see graphite dust in very small concentrations, but it can still cause irritation, tearing and redness. Contact with dust may be abrasive and slightly irritating, but it is unlikely to be absorbed. The time-weighted average (TWA) exposure guideline for graphite dust in 8 hours is 10 mg/m3, which is a visible concentration and will never appear in the dust collection system in use.
Excessive exposure to graphite dust for a long time can cause the inhaled graphite particles to stay in the lungs and bronchi. This can lead to severe chronic pneumoconiosis called graphite disease. Graphitization is usually related to natural graphite, but in rare cases it is related to synthetic graphite.
Dust that accumulates in the workplace is highly flammable, and (in the fourth article) Mercer says it can explode under certain conditions. When the ignition encounters a sufficient concentration of fine particles suspended in the air, a dust fire and deflagration will occur. If the dust is dispersed in a large amount or is in a closed area, it is more likely to explode. Controlling any kind of dangerous element (fuel, oxygen, ignition, diffusion or restriction) can greatly reduce the possibility of dust explosion. In most cases, the industry focuses on fuel by removing dust from the source through ventilation, but stores should consider all factors to achieve maximum safety. Dust control equipment should also have explosion-proof holes or explosion-proof systems, or be installed in an oxygen-deficient environment.
Mercer has identified two main methods for controlling graphite dust: high-speed air systems with dust collectors—which can be fixed or portable depending on the application—and wet systems that saturate the area around the cutter with fluid.
Shops that do a small amount of graphite processing can use a portable device with a high-efficiency particulate air (HEPA) filter that can be moved between machines. However, workshops that process large amounts of graphite should usually use a fixed system. The minimum air velocity to capture dust is 500 feet per minute, and the velocity in the duct increases to at least 2000 feet per second.
Wet systems run the risk of liquid “wicking” (being absorbed) into the electrode material to flush away dust. Failure to remove the fluid before placing the electrode in the EDM can result in contamination of the dielectric oil. Operators should use water-based solutions because these solutions are less prone to oil absorption than oil-based solutions. Drying the electrode before using EDM usually involves placing the material in a convection oven for about an hour at a temperature slightly above the evaporation point of the solution. The temperature should not exceed 400 degrees, as this will oxidize and corrode the material. Operators should also not use compressed air to dry the electrode, because the air pressure will only force the fluid deeper into the electrode structure.
Princeton Tool hopes to expand its product portfolio, increase its influence on the West Coast, and become a stronger overall supplier. In order to achieve these three goals at the same time, the acquisition of another machining shop became the best choice.
The wire EDM device rotates the horizontally guided electrode wire in the CNC-controlled E axis, providing the workshop with workpiece clearance and flexibility to produce complex and high-precision PCD tools.
Post time: Sep-26-2021