This article takes an in depth look at graphite machining.
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Graphite machining involves the shaping, molding, configuring, and precision cutting of graphite materials to fabricate a diverse array of parts and components for industrial uses. The effectiveness of graphite machining largely depends on the tooling employed. Manufacturers utilize specially crafted cutting tools that minimize the risk of chipping and breakouts. Oftentimes, indexable carbide cutters are implemented due to their highly efficient design, which is ideal for roughing at elevated speeds.
Graphite is categorized mainly into two types: natural and synthetic. Natural graphite originates from igneous and metamorphic rock formations and is extracted from several places worldwide. Conversely, synthetic graphite is manufactured by subjecting carbon-rich materials, such as pitch, coal, and acetylene, to extremely high temperatures. This heating process causes the carbon atoms to realign into layered structures that constitute graphite.
The process of machining graphite resembles the methods used for machining cast iron. In this process, fine chips, or swarf, are removed as a fine powder. The tools used do not grip the workpiece but cut it using a technique similar to plowing snow.
Graphite has high compressive strength and can be secured with clamping force. It is crucial to determine the appropriate amount of clamping force before working on the piece. Testing the workpiece to the point of compressive failure helps identify the required clamping force.
The initial consideration when preparing to machine graphite is selecting the appropriate tools. Graphite is abrasive and can cause significant wear on uncoated metallic tools. Diamond-edged tools are preferred, though tungsten carbide tools are also suitable. High-speed steel tools can be used, but they wear out quickly, limiting their effectiveness. Using incorrect tools, speeds, or feeds can lead to chipping and breakouts.
A crucial consideration when working with graphite is ensuring that it is dry. Exposure to water or moisture can turn graphite into an abrasive slurry during machining, which can severely damage tools. This is particularly noticeable when cutting damp graphite on a band saw, as the dust created packs the kerf, causing the tool to repeatedly recut the same space.
Graphite materials use a temporary binder, typically a form of pitch, that carbonizes during the baking and graphitization process. Pitch can be pressured into the residual porosity of a baked shape, which results from the carbonization of the temporary pitch binder in the initial baking. Adding more carbon into the porosity and refurnacing fills the voids with additional carbon.
The machining process for graphite generates a significant amount of dust and chips. While dust is common in machining other metals, graphite dust is electrically conductive and tends to adhere to machines, other metals, and every possible crack and opening. The static electricity from circuit boards attracts this dust, potentially causing shorts and building electrical contacts.
The Occupational Safety and Health Administration (OSHA) has established standards for handling synthetic and natural graphite dust, expressed as permissible exposure limits (PELs) of 15 mppcf or 1.5 mg/m³. To control dust emissions, machining centers employ high air velocity equipment with dust collectors.
Special methods are required when milling graphite. The two primary milling methods are climb milling and conventional milling. The distinction between them lies in the direction of the cutter's rotation relative to the feed. In conventional milling, the cutter rotates against the feed direction, whereas in climb milling, the cutter rotates with the feed direction.
Climb milling is preferred for graphite work because it starts with maximum chipping, which decreases as heat is transferred to the chips. This method produces a cleaner shearing plane, reduces tool wear, and ensures that chips are removed behind the cutter—an essential factor in milling graphite.
Chip formation is crucial in the milling process and depends on the cutter's position. Thick chips should be formed when the cutter is cutting in, and thin chips when cutting out. Adhering to this guideline ensures a stable milling process. For graphite milling, it is advisable to mill from the outside into the material.
When drilling graphite, the primary concern is the buildup of dust within the holes. Proper drilling techniques allow for the use of higher spindle speeds, which reduces drill wear. There are no size limitations for drilling as long as diamond-coated drills are employed. As with all graphite machining, the grade of graphite will influence the conditions, parameters, and dust removal practices.
During the turning process, it's important to use minimal force to avoid damaging the workpiece. Employing a collet chuck can save time during loading and unloading. It is crucial to avoid extending too far from the tailstock and to adjust the pressure on the ends of the workpiece. For bars under 20 mm (0.79 in), there is a risk of bending.
The density and surface strength of graphite generate cutting force during turning. To maintain rigidity, the turning tool should not extend beyond the tool holder and must be securely clamped.
When sawing graphite, controlling the dust is crucial. Whether the sawing is done with a CNC machine or by hand, it's essential to collect and remove dust to prevent damage to tools and equipment. Use blades with tungsten carbide or diamond grit, regardless of whether a band saw or round blade is employed.
Graphite parts and components can be machined using various processes depending on the specific requirements of the piece. Common methods include extruding, isostatic pressing, vibrating, and molding.
Extruding, commonly used in plastics manufacturing, involves mixing graphite powder with a binder. The mixture is poured into a hopper, then fed into the extruder's barrel by a piston. The material moves down the barrel to the die, where it takes shape. After extrusion, the shape is fired, impregnated, fired again, and graphitized at 2000° C or 3632°. Extruding graphite parts is a cost-effective method.
Isostatic pressing involves applying uniform pressure to fine grain graphite powder. After pressing, the workpieces are heat-treated to solidify, densify, modify, and purify them, resulting in the final crystalline structure. This process can be carried out either cold or hot.
The vibrated graphite process is used for producing large quantities of less dense graphite. It is a cost-effective method suited for applications where high strength is not a primary requirement. The final product features a uniform structure and low ash content.
During this process, a pasty mixture is placed into a mold, and a heavy plate is set on top. The mold is then vibrated to compact the material until it solidifies.
The molding process for graphite machining creates components with properties akin to those produced by isostatic pressing. In this method, a graphite powder mixture is uniformly pressed into a mold and held there for an extended period. Although the resulting products do not achieve the same high quality as those made by isostatic pressing, they are well-suited for high-volume production runs of small parts, such as washers.
Graphite is a form of carbon with its atoms arranged in layers, giving it distinctive properties. Natural graphite is mined in several countries, with major sources in China, Brazil, Canada, and Madagascar. It forms in metamorphic and igneous rocks when carbon undergoes high pressure and temperature in the Earth's crust.
Synthetic graphite, known for its high purity and resistance to high temperatures and corrosion, is produced using calcined petroleum coke and coal tar pitch, which contains graphitizable carbon. The manufacturing process involves mixing, heat treating, molding, and baking these materials to create synthetic graphite.
The method used for mining graphite depends on the ore's weathering and proximity to the surface. The two primary mining techniques are open pit and underground mining. Graphite can be classified into microcrystalline, macrocrystalline, and lump types, each with unique characteristics based on their location.
Open pit mining is employed when graphite ore is near the Earth's surface. This method involves excavating large pits and can include various techniques such as drilling, blasting with dynamite, and using compressed air and water. Drilling and blasting are specifically used when the surface rock is particularly hard.
Underground mining is used when graphite ore is not near the surface. This method involves creating shafts large enough to accommodate miners and equipment. Slope mining requires less depth, with ore extracted using conveyors. Drift mining involves cutting a horizontal shaft below the ore vein, allowing gravity to assist in the extraction process.
The manufacturing process for synthetic graphite includes several key steps: powder preparation, shape forming, baking, densification, rebaking, and graphitization.
The raw materials for synthetic graphite include petroleum coke, pitch coke, carbon, natural graphite, and graphite scrap. These materials are ground and crushed to produce a powder, which is then blended with a binder to form a paste. The binders used are typically petroleum or coal tar pitch.
Three methods are used to shape the powder: extrusion, vibration, and isostatic pressing.
The compacted material undergoes heat treatment in a furnace at temperatures ranging from 900°C to 1200°C (1650°F to 2200°F). This process carbonizes the material, binding the powder particles together. Due to the higher volume of the binder compared to the material, pores form in the baked mass.
During pitch impregnation, a lower viscosity pitch fills the gaps left in the material. For high-density graphite grades, this impregnation and rebaking process is often repeated several times.
Graphitization transforms amorphous carbon into crystallized graphite. At temperatures between 2700°C and 3000°C (4900°F to 5450°F), the carbon crystallites grow and align into a pattern of stacked parallel planes.
Additionally, graphitization purifies the graphite by vaporizing impurities such as binder residues, gases, oxides, and sulfur. This process takes place in an Acheson furnace, which uses direct heating with graphite as a heat conductor.
The complete process is illustrated in the image below.
Graphite is utilized across various industries due to its unique chemical and physical properties. It can be machined to tight tolerances, resists thermal shock, has a low thermal expansion coefficient, and maintains excellent stability at high temperatures. These attributes make graphite an ideal material for specific manufacturing applications.
Graphite’s applications are diverse, ranging from the material in pencils to linings for nuclear reactors. Crystalline flake graphite is employed in making electrodes, brushes, and plates for dry cell batteries. A significant new development is the use of graphite in the production of electric vehicles.
Bearings are designed to reduce friction between two surfaces. They support a load while in contact with another moving part. Graphite is ideal for bearings due to its self-lubricating qualities, long service life, and ability to withstand harsh environments.
Blades are affixed to rotating wheels to harness wind or water power. Graphite's strength, durability, and resistance to water absorption make it an ideal material for constructing vanes.
Lubrication blocks are employed in situations where wet lubricants are unsuitable. Commonly used in rotary equipment such as trunnion rolls, riding rings, tires, and insert seals, these blocks rely on their weight to maintain continuous contact with the rolling surface, thereby depositing a thin layer of graphite. Graphite's durability and resistance to wear contribute to the long service life of these lubrication blocks.
Graphite brushes are square and used for carrying current through electric motors. They allow for a uniform shift of current between commutator segments and wear to save the condition of the commutator. Natural or synthetic graphite is used to produce them using a pitch or resin as a binding agent. Graphite brushes have a low porous quality and high density and will not be contaminated by environmental factors.
Graphite anodes are used in cathodic protection systems. They are an electrode that is used in a mercury cell to produce chlorine. As the anode is inserted into a mercury pool cathode of an ignitron, an electrical current begins because the anode is a collector of electrons. Anodes are a positive polarity in an electrolytic cell where oxidation occurs. Graphite is ideal as cathodic protection because of its chemical inertness, good conductivity, and low cost.
High-temperature gas-cooled nuclear reactors have graphite components for core and moderator material. Graphite blocks in a nuclear reactor serve as a safety measure to help keep the reactor operating. Reactor cores are 10 meters or 32 feet high with a diameter of 10 meters or 32 feet and weigh 1400 tons. Uranium fuel and control rods are inserted into the reactor through channels in the graphite core.
Graphite bricks function as moderators in nuclear reactors, slowing down neutrons to sustain the nuclear reaction. They also play a crucial safety role by providing a pathway for CO2 gas to flow through, which helps remove heat from the fuel. The longevity of a reactor's operation is influenced by the aging of the graphite bricks.
Fluxing tubes are used in aluminum processing applications such as transfer ladles, melting furnaces, and holding furnaces to introduce fluxing gases that remove hydrogen, aluminum oxide, and other impurities from molten aluminum. Graphite fluxing tubes are valued for their resistance to corrosion and thermal shock and are made from various grades of graphite treated with anti-oxidation coatings. They come in different sizes or can be customized to meet specific application needs.
Graphite crucibles are used for melting materials at temperatures up to 1600° C or 2900° F and are suitable for refining precious and base metals. They are used in every form of casting and melting production operation. Graphite crucibles are made from materials that allow a variety of metals with different melting temperatures to be prepared for processing.
Graphite crucibles are available in various shapes—such as barrel, cylinder, and cone—to suit different applications. They offer a cost-effective alternative to crucibles made from copper, platinum, quartz, and porcelain. Graphite’s chemical inertness and temperature resistance make these crucibles suitable for use in melting furnaces.
Graphite can be synthesized in various ways, each method affecting its grade and intended use. The diverse production methods result in a wide range of graphite grades, each tailored to specific applications. This variety has led to a multitude of grade types.
For those new to the industry, it can be helpful to categorize the various graphite grades into compact groups to facilitate discussion and understanding.
Fine grain graphite is processed through grinding, resulting in a grain size of less than one millimeter, with some particles being as small as one micrometer (µm). Its extremely fine structure makes it suitable for producing highly precise details and exceptional surface finishes. To be classified as fine grain graphite, the particle size ranges from 0.0001 inches to 0.005 inches (0.00254 mm to 0.127 mm).
The grains are milled to the desired particle size, blended, and isostatically pressed. Fine grain graphite typically has a porosity of 15%, which is not easily visible due to the small grain size. It is used in a variety of applications, including rocket nozzles, brushes, and heating elements.
Medium grain graphite features particles with sizes ranging from 0.020 inches to 0.062 inches (0.508 mm to 1.578 mm) and has a porosity of 20%. Unlike fine grain graphite, the openings in medium grain graphite are visible due to the larger particle size. It has a dense, uniform structure with high temperature and oxidation resistance, and low electrical resistivity. Medium grain graphite is commonly used for applications such as anodes, pallets, and heat shields and elements.
Coarse grain graphite has particles larger than 25 mm (0.984 inches) and is produced through extrusion. Its significant porosity and large particle size contribute to its resistance to thermal shock, allowing it to handle drastic temperature changes during melting processes. The porosity and openings between particles are easily visible. Coarse grain graphite's strength, durability, and resilience make it well-suited for manufacturing large components.
The lattice structure of a material defines its properties. Graphite is a soft black mineral characterized by atoms that are easily separable. It consists of layers of hexagonally bonded rings, where each layer is held together by strong bonds while being weakly linked to adjacent layers. This structure allows the layers to slide past one another. The spacing between these layers also enables other molecules to intercalate, which accounts for graphite's absorbency and catalytic properties.
Both natural and synthetic graphite feature hexagonal lattices. The characteristics of these lattices are determined by the crystalline arrangement of the solid particles. Therefore, the particle size plays a crucial role in defining the properties of graphite.
Graphite’s unique combination of properties has made it indispensable in various industrial applications. Its physical, chemical, and mechanical characteristics contribute to its widespread use. Graphite is an excellent conductor of electricity and heat and can endure extreme temperature fluctuations.
For centuries, graphite has been utilized in the production of numerous products and materials. Advances in modern technology have significantly expanded its applications, particularly in metallurgy and nuclear reactors.
Graphite is widely recognized for its lubrication properties. Its molecular structure creates a thin film on moving parts, making it ideal for use as brushes and block lubricants. This indestructible film reduces friction at both slow and high speeds, preventing galling and material transfer between metals.
One of the major advantages of graphite machined parts is their resistance to corrosion and rust. Graphite is unaffected by acids, alkalis, solvents, and similar chemicals, making it a suitable material for applications in food processing, chemical handling, fuels, pumps, vanes, and valves.
Graphite’s mechanical properties allow it to maintain flatness during operation. While no material is perfectly flat, graphite provides excellent flatness for creating effective seals in various applications.
Graphite boasts a compressive strength ranging from 11,000 to 38,000 lbs/sq in. This makes it ideal for applications requiring materials with high compressive strength to withstand heavy stress. However, graphite can be weak in tension and brittle, which may lead to chipping during machining.
Graphite’s machinability is crucial for achieving exceptionally close tolerances in designs and engineering. This property ensures that components fit seamlessly into mechanisms, even with the most precise and demanding specifications.
Graphite’s inherent porosity can be a concern, but this can be addressed by impregnating the material with various substances to fill the gaps. Not all types of graphite require impregnation, as some have smaller pores. Selecting the appropriate material for impregnation is essential to meet specific application needs.
Graphite is favored for applications involving metal melting due to its high thermal conductivity. It efficiently conducts heat and is resistant to thermal shock, making it a valuable material for such processes.
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