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Introduction
This article takes an in depth look at graphite machining.
You will learn more about topics such as:
What is graphite machining?
How graphite is machined
How graphite is made
Uses for graphite machined parts
And much more…
Chapter One – What is Graphite Machining?
Graphite machining is a method for shaping, forming, configuring, and cutting graphite material to produce a wide selection of parts and components for industrial applications. The success of graphite machining is dependent on the types of tools used. Manufacturers use specially designed cutting tools to prevent chipping and breakouts. In most cases, indexable carbide cutters are used, which have the most efficient shapes for roughing at high rates of speed.
The types of graphite are natural and synthetic. Natural graphite is formed from igneous and metamorphic rock and is mined in several places in the world. Synthetic graphite is made from superheating carbon-containing materials, such as pitch, coal, and acetylene. When the carbon materials are heated, the carbon atoms rearrange into layers to form graphite.
Chapter Two – How Graphite is Machined
The process for machining graphite is similar to the methods used to machine cast iron. The fine chips, or swarf, are removed as a fine powder. Tools used in the process do not grab the workpiece but cut using a process much like plowing snow.
Graphite material has high compressive strength and can be held in place with clamping force. It is important to determine the amount of required clamping force prior to working the piece. Testing a workpiece to the point of compressive failure indicates the amount of sufficient clamping force.
Methods For Graphite Machining
Specialized Tools
The first consideration when preparing to machine graphite is the types of tools that can be used. Graphite is abrasive and will cause severe wear on uncoated metallic tools. Preferable tools are diamond-edged, though tungsten carbide tools may be used as well. High-speed steel can be used but will wear out quickly, which limits its use. Using the wrong tool, speed, or feed results in chipping and breakouts.
Baking Graphite
A key issue when working with graphite is to be sure that it is dry. If graphite material is exposed to water or moisture, it will become an abrasive slurry when being worked. This dramatically damages tools, which can easily be observed when cutting damp graphite on a band saw. The dust created packs the kerf, cutting slit, causing the tool to recut the same space repeatedly.
All graphite materials use a temporary binder, a form of pitch, that carbonizes during baking and the graphitization process.Pitch can be pressured into the residual porosity of a baked shape that resulted from the carbonization of the temporary pitch binder in the initial baking. Placing more carbon in the porosity and refurnacing fills the porosity with more carbon.
The Importance of Ventilation
The machining process for graphite produces a tremendous amount of dust and chips. Though this may be common in machining other metals, dust from graphite is electrically conductive and adheres to machines, other metals, and every possible crack and opening. The static electricity from circuit boards will attract the dust which will short out the boards and build electrical contacts.
The Occupational Safety and Health Administration (OSHA) has standards regarding synthetic and natural graphite dust handling. The standards are expressed as permissible exposure limits (PELs), which are 15 mppcf or 1.5 mg/m3. Machining centers use high air velocity equipment with dust collectors to control dust emissions.
Climb Milling and Conventional Milling
Special methods should be used when milling graphite. The two primary methods for milling are climb and conventional. The difference between them is the rotation of the cutter in relation to the feed. With conventional milling, the cutter rotates against the direction of the feed, while with climb milling, the cutter rotates with the feed.
For graphite work, climb milling is preferred since chipping begins at maximum and decreases with the heat generated being transferred to the chips. The process creates a cleaner shearing plane, which lessens the wear on the tool, and chips are removed behind the cutter, an essential element for milling graphite.
The key to the milling process is chip formation, which is determined by the position of the milling cutter. Thick chips should be formed when the cutter cuts in and thin chips when it cuts out. Adherence to this factor leads to a stable milling process. In the case of milling graphite, it is advisable to mill from the outside into the material.
Drilling Graphite
The main concern when drilling graphite is the buildup of dust in holes. When completed properly, a higher spindle speed can be used, which reduces wear on the drill. There are no limitations on size for drilling as long as diamond-coated drills are used. As with all forms of graphite machining, the grade of graphite determines the conditions and parameters and the dust removal practices.
Safety and Turning
In the turning process, minimal force should be used to prevent clipping the workpiece. The use of a collet chuck saves time for loading and unloading. During turning, it is essential not to extend too long from the tailstock and adjust the pressure on the ends of the workpiece. When working on bars under 20 mm or 0.79 in, bending can occur.
The density and surface strength of graphite generates cutting force during turning. The turning tool should not protrude from the tool holder and must be clamped to keep its rigidity.
Sawing Graphite
As with every aspect of machining graphite, the first concern in sawing graphite is controlling the dust. If the sawing is being completed by a CNC machine or by hand, dust from the process must be collected and removed before it can damage tools and equipment. The blade should be tungsten carbide or diamond grit, whether a band saw or round blade is being used.
Types of Machined Graphite Processes
There are a wide variety of graphite parts and components that are machined. The process used to machine them depends on the requirements of the piece being produced. The various methods for machining graphite include extruding, isostatically pressing, vibrating, and molding.
Extruding Process
Extruding is a process that is commonly used in the manufacture of plastics. In graphite manufacturing, graphite powder is mixed with a binder, poured into a hopper, fed into the barrel of the extruder, and moved down the barrel to the die by a piston. After the shape exits the extruder, it is fired, impregnated, fired again, and graphitized at 2000° C or 3632° F. Extruding of graphite parts is very economical.
Isostatic Pressing
Isostatic pressing takes fine grain graphite powder and equally applies pressure. Once the pressing is completed, workpieces are heat-treated to fully solidify, densify, modify, and purify them to achieve the final crystalline structure. The process can be performed cold or hot.
Vibrated Graphite and Solidification
Vibrated graphite processes large sizes of less dense graphite, which is a cost-effective production method when high strength is not required. The final product has a uniform structure with low ash content.
The process includes placing a pasty mixture in a mold with a heavy plate placed on top. To compact the material, the mold is vibrated until the pasty mass solidifies.
Molding Graphite
The molding process for graphite machining produces components with similar properties to those made by isostatic pressing. A graphite powder mix is uniformly pressed into a mold where it is held for an extended length of time. The resulting products do not have the same high quality properties as those produced by isostatic pressing but are ideal for high volume production runs such as small parts like washers.
Leading Manufacturers and Suppliers
Chapter Three – How Graphite is Made
Graphite is one of the many forms of carbon and has carbon atoms arranged in layers, which gives it its unique properties. Natural graphite is mined worldwide but mainly in China, Brazil, Canada, and Madagascar. It is found in metamorphic and igneous rocks and is formed when carbon is subjected to high pressure and temperature in the earth’s crust.
Synthetic graphite has high purity carbon and can withstand high temperatures and corrosion. Primary ingredients for making synthetic graphite are calcined petroleum coke and coal tar pitch, which contains graphitizable carbon. The manufacturing process consists of mixing, heat treating, molding, and baking of the mixture materials.
The Graphite Creation Process
Graphite Mining
How graphite is mined is determined by the weathering of the ore and how close it is to the surface. The two most common methods for mining graphite are open pit and underground. Classifications for graphite include microcrystalline, macrocrystalline, and lump. Each type has its own characteristics and properties as a result of where they are located.
Open Pit Graphite Mining
The open pit process involves removing the graphite by digging large pits and is used when the ore is close to the earth’s surface. The process includes several methods of digging, including drilling, using dynamite, compressed air, and water. Drilling and blasting are used when the surface rock is hard.
Underground Graphite Mining
Underground mining is necessary when the ore is not located close to the surface and involves creating shafts large enough to hold miners and equipment. Slope mining does not require a very deep shaft, and the ore is removed using conveyors. With drift mining, the shaft is cut horizontally below the vein of ore to allow for gravity extraction.
Synthetic Graphite Creation
There are several steps to manufacturing synthetic graphite, which include powder preparation, shape forming, baking, densification, rebaking, and graphitization.
Powder Preparation
The raw materials for synthetic graphite include petroleum coke, pitch coke, carbon, natural graphite, and graphite scrap, which are ground and crushed. The produced powder is blended with a binder to form a paste. Binders are either petroleum or coal tar pitch.
Powder Shaping
There are three methods used to shape the powder: extrusion, vibration, or isostatic pressing.
Powder Baking
The compacted material is heat-treated in a furnace at between 900° C and 1200° C or 1650° F and 2200° F, the result of which is the carbonization of the material. The carbonization process binds the particles of the powder. Since the volume of the binder is higher than that of the material, pores form in the baked mass.
Pitch Impregnation
The impregnation material is a form of pitch that is lower in viscosity than that of the original pitch, which will fill in the gaps. For high-density graphite grades, the process of impregnation and rebaking is repeated several times.
Graphitization
Graphitization crystallizes the amorphous carbon to create crystallized graphite. Under the high temperature of graphitization, 2700°C to 3000°C or 4900°F to 5450°F, the crystallites grow and rearrange to form a pattern of stacked parallel planes.
A further result of the graphitization process is the graphite purification since the impurities such as the binder residue, gases, oxides, and sulfur are vaporized in the process. Graphitization is completed in an Acheson furnace, which uses a direct heating method using graphite as a heat conductor.
The complete process can be seen in the image below.
Chapter Four – Uses for Graphite Machined Parts
Several industries use graphite parts because of their chemical and physical properties. It can be machined to tight tolerances, is resistant to thermal shock, has a low thermal expansion coefficient, and has excellent stability at high temperatures. Each of these characteristics makes it the perfect material for specific manufacturing applications.
The number of uses of graphite varies widely, from the material in a pencil to the lining for nuclear reactors. Crystalline flake graphite is used to make electrodes, brushes, and plates for dry cell batteries. A major new development for graphite use is in the manufacture of electric cars.
Graphite Machined Parts
Graphite in Bearings
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.
Vane Construction
Blades are attached to rotating wheels to push or be pushed by wind or water. The strength and endurance of graphite and its inability to absorb water making it the perfect choice for the construction of vanes.
Graphite Lubrication Blocks
Lubrication blocks are used where wet lubricants can’t be used. They are mainly used in rotary equipment such as trunnion rolls, riding rings, tires, and insert seals. Their weight keeps them in constant contact with the rolling surface, depositing a thin film of graphite. Graphite’s resistance to wear and long service life are the main reasons it is used for lubricating blocks.
Low Porosity Graphite Brushes
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 in Cathodic Protection Systems
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.
Nuclear Graphite Cores
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 act as moderators and reduce the speed of the neutrons to help sustain the nuclear reaction and serve a safety measure by providing a structure for CO2 gas to flow through as it removes heat from the fuel. How the graphite ages determines how long the reactor will remain in operation.
Graphite Fluxing Tubes
Fluxing tubes are used in aluminum applications for transfer ladles, melting furnaces, and holding furnaces to add fluxing gases to remove hydrogen, aluminum oxide, and other materials from molten aluminum. Graphite Fluxing tubes are resistant to corrosion and thermal shock. They are produced using various grades of graphite that have had an anti-oxidation treatment. Since not every aluminum processing operation is the same, fluxing tubes are available in various sizes or can be customized to fit any application.
Graphite Crucibles For Material Melting
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.
Since graphite crucibles are used for different applications, they are available in a variety of shapes to fit the needs of the process. Graphite crucibles come in barrel, cylinder, and cone form and are a cost-saving alternative to copper, platinum, quartz, and porcelain crucibles. The graphite material is chemically inert and temperature resistant, which allows them to be placed in melting furnaces.
Chapter Five – Graphite Grades
Graphite can be synthesized in several different ways. How it is produced determines its grade and use. The widely variant methods for producing graphite can be seen in its many grades. Each grade is formulated to serve the needs of a particular application. The wide assortment of grades has created an endless number of grade types.
When examining the graphite grades, especially with people new to the industry, it is best to break the various grades into compact groups for ease of discussion and understanding.
Grades of Graphite
Fine Grain Graphite
Fine grain graphite is processed by grinding and has a grain size of less than one millimeter, with some sizes being less than one micrometer (µm). The very fine structure of fine grained graphite allows it to be used to produce very precise details and exceptional surface finishes. To be defined as fine grain graphite, the particles must range in size from 0.0001 in up to 0.005 in or 0.00254 mm up to 0.127 mm.
The grains are milled to the particle size, blended, and isostatically pressed. Fine grain graphite has a 15% porosity, which is difficult to see due to the size of the grains. There are innumerable applications where fine grain graphite is used including rocket nozzles, brushes, and heating elements.
Medium Grain Graphite
Medium grain graphite has particles with sizes ranging from 0.020 in up to 0.062 in or 0.508 mm up to 1.578 mm with a porosity of 20%. Unlike fine grain graphite, the openings in medium grain graphite are visible due to the size of the particles. Medium grain graphite has a dense uniform structure, high temperature, and oxidation resistance, and low resistivity. It is mainly used for anodes, pallets, and as heat covers and elements.
Coarse Grain Graphite
Coarse grain graphite has a grain size that is less than 25 mm or 0.984 in and is manufactured using extrusion. The porosity and large particles of coarse grain graphite make it resistant to thermal shock and gives it the ability to withstand drastic temperature changes that occur during melting processes. The porosity and openings between the particles are easily visible. The strength, endurance, and resilience of coarse grain graphite make it an ideal material for the production of large parts.
Hexagonal Lattice Structure of Graphite
The lattice of a material determines its properties. Graphite is a soft black mineral with atoms that are easy to separate. It is made of sheets of strongly bonded hexagonal rings. Each of the various sheets are far from each other and weakly linked, which allows them to slide past each other. The distance between the sheets allows other molecules to enter and is why graphite is absorbent and has catalytic properties.
Natural and synthetic graphite have hexagonal lattices. The formation of the lattices is determined by the crystalline order of the solid particles. Thus the size of the particles is the determining factor of its properties.
Chapter Six – The Benefits of Graphite Machining
Graphite’s unique combination of properties has made it an essential part of several industrial applications. Its physical, chemical, and mechanical characteristics are why graphite is so widely used. Graphite is an excellent conductor of electricity and heat and can withstand extreme temperature changes.
For centuries, graphite has been used to produce products and materials. Modern technology has greatly enhanced and expanded its use especially in the fields of metallurgy and nuclear reactors.
Advantages to Graphite Machining
Constant Lubrication
Every graphite manufacturer emphasizes the use of graphite as a lubricant. Its molecular structure forms a thin film on moving parts, making it ideal for use as brushes and block lubricants. It produces an indestructible film, which can combat friction at slow and fast speeds. The lubricant qualities of graphite prevent it from galling or transferring material between metals.
Corrosion Resistance
Of the many benefits of graphite machined parts, their inability to corrode or rust makes them ideal for product production. Graphite is unaffected by acids, alkali, solvents, and similar chemicals. This characteristic makes graphite an excellent material for food processing, handling chemicals and fuels, pumps, vanes, and valves.
Seal Maintenance Advantages
The mechanical properties of graphite allow it to maintain flatness during the operation of a device. Of course no material is perfectly flat, but many applications require it to meet the needs of an application. Of the various types of metals, graphite provides excellent flatness to form a perfect seal.
High Compression Strength
The compressive strength of graphite ranges between 11,000 lbs./sq in up to 38,000 lbs./sq in. In designing mechanical components, it is advisable to take advantage of materials with high compressive strength. This characteristic protects a part when it is placed under heavy stress. Graphite can have exceptionally high compression strength but be weak in tension and brittle, which leads to chipping during machining.
Easy Machinability
Having exceptionally close tolerances is critical to designs and engineering. This particular characteristic ensures that a component will fit easily into the overall structure of a mechanism. Machined graphite can be shaped to fit the most precise and demanding tolerances.
Porosity of Graphite
A characteristic of graphite that causes a great deal of concern is its porosity. To overcome this trait, manufacturers impregnate graphite with various materials to fill the gaps. Not all forms of graphite require impregnation since they have small pores. It is important to select the correct material for the impregnation process.
Excellent Thermal Conductivity
Graphite machined parts are chosen for applications that require the melting of metals because of its high thermal conductivity. Graphite is an excellent heat conductor and is resistant to thermal shock.
Conclusion
Graphite machining is a method for shaping, forming, configuring, and cutting graphite material to produce a wide selection of parts and components for industrial applications.
Tools used in the graphite machining process should not grab the workpiece since it is more like plowing snow than cutting.
Graphite material has high compressive strength and can be held in place with clamping force.
Graphite’s unique combination of properties has made it an essential part of several industrial applications.
Graphite can be machined to tight tolerances, is resistant to thermal shock, has a low thermal expansion coefficient, and excellent stability at high temperatures.
Leading Manufacturers and Suppliers
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