Carbon Machining
Carbon machining, known as carbon fiber machining, is performed using an assortment of machining methods. The process for machining carbon fabric is very carefully executed due to the expense of the process. The use of carbon fiber products is highly valued for their high tensile strength, which is twice as stiff and five times as strong as steel. In addition, carbon fibers are resistant to chemicals and have an exceptionally high tolerance to extremes in temperature.
How Carbon Fibers are Made
Carbon fibers are composed of long strings of polymer molecules that are held together by carbon atoms produced using a polyacrylonitrile (PAN) process. Other manufacturing processes involve rayon or petroleum pitch in place of the polymer molecules. The addition of gases, liquids, and various other materials changes the properties, quality, and grade of carbon fibers to make it applicable for a wide range of products. This aspect of the process is designed to create a specific effect, such as a reaction goal or the prevention of a reaction.
The manufacture of carbon fiber begins with raw materials referred to as precursors, which is normally a polymer. Precursor composition varies from manufacturer to manufacturer, which is a closely guarded proprietary secret. In the initial phase of the process, precursors are drawn into long strands of fibers that are woven into fabric. In addition, the woven fibers can be combined with other materials such as filament, which can be wound or molded into different shapes.
Spinning – During the spinning phase, the precursor is mixed with other materials and spun into fibers. In preparation for the next phase, the fibers are washed, stretched, and shaped.
Stabilizing – Before the carbonization process, carbon fibers are chemically altered to make them thermally stable by changing their linear bonds to ladder bonds. To accomplish the change, the fibers are heated to force the carbon atoms to pick up oxygen atoms to form molecules with a stable bonding pattern. In order to achieve the best results, the heating process is closely monitored to prevent overheating.
Carbonizing – To begin carbonization, the fibers are heated in an oxygen free environment at temperatures exceeding 1000o C. The absence of oxygen is necessary to prevent the fibers from burning. The increased heat causes the fiber material to release its non-carbon atoms such that the remaining atoms form a tightly bonded carbon crystalline material that aligns parallel to the axis of the carbon fibers.
Surface Treatment – After carbonization, the fibers have a smooth even surface that does not bond well, which necessitates the surface being slightly oxidized to improve the surface’s ability to chemically bond. As with many parts of the process, surface treatment requires expertise to prevent defects in the surface of the fibers.
Sizing – After full oxidation is achieved, the fibers are wound onto a bobbin or woven into fabrics, during which they are coated to prevent any possible damage. The coating process, known as sizing, includes several different types of materials that are selected for their compatibility with the adhesives used to form composite structures. The various types of coating materials include polyester, nylon, urethane, and epoxy.
To prepare carbon fiber for machining and the manufacture of various products, they are wound onto bobbins and placed into spinning machines where they are twisted into yarn that is woven into fabrics or formed into composites. The final woven fabrics are ready for creating carbon fiber parts and products.
Carbon Fiber Machining
The machining of carbon fiber requires specialized tools due to its abrasive nature. The need for precision control of the process necessitates the use of computerized systems that can complete cuts and shaping without damaging the fabric. The types of specialized tools include diamond coated and polycrystalline diamond (PCD) tools that are efficient and sturdy enough to perform high quality machining.
Computer Numerical Control (CNC) Machining – CNC machining is ideal for the machining of carbon fiber due to its precision, limited waste, control, and adherence to tolerances. The CNC machining methods used on carbon fabric are turning and milling. Turning is used on components that have circular surfaces using tools made of ceramic material, cemented carbide, PCD, and cubic boron nitride. The parameters of turning are cutting speed, feed rate, and cut depth, each of which influences tool life and surface quality. The feed rate is the most significant attribute of turning since it determines the roughness of a component’s surface.
Unlike traditional milling, carbon fiber milling requires exceptionally high spindle speeds with a lower feed rate since too much heat affects the quality of the fibers. During milling, the feed rate is carefully controlled and monitored to prevent the generation of heat. Since carbon fiber has low thermal conductivity, workpieces retain any heat that is generated. No chips are generated by carbon fiber machining, which, under normal circumstances, disperses heat during machining. Tools and tool paths are used to manage any heat since coolants cannot be used.
Drilling – As with CNC machining, drilling carbon fiber is a very complicated process due to the conductive dust the process generates that can cause skin irritations and damage electronics. A factor used to control the creation of dust is the speed of a drill, which should be calibrated to the size and depth of the drilled hole. Drill bits are specially made for drilling carbon fiber such that they prevent delamination of the layers of the carbon fabric during drilling. Although delamination is a major concern during drilling, it is a general concern for all forms of carbon fiber machining.
Waterjet Machining – Unlike the other methods used to machine carbon fiber, waterjet machining does not require any form of special tools. The process effectively controls dust contamination by trapping dust in the waterjet water. As with drilling, delamination may occur during waterjet machining, which is prevented by placing a pilot hole in the fabric. Waterjet carbon fabric machining is chosen for parts that require high tolerances since it can be adjusted to meet the requirements of the final dimensions.
Waterjet machining is an erosion process where pressurized water is forced through a diamond or ruby nozzle. The pressure of the water creates a vacuum that pulls garnet sand into the water stream that cuts into the carbon fabric. The key to waterjet machining is the pressure that is converted into velocity as it passes through the orifice of the nozzle. The introduction of the garnet sand increases cutting power by 1000 times, making waterjet machining a supersonic process.
Factors Regarding Carbon Fabric Machining
Machining of carbon fabric requires expertise and know-how, which necessitates that it be completed by a trained professional with the required knowledge and skills. During the machining process, there are factors to consider to ensure the success of the machining.
High Hardness Tools – Carbon fiber is high hardness material that necessitates the use of tools that can endure the hardness. Each type of carbon fiber has a different Rockwell hardness, with hardnesses varying between HRC 53 and HRC 90 and above. Tools used for carbon fiber machining must have a hardness that matches that of the carbon fabric.
Dust Control – The control of dust is essential since it is a health hazard and can be damaging to equipment. Carbon machining experts rely on clothing, masks, and gloves to protect their skin and eyes. In addition, when machining carbon fabric, they advance the process at a controlled speed to manage the generating of dust.
Reaming – Reaming is a finishing process that is used to enlarge, refine, and smooth drilled holes. A reamer is a multi-fluted cutting tool that is used to precision bore holes to achieve tight tolerances. It is widely used in applications where holes require a tight fit and accurate placement.