Graphite is a kind of engineering material, and it has been widely used as an electrode in EDM machining. How to machine graphite is a concern for mechanical technologists. It is not similar to metal processing technology and has special characteristics. This article will makes an introduction of graphite machining technology.
Usually, there are few special equipments for graphite machining in the metalwork shop, especially in the process of graphite processing, which will cause serious dust flying. It is necessary to install strong vacuum equipment, because graphite is conductive material, dust will make electrical equipment cause “Short circuit” failure, the current environmental protection department has considered the graphite powder as “harmful substances” and harmful to the human body.
The processability of graphite has a great relationship with the grade of graphite. For general high-quality graphite materials, thin materials with a thickness of 0.15±0.01 mm can be machined. The processing tolerance of holes is within ±0.05mm, and the processing diameter of graphite rods is at least 0.25. ±0.05mm, of course, none of the graphite can process holes that are less porous than it‘s porosity.
First, the sawing of graphite
The graphite material is a giant block, plate, or bar when it is shipped from the factory. When manufacturing the electrode, the cutting process must be carried out. The cutting can use the high efficiency band saw, the saw blade is made of 10mm wide, carbon steel with 2.5mm pitch, the saw blade line speed can reach 900m/min. Before sawing, the distance of the guide block of the machine tool should be adjusted, and feed is adjusted. Pushing device to prevent the feed from slipping and breaking the saw blade, strictly according to the operating procedures of the sawing machine, safe operation.
Second, graphite turning
Graphite is composed of abrasive material. Its turning performance is good, but the texture is very brittle. When turning and clamping, the force should not be excessive, so as to prevent the workpiece from being clipped, it is better to use a collet chuck to save the loading and unloading time, the accuracy of the lathe spindle head is higher , Front vibration will affect the accuracy of the workpiece surface.
When turning, use live thimble as far as possible, do not extend too long from the tailstock, and properly adjust the pressure between the two ends of the workpiece. If using a dead ejector pin, graphite can easily wear the tip when it is turning.
When the diameter of graphite bar is under 20mm, the bending of the workpiece is often caused by the pressure of the rear ejector pin. Especially when the length of the workpiece increases, the bending is very strong. At this time, the spindle speed should be 980r/min~1500r/min. The feed rate is 0.05~0.08mm/r. When turning rough bars, the bending phenomenon is rare. In this case, the spindle speed can be increased to 1500 r/min or more, and the feed rate can also be as high as 0.1 mm/r.
Graphite material has high density and surface high strength properties, and it generates a large cutting force when turning. Therefore, the turning tool cannot protrude too long from the tool holder, and the tool must be clamped to maintain rigidity so as to avoid vibration during cutting.
In the machining of graphite, the depth of cut is inversely related to the surface roughness. For example, if the depth of cut is 0.4 to 0.5 mm, the resulting machined surface roughness is much smaller than the cutting depth of 0.1 mm, which produces this abnormality. The reason is that since graphite chips are like abrasives and do not form a chip shape, no chip breaker is needed when turning graphite. When turning graphite, rectangular cutters should be avoided because they are produced during turning. The cutting force is sufficient to break the workpiece and a tool with an inclination of 20° is used instead. In general turning, high-quality cemented carbide tungsten carbide inserts should be used to sharpen the front rake, and the radius of the tool nose should be 0.4mm. It is not appropriate to use low-quality carbon steel cutters.
Third, graphite milling
In order to prevent vibration during milling, the tool cannot protrude too long from the spindle hole. When the workpiece must be clamped on the milling plane, a silicon carbide cup-shaped grinding wheel should be used instead of a face milling cutter or other type of milling cutter. The grinding wheel should be medium granularity, medium hardness, open structure and ceramic binder composition, grinding wheel diameter 150mm, spindle speed 3000r/min, face milling cutter will produce chipping when milling graphite, they are only used when processing small parts , Or use face milling carbide tungsten carbide cutter to improve milling.
Cutting depth and milling force directly depend on the three factors of power, stiffness and clamping force of the equipment. Therefore, when selecting the machine tool, it must be considered in order to obtain the best results. The workpiece is fractured during milling. It is always caused by improper tool vibration or clamping force. If the rigidity is good and the clamping force is proper, the machine tool will be blocked before the material breaks to force it to stop.
When milling graphite, it is better to use high-quality tungsten carbide tools and apply low stress to the tool. Practice has proved that the cutting depth is 0.05~0.15mm, and the total area of milling is 16~20m2/min.
When milling certain high-density and high-hardness graphites, smaller-sized end mills are often overloaded. Three-groove end mills are used at this time to increase stiffness, while up-milling is often used for small tools and reduces tool wear.
Fourth, graphite grinding
Almost all plane grinding wheels can be used to process graphite. The surface of the grinding wheel must be sharp. When machining large workpieces with high accuracy, it is recommended to use a diamond grinding wheel, which can make the workpiece edge smooth, improve the surface accuracy, and after the grinding wheel grinding , Durable for a long time. Although, compared with silicon carbide grinding wheels, diamond grinding wheels are slow and costly.
The green silicon carbide grinding wheel can use medium particle size, medium hardness, open structure and ceramic binder. The diameter of the grinding wheel can be 300~350mm, the spindle speed is 1500r/min, and the grinding depth is 0.3mm/stroke in rough machining. The processing is 0.05~0.07mm/stroke and the wheel speed is about 17m/min.
In order to prevent grinding clogging, a strong vacuum pump must be used to grind graphite, and the abrasive debris and fine particles around the grinding wheel must be collected and discharged. When the thickness of the graphite to be processed is 0.2mm,In order to meet the required tolerances. first, the graphite should be twisted into a thickness of about 1mm, and the workpiece is sucked by a vacuum suction table, First roughen the two sides, eliminate the rough surface of the blank, and then the two surfaces are refined to improve the accuracy.
Fifth, graphite drilling and hanging
Before drilling a hole in graphite, two points must be considered. One is how many holes to drill, and the other is how big holes to drill. Graphite materials are more likely to wear the edges of the drill than any other material. Once the cutting edge is worn, it will reduce the diameter or form a tapered hole. Therefore, when high-speed steel drills are used to process the graphite material, it can only be performed at a relatively lighter workload. In the case of a small number, the material for making the drill bit is appropriately selected according to the diameter of the drill hole. When processing holes with a diameter of 1.5mm or less, high-speed steel drills can be used only in a small amount of production because of its low cost and good toughness; tungsten carbide drills are brittle and can be easily broken when processing small holes.
When the hole is drilled in a graphite material with a thickness of 25mm or less, the smaller the hole diameter is, the higher the spindle speed is, and the bit feed rate should be slower. If the processing capability of the machine allows, the typical data is drilled hole of 0.8mm, and the rotation speed 4000r/min or so should be selected.
When drilling in the drilling machine, the larger the manual feed, the higher the cutting efficiency, but when the manual feed is large, the spindle speed is slower, for example, in the drilling diameter of 12mm hole, the spindle speed is 900r/min, the best cutting conditions, when the manual feed slower and the spindle speed is too fast, resulting in fewer chips, will make the chip packing, friction at this time increased sharply, the workpiece temperature increased significantly, resulting in the graphite workpiece burst.This bursting phenomenon often occurs when processing graphite materials with a thickness of 25mm or more. The manual feed rate is faster, and the slower spindle speed is the best method to overcome the bursting of processed graphite workpieces. For deep hole drilling with a diameter of less than 1.5 mm, a dense spiral drill is recommended. It is a special drill with a much more dense pitch than the usual spiral drill.
High-quality tungsten carbide carbide drills are preferred when the diameter of the graphite drilled hole is more than 1.5mm, because it is durable and has few re-grinding times. The typical process data is 28 to 12mm in bore diameter, and the spindle speed is 500 to 900r/min. For the aperture of 6mm, the spindle speed is generally above 1000r/min.
When the drilling diameter is greater than 12mm, the borehole is much more accurate than the one-time drilling. When boring, the spindle speed and feed rate are about half of the drilling data. The exact data should be selected according to the machining accuracy, machining diameter, depth and the use of tools. It can also use insert boring tool to insert the graphite into the drill sleeve. For boring or reaming, for a bore diameter of 15 mm, a spindle speed of 400 to 500 r/min and a feed rate of 0.01 mm/r are suitable.
Graphite can also be used to drill holes. It uses a diamond flat nesting bit, drills with a pressure of 5.6 kgf/cm2, and uses a vacuum pump to collect waste. At the beginning, manual feed is performed at a spindle speed of 125 r/min, and the spindle speed is gradually increased until a normal drilling process is formed. For a hole with a diameter of 50mm, the spindle speed can reach 600r/min. For 125mm diameter holes, the spindle speed is 500r/min. In the boring process, vacuum pumps must be used to collect waste, to prevent contamination of the environment and to fill the holes in the waste.
Sixth, other
When graphite is processed, since graphite is composed of carbon material and the like, it has porosity, and if the solution is used for cooling during the processing, the impurities in the solution can be easily injected into the graphite body when the lubricant is washed. Therefore, when the graphite is used as the electrode, , Which impurities will bring harm, in general, any fluid, even pure water will bring pollution to graphite.
The coolant is not required for graphite processing because it is a high temperature resistant material that does not change its hardness by the quenching or annealing process. Graphite is also a good solid lubricant, but its lubrication properties vary with the grade of graphite.
However, for some graphite workpieces, the aqueous solution is indispensable when processing on a centerless grinding machine. In this case, it is better to use a rust-proof aqueous solution instead to clean the surface of the parts. Once the workpiece is contaminated by the solution, it is best to place the processed parts at 100°F and bake for 1 hour to achieve the purpose of decontamination. However, the operating temperature must be strictly controlled.Excessive temperatures may cause the product to be oxidation, but the temperature is too low, can not achieve the purpose of removing water.
Author: Ruishun Fan
