Graphite is a kind of black non-metallic material that is very common in production and life, and it has excellent properties such as high temperature resistance, electrical and thermal conductivity, lubrication, chemical stability, plasticity, and thermal shock resistance. And because it belongs to the hexagonal crystal, the bonding force between the layers is weak, the internal porosity is high, and the tensile strength is low, which makes the machining performance of graphite very good. According to its different characteristics, its use is also very wide, for example: according to its chemical stability can be made into ink, pencil core, etc.; according to its high temperature resistance, it can be made into a grinding tool for refining gold. according to its electric conduction and thermal conductivity can be made into the electrode of the machine tool.
The structure of graphite parts is relatively complex and requires high precision. Generally, the quality requirements of graphite parts are relatively high. Because of the above characteristics, the quality of the sawing, scumming, and chipping of the edges are easily unqualified during processing.
2, the choice of tools
The correct choice of tool directly determines the efficiency and quality of the machining. Therefore, if we want to effectively control the quality of the processing, we must choose a reasonable and effective tool. For example: Graphite material itself is more convenient for cutting, so for some products with low surface accuracy and cost control, high steel tools can be used. The cutting performance of the tool directly determines the machining quality, the machining efficiency, and the tool durability. Therefore, the tool material must be reasonably selected. The hardness and toughness of the cutter are contradictory. The higher the hardness, the better the wear resistance, but the lower the toughness and the more brittle the texture. At the same time, graphite is made by compressing graphite powder. It only started with the dry cutting method. However, the wear resistance of high-speed steel is very poor, and it is very easy to produce severe wear during processing. Therefore, graphite products with high precision and high quality requirements cannot be processed with high speed steel tools, but should be used with coated tools or tools with good abrasion resistance.. For example: Hard alloys are mainly made of refractory metal carbides such as TiC, TaC, WC, and iron powder refined by metallurgical methods. Compared with ordinary high-speed steel, carbide steel has the advantages of high hardness and good wear resistance; at the same time, it has the advantage of good toughness compared with superhard materials. The influx of cemented carbide steel is very extensive, not only improving the production efficiency, but also improving the quality of graphite products. Therefore, the most appropriate choice must be made in the selection of the tool.
3, Choose the geometric Angle of the cutter tool reasonably
The selection of a suitable geometrical angle for the graphite cutter tool helps to reduce the vibration of the tool and the graphite workpiece is not easy to collapse.
3.1 The front corner. when the negative rake angle is used to process graphite, the cutting edge strength is good, and the impact resistance and friction performance are good. With the decrease of the absolute value of the negative rake angle, the flank wear area does not change much, but the overall It is a less frequent trend. When machining with a positive rake angle, as the rake angle increases, the cutting edge strength of the tool is weakened, resulting in increased flank wear. When the negative rake angle is processed, the cutting resistance is large, and the cutting vibration is increased. When the big positive rake angle is used for machining, the tool wears badly and the vibration is also large.
3.2 The rear corner, if the relief angle increases, the cutting edge strength decreases and the flank wear area gradually increases. When the tool clearance angle is too large, the cutting vibration increases.
3.3 Helix Angle.When the helix angle is small, the longest blade length of the graphite workpiece cut into the same cutting edge is the longest, the maximum cutting resistance, and the maximum cutting impact force the tool bears. As a result, tool wear, milling forces and cutting vibrations are the greatest. When the helix angle is large. The direction of the milled resultant force deviates from the working surface, and the cutting impact caused by the disintegration of the graphite material is aggravated, so that the tool wear, the milling force and the cutting vibration also increase.
Therefore, the influence of tool angle change on tool wear, milling force, and cutting vibration is a combination of rake angle, relief angle, and helix angle. Therefore, we must pay attention to the choice.
4, the coating of the cutter tool
Diamond-coated tools have the advantages of high hardness, good wear resistance and low friction coefficient. At this stage, diamond coating is the best choice for graphite machining tools, and it can best reflect the superior performance of stone grinding tools. The advantage of diamond-coated carbide tools is that they combine the hardness of natural diamonds with the strength and fracture toughness of cemented carbides.
5, the strengthening of the cutting edge
Passivation of tool edges is also a very important issue. The grinding edge of the cemented carbide cutting tool after the diamond grinding wheel has a small chipping and sawing mouth with different degrees. In the high-speed cutting of graphite, the stability of the tool performance is required to be higher. In particular, the diamond-coated tool must be passivated before the coating to ensure the firmness and service life of the coating. The purpose of the passivation of the tool is to solve the defect of the edge of the cutting edge and reduce or eliminate its frontal value to achieve smooth and smooth surface, which is both sharp and sturdy and wear-resistant.
6, the tool machining conditions
Choosing the right processing conditions has a considerable influence on the life of the tool and the quality of the machining.
6.1 Select the appropriate speed and feed. In general, because the graphite material is easy to cut, in the machining process, cutting parameters such as high rotation speed, rapid feed, and large knife can be selected to effectively improve the processing efficiency. However, due to the graphite, the chipping and the dregs are likely to occur during processing. In particular, serrations are easily formed at edges and other locations. In these locations, the speed and feed speed should be appropriately reduced.
6.2 The timely removal of graphite powder on the surface of the workpiece is beneficial to reduce the secondary wear of the tool, prolong the service life of the tool, and reduce the influence of graphite dust on the screw and guide rail of the machine tool.
6.3 Cutting mode: The milling mode is divided into climb milling and up milling. Judging whether crush milling or up-milling can be based on judging whether the feeding direction of the workpiece is the same as the rotating direction of the cutting tool, which is the same as crush milling, and vice versa.
Since the cutting vibration during climb milling is less than the cutting vibration during up milling, climb milling should be used to machine the inside profile. Since the cutting depth of the cutter is reduced from the maximum to zero, the cutter phenomenon caused by cutting cannot be seen at the same position. Conversely, the starting thickness of the inverse milling is very small. Once the edge comes into contact with the hard point, or In the case of cutting particles, tremors or projectile marks occur. At the same time, we must try to avoid eating a large engagement to prevent edge collapse or dregs.
7, a reasonable way to mill
Because different machining surfaces require different cutting methods, we need to determine different tool paths according to different conditions.
7.1 When machining planes, spiral feeds or slashed feeds should be used to avoid damage to the surface or breakage of the parts.
7.2 The method of pull milling should be avoided when machining the inclined surface. Drilling and milling are the main methods. Because the pull-milling is easy to form a corrugated surface on the surface of the product, especially when the working time is too long, there is a great chance that the tool will directly vibrate and form a wavy appearance.
In short, in order to avoid damage to the surface of the parts, it is important to avoid tripping away from the workpiece on the working face of the part and forming islands on the working surface by means of reasonable arrangements to go to the path.
In summary, with the popularization of graphite products, we are faced with tremendous opportunities and challenges. How can we seize opportunities and increase product quality while increasing production efficiency has become our main topic.
Author: Zhigang Liu