Compared with copper electrode, graphite electrode has the advantages of small electrode consumption, fast processing speed, good machinability, high processing precision, small thermal deformation, light weight, easy surface treatment, high temperature resistance, high processing temperature and electrode bonding. . Although graphite is a very easy to cut material, graphite materials used as EDM electrodes must have sufficient strength to avoid damage during handling and EDM processing, while electrode shapes (thin walls, small rounded corners, sharp changes) Etc.) also imposes high requirements on the grain size and strength of the graphite electrode, which results in the graphite workpiece being easily broken during the processing and the tool being easily worn.
Tool wear is the most important issue in graphite electrode processing. The amount of wear not only affects the tool loss cost, processing time, processing quality, but also affects the surface quality of the workpiece material processed by the electrode EDM, which is an important parameter for optimizing high-speed machining. The main tool wear areas for graphite electrode material processing are the rake face and the flank face. On the rake face, the impact contact between the tool and the broken chip area produces impact abrasive wear, and the sliding debris along the tool surface produces sliding friction wear.
The main factors affecting tool wear include:
1 tool material
Tool material is the fundamental factor determining the cutting performance of the tool, and has a great influence on machining efficiency, machining quality, machining cost and tool durability. The harder the tool material is, the better its wear resistance is. The higher the hardness, the lower the impact toughness and the brittle material. Hardness and toughness are a pair of contradictions and a key issue that should be addressed by tool materials. For graphite tools, ordinary TiAlN coatings can be selected to have a relatively better toughness in the selection of materials, that is, the cobalt content is slightly higher; for diamond-coated graphite tools, the hardness can be appropriately selected on the material selection, That is, the cobalt content is slightly lower.
2 tool geometry angle
The choice of a suitable geometry for the graphite-specific tool helps to reduce the vibration of the tool and, in turn, the graphite workpiece is not easily collapsed.
2.1 Front angle
When the graphite is processed by the negative rake angle, 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 wear area of ​​the flank face does not change much, but the overall trend is decreasing. When the positive rake angle is used, the tool edge strength is weakened as the rake angle increases, which in turn causes the flank wear to increase. When the negative rake angle is processed, the cutting resistance is large, and the cutting vibration is increased. When the large positive rake angle is used, the tool wear is severe and the cutting vibration is large.
2.2 Back angle
If the relief angle is increased, the tool edge strength is reduced and the flank wear area is gradually increased. When the back angle of the tool is too large, the cutting vibration is strengthened.
2.3 helix angle
When the helix angle is small, the cutting edge of the graphite workpiece at the same cutting edge has the longest blade length, the maximum cutting resistance, and the maximum cutting impact force the tool bears, so the tool wear, milling force and cutting vibration are the largest. When the helix angle is large, the direction of the milling force is larger than the surface of the workpiece, the cutting impact of the graphite material is increased due to the collapse, and the tool wear, the milling force and the cutting vibration are also increased. Therefore, the influence of the tool angle change on the tool wear, milling force and cutting vibration is the combination of the rake angle, the back angle and the helix angle, so it is necessary to pay more attention when selecting.
3 tool coating
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 also best reflects the superior performance of graphite tools. The advantages of diamond coated carbide tools are: the hardness of natural diamond and the strength and fracture toughness of cemented carbide. However, at present, diamond coating technology is still in its infancy, and there is also a large investment in cost. Therefore, diamond coating will not develop much in the near future. However, we can optimize the angle of the tool, select materials and improve the structure of the ordinary coating on the basis of ordinary tools, and to some extent, it can be applied in graphite processing.
The geometrical angle of the diamond coated tool is different from that of the ordinary coated tool. Therefore, when designing the diamond coated tool, due to the particularity of the graphite processing, the geometric angle can be appropriately enlarged, and the chip pocket becomes larger without Reduce the wear resistance of the cutting edge of the tool. For ordinary TiAlN coatings, although the wear resistance is significantly improved compared with uncoated tools, the geometric angle of the graphite coating should be appropriately reduced compared with the diamond coating to increase its wear resistance. Sex.
For diamond coatings, many coating companies in the world have invested a lot of manpower and resources to research and develop related technologies, but so far, coating companies with mature foreign technology and good economics are limited to European companies.
4 tool edge reinforcement
Passivation of the cutting edge of the tool is a problem that has not been widely recognized and is very important. The cutting edge of the carbide tool sharpened with a diamond wheel has microscopic notches of different degrees (ie, micro chipping and sawtooth). The high-speed machining of graphite puts higher demands on the performance and stability of the tool. Especially the diamond-coated tool must be passivated before the coating to ensure the firmness and service life of the coating. The purpose of tool passivation is to solve the problem of micro-notch of the cutting edge after the tool is sharpened, reduce its sharpness, achieve a smooth and flat, and is both strong and durable.
5 Cutting conditions
The choice of machining conditions has a considerable impact on tool life.
5.1 Cutting method (shun milling and up milling)
The cutting vibration of the down milling is smaller than the up milling. When milling, the cutting thickness of the tool is reduced from the maximum to zero. After the tool is cut into the workpiece, there is no knives caused by cutting the chips. The rigidity of the process system is good and the cutting vibration is small. When cutting the thickness of the tool during up-cut milling From zero to maximum, at the beginning of the cutting of the tool, due to the thin cutting thickness, a path will be scratched on the surface of the workpiece. At this time, if the cutting edge encounters hard spots in the graphite material or chip particles remaining on the surface of the workpiece, Causes the tool's knives or flutter, so the cutting vibration of the up-cut milling is large.
5.2 Blowing (or vacuuming) and impregnating EDM
Clean the graphite dust on the surface of the workpiece in time, which helps to reduce the secondary wear of the tool, prolong the service life of the tool, and reduce the influence of graphite dust on the lead screw and guide rail of the machine.
5.3 Other matters
Choose the right high speed and the corresponding large feed.
Combining the above points, the tool material, geometric angle, coating, edge reinforcement and cutting conditions play different roles in the life of the tool, which are indispensable and complementary. A good graphite tool should have smooth graphite powder flutes, long service life, and deep engraving processing, which can save processing costs.
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Label: Features and applications of special tools for graphite processing
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