Causes of tool wear

Tool material

Tool material is the fundamental factor that determines the cutting performance of the tool, and has a great influence on processing efficiency, processing quality, processing cost and tool durability. The harder the tool material, the better its wear resistance, the higher the hardness, the lower the impact toughness, and the more brittle the material. Hardness and toughness are a pair of contradictions, and it is also a key that tool materials should overcome. For graphite tools, ordinary TiAlN coatings can appropriately select materials with relatively better toughness, that is, slightly higher cobalt content; for diamond-coated graphite tools, materials with relatively better hardness, that is, slightly lower cobalt content can be appropriately selected;

 

Tool geometry

Selecting the appropriate geometry of the graphite tool helps to reduce the vibration of the tool, and conversely, the graphite workpiece is not easy to break; 1. Rake angle: When using negative rake angle to process graphite, the tool edge strength is better, and the impact and friction resistance is good. As the absolute value of the negative rake angle decreases, the wear area of the back tool face does not change much, but generally shows a decreasing trend. When using positive rake angle, as the rake angle increases, the tool becomes sharper, but the strength of the tool edge is weakened, which leads to increased wear of the back tool face. When using negative rake angle for processing, the cutting resistance is large, which increases the cutting vibration. When using large positive rake angle for processing, the tool wear is serious and the cutting vibration is also large. Generally, a tool with a smaller rake angle or a negative rake angle tool should be selected for rough processing. 2. Back angle: If the back angle increases, the tool edge strength decreases and the wear area of the back tool face gradually increases. When the tool back angle is too large, the cutting vibration is enhanced. The smaller the back angle, the greater the friction contact length between the elastic recovery layer and the back tool face, which is one of the direct causes of wear of the cutting edge and the back tool face. In this sense, increasing the back angle can reduce friction and improve the quality of the processed surface and the service life of the tool. 3. Helix angle: When the helix angle is small, the blade length of the same cutting edge that cuts into the graphite workpiece at the same time is the longest, the cutting resistance is the largest, and the cutting impact force borne by the tool is the largest, so the tool wear, milling force and cutting vibration are all the largest. When the helix angle is large, the direction of the milling force deviates greatly from the workpiece surface, and the cutting impact caused by the graphite material breaking is aggravated, so the tool wear, milling force and cutting vibration are also increased. Therefore, the effect of tool angle changes on tool wear, milling force and cutting vibration is a combination of the front angle, back angle and helix angle, so you must pay more attention to the selection. Through a large number of scientific tests on the processing characteristics of graphite materials, PARA tools have optimized the geometric angles of related tools, thereby greatly improving the overall cutting performance of the tools.

 

Tool coating

Diamond coated tools have the advantages of high hardness, good wear resistance, and low friction coefficient. At present, diamond coating is the choice of graphite processing tools, and it can best reflect the superior performance of graphite tools. The advantage of diamond coated carbide tools is that they combine the hardness of natural diamonds with the strength and fracture toughness of carbide. However, diamond coating technology is still in its infancy in China, and the cost investment is very large, so diamond coating will not have much development for the time being. However, we can optimize the angle of the tool, material selection and other aspects and improve the structure of ordinary coatings on the basis of ordinary tools, which can be applied in graphite processing to a certain extent. The geometric angles of diamond coated tools and ordinary coated tools are essentially different. Therefore, when designing diamond coated tools, due to the particularity of graphite processing, the geometric angles can be appropriately enlarged, the cutting groove becomes larger, and the wear resistance of the tool edge will not be reduced. For ordinary TiAlN coatings, although their wear resistance is significantly improved compared to uncoated tools, compared with diamond coatings, their geometric angles should be appropriately reduced when processing graphite to increase their wear resistance. Tool surface treatment technology has made new developments. The latest news from abroad released by Mobile Spinach: Using solid nanostructured boron atoms to modify the tool surface can significantly increase the tool life. For diamond coating, many coating companies in the world have invested a lot of manpower and material resources to research and develop related coating technologies, but so far, mature and economical coating companies abroad are limited to Europe; PARA, as an excellent graphite processing tool, also uses the world's most advanced coating technology to treat the tool surface to ensure the processing life while ensuring the economic and practical use of the tool.

 

Tool edge reinforcement

Tool edge passivation technology is a very important issue that is not widely valued by people. After the carbide tool edge is sharpened by the diamond grinding wheel, there are microscopic gaps (i.e., tiny chipping and sawing) of varying degrees. Graphite high-speed cutting tool performance and stability have higher requirements, especially diamond-coated tools must be passivated before coating to ensure the firmness and service life of the coating. The purpose of tool passivation is to solve the defects of the micro-notch of the tool edge after the above-mentioned sharpening, so as to reduce or eliminate its edge value, and achieve the purpose of smoothness, sharpness, firmness and durability.

 

Processing conditions

Choosing appropriate processing conditions has a considerable impact on the life of the tool. 1. Cutting mode (down milling and reverse milling). The cutting vibration during down milling is less than the cutting vibration of reverse milling. The tool cutting thickness during down milling is reduced from the maximum to zero. After the tool cuts into the workpiece, there will be no tool bounce caused by the failure to cut the chips. The rigidity of the process system is good and the cutting vibration is small. During reverse milling, the tool cutting thickness increases from zero to the maximum. In the initial stage of tool cutting, the tool will scratch a path on the surface of the workpiece due to the thin cutting thickness. At this time, if the cutting edge encounters hard points in the graphite material or chip particles remaining on the surface of the workpiece, it will cause the tool to bounce or vibrate. Therefore, the cutting vibration of reverse milling is large; 2. Blowing (or vacuuming) and immersion in EDM liquid machining, timely cleaning of graphite dust on the workpiece surface is conducive to reducing secondary wear of the tool, extending the service life of the tool, and reducing the impact of graphite dust on the machine tool screw and guide rail; 3. Choose a suitable high speed and corresponding large feed rate. Summarizing the above points, the material, geometric angle, coating, edge reinforcement and machining conditions of the tool play different roles in the service life of the tool, and they are indispensable and complementary. A good graphite tool should have a smooth graphite powder chip groove, a long service life, the ability to perform deep engraving, and save processing costs.