Several reasons affecting tool wear

1. Tool material

NC tool material is the fundamental factor to determine the cutting performance of the tool, which has a great impact on the 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 is, the lower the impact toughness is, and the more brittle the material is. Hardness and toughness are a pair of contradictions, and also a key to overcome for tool materials. For graphite tools, common TiAlN coating can be used to select materials with relatively good toughness, that is, with slightly higher cobalt content; For diamond coated graphite tools, it is appropriate to select materials with relatively good hardness, that is, with slightly lower cobalt content;

2. Geometric angle of the tool

The proper geometric angle of the graphite tool is helpful to reduce the vibration of the tool. On the contrary, the graphite workpiece is not easy to crack;

(1) Front angle: When machining graphite with negative front angle, the cutting edge strength of the tool is good, and the performance of impact resistance and friction resistance is good. With the reduction of the absolute value of the negative front angle, the wear area of the rear tool surface does not change much, but the overall trend is decreasing. When machining with positive front angle, the cutting edge strength of the tool is weakened with the increase of the front angle, which leads to increased wear of the rear tool surface. When machining with negative rake angle, the cutting resistance is large and the cutting vibration is increased. When machining with large positive rake angle, the tool wear is serious and the cutting vibration is also large.

(2) Rear angle: If the rear angle increases, the cutting edge strength decreases and the wear area of the rear tool surface gradually increases. When the back angle of the tool is too large, the cutting vibration will be strengthened.

(3) When the helix angle is small, the cutting edge that cuts into the graphite workpiece at the same time with all cutting edges is the longest, the cutting resistance is the largest, and the cutting impact force that the tool bears is the largest, so the tool wear, milling force and cutting vibration are the largest. When the helix angle is larger, the direction of the milling resultant force deviates from the workpiece surface to a large extent, and the cutting impact caused by the fragmentation of graphite materials is intensified, so the tool wear, milling force and cutting vibration are also increased. Therefore, the influence of tool angle change on tool wear, milling force and cutting vibration is a combination of rake angle, rake angle and helix angle, so more attention must be paid to the selection.

Through a large number of scientific tests on the processing characteristics of graphite materials, the PARA tool optimizes the geometric angles of related tools, thus greatly improving the overall cutting performance of the tool.

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 cutting tools, which can also best reflect the superior performance of graphite tools; The advantage of diamond coated cemented carbide tools is that they combine the hardness of natural diamond with the strength and fracture toughness of cemented carbide; However, the diamond coating technology in China is still in its infancy, and the cost investment is very large, so the diamond coating will not have much development in the near future. However, we can optimize the tool angle, material selection and other aspects and improve the structure of the common coating on the basis of ordinary tools, which can be used in graphite processing to some extent.

The geometric angle of diamond coated tools is essentially different from that of ordinary coated tools. Therefore, when designing diamond coated tools, due to the particularity of graphite processing, the geometric angle can be appropriately enlarged, the cutting groove can also be enlarged, and the wear resistance of the tool tip will not be reduced; For ordinary TiAlN coating, although its wear resistance is significantly improved compared with that of uncoated tools, its geometric angle should be appropriately reduced when machining graphite compared with diamond coating to increase its wear resistance.

For diamond coating, many coating companies in the world have invested a lot of manpower and material resources to research and develop relevant coating technologies, but so far, mature and economic coating companies abroad are only limited to Europe; As an excellent graphite processing tool, PARA also uses the world’s most advanced coating technology to surface treat the tool, so as to ensure the tool’s economic and practical use while ensuring its processing life.

4. Strengthening of cutting edge

Cutting edge passivation technology is a very important problem, which has not been paid much attention to. The cemented carbide tool edge after grinding with diamond wheel has micro notches of different degrees (i.e. micro chipping and sawing). The performance and stability of graphite high-speed cutting tools put forward higher requirements, especially the diamond coated tools must be passivated before coating to ensure the fastness and service life of the coating. The purpose of tool passivation is to solve the micro notch defects of the tool edge after grinding, reduce or eliminate its edge value, and achieve the goal of smooth, flat, sharp, firm and durable.

5. Machining conditions of tools

The selection of appropriate machining conditions has a considerable impact on the tool life. (1) In the cutting mode (down milling and up milling), the cutting vibration in down milling is less than that in up milling. The cutting thickness of the cutter in the forward milling process is reduced from the maximum to zero. After the cutter cuts into the workpiece, there will be no tool spring due to the inability to cut chips. The process system has good rigidity and low cutting vibration; During up milling, the cutting thickness of the tool increases from zero to the maximum. At the beginning of cutting, the tool will scratch a path on the workpiece surface due to its thin cutting thickness. At this time, if the cutting edge encounters hard points in the graphite material or chip particles left on the workpiece surface, it will cause the tool to snap or vibrate, so the cutting vibration of up milling is large;

(2) Blowing (or vacuuming) and dipping EDM fluid processing, timely cleaning the graphite dust on the workpiece surface, is conducive to reducing the secondary wear of the tool, extending the service life of the tool, and reducing the impact of graphite dust on the machine tool lead screw and guide rail;

(3) Select appropriate high speed and corresponding large feed rate.

Summarize the above points. The material, geometric angle, coating, edge strengthening and machining conditions of the tool play different roles in the service life of the tool. A good graphite cutter should have a smooth graphite powder chip removal groove, long service life, deep engraving and processing, and can save processing costs.