Large precision part machining tools: As the usage time extends, there will be some wear on the tool. What are the several factors that affect tool wear? Through summarization, a few reasons have been identified.
a) Tool material
The tool material is the fundamental factor that determines the cutting performance of the tool, and it has a significant impact 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 becomes. Hardness and toughness are a pair of contradictions and also a key point that the tool material needs to overcome. For graphite tools, ordinary coatings can choose materials with relatively better toughness, which means slightly higher cobalt content for large CNC machining. For diamond-coated graphite tools, materials with relatively better hardness can be selected, meaning slightly lower cobalt content.
b) Geometric angles of the tool
Choosing appropriate geometric angles for graphite tools helps reduce tool vibration, and in turn, graphite workpieces are less likely to break.
* Rake angle: When using a negative rake angle to machine graphite, the tool edge strength is better, and the tool has good resistance to impact and friction. As the absolute value of the negative rake angle decreases, the wear area on the back face does not change much, but overall shows a decreasing trend. When using a positive rake angle, as the rake angle increases, the tool edge strength weakens, leading to increased wear on the back face. When using a negative rake angle, the cutting resistance is high, increasing cutting vibration. When using a large positive rake angle, tool wear is severe, and cutting vibration is also high.
* Clearance angle: If the clearance angle increases, the tool edge strength decreases, and the wear area on the back face gradually increases. After the clearance angle becomes too large, cutting vibration intensifies.
* Helix angle: When the helix angle is small, the longest cutting length on the same cutting edge enters the graphite workpiece at the same time, resulting in the highest cutting resistance and the greatest cutting impact force on the tool. Therefore, tool wear, milling force, and cutting vibration are all the highest. When the helix angle is larger, the direction of the milling resultant force deviates from the workpiece surface to a greater extent, exacerbating the cutting impact caused by the fragmentation of graphite material. Thus, tool wear, milling force, and cutting vibration all increase. Therefore, the effect of changes in tool angles on tool wear, milling force, and cutting vibration is a comprehensive result of the rake angle, clearance angle, and helix angle. Attention must be paid when selecting these angles.
Through extensive scientific testing of the machining characteristics of graphite materials, the tool geometry has been optimized, significantly improving the overall cutting performance of the tool.
c) Coatings on the tool
Diamond-coated tools have advantages such as high hardness, good wear resistance, and low friction coefficient. Currently, diamond coating is the best choice for graphite machining tools and best demonstrates the superior performance of graphite tools. Diamond-coated carbide tools combine the hardness of natural diamonds with the strength and fracture toughness of carbide. However, diamond coating technology in China is still in its infancy, and the cost investment is substantial. Therefore, diamond coatings will not develop significantly in the near future. However, we can optimize the angles and material selection of ordinary tools and improve the structure of common coatings to make them applicable in graphite machining to some extent. There are essential differences in the geometric angles between diamond-coated tools and ordinary coated tools. Due to the particularity of graphite machining, the geometric angles of diamond-coated tools can be appropriately enlarged, and the chip clearance groove can also be increased without reducing the wear resistance of the tool's cutting edge. For ordinary coatings, although they significantly improve the wear resistance compared to uncoated tools, their geometric angles should be appropriately reduced during graphite machining to enhance their wear resistance.
For diamond coatings, many coating companies worldwide invest considerable human and material resources in researching and developing related coating technologies. To date, only European companies offer mature and economical coatings. Excellent graphite machining tools use the most advanced coating technology currently available to treat the tool surfaces, ensuring both long service life and economic practicality.
d) Reinforcement of the tool edge
Tool edge dulling technology is an issue that is not yet widely recognized but is very important. After grinding with a diamond wheel, the edges of carbide tools have microscopic notches (i.e., tiny chipped edges and saw-like notches). High-speed graphite machining places higher demands on the performance and stability of the tools, especially for diamond-coated tools, which must undergo edge dulling treatment before coating to ensure the adhesion and lifespan of the coating. The purpose of dulling the tool is to address the defects of microscopic notches on the tool edge after sharpening, reducing or eliminating their sharp points, achieving smoothness and flatness, making the tool both sharp and durable.
e) Mechanical machining conditions
Selecting appropriate machining conditions has a significant impact on the tool's lifespan.
* Cutting method (climb milling vs. conventional milling): The cutting vibration in climb milling is less than in conventional milling. In climb milling, the cutting thickness decreases from maximum to zero, and after the tool cuts into the workpiece, there is no phenomenon of bouncing due to inability to cut chips, providing better rigidity and reducing cutting vibration. In conventional milling, the cutting thickness increases from zero to maximum, and during the initial cutting phase, the thin cutting thickness causes the tool to scratch the workpiece surface. If the edge encounters hard particles in the graphite material or chips left on the workpiece surface, it will cause tool bouncing or chatter, thus increasing cutting vibration.
* Blowing (or dust extraction) and immersion EDM fluid machining: Timely cleaning of graphite dust from the workpiece surface helps reduce secondary tool wear, extend tool life, and minimize the effects of graphite dust on the machine's screws and guides.
* Choosing appropriate high spindle speeds and corresponding large feed rates.
In summary, the tool's material, geometric angles, coating, edge reinforcement, and mechanical machining conditions play different roles in the tool's lifespan. None of these aspects can be missing, and they complement each other. A good graphite tool should have smooth graphite powder chip evacuation grooves, a long service life, the ability to perform deep engraving, and the ability to save machining costs.
Dongguan Wanjun Large Computer Milling Processing Center, Large Precision Parts Machining, Large CNC Machining, Large Computer Milling Processing, Large Mold Machining, Large Aluminum Plate Machining, Machinery Equipment Panel Machining, Precision Sand Casting. Website: http://www.dgwanjun.com