Large precision part machining tools will experience a certain degree of wear as their usage time increases. What are the several factors that influence this wear? Through summarization, we can identify a few reasons:
a) Tool Material
The material of the tool is the fundamental factor determining its cutting performance and significantly affects processing efficiency, quality, cost, and durability. The harder the tool material, the better its wear resistance; however, the higher the hardness, the lower the impact toughness, making the material more brittle. Hardness and toughness are a pair of contradictions and key aspects for tool materials to overcome. For graphite tools, regular coatings allow for the selection of relatively tougher materials with slightly higher cobalt content in large CNC machining. For diamond-coated graphite tools, materials with relatively better hardness and slightly lower cobalt content can be selected.
b) Geometric Angles of Tools
Choosing appropriate geometric angles for graphite tools helps reduce vibration and prevents damage to graphite workpieces.
- *Rake Angle: When using a negative rake angle to machine graphite, the edge strength is good, and the tool has better 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 generally shows a decreasing trend. When using a positive rake angle, as the rake angle increases, the edge strength weakens, leading to increased back face wear. Negative rake angle machining results in high cutting resistance, increasing cutting vibration. Large positive rake angle machining leads to severe tool wear and significant cutting vibration.
- *Clearance Angle: If the clearance angle increases, the edge strength decreases, and the wear area on the back face gradually enlarges. Excessive clearance angle amplifies cutting vibration.
- *Helix Angle: A smaller helix angle means the longest cutting length along the same cutting edge enters the graphite workpiece simultaneously, resulting in maximum cutting resistance and cutting impact force, leading to maximum tool wear, milling force, and cutting vibration. A larger helix angle causes the resultant milling force direction to deviate further from the workpiece surface, exacerbating cutting impact due to chipping of graphite material, thus increasing tool wear, milling force, and cutting vibration. Therefore, the impact of changes in tool angles on tool wear, milling force, and cutting vibration is a comprehensive effect of the rake angle, clearance angle, and helix angle, requiring careful attention when selecting.
Through extensive scientific testing on the machining characteristics of graphite materials, the relevant tools' geometric angles have been optimized, greatly improving the overall cutting performance of the tools.
c) Tool Coatings
Diamond-coated tools have advantages such as high hardness, excellent wear resistance, and low friction coefficients. Currently, diamond coating is the best choice for graphite machining tools and best demonstrates the superior usability 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, with significant costs involved, so diamond coatings will not develop significantly in the near future. Nevertheless, improvements can be made by optimizing the angles and materials of regular tools and enhancing the structure of standard coatings, which can be applied in graphite machining to some extent. There are essential differences in the geometric angles between diamond-coated tools and standard coated tools. Due to the special nature of graphite machining, the geometric angles of diamond-coated tools can be appropriately enlarged, increasing the chip clearance groove without reducing the wear resistance of the cutting edge. For standard coatings, although they significantly improve wear resistance compared to uncoated tools, their geometric angles should be appropriately reduced during graphite machining to enhance wear resistance.
For diamond coatings, numerous coating companies worldwide invest substantial resources into research and development of related technologies. To date, only European companies offer mature and economical coatings. High-quality graphite machining tools use the world's most advanced coating technology for surface treatment to ensure both long service life and economic practicality.
d) Edge Reinforcement of Tools
Edge blunting technology is an important issue that hasn't yet received universal attention. After grinding with a diamond wheel, the edges of carbide tools exhibit varying degrees of microscopic notches (i.e., small chips and saw-like edges). High-speed graphite machining places higher demands on the performance and stability of tools, especially for diamond-coated tools, which must undergo edge blunting before coating to ensure the adhesion and service life of the coating. The purpose of tool blunting is to address the defects of microscopic notches on the edges after grinding, reducing or eliminating sharp points to achieve smoothness and flatness, making the tools sharp, strong, and durable.
e) Mechanical Machining Conditions
Selecting appropriate machining conditions significantly affects the tool's lifespan.
- *Cutting Method (Climb Milling vs Conventional Milling): Climb milling produces less cutting vibration than conventional milling. In climb milling, the tool’s cutting thickness decreases from maximum to zero, preventing bouncing caused by inability to cut chips once the tool enters the workpiece, ensuring good system rigidity and minimal cutting vibration. In conventional milling, the cutting thickness increases from zero to maximum, causing the tool to scratch the workpiece surface initially due to thin cuts. If the edge encounters hard points in the graphite material or residual chips on the workpiece surface, it may cause bouncing or chatter, increasing cutting vibration.
- *Blowing Air (or Dust Extraction) and EDM Fluid Immersion Processing: Promptly cleaning graphite dust from the workpiece surface reduces secondary wear on the tool, extends its lifespan, and minimizes the impact of graphite dust on the machine's lead screws and guideways.
- *Selecting appropriate high spindle speeds and corresponding large feed rates.
In summary, the material, geometric angles, coatings, edge reinforcement, and mechanical machining conditions of the tool play different roles in the tool's lifespan, each indispensable and complementary. A good graphite tool should have smooth graphite powder discharge grooves, a long lifespan, deep engraving capabilities, and cost savings in processing.
Dongguan Wanjun specializes in large computerized milling centers, large precision part machining, large CNC machining, large computerized milling processing, large mold machining, large aluminum plate machining, machinery equipment panel processing, and precision sand casting. Visit: http://www.dgwanjun.com