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简体中文 Cutting tool wear is the most critical issue in the machining of graphite electrodes. The extent of cutting tool wear not only affects tool consumption costs, machining time and machining quality, but also impacts the surface quality of workpiece materials processed by electrode EDM, making it a key parameter for optimizing high-speed machining of graphite.
The main areas where tool wear occurs during the machining of graphite electrode materials are the rake face and the flank face of the tool. On the rake face, impact contact between the cutting tool and the broken chip zone leads to impact abrasive wear, and the chips sliding along the tool surface cause sliding friction wear, both of which are main forms of tool wear in graphite machining.
Selection of Cutting Tools for Graphite Machining
The core difficulty of graphite machining also lies in cutting tools, as cutting tool wear proceeds extremely rapidly in this process. Domestic ordinary cutting tools only last about 3–4 hours in graphite machining, and better-quality domestic ones can last up to 5 hours, while imported foreign cutting tools have a service life of approximately 6–8 hours.
To reduce cutting tool wear and improve results, prioritize flat-bottom tools with a small radius (R-tools, e.g., 0.5R or 0.2R) over sharp flat-bottom tools for both roughing and finishing. Graphite’s hardness and brittleness make it prone to chipping with sharp edges, which accelerates cutting tool wear—R-tools mitigate this risk and prevent workpiece defects.
Ball end mills are used for finishing, but R-tools experience the most severe cutting tool wear; machining one mold takes 2–3 hours, increasing tool replacement frequency. Domestic manufacturers are now developing better tool materials and coatings to slow cutting tool wear, extend tool life, and lower overall costs.
Setting Optimal Parameters for Different Machining Stages
Rational parameter setting and tool selection for each machining stage are effective means to control tool wear and improve machining efficiency of graphite.
Sgrossatura
Roughing Tool Path for Graphite Machining
The figure below shows a basic roughing tool path for graphite machining, with an enlarged view of the roughing cutting depth and the machining amount per pass—scientific setting of these parameters is the first step to reduce tool wear in the roughing stage.
Parameter Settings:
- Cutting amount: 2~3mm
- Machining speed: 3~4m/min
- Spindle speed: 10000~12000rpm
Tool Selection:
- Roughing tool: 4-flute, 10~12mm in diameter
(Excessively small diameter: Low machining speed and efficiency, prolonging machining time and increasing cumulative cutting tool wear)
(Excessively large diameter: Severe tool runout, affecting machining accuracy and causing uneven cutting tool wear)
Roughing Effect Diagram
Semi-Finishing
Semi-Finishing Tool Path for Graphite Machining
The figure above shows the semi-finishing tool path for graphite machining. Typically, an R-tool is used for semi-finishing before the finishing process; this step can reduce the finishing allowance, thereby reducing the processing load of the finishing tool and effectively alleviating cutting tool wear in the subsequent finishing stage.
Parameter Settings:
- Cutting amount: 0.5mm
- Machining speed: 3m/min
- Spindle speed: 15000rpm
Tool Selection:
- R-tool: R3 (with a relatively large curved surface, suitable for roughing of curved surfaces and reducing localized cutting tool wear)
Semi-Finishing Effect Diagram
Finitura
Finishing Tool Path for Graphite Machining
The figure above shows the finishing tool path for graphite machining. There are various climbing tool paths for curved surface finishing, including 0°, 45°, 90°, and spiral paths. At present, the 90° path achieves the best cutting effect and the most uniform cutting force, which can minimize cutting tool wear during finishing. Finishing with an R-tool also yields excellent results with low cutting tool wear, while foreign machine tools adopt bull-nose tools (flat-bottom tools with a radius) for finishing—this requires extremely high machine tool precision. High-precision machine tools such as Makino and Mikron can achieve ultra-high machining accuracy; using bull-nose tools on such machines can produce a bright surface that generally requires no polishing, and the stable cutting process also reduces cutting tool wear significantly.
Parameter Settings:
- Cutting amount: 0.03mm
- Machining speed: 3m/min
- Spindle speed: 22000rpm
Tool Selection:
- Male mold: R-tool (R2, small curved surface, compatible with high spindle speed, low vibration, improved machining effect and reduced cutting tool wear)
- Female mold: Bull-nose tool (flat-bottom with radius) (6R0.5 / 4R0.2)
(6 & 4 represent the diameter, 0.5 & 0.2 represent the radius. A smaller radius means a smaller machining range, higher machining precision, and suitability for machining corner positions in female molds, with controlled cutting tool wear in complex areas)
Finishing Effect Diagram
Main Factors Affecting Cutting Tool Wear in Graphite Machining
Cutting tool wear in graphite machining is a comprehensive result of multiple factors; mastering the influence rules of each factor is the key to reducing cutting tool wear and improving tool service life.
Cutting Tool Material
The harder the cutting tool material, the better its wear resistance; however, higher hardness leads to lower impact toughness and increased brittleness of the material.
Hardness and toughness are a pair of contradictions, and balancing them is a key issue to solve for cutting tool materials to reduce cutting tool wear. For graphite cutting tools with ordinary TiAlN coatings, materials with relatively better toughness (i.e., slightly higher cobalt content) should be selected—fine-grain cemented carbide with 8%–10% cobalt is recommended, which can reduce cutting tool wear caused by brittle fracture.
Cutting Tool Geometric Angles
Selecting appropriate geometric angles for graphite-specific cutting tools helps reduce tool vibration, prevent graphite workpiece chipping, and fundamentally reduce abnormal cutting tool wear caused by vibration and chipping.
Rake Angle
When machining graphite with a negative rake angle, the cutting tool edge has high strength and excellent impact and friction resistance, which can effectively reduce cutting tool wear caused by edge damage. As the absolute value of the negative rake angle decreases, the wear area of the flank face changes little but shows an overall decreasing trend in cutting tool wear. When machining with a positive rake angle, the strength of the cutting tool edge is weakened with the increase of the positive rake angle, which instead leads to aggravated flank face wear and accelerated cutting tool wear.
Machining with a negative rake angle results in large cutting resistance and increased cutting vibration, which may cause periodic cutting tool wear; machining with a large positive rake angle leads to severe cutting tool wear and also large cutting vibration.
Flank Angle
If the flank angle is increased, the strength of the cutting tool edge is reduced, and the wear area of the flank face gradually increases, directly accelerating cutting tool wear. When the flank angle of the cutting tool is excessively large, cutting vibration is intensified, which further causes uneven cutting tool wear and shortens tool service life.
Helix Angle
When the helix angle is small, the length of the cutting edge that cuts into the graphite workpiece at the same time on the same cutting edge is the longest, resulting in the largest cutting resistance and the maximum cutting impact force borne by the tool—thus, cutting tool wear, milling force and cutting vibration all reach the maximum.
When the helix angle is large, the direction of the resultant milling force deviates greatly from the workpiece surface, and the cutting impact caused by the chipping of graphite material is aggravated, which also increases cutting tool wear, milling force and cutting vibration to a certain extent. Therefore, the influence of cutting tool angle changes on cutting tool wear, milling force and cutting vibration is a comprehensive effect of rake angle, flank angle and helix angle; careful selection is essential to control cutting tool wear.
Cutting Tool Coating
Coating is an important protective layer for cutting tools, and a suitable coating can significantly reduce cutting tool wear in graphite machining. Diamond-coated cutting tools have the advantages of high hardness, excellent wear resistance and low friction coefficient. At this stage, diamond coating is the optimal choice for graphite machining tools, and it can best reduce cutting tool wear and reflect the superior service performance of graphite cutting tools.
Diamond-coated cemented carbide cutting tools combine the high hardness of natural diamond (which resists cutting tool wear) with the high strength and fracture toughness of cemented carbide (which prevents tool breakage). However, in China at present, diamond coating technology is still in the initial stage, and the cost input is relatively large, so diamond coating will not develop significantly in the near future.
Cutting Tool Edge Enhancement
The passivation of the cutting tool edge is a very important issue that is not yet universally valued, and edge quality is closely related to cutting tool wear in graphite machining. The cutting edges of cemented carbide cutting tools ground with diamond grinding wheels have micro-notches of varying degrees (i.e., micro-chipping and serrations), which are the main sources of initial cutting tool wear and will rapidly expand during high-speed cutting.
High-speed cutting machining of graphite puts forward higher requirements on the performance and stability of cutting tools; especially for diamond-coated cutting tools, edge passivation treatment must be carried out before coating to ensure the firmness of the coating and the service life of the tool, and to avoid coating peeling and accelerated cutting tool wear caused by micro-notches.
The purpose of cutting tool edge passivation is to solve the problem of micro-notches on the tool edge after grinding, reduce its sharpness, achieve a smooth and flat edge, and make the tool both firm and durable, thereby fundamentally reducing cutting tool wear.
Cutting Processing Conditions
The selection of cutting processing conditions has a considerable impact on tool life, and scientific and reasonable conditions can effectively reduce cutting tool wear in graphite machining.
Cutting Method (Climb Milling and Conventional Milling)
The cutting vibration of climb milling is smaller than that of conventional milling, and low vibration can significantly reduce cutting tool wear caused by periodic impact. In climb milling, the cutting thickness of the tool decreases from the maximum to zero; after the cutting tool cuts into the workpiece, there will be no tool chatter phenomenon caused by inability to cut chips, the rigidity of the process system is good, and the cutting vibration is small, so the tool wear is uniform and mild. In conventional milling, the cutting thickness of the tool increases from zero to the maximum; at the initial stage of cutting, the tool will scratch a section of the workpiece surface due to the thin cutting thickness. At this time, if the edge encounters hard particles in the graphite material or chip particles remaining on the workpiece surface, it will cause tool chatter or flutter, resulting in large cutting vibration and severe and uneven tool wear.
Air Blowing (or Dust Suction) and Immersion EDM Fluid Machining
Timely cleaning of graphite dust on the workpiece surface is an important measure to reduce secondary cutting tool wear in graphite machining. Graphite dust will adhere to the tool edge and the workpiece surface, causing abrasive wear between the tool and the workpiece during the cutting process; removing dust in time can not only extend the service life of the cutting tool by reducing tool wear, but also reduce the impact of graphite dust on the machine tool lead screw and guide rail.
Other Matters
Selecting an appropriate high spindle speed and the corresponding large feed rate can make the cutting process more stable, avoid the tool staying on the workpiece surface for a long time and causing excessive cutting tool wear, and improve machining efficiency while controlling cutting tool wear.
Common Problems and Solutions in Graphite CNC Machining
Abnormal cutting tool wear is often accompanied by various machining defects in graphite CNC machining; solving these problems can also reduce tool wear and improve machining quality.
Tool Mark Problems
- Cutting tool problems: High-quality cutting tools are recommended to ensure stable cutting performance and reduce cutting tool wear caused by poor tool quality. At present, imported cutting tool materials and coating formulations are preferred for graphite machining, as domestic cutting tool materials and coatings are still under in-depth research, and their performance is yet to be improved to effectively control cutting tool wear.
- Post-processing: Use a special post-processing program matching the CNC system to ensure the smoothness of the tool path and reduce abnormal tool wear caused by sudden changes in the tool path.
Dark Line Problems
Machine tools, cutting tools and other factors will affect machining precision and produce dark lines, and the dark line problem is often related to uneven cutting tool wear and unstable cutting.
- System problems: The system settings need to be adjusted, select the “high-precision finishing” mode, and match the special post-processing of the system to make the cutting process more stable and reduce cutting tool wear and dark lines caused by unstable cutting.
- Tool path method: It is recommended to move two axes at the same time as much as possible, rather than three axes at the same time. The 45° path generally uses three-axis simultaneous movement, and the effect of three-axis simultaneous movement is not good, which is prone to tool vibration and uneven tool wear, thus producing tool marks and dark lines.
Conclusione
To sum up, cutting tool material, cutting tool geometric angles, cutting tool coating, cutting tool edge enhancement and cutting processing conditions all play different, indispensable and complementary roles in the service life of cutting tools and the degree of cutting tool wear in graphite machining. A high-quality graphite cutting tool that can effectively control tool wear should have smooth graphite chip flutes to avoid chip accumulation and aggravated tool wear, a long service life to reduce tool replacement frequency, the ability to perform deep engraving machining, and can save overall machining costs by reducing tool wear and improving machining efficiency. Controlling tool wear is the core of graphite machining technology research, and the comprehensive optimization of the above factors is the fundamental way to solve the problem of excessive cutting tool wear in graphite machining.
