Flank wear is the most ideal wear state because it is quite predictable and reliable. At the same time, a clear relationship is established between flank wear and the achievable tool life. However, if flank wear occurs too quickly – similar to typical flank wear but in a very short time – it may cause problems.
At lower cutting speeds, the main causes of flank wear are abrasion and erosion. Hard tiny carbide inclusions or work – piece material particles that have undergone strain hardening cut into the cutting tool. Then, small pieces of the coating come off and cut into the tool flank. Eventually, the cobalt leaches out from the matrix. This reduces the adhesion of carbide grains, causing them to come off as well. At higher cutting speeds, diffusion wear is the main cause of flank wear because higher cutting speeds generate higher temperatures on the cutting edge, creating favorable conditions for the occurrence of diffusion wear.
Flank wear is similar to relatively uniform wear on the cutting edge of the tool. Sometimes, the metal from the work – piece covers the cutting edge, increasing the visible size of the wear mark. Flank wear occurs in all materials. If the cutting edge does not fail first due to other types of wear, it usually fails due to flank wear.
Some corrective measures to reduce flank wear are to lower the cutting speed (in some cases, increasing the feed rate can also help), select a more wear – resistant and harder carbide grade, and use coolant correctly.
Improvement methods:
Reduce the cutting speed.
Select a carbide material grade that is more wear – resistant.
Use the correct cooling method.
2. Crater wear
Crater wear is the combined effect of diffusion – decomposition wear (at higher cutting speeds) and abrasive wear (at lower cutting speeds). The heat generated by the workpiece chips decomposes the tungsten carbide particles in the matrix, and carbon infiltrates into the chips (diffusion), thus forming a “crater” on the rake face of the cutting insert.
Ultimately, the crater will become large enough to cause the flank face of the insert to crack or may lead to rapid flank wear.
Crater wear refers to the shape/appearance of a crater or pit that appears on the rake face of the cutting insert. Crater wear is most common when machining abrasive workpiece materials or materials with a hard surface.
To minimize crater wear, it is advisable to use coatings containing a thick alumina layer, apply coolant, use a free – cutting geometry that can reduce heat, and decrease the cutting speed and feed.
Improvement methods:
It is advisable to use a thicker alumina coating.
Use cutting fluid. Reduce chip interference.Reduce heat.
Lower the cutting speed and feed rate.
3.Cutting edge breakage
Any overview of basic wear patterns must cover cutting – edge fracture. Catastrophic fracture of the cutting edge is not a wear pattern but a harmful and dangerous phenomenon caused by improper use of the tool.
Cutting – edge fracture means that the selected cutting conditions are too severe, resulting in excessive mechanical loads acting on the cutting edge, which the tool cannot withstand.
Improvement methods:
Apart from common flank wear, other wear failure mechanisms should also be noted.
Reduce the feed rate and cutting depth. Increase the stiffness of the technological system.
Select cemented carbide materials with better toughness and strong cutting – edge groove shapes.
Use inserts with chip – breaking grooves during high – feed cutting.
4.Built – up edge
Built – up edge (BUE) is formed when workpiece material is pressure – welded onto the cutting edge. This occurs when there is chemical affinity, high pressure, and a sufficiently high temperature in the cutting zone. Eventually, the BUE breaks off and takes fragments of the cutting edge with it, resulting in chipping and rapid flank wear.
The BUE appears as shiny fragments of material on top of the cutting edge or on the flank face, creating small pits or craters on the rake face of the tool, ultimately leading to the chipping of the cutting edge. BUE typically occurs in viscous materials such as non – ferrous metals, superalloys, and stainless steels, as well as during machining processes with low cutting speeds and feed rates.
To prevent BUE – related wear, the cutting speed and/or feed rate should be increased, inserts with a sharper geometry and a smoother rake face should be selected, and coolant with a higher concentration should be used correctly.
Improvement methods:
Increase the cutting speed and feed rate.
Select insert groove shapes that are sharp and a smoother rake face.
Use cutting fluid correctly or increase the concentration of the cutting fluid.
5.Notch wear
Notch wear occurs when the workpiece surface is harder or more abrasive than its underlying material. This can be due to work – hardening during previous cutting processes (strain – hardened materials such as stainless steel and superalloys), or it may stem from forged or cast surfaces with scale. All of these cause the cutting edge to wear more quickly at the point of contact with the hard layer.
This local concentrated stress can also lead to notch wear. What happens is that compressive stress develops along the cutting edge in contact with the workpiece material, but not where the cutting edge is not in contact. This subjects the cutting edge to high stress at the point of direct contact between the two (cutting – line depth). Any form of impact, such as hard micro – inclusions in the workpiece material or minor interruptions, can also cause notch wear.
Some corrective measures include reducing the feed rate and varying the cutting depth when using multiple passes, increasing the cutting speed when machining superalloys (which will result in more flank wear), selecting a harder carbide grade, and using chip – breaking geometries for high – feed to prevent built – up edge, especially in stainless steels and heat – resistant alloys.
Improvement methods:
Reduce the feed rate and adopt variable cutting depths in multi – pass cutting.
When machining superalloys, increase the cutting speed (which will cause more flank wear).
Select cemented carbide materials with better toughness and use chip – breaking grooves during high – feed cutting.
Avoid the formation of built – up edge when cutting stainless steels and superalloys.
6.Chipping wear
Chipping is caused by the mechanical instability or cracks in the cutting material. Chipping of the cutting edge is usually caused by the vibration of the workpiece, machine tool, or the tool itself.
Hard inclusions on the surface of the workpiece material, as well as interrupted cutting, can lead to local stress concentration, which may cause cracks and chipping.
Chipping appears as small fragments falling off from the cutting edge and is common in non – rigid machining. Workpiece materials containing hard particles (such as precipitation – hardened workpiece materials) can also cause chipping of the cutting edge.
Corrective measures include: properly setting up the machine tool and minimizing deflection; using tougher cemented carbide materials and more robust cutting – edge geometries; reducing the feed rate (especially at the cutting entry or exit); and increasing the cutting speed.
Improvement methods:
Ensure the machine tool has good accuracy and reduce errors.
Select a cemented carbide grade with better toughness and a strong cutting – edge groove shape.
Reduce the feed rate (especially during tool engagement and disengagement).
Increase the cutting speed. Refer to the section on built – up edge in the text.
7.Plastic deformation
Thermal overload is the main cause of plastic deformation. Overheating can cause the cemented carbide binder (cobalt) to soften. Then, due to mechanical overload, the pressure on the cutting edge causes it to deform or the tip to droop, ultimately leading to fracture or rapid flank wear. Plastic deformation appears as the deformation of the cutting edge. Careful observation is required because plastic deformation looks very similar to the flank wear of the cutting edge.
Plastic deformation occurs when the cutting temperature is high (with high cutting speeds and feeds) and the workpiece material is inherently of high strength (hard steels or strain – hardened surfaces and superalloys).
Some corrective measures include using coolant correctly, reducing the cutting speed and feed, using inserts with a larger nose radius, and selecting a harder and more wear – resistant cemented carbide grade.
8.Thermal crack
Thermal cracks are caused by a combination of thermal cycling (temperature variations of the cutting edge), thermal loading (the temperature difference between the hot and cold zones of the cutting edge), and mechanical shock.
Stress cracks form along the cutting edge, ultimately causing parts of the carbide to be pulled out and the cutting edge to break.
Thermal cracks mainly occur in milling and interrupted – cutting turning. Intermittent coolant flow can also lead to thermal cracks.
Some corrective measures include using coolant correctly, choosing a harder carbide grade, reducing the cutting speed and feed rate, using free – cutting geometries that reduce heat, and considering different processing methods (the ratio of cutting time to cutting time).
Improvement methods:
Use cutting fluid correctly. Select alloy inserts with better toughness.
Reduce the cutting speed and feed rate. Minimize chip interference to reduce heat.
Adopt different processing methods (cutting time / cutting – out time).
Summary
Tool degradation refers to the process in which the condition of a cutting tool deteriorates increasingly, gradually causing the tool to lose its ability to perform as expected. Tool degradation is caused by aging wear, sudden impact phenomena such as chipping, and chemical interactions between the workpiece material and the cutting material. Aging wear is a progressive surface damage process that leads to the detachment of material from one or both of the two solid surfaces in contact in a solid state. This occurs when the two solid surfaces are in sliding or rolling contact under environmental conditions of pressure and temperature. An overview of the basic and unique wear patterns provides fundamental remedial measures to address tool wear issues that are unacceptable to machinists in terms of form or rate of development.
