The failure of cutting tool may be result of:
1. Wear on the flank of the tool
Flank wear is a flat portion worn behind the cutting edge which eliminate some relief. Flank wear takes place when machining brittle materials like CI . The worn region at the flank is called the wear land.
2. Wear at the tool chip interface
It occurs in the form of a depression or crater. This is caused by the pressure of the chip as it slides up the face of the cutting tool. Actually a limited amount of cratering may improve the cutting action. But when it is further enlarged cause the cutting edge to be weakened. This type of failure occurs when high speed steel, stellite or sintered carbide tools turn ductile metals.
3. A combination of flank wear and cratering
4. The spalling or crumbling of the cutting edge as when cutting extremely hard materials. A cutting tool that has improperly ground relief angles will either rub on the material or be weak because of excessive clearance angle. The other factors that cause the tool to chip or spall are excessive chip load, intermittent heating and cooling, interrupted cutting.
5. The loss of hardness because of excessive heat generated at the cutting. Various tool materials can withstand various heating temperatures before they loose the required hardness- 200 to 2500C for carbon tool steel, 500 to 6000 C for high speed steel and 800 to 10000 C for cemented carbides.
6. Fracture by a process of mechanical breakage when the cutting force is very large or by developing fatigue cracks under chatter conditions.
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