Tool wear and breakage

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Tool wear and breakage
Tool wear is defined as a gradual loss of tool material at workpiece material and tool contact zones.
Tool Wear
There is several wear mechanism that may occur simultaneously, or one of them may dominate the process. They can be listened as:
* Abrasion
* Adhesion
* Diffusion
* Fatigue
* Chemical wear
Abrasion Wear
Abrasion occurs when a hardermaterial shears away small particles from the softer material. However, softer work material also removes small particles from the tool material although at a smaller rate. The hard tool particles are caught between the hard tool and soft work material and this causes additional abrasion wear. Tool and work materials contain carbides, oxides and nitrides with hard microstructures; this cause abrasionwear during machining.
Adhesion Wear
When there is a relative motion between the two bodies that are under the normal load, fragments of soften work material adhere to the harder tool. The adhered material is unstable, and it separates from the cutting tool and tears small fragments of the tool material. The typical example in metal cutting is a build up edge, which usually occurs at low cuttingspeeds when part of the chip material welds to the cutting edge.
Depending on the size and stability of the built-up edge, either the forces decrease because the effective rake angle becomes positive or the lumped (aglomerado) built-up edge dulls the tool and increases the forces. An unstable built-up edge occurs close to the cutting edge at low speeds where the tool-chip interface temperature islow (i.e., less than austenitic temperature.)
When the cutting speed is increased, the magnitude and length of the built-up edge becomes smaller and localizes close to the cutting edge. Predicting the tool cheap interface temperature is therefore important in identifying cutting speeds where the built-up edge is minimum.

Diffusion Wear
When the temperatures of the tool and work materialsincrease at the contact zones, the atoms in the two materials become restive and migrate to the opposite material where the concentration of the same atom is less. Typically, in a tool material such as tungsten carbide (WC), where carbide (C) provides the hardness while cobalt (Co) binds the WC grains, carbon diffuses to the moving chips, which have less concentration of the same atoms. Progressivediffusion of tool materials into the chip gradually leads to a weakened cutting edge and eventual chipping or breakage of the tool.
Oxidation Wear
The atoms in the cutting tool and/or work material form new molecules at the contact boundary where the area is exposed to the air (i.e. oxygen). Tungsten and cobalt in the cutting tool are oxidized close to the work surface-cutting tool flank, whichleads to a notch wear on the cutting tool.
Depending on the tool work materials, tool geometry, and cutting conditions, one wear mechanism may be dominant, but all of them may occur simultaneously but at different rates.
Tool Wear Zones
Crater wear
Crater wear occurs at the tool-chip contact area where the tool is subject to friction force of the moving chip under heavy loads and hightemperatures. At higher speeds (i.e. turning P20 mold steel at v = 250 m/min cutting speed), the temperature on the rake face of a carbide tool may reach over 1000°C. At these higher temperatures, the atoms in the tool continuously diffuse to the moving chip. As the crater wear approaches the cutting edge, it weakens the wedge and causes chipping of the tool. Crater wear can be minimized by selecting a toolmaterial that has the least affinity to the workpiece material in terms of diffusion. The use of lubricants also reduces the wear.
The chemical affinity between the workpiece material (mostly iron Fe) and the tool material (mostly tungsten-carbide WC) can be reduced using Al2O3, TiN, and TiC coatings on the tools. The use of coated tools may significantly increase productivity by reducing tool...
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