Tool Wear

A brief overview of tool wear

Due to the interaction between the chip and the tool occurring under high thermal and mechanical stresses, the tool wears. The mechanism of tool wear is extremely complicated and depends on the material of the tool, workpiece to be machined, cutting variables including speed and chip cross-sectional area, type of coolant, tool geometry, and the condition of the machine tool and its rigidity. Four basic mechanisms operate singly or in various combinations to produce diffusion, adhesion, abrasion, and electrochemical wear.

Different mechanisms of tool wear
Different mechanisms of tool wear

Tool wear mechanisms

  • Diffusion wear:

Diffusion wear occurs when atoms in a metallic crystal lattice move from an area of high atomic concentration of that particular metallic atom to an area of low concentration. The rate of diffusion increases exponentially with temperature. During machining by cutting, the high temperature generated at the tool–workpiece and the chip–tool interfaces creates favorable conditions that promote fusion of tool material to the chip and the workpiece.

  • Adhesion wear:

Adhesion wear is caused by the fracture of welds that are formed as part of the friction mechanism between the chip and the tool. When these minute particles are fractured, small bits of the tool material are torn out and carried away to the underside of the chip or to the machined workpiece surface.

  • Abrasion wear:

Abrasion wear occurs when hard particles on the underside of the chip pass over the tool face and remove the tool material by mechanical abrasion action. These particles could be abrasive inclusions in the workpiece, fragments of the built-up edges (BUEs), or particles of tool material that have been removed by adhesion.

  • Electrochemical wear:

This type of wear occurs when ions are passed between the tool and the workpiece causing oxidation of the tool surface. It is important to note that the tool wear in metal-cutting processes is mainly due to adhesion and abrasion.

Types of tool wear

  • Flank wear

It is due to work hardening. Flank wear occurs at the tool flanks, where it contacts with the finished surface, as a result of abrasion and adhesion wear. The cutting force increases with flank wear. It affects the great extent of mechanics of cutting. The flank wear region is known as wear land and is measured by the width of wear land. If the width of wear land exceeds 0.5-0.6mm the excessive cutting forces cause tool failure.

  • Crater wear

Crater wear happens on the tool face at a short distance from cutting edge by the action of chip flow over the face at very high temperature. The crater wear is mainly due to diffusion and abrasion. They are commonly observed where the continuous chip is formed (usually in the ductile material). In the brittle material, the chip formation in the shape of a small segment, this loosely fragmented chip has low abrasive action on the face as compared to the continuous chip formation. The depth of crater measures the crater wear; the surface measuring instrument can measure it. The cutting edge may break from tool due to excessive cratering.

  • Corner wear (nose wear)

It occurs at tool nose radius. Corner wear shortens the cutting tool, cause a significant dimensional error in machining. It is considered as part of flank wear since there is no distinguishing boundary between them.

  • Flank wear is predominant in low-speed cutting.
  • The crater is predominant in high-speed cutting.

Effects of Tool Wear

Some General effects of tool wear include:

  • increased cutting forces
  • increased cutting temperatures
  • poor surface finish
  • decreased accuracy of finished part
  • May lead to tool breakage
  • Causes change in tool geometry

Causes or Reasons of tool wear

  • Increased cutting speed causes flank to wear grow rapidly.
  • Increase in feed and depth of cut can also result in larger flank wear.
  • Abrasion by hard particles in the work piece.
  • Shearing of micro welds between tool and work-material.
  • Abrasion by fragments of built-up edge, which strike against the clearance face (Flank face) of the tool.
  • Severe abrasion between the chip-tool interfaces, specially on rake face.
  • High temperature in the tool-chip interface.
  • Increase in feed results in increased force acting on tool interface, this leads to rise in temperature of tool-chip interface.
  • Increase in cutting speed results in increased chip velocity at rake face, this leads to rise in temperature at chip-tool interface and so increase in crater wear.

Remedies of tool wear

  • Use of proper lubricants, can decrease the abrasion process, and so decrease in crater wear.
  • Proper coolant for rapid heat dissipation from tool-chip interface.
  • Reduced cutting speeds and feed rates.
  • Use tougher and hot hardness materials for tools.
  • Use positive rake tool.
  • Reduce cutting speed.
  • Reduce feed and depth of cut.
  • Use hard grade of carbide if possible.
  • Prevent formation of built-up edge, using chip breakers.

Enjoyed reading, also read Lapping

Types of tool wear, reference mecholic

Effects of tool wear, reference wikipedia

Reasons and remedies of tool wear, reference yourarticlelibrary

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