Failure of Cutting Tools

Whenever the tool is not performing the machining operation satisfactorily, there is a cutting tool’s failure.

The following drawbacks are observed if the tool failure occurs.

  1. The tool ceases to produce the workpiece according to the required dimensions.
  2. The tool gets overheated.
  3. Excessive surface roughness is observed.
  4. Forces and power consumption increase.
  5. Sometimes the burnishing brand will appear on the workpiece.
  6. Produces vibrations during machining.

The parameters used for measurement of satisfactoriness of machining is

1. Surface finish produced on the workpiece

  • At the beginning of the machining operation, it is found that an excellent and mirror-like finish is produced on the workpiece.
  • Sometimes, when the lines are produced on the machined surface, it is assumed that the surface finish is reduced and the tool is failed.

2. Forces induce during machining:

  • By connecting the dynamometer to the work table, the forces in machining will be measured online.
  • Whenever the increase in forces occurs, it is assumed that the tool has failed.

3. Power consumption:

  • By connecting the Ammeter to the input of the electrical motor, the current drawn by the motor will be measured online.
  • Whenever the increase in current drawn by the motor is more significant, it is assumed that the tool has been failed.

4. Temperature of chip:

  • The chip’s temperature can be measured by observing the color of the chip formed.
  • During normal satisfactory machining conditions, the color of the chip is a light Blue or metallic color.
  • When the machining is done with a failure tool, because of higher heat generation, the color of the chip is turned to black or burnt color.
  • From the above, whenever the color of the chip is observed to be black or burnt color, the tool is assumed to be failed.
  • During machining of high carbon workpieces, it is assumed that the tool is failed whenever white-colored gases are observed.

Modes of tools failure:

  1. Failure through plastic deformation
  2. Failure through mechanical breakage
  3. Failure through gradual mechanical wear

1. Plastic Deformation Failure:

Whenever the tip of the tool is experiencing a temperature more significant than the hot hardness temperature of the tool material, it loses its hardness considerably. The tip of the tool is deforming plastically, called plastic deformation failure of the tool.
Reasons for plastic deformation failure:

  1. Wrong selection of tool material.
  2. Wrong selection of process parameters.

2. Mechanical Breakage Failure:

A cutting tool gets broken due to the following factors:

  1. Large cutting force.
  2. By developing fatigue cracks under chatter conditions.
  3. Soft tool materials.
  4. High temperature and high stress.

In this also failure duration is repeatable.
Therefore it is also considered an abnormal failure of the tool.

3. Mechanical Gradual wear failure:

During machining operation, the tool is wearing out slowly and whenever the wear becomes considerable, it can’t perform the machining satisfactory called gradual wear failure.
The gradual wear takes place due to
1. Crater wear and
2. Flank wear
Crater wear:

  • The central tendency for wear is due to the abrasion between the chip and the face of the tool, a short distance from the cutting edge.
  • The crater  (a shallow spherical depression present on the surface) is formed on the tool’s surface by the nation of chip particles flowing over it because of the very high temperature.
  • When Cratering becomes excessive, the cutting edge may break from the tool.
  • Cratering is commonly observed while machining ductile materials, which produce continuous chips.

Reasons for crater wear:

  • Presence of friction between chip-tool interference.
  • The abrasive action of microchips present at chip-tool interference.
  • The abrasive action of fragments of built-up edge present at chip-tool interface diffusion wear.

Flank wear:
Wear taking place on the flank face of the tool is called flank wear.

Reasons for flank wear:

  1. Presence of friction at tool interface.
  2. Abrasion action of microchips present at tool work interface.
  3. Diffusion wear.
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