The grind crack may not be so obvious to see visually because it will usually occur at right angles to the direction of the grinding pass and at right angles to the surface of the gear tooth. The depth of the crack is not known unless a cross-sectional examination of the gear tooth is made.  Because cross sectioning a gear results in scrapping a part, this is not a practical solution. Also, we don’t know whether the sectioned gear represents the entire load. That gear may or may not be representative of the results of an entire batch.

The depth of the grind burn may only be microns deep from the ground surface where the grind burn is, but the crack can be quite deep into the formed case from the surface. A simple saying is, “From little acorns doth great oak trees grow.”  This means that if a small crack is present only a few microns deep, that crack can propagate into the case to form what might be a catastrophic crack that could lead to a premature failure of that particular tooth or multiples of teeth.

To establish if grind cracks are present, a simple test might be a magnetic particle test.

The writer does not believe that there is any relationship between the depth of the grind burn and the depth of the potential crack that might form from the surface of it. It is most likely that the crack begins to form when the grinding wheel is in direct contact with the carburized and hardened surface when the surface is being “burned” (overheated because of the grinding conditions).

It is also critical (in the writer’s opinion) that the amount of retained austenite is minimized because small cracks might form between the martensite and the austenite.

Please do not interpret from the above that cracks will always form as a result of grind burn. That will be dependent on:
  • The existing metallurgical conditions (retained austenite, etc.)
  • Surface hardness
  • Depth of grind cut
  • Type of grind wheel (hardness of the grind wheel)
  • The grind-face conditions of the grind wheel (Is the grinding wheel regularly dressed?)
  • The absence or lack of grinding fluid to act as both a coolant and a lubricant to also control the developing frictional heat of the grinding conditions and to wash away ground metal fines from the grinding action
  • Traverse speed of the component (or wheel speed traverse)
The next blog (part 3) will continue with the effects of retained austenite and residual cracks that can be caused by gear-face grinding.