Gear-tooth grinding is a very contentious subject to engineers, metallurgists and heat treaters alike. It is important to understand the root cause of grind cracks.

Case-hardened steel gear teeth are ground using very small yet very soft abrasive material particles, bonded together with a binder to form the grinding wheel. The simple rule of grinding is to use a hard wheel for very soft steels and soft wheels for very hard steels. That simple rule of thumb has stood the test of time.

Taking too large a grind cut can also cause grind burn. In other words, grind feeds and speeds that are too high.

The grinding wheel cuts the steel surface simply by the tiny sharp stones that are bonded together. Once the wheel has cut the steel surface, the metal fines that are removed are washed away by the grinding coolant. Thus, the next rotation loads the wheel with metal fines. As long as the grinding wheel is rotating and removing the metallic fines, the grind coolant should be removing the ground-metal fines.

If the grinding wheel is not being correctly dressed with the diamond grindstone, the grinding-wheel face is loading up with the metallic fines, thus grinding metal with metal. This leads to surface frictional forces set up by the contact of the metal-containing wheel face with the steel surface causing it to overheat, resulting in a burn. The grind burn can usually be seen as fine brown marks on the steel surface when etched lightly with 5% nital (5% nitric acid and the balance alcohol). It is usually accompanied with very fine surface cracks.

The discoloration of the grind burn is usually a tempering effect and will most likely cause localized carbide precipitation and a change in volume, which results in the fine surface cracks. In addition, the “burn” can cause localized transformation to austenite, and because of the mass of the gear, there is the potential for a rapid heat sink (quenching) thus transforming the local austenite to martensite.

Keep the following in mind when gear grinding:

• Well-dressed grinding wheel
• The appropriate grinding wheel
• Small grinding cuts
• Slow traverse feeds
• Appropriate grinding coolant