wood working machines
Blades dull with use and abuse. This is particularly true of acute blades and those made of soft materials. Dulling usually occurs due to contact between the blade and a harder substance such as a ceramic, stone or a tougher metal. To a first approximation, a harder material cannot be deformed by a softer material at their interface because the stress on both materials is the same at the interface and so the softer material will yield first. One exception to this is when the highest stress is not at the contact point; this is why a steel paper clip can easily be bent even though the end of the same paper clip could scratch the skin.
