Recently I had my leg in plaster and I had reason to encounter a very neat piece of technology. In order to cut off the plaster without cutting the patient an oscillating saw is used.

 

I had a personal demonstration of how it cuts plaster but not skin in a way which seems almost magical and it set me off down a TRIZ thinking path. Before the saw was invented, the problem, in TRIZ terms, could be stated as a physical contradiction:

In order to cut the plaster the saw must have movement relative to the cutting surface

In order to prevent the skin being cut the saw must have no movement relative to the cutting surface

The clever thing about the saw is that it separates the physical contradiction on condition of the surface contacted. For a rigid surface such as the plaster, the saw cuts due to the fact that the surface stays put and doesn’t move with the saw, creating the required relative movement. When the saw contacts skin, the flexibility of the skin surface is exploited and the saw and skin move together, eliminating any relative movement between the saw and skin. A similar principle is used in some fabric cutting table designs to prevent damage to the table. Simple, cool and clever.�