KnifeSol metal spacers are in 9CrSi material and have similar tolerances (flatness and parallelism) with circular knives. We believe that spacers should have the same core hardness, as circular knives, especially to maintain the same features on the shaft. What’s more, the tools (knives, spacers, rubber bonded steel spacers) are mounted and fixed to the shaft with a very high tightening load. Our spacers must therefore resist these loads. But the most obvious reason is that spacers must have a long service life, so we also consider the possibility of improper handling by workers that could easily damage the spacer, if they were not hardened at core.