A planer knife is used in a planer to shape or smooth out wood, and getting the right knife depends on looking into several factors about the blade. Nearly all planer knives are made from tungsten steel, and tungsten will affect the durability of the planer knife. The size of the knife must be exact, because blades that are too short or too long cannot fit into the appropriate planer. Reversible knives will be useful for much longer because, when one side goes dull, the user can just reverse the blade and have another sharp edge ready for cutting. Sharpness also is important, because a sharper knife will usually keep its edge longer and will shape wood quicker.
Most planer knife blades are made from steel with a significant amount of tungsten, from 10 percent to 20 percent or more. Tungsten is a hard metal that increases the durability and strength of the blade. This means knives with higher tungsten amounts will typically last longer and will be better for cutting through hard woods. This also helps the blade keep its edge, because the metal will be slower to degrade from use.
The sizing of the planer knife is important, because getting the wrong size blade will keep the blade from fitting into the planer. Most blades' packaging will say which machines they work with, but sometimes the operator’s machine may not be listed. The operator should measure the last knife used, or the bay where the knife is held in the planer, so he or she knows the exact size required, both in length and width.
Reversible blades are not required to make a planer work, but they provide operators with a planer knife that will last twice as long. With a normal knife, once the edge is gone, the operator has to either sharpen the blade or buy a new one. If a reversible knife is used, the operator just removes the knife, flips it over and has a new blade ready for cutting.
Blades may have different sharpness levels, depending on how they are created. If the operator is working with soft woods, then a super-sharp knife is not required, though it may be useful; hard-wood workers will need the sharpest blades or the wood may not cut correctly. Sharper blades tend to keep their edge longer, as well, increasing the knife’s overall lifespan.