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JANKA WOOD HARDNESS (Use as a general guide only)
The Janka Hardness test measures the resistance of a sample of wood to denting and wear. It measures the force required to embed an 11.28mm (.444inch) steel ball into wood to half the ball's diameter.
The Janka Hardness Scale determines the hardness of a particular type of wood over another. Generally speaking, the higher the number on the scale, the harder and more scratch resistant a wood is and the more difficult it will be to saw or turn the wood. This may just mean tools must be sharpened more often to get a good cut.
The results are stated in various ways in different countries, which can lead to confusion, especially since the name of the actual unit employed is often not attached. In the United States, the measurement is in pounds-force. In Sweden it is apparently is in kilogram-force (kgf), and in Australia, Janka hardness ratings are either in newtons (N) or kilonewtons (kN). For example, Post Oak is 6.0kN and 1360 pounds-force. The hardness of wood usually varies with the direction of the grain. If testing is done on the surface of a plank, with the force exerted perpendicular to the grain, the test is said to be of “side hardness.” Side hardness of a block of wood measured in the direction of the tree's center (radially), and on a tangent to the tree's rings (tangentially), are typically very similar. These are the most commonly used ratings published. Wood shrinks most in the direction of annual growth rings (tangentially), about one-half as much across the rings (radially), and only slightly along the grain (longitudinally). Tangential dimensional change has the highest rate of change due to parallel orientation of microfibrils along the axis of the cell wall. Shrinkage in the radial direction is the second largest, while longitudinal shrinkage is negligible for most practical applications. This information is very general in its presentation. For much more detailed explanation of shrinkage and Janka Hardness you may want to read the information available at: http://www.fpl.fs.fed.us/documnts/fplrn/fpl_rn303.pdf http://www2.wwpa.org/Techguide/shrinkage/tabid/858/default.aspx
The accompanying charts on Janka Hardness and Tangential Shrinkage were first assimilated and presented to the Central Oklahoma Woodturners Club by Michael Reggio, a current member of the Northwest Oklahoma Woodturners Association. Ron Eades has expanded the wood types listed in the charts by a recent web search for additional data. There are still many blanks in the charts. If anyone has additional information to make the charts even more complete, please provide the information to the club secretary for inclusion on the charts. |