Machinists-turned-woodworkers occasionally ask me why the jointer knives must be set slightly higher than the outfeed table. In their minds it makes more sense to set the knives dead even with the table to get a straight, true cut.

The answer is that unlike metal, wood is compressed when it is cut.  At a microscopic level, cutting wood is a "stress-fracture process." The cutting edge compresses the wood fibers until the stress causes them to fracture. When the cutting edge moves on, the fibers decompress. This slight movement is called springback, and the amount of springback for jointing and planing operations is usually 0.001 inch to 0.003 inch.  You set the jointer knives slightly higher than the outfeed table to compensate for this springback.

Springback is also the reason you can cut a perfectly flat surface with a properly tuned hand plane. As long as the distance which the plane iron protrudes from the sole is equal to the springback, the planed surface will be true.

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