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Milling is the process of machining rough lumber to the desired dimension. Usually, the hobbyist is limited to buying dimensioned lumber (that is, lumber already milled)

As your skills increase you will realize that buying dimensioned lumber presents some risk. With dimensioned lumber, you are at the mercy of the way the lumber yards mill the lumber. Lumber thickness is measured in quarters, for example, a one-inch board is called four-quarter, a two-inch board is called eight quarter, and so on. These dimensions are taken before the lumber is milled. So, a 1 inch (four-quarter) board dresses down to 13/16 after milling. If the board is warped going into the machines it comes out smooth but is still warped. The most troublesome of the warps is “twist”, for example, if a board has, let’s say, 1/16″ of twist that board will have to be face jointed and then planed to remove the twist, you will lose a minimum of 1/16 inch on each face leaving a board thickness of about 11/16″. A larger defect results in an even thinner board. If the board starts out in the rough, then losing 1-inch results in a board about 7/8″ thick. This points out the value of being able to mill your own lumber. Almost always a rough board can be milled to the desired dimension