After ripping, the slats are still slightly curved and the surfaces are rough sawn. Planing removes this curvature and levels the surfaces. In addition, an exactly rectangular cross-section is created.
Sometimes a gluing profile is also milled on the long sides: the so-called crown joint. This tooth-like gluing profile makes the joint more resilient, as the glued area is larger than when smooth surfaces are butted together. A crown joint is typical for traditional, handcrafted production. Here the wood is usually intended from the outset to be used for a special application. This means that the lamellas are not stored for a longer period of time, during which another slight swelling or shrinkage is possible, which would jeopardise the accuracy of fit of the crown joint.
The following machine types are suitable for planing and milling the crown joint:
The four-sided planing machine smoothes all four long sides of the lamella in one pass.
The four-side moulder also planes all four long sides. Unlike the four-sided planing machine, a moulder can also be equipped with profile knives to produce any desired profile, such as a crown joint.
The single-spindle moulder can only machine one side of the workpiece per pass, whereby both longitudinal and transverse machining is possible. In addition to straight parts, profile milling can also be carried out on curved parts using a thrust ring. In traditional staircase construction, a bench milling machine is used to mill a crown joint after planing the lamellas.
