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A spiral upcut bit lifts all of the swarf and dust created out of the cut toward the router base, but in the process is also prone to lift and chip the upper surface of the material. Depending on what you want best protected, and whether you have both faces of the ply visible, you might want to work from the back of the sheet to get the best finish on the face. A spiral down works opposite, pushes swarf and dust down into to cut, leaves a good surface at the router base side, but can chip out on the underside as the spiral is pushing down. The absolute best solution is a compression bit, which is upcut at the tip and downcut for the rest of the length. Used with a sacrificial board under the work material, it can give perfect faces on both sides, but generally need to do the cut in a maximum of 2 passes, so needs a powerful machine. The 2 cut issue arises because the initial depth of cut has to place the point where the bit changes from upcut to downcut at least 1 ply layer below the surface so that the upper layer is being cut with the downcut section of the bit. If cut with the upcut section, the top layer is as prone to chipout etc as it would be with a spiral upcut bit.
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