Build a Foamcore CNC Machine - Instructables
Technique 1: Marking a Square. 1) Mark along the lower edge of the foamcore at 14-1/2". 2) Similarly mark along the left edge of the foamcore at 14-1/2". 3) Align the ruler perpendicular to the left edge of the board at the vertical mark which you just created. Now mark 14-12" out into the center of the board.