Abstract: A method/system of mechanical holdowns allows a substrate sheet in a CBAM (composite-based additive manufacturing technology) process to lie flat during printing. The invention includes a process of mechanically clamping sheets to be printed by a print head to a flat platen using a set of barrel cam driven clamping fingers. The finger supports are attached to the platen and the fingers can be raised and lowered with respect to the platen. Each finger can rotate while swinging downward toward a sheet at the edge of the platen. To clamp the sheet, the fingers are rotated to the perpendicular position and swung lower down to pinch the sheet to the platen. The process can include additional steps that release some, but not all, of the fingers to allow the sheet to relax before re-clamping them.