Board clamped across a panel glue-up to hold the panel flat.
Although commercial plywood is habitually glued under pressures up to 200 lbs. per sq. in., the home craftsman can turn out a thoroughly satisfactory panel or veneer job if he allows one hand clamp for each 40 sq. in. of surface. If press screws are used, the area can be doubled because of the extra pressure exerted. By using bench screws instead of the rotary handled press screws, a veneer press can be easily built up from a series of simple frames like the one illustrated in Figure 6.15. To resist a pressure of about 4500 lb, the two crossbars for an r8-in. span with two screws should be of hardwood not less than 3 in. by 3/ in.; for a three-screw span of 30 in., the dimensions of the cross-bars should be increased to 3/ in. by 4 3/4 in.
For small work a satisfactory veneer press can be improvised by placing the work on a flat board on the floor, directly under a floor beam. Using a 4 X 4 or two 2 X 4's as a bearer, an automobile jack under a long a X 4 reaching to the floor beam will exert the necessary center pressure. C clamps can be used to hold down the edges, and folded newspapers under the plank "bearer" will compensate for uneveness in the floor.
In an emergency, sand bags will give satisfactory results on small jobs, or piles of bricks, or a washboiler filled with water.
In using a veneer press or any sort of pressure screws or clamps, the veneer is laid on a flat "caul" or board and covered with another caul. Cauls of /-in, plywood are now finding favor in many home workshops. The lower caul rests on solid stock bearers, or the lower crossbars of the veneer press. Directly above the bottom bearers are the bearers upon which the pressure is exerted. As indicated in Figure 6.16, these top bearers are crowned or slightly arched in the center, so that pressure will be exerted upon the center of the glued area first, forcing the glue out toward the edges. For the same reason the pressure screws, bench clamps, hand clamps, or C-clamps controlling the center bearer are screwed down first. It is wise to insert a folded newspaper between the veneer and its caul, to take up the squeezed-out glue.
Source: B W Pelton Furniture Making and Cabinet Work NY Van Nostrand 1949, pages 364-366: