I have a 6-inch jointer and a 12-1/2-inch thickness planer. I have roughsawn lumber of varying widths. I am (trying to) make panels (sides and top of a cabinet) measuring 22 x 21 inches. I am ripping to something less than six inches to plane the lumber flat on one side, jointing one edge, ripping the opposite edge on the table saw, jointing that edge, and gluing up a 1/2 panel that will fit in the thickness planer, planing the half panel to flat and to thickness, then gluing the half panels together. I cannot get a perfectly flat panel. I have attempted to check flat, straight and square at all steps, and I still end up with a bow from half panel to half panel. I glue the half panels on a flat surface and clamped to that surface as well as clamped panel to panel. The bow is in the direction of the panel to panel clamps. Could they be too tight?
Chris Marshall: I would double-check those final jointed edges of the half panels to make sure that they are indeed square (or at least complementary to one another) before gluing them up. The wider a panel gets, the easier it is to accidentally tip it a tad off the jointer fence and create an unsquare edge. But, if those edges are on the money, my next guess would be that since you are clamping the half panels to the work surface as well as to one another, you are applying too much clamping force on one side of the panel with no way to tell how those panels are responding to the clamping pressure. I would suggest elevating your panel glue-ups on two short stands and alternating the clamps above and below the workpieces to distribute the pressure evenly. Then, check your freshly glued panel for flatness with a straightedge before the glue dries. You should know in a hurry whether the clamps are too tight (bowing starting to happen) or just right.
Rob Johnstone: If I am understanding your problem correctly, it is at the point that you are gluing up the two 12-inch wide panels that you are getting the bowed result. At first blush, I agree with Chris that the wide panels are getting away from you as you joint a fresh edge. The short fence on your planer makes it challenging (but not impossible) to hold a wide panel perfectly aligned. Another option might be to rout the edge square using a handheld router and a good straightedge. Remember, there is usually more than one way to skin a woodworking cat.