Edge joining is the process of bringing two or more boards together edge-to-edge, and is done in order to create a wider board. There are many reasons a woodworker might edge join boards. These include:
Making a table top
Making wider furniture carcases
Making a panel with book-matched pieces
In the first examples, the wood available is not wide enough to satisfy the design requirements. In the third example, book-matching is a technique based on the process of re-sawing a board and then folding out the resawn faces (like pages of a book) to create a mirror image.
The first thing to consider when planning to edge join two non-book-matched boards together is how the boards look next to each other. This is a two stage process. First, look at the boards in their rough sawn state before milling them. Look at the rough sawn boards to see potential matches of grain pattern. Use chalk to mark the significant patterns in the boards, such as cathedrals and knots. Do the grain patterns of the boards have anything interesting I want to showcase? What direction does the grain run? How do the cathedrals (arched shape of the grain line) run in the boards?
After jointing and planing the boards, lay the boards side-by-side and carefully examine them. Look for patterns that look nice together. What happens when you offset a board - move one or more of the boards to the right or left? Does the overall flow of grain patterns look more integrated, like they belong together? What happens when you flip a board end-for-end or onto the opposite face? Does the edge from one board seem to flow into the edge of the next board? Should you cut one of the boards on an angle (band saw and re-joint) to get the grains to better match up? This is part of the fun and enjoyment of working with wood. Every piece is different. Each piece is a discovery.
Another consideration, but one that is secondary to the flow of the grain pattern on the faces of the boards is the grain pattern on the ends of the board. Wood wants to warp away from the center of the tree (in this case, called cup). If all the boards are arranged with the end grain in the same direction, you would get significantly more cupping than if you alternated the end grain (one board with the interior of the tree up and the next with the exterior up).
There are design techniques that you can use to overcome the warping problem (even if you find the grain patterns more appealing without alternating the end grain patterns). The use of breadboard ends is one technique for a tabletop. Here you create a tongue along each end of the glued up panel and insert it into a mortise in the edge of a board that is glued onto each end. Another technique is the method you use to fasten the panel to your piece. Buttons and figure-eight fasteners are two methods that work really well. A figure-eight fastener is a piece of metal with two holes; one that screws into the panel and the second that screws into the frame.
This problem can also occur with quartersawn wood. Here you want the edges to match up. The edge of a board that is toward the interior of the tree should mate to another edge from the interior. Edges toward the exterior of the tree should be matched. The reason for this is that the expansion at the interior edge will be different from that edge toward the exterior of the tree. If an exterior edge were glued up with an interior edge, the difference in expansion and contraction could create a noticeable edge line that could be felt with the hand.
Edge Gluing Preparation
Once you have decided on your layout of the boards, you can now prepare for glue-up. This includes marking up the boards to maintain their order and alignment during jointing of the edges on a jointer, and glue-up.
The first thing to do once you have determined the order of the boards is to mark them. With the boards arranged as you want them, draw an upside down "V" across the boards in either a heavy pencil or chalk. The inverted "V" keeps the boards in order and aligned (the boards may not be the same length). The sides of the upside down "V" get farther part with each successive board. So, if they get mixed-up, only the proper order will yield the boards with the lines matched-up. Not only will they be in the proper order, but if the lines touch at each board edge, the alignment of the grain patterns will be as you earlier arranged.
The next thing to do is to mark the boards with an "I" or an "O", one on each side of each joint line, where the boards meet. The reason for this is to denote whether the face of the board should be turned "I" inside towards the fence of the jointer or "O" outside away from the fence of the jointer. This final pass over the jointer will mate the edges of each board flush at the joint, regardless of whether the jointer fence is set at 90 degrees or not.
Now, with the boards marked and the edges jointed, we are ready to glue up the boards. The secret to success for glue up is the use of cauls. Cauls are clamping boards that stretch across the width of the boards to be glued up. They hold the boards flat as the glue dries. Cauls have a slight camber or arc, and are placed so the camber is to the middle of the width, both above and below the boards. Make sure you cover the cambered side with plastic packing tape to keep the cauls from sticking to the panels during glue up. Clamps are applied to the outside of the cauls. When pressure is applied the cauls flatten out, giving even pressure across the width of the glued up panel. This helps keep the panel joints flush as the glue dries. You will also need clamps across the panel width in order to apply pressure to the edge joints.
Dry Run and Glue-Up
Before gluing up, make a dry run and keep track of how long it takes. Lay out the bottom cauls. Place the panel boards on the cauls, pretending to apply the glue. Put the top cauls on and clamp the cauls with light pressure. Having three pairs of cauls which is enough for most purposes. Start clamping the center caul, then move to the outside ones.
Once the cauls are clamped lightly, I then apply clamps across the width of the panel. Again, apply light pressure. Alternate tightening the caul clamps a little more, and then the clamps across the panel. This keeps the joints even and the panel flat.
So, how long did it take to do the glue up? Yellow woodworker's glue (aliphatic resin or polyvinyl acetate) starts to set up in five to ten minutes. If you didn't get done in that time, then practice until you can. The alternate is to get a slower setting glue, like Titebond Extend which will give you ten to twenty minutes. Polyurethane glue, such as Gorilla Glue, is another choice which can give you up to forty minutes of assembly time.
Once you've practiced and chosen the glue, we are ready to glue up. Again lay out the cauls. Apply the glue. Start from the joint on one side of the panel and work towards the other side. I sometimes use a brush to spread the glue or my finger works faster. Wipe off with a damp rag any glue that runs over the side. Once you finish applying the glue to all the joints, go back to the ones you've finished and check to see if the glue has absorbed into the wood. You may have to apply more glue. Now complete the clamp up like the dry run(s). You should have some squeeze-out of the glue along all the joints.
Twenty to forty minutes after assembly, scrape off as much of the squeezed-out glue as you can get to. If you wait until after the glue has fully cured, it will be a lot more difficult to remove. Then leave the panel clamped up for about 24 hours while the glue cures. After removing the clamps and cauls you can scrape off the glue and cut it to size.
The end result should be a flat panel with a grain pattern to be truly admired.