Mating hardwood boards together into panels is the beginning of most fine woodcraft projects. This is hardly a random process and there can be a steep price to pay later for failing to do everything correctly at this point. It’s like building a beautiful house on a poor foundation: It does not matter how much care and skill goes into the house itself, if what’s holding it off the ground suffers from inferior workmanship.
There are several steps to consider in the process of edge-gluing lumber including (1) lumber selection, (2) cutting lumber to rough length, (3) ripping boards straight, (4) use of the jointer, (5) grain matching, (6) use of a biscuit joiner, (7)edge gluing, (8)using wood clamps and (9) thickness sanding. Just how you go about these steps depends on the condition of the lumber, the capacity of your machinery and the final size of the glue-up.
If at all possible, try to have all boards in the glue-up out of the same tree. If that is not possible, select lumber that is of similar color and grain pattern. The ideal glue-up looks like one, extremely wide board with the glue joints barely visible to the naked eye. Since this only an ideal, I try to get as close to it as possible.
Another, less-important goal would be to have all boards in the glue-up of the same approximate width. I am not suggesting ripping the wider boards down to match the narrowest board as this would be a terrible waste of expensive lumber. I do suggest, however, ripping extremely wide boards in two to minimize the possibility of cupping due to changes in humidity after delivery.
Excellent final results in your woodcraft will be based in part on how much attention you give to grain selection and matching. Straight or ribbon grain makes the best homogeneous final appearance while wavy or swirly grain makes for an interesting but more difficult grain matching process. Swirly grain will require orientation of the individual boards to minimize the number of places that the grain line suddenly stops at the glue line rather than appearing to continue into another swirl in the adjacent board. This orientation is highly subjective yet very important.
CROSS-CUTTING TO ROUGH LENGTH
I always rough-cut my lumber into lengths an inch longer than the length of the final product. This allows the entire glue-up to be neatly trimmed to size after the glue is dry. It also makes the ripping and jointing process a lot easier as I will explain below. The same is true for the width of the glue up: Make sure it is at least an inch wider than the final product will be after trimming.
Kiln or air-dried lumber often decides to bow into a curve as it dries and this must be corrected before a glue-up can be accomplished. If my finished glue-up is only 3 feet long and it is coming out of a 14-foot bowed or curved board, it will be far easier and economical to get the curve and/or bow out of the 3-foot pieces than it would to remove the curve from the entire 14-foot board before cross cutting. This is one reason that you should always do your rough cross-cutting before ripping and jointing. Another reason is that a 14-foot, 2 thick x 12 wide board is pretty difficult to control on a jointer or table saw.
If there is a bow in one or more of your rough-cut pieces, those pieces should first have the curved edges ripped off on the table saw. The concave side of the board should always be towards the table saw fence. Measure from the fence out to the outside of the end of the board that is nearest the fence and set the fence to cut this width. Once you have trimmed off the convex side of the board, flip it over side-to-side and find the point where the outer edge of the board is closest to the fence (somewhere near the middle) and rip the board to that width. When all boards have been ripped straight, take them to the jointer.
The process at the jointer should now be fairly easy in that the boards have been ripped straight. Take shallow depth jointer cuts to minimize the possibility of tear-out. In loose-grained lumber with a lot of swirls on the face side, tear-out on the jointer is sometimes unavoidable. If this happens, try running the board over the jointer head in the opposite direction. If the jointer tear-outs persist, you will have no other option than to rip the jointer tear-outs away on the table saw. You will then have a sawn edge in your glue-up. If you have a clean-cutting table saw blade like a recently sharpened Forrest Woodworker II, this should not be much of a problem, especially if you plan on using a biscuit joiner to secure your glue-up. You probably wont be able to tell which glue lines are jointed and which are ripped in the final product.
FINAL GRAIN MATCHING
Lay out all the boards on your work bench and arrange them for best appearance. Obviously, if one side of the final product will show more than the other in a piece of furniture, then you will generally want to have the best-looking sides all on that side of the glue-up. Examples of this would be table tops and cabinet doors. You also must orient the boards so that the glue-lines are not accentuated, as discussed in the paragraph on lumber selection above. That may require breaking the rule that the best sides of all board be on the same side of the glue-up.
Whenever possible, make sure that you biscuit-join your glue-ups. I say, whenever possible because you will not be able to use a biscuit joiner on very thin lumber. On the other hand, very thin lumber (3/8, for instance) does not usually have enough strength to pop open a joint. So, with very thin lumber, you will simply be using glue without biscuits. With regard to lumber ¾ or thicker, I have seen a number of table tops, cabinet doors and cabinet casings open up along a glue line after delivery. At this point, repairs are difficult or impossible so the extra step of biscuit joining is well worth the minor time and expense. Look on it as major headache insurance! If you dont yet own a biscuit joiner, there are a number of great machines out there including Porter Cable, Lamello and Freud. There are also two good alternatives to using a biscuit jointer: Those are the Festool Domino floating tenon joiner and the Freud Doweling Joiner. Different methods, same result.
When you have your boards laid out the way you want them in the glue-up, make sure all the ends are flush and the edge joints are touching. Double-check to make sure the glue-up will be about an inch wider than the final product after trimming. With a builders square or a straightedge mark a pencil line in 4 in from each end of the rough glue-up across the grain, crossing all glue lines but not continuing over the side edges of the glue-up. Make a similar pencil line across the grain at the mid-point of the boards. Make additional pencil lines half-way between the other pencil lines until all pencil lines are about 6 apart.
Mark the boards on one end A,B,C or 1,2,3, etc. so that you can put them back together in the same order and orientation when it is time to glue them up. Put the boards aside and nail, screw or clamp a stop board (scrap) to the bench top, left to right in front of you and about a foot in from the edge of the bench. As you are applying pressure with the biscuit jointer, while making mortises for the biscuits, this stop board will keep the board you are mortising from moving away from you. Make a mortise wherever a pencil line touches a board edge on every board.
GLUE-UP AND CLAMPING
There are two ways to clamp up a glue-up: horizontally on the bench top and vertically with the first board clamped mortised-edge-up in a woodworking vise on the end or side of the bench. In the case of horizontal glue up, place pipe or bar wood clamps about 2 feet apart on the bench top with the wood clamp handles hanging slightly over the edge of the bench. Pre-adjust the wood clamps to an inch larger opening than they will be when tightened. Place the first board on edge on top of and across the wood clamps with the mortises facing up. Do the same with all the boards, in order. Make sure you have sufficient biscuits for the job ready. A small dispensing glue bottle with sufficient glue for the job should be within easy reach. The type of glue is important: If the glue dries too quickly you will have big problems and if the glue dries too slowly, you will be losing valuable production time. I like to use Franklin Titebond Glue or Franklin Titebond II for outdoor applications. These are aliphatic resin type glues that can be easily cleaned up with water. Ether formula gives a very strong joint and has a reasonable, 45 minute clamping time. Both of these glues are widely available in hardware stores, home improvement centers and woodworking stores.
Run about a 1/8-thick glue line down the center of the edge of the first board, making sure that the glue drops into every biscuit mortise along the way. Then apply short glue lines on both sides of every mortise. This should result in sufficient glue so that it appears squeezed out of both sides of every glue joint after clamping. Insert a biscuit into each mortise. With 2 lumber you may need an extra glue line for the full length of the joint. There is no such thing as too much glue because you can wipe up the excess with a wet rag. There is, however such a thing as not enough glue and you will recognize that condition when you see that glue is not being squeezed out of the full length of both sides of the glue joint. That is called starving the joint and starved joints often open up later.
Lay down the first board with the letter or number up and the mortised edge away from you. Apply glue in the same manner to each succeeding board wherever there are mortises and place biscuits in the far edge of each board, except, of course the last board.
The board ends should be flush and the left clamp should be about 6 in from the end. The right clamp should be about 1-foot six inches in from the right end. This is because you will be placing alternately spaced clamps on the top side of the glue-up so that there is a clamp (top or bottom) about every foot. The top, right clamp will be in about 6 from the right end.
Once you have all of this in place, start tightening the clamp handles. Clamp all the bottom clamps finger tight, then the top clamps finger tight. Then, go down the row of clamps tightening them fully, bottom, top, bottom, top, etc. With a wet rag, wipe off most of the excess glue. Turn over the entire glue-up and wipe the other side. Look at your watch or clock and add 45 minutes to the time. This will be the minimum clamping time, any time after which you may remove the glue-up from the clamps. Mark this time on the glue-up with a felt pen which will sand off later. If you have multiple glue-ups, you can stand this glue-up against a wall to get it out of the way while it dries.
If you have been paying attention to the above, then you can figure out how to do a vertical glue-up in a vise which is suitable for smaller glue-ups and is easier to manage. The difference is that when it comes time to apply the glue, you will clamp the first board at its center in the vise with the mortises facing up. Apply the glue and biscuits. Apply glue to the mating edge of the second board and place it in correct orientation on top of the first board, and so on. Place the first clamp 6 in from the end, in front, the second clamp a foot away from the first clamp, in back and so on.
Once your glue-up is out of the clamps, it is ready to be thickness sanded either in a drum sander or wide-belt sander. If you don’t have either of these machines, don’t worry. Most professional furniture-manufacturing shops in your area will be happy to thickness sand your glue-ups for an hourly rate. You might want to consider buying your own drum sander or wide-belt sander, if you can justify the expense.
It is best to know the maximum width capacity of the sanding machine you will be using: 48-wide glue-ups will not pass through a 36-wide sander. If you know that you will have this limitation in advance, simply make two, 24 glue-ups and glue those together with biscuits after the thickness sanding is complete. The glue line wont be perfectly even and so it will have to be sanded true with a random orbit sander. Your glue-up should be sanded to at least 150 grit. 220 grit is even better. Trim the glue-up on the table saw to its final dimensions (rip first, crosscut second), rout the edges, if appropriate, and then random orbit sand the final piece to 220 or 320 grit before finishing.