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Efficient Glue-Ups
22nd in a series of articles by Barb Siddiqui
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Efficient glue-ups are something better learned by experience than by reading about them, but some basics are worth understanding before you begin. First off, preparation of the surfaces being joined is a major factor in the success of any glue-up, and there is no glue made than can compensate for a sloppy fit in the joinery. Tenons must fit into mortises with hand pressure only, and not 'bottom out,' but have enough depth for a slight space at the bottom where excess glue can flow. The tenon cheeks and shoulders must be square, with no tiny ridges left from poor cutting.
Surface preparation also includes squaring up surfaces to be joined. When boards are to be edge glued, hold the joint in front of a strong light and look for any gaps before clamping. Check that a new table saw blade has not left a shiny, burnished edge to the cuts, as that condition may interfere with glue absorption and bonding properties. Be sure gluing surfaces are free of any sanding dust, finish or contaminants.
If joints are too tight or out of line, don't count on clamp pressure to straighten them. After the clamps are removed, the wood will have its way, and the project will show it. If joints are loose, shim them by gluing on a thin veneer piece to the face of a tenon, or plugging a wide dowel hole and re-drilling it to the correct size and vertical angle. Remember, clamping pressure will not correct sloppy joinery. Fix it before going on to assembly.
Glue-squeeze out seems to confuse many beginners, but it is really quite simple: "just enough" means you can see it beading up uniformly along the full line of the joint, on both sides. If, after medium pressure with clamps, no glue is visible at all, you probably have a glue-starved joint that will be structurally unsound. If the clamps are applied and glue drips and runs over the wood surfaces, you used too much and it will be a problem to remove before applying most finishes.
To supply "just enough" glue, first read the directions available on your adhesive. Some require applications to both pieces, some to only one. Some recommend wetting a surface before applying glue. You will also need to know the available open time, closed joint time and cure time for the product you are using.
Apply any glue product evenly to the surfaces. Large globs don't even out as well as you might expect under clamping pressure. Use a flexible plastic credit card, cut with pinking shears on one edge, to spread glue over wide surfaces. Small throwaway acid brushes or Popsicle© sticks can assist in spreading glue on a board's edge. Move the glue all the way to the ends and into any corners necessary. Every bit of the surface to be glued should receive a light coating of adhesive. Any squeeze-out can be allowed to dry to a rubbery consistency, and then flicked off wood surfaces with a chisel or card scraper.
It is generally wise to think of glue-ups completed in small stages of assembly such as vertical end frames, then legs and aprons, or frames and panels, instead of whole units of assembly at one time. This allows plenty of open and clamping time and, better than that, time to think before moving on to the next unit of assembly.
Before the final glue-up, it is best to do a dry fit with all the clamps in place. This will allow you to check each joint and be certain the assembly is truly square. It also means that backing the clamps off a few turns to disassemble the work will leave all the clamps adjusted to the needed length and resting near their assigned places awaiting the glue.
Most glue makes joints quite slippery, especially if you've been a little generous in applying it. Then, when pressure is applied, parts can slide out of alignment quickly. On a wide panel, you can use end battens of thick scrap lumber to hold multiple boards in place. Simply cut a long groove the width of the edge thickness being glued, in two scrap pieces. Line them with waxed paper and lightly clamp them like temporary breadboard ends over the joined panel pieces. These will keep things aligned while you begin clamping the panel at the center, working outward toward each end. It also helps to alternate the clamps, one above and one below the work, and remember that excessive clamp pressure does more harm than good.
How you align the clamp pads on the boards' edges matters, too. During the dry run, experiment with how high to set the clamp jaws and see where they apply force to the workpiece with pressure. It is important to align the pressure in a direct line from front to rear jaw, as offsetting them will skew the clamp pressure and throw the assembly out of line. Scrap blocks of wood laid between the clamp jaws and the workpiece can help immensely in distributing clamp pressure more evenly.
Most woodworkers approach a large glue-up with trepidation, but it doesn't have to be a nightmare experience. Prepare the wood and the joinery with care, always do a dry run clamp-up to get ahead of unforeseen problems, and just take it a step at a time.