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07.09.2015 admin
A weak joint will split in the glueline, either because the glue was too thick or the glue didn’t penetrate the wood correctly. Wet both surfacesIt is important to get even, continuous glue coverage on the surfaces to be bonded, so apply yellow glue to both surfaces when you can. Instead of dovetails or other right-angle joinery I simply miter the joint, and glue end grain to end grain.
From my own gluing experiences, I think that how well the joint fits before the glue is applied, along with proper glue application technique, are both likely to affect the long-term strength of the joint much more than the kind of glue that was used. In my limited experience, the 2-layer method of glue application matters mostly in situations where the wood tends to "wick" away the glue, usually only in end-grain glue-ups or rotten wood. Also, in most cases I have seen, two layers of glue on an edge glueline is just a liability; the first drier layer of glue is more likely to make bumps and high spots to hold your joint apart than it is to stabilize your glue surface. I have seen some people apply a light layer of glue; then they wait a few minutes for it to start to dry and then they apply a second heavier layer. Mark Schofield, a seasoned woodworker, details his strategies for avoiding glue squeeze-out, planning for squeeze-out, and removing glue when it does squeeze-out.
Every woodworker experiences the trauma of discovering an errant glue splotch on their project when they apply a finish. Bevel the edges of the mortise with a chisel to leave an area for any excess glue to hide in. Apply finish before glue-up-With any workpiece you always have the option of prefinishing, but it's my favorite technique for multislatted pieces or where there are a lot of complicated areas to finish. Rather than grabbing any old wet rag to remove glue, I take a more systematic approach: First, use distilled water, as tap water may contain dissolved iron salts that will cause little gray spots on tannin-rich woods like oak. On large panels, let the glue dry six to 12 hours and then use a cabinet scraper to remove the surplus glue.
PVA glues will set (dry) too fast unless you are superman & work at the speed of light. IMO & IME, that leave phenol-resorcinal, and epoxy designed for wood such as WEST system.
The last clock case I built only had fasteners in the door hinges, all other joints were only glued and they survived the planer without any problems. The wood magazines have done extensive studies in the joint strenght and the results are a little surprising. If not fresh of the rack run the to be glued side across a jointer to make it flat and also to remove any loose fibers. Tite Bond is stronger than the wood, will tollerate some moisture and is not so picky about clamping pressure. Hold the brush at a shallow angle and push firmly to squeze the glue into the fibers of the boards. The grip is from the working of the glue into the fibers when you applied the glue, there is no need to clamp any tighter than the amount of force required to remove the excess glue. My plan was to use a 7 foot length of 2x3 heavy wall rectangular steel tubing on either side of the wood stack to even out the pressure of the clamps.
The wood was milled to my order by a professional wood shop so right now the wood is very straight and the surfaces are very flat and smooth. The shims need to be slathered up with Johnson or Butcher's Paste wax , and the glue should be wiped off with a wet rag before shimming or you'll have to chisel them off later even if they are waxed. I always used my index finger as a squeegie glue spreader following right behind the bottle being squeezed by my other hand - the joint between the first and second phalange in the middle of the board edge.


A couple guys on my crews that didn't like glue on their fingers used either flexible putty knives or shims to spread with, following up after the bead.
The nice thing about wood, it grows on trees, if you mess up the job you can get some more! As for glue, the Tite Bond 3 has a longer open time, but you will be pushing your 20 minute open time with this much surface. Put the faces together and rub them against each other until the surfaces are 100% covered with a thin film of glue. Wait a few minutes for the glue to set then put the two pieces together and clamp with sufficient number of clamps (about 1 every 6 inches). Why correct clamping pressure matters Optimum clamping pressure creates strong glueline joints in several ways. The chart simplifies the scienceThe chart above shows the recommended glueline pressure for selected furniture woods. For most hardwoods, however, normal woodworking clamps can’t get close to these levels of force. Wood OrientationWhen determining whether a joint is flatsawn or quartersawn, consider the two surfaces to be glued rather than the visible surface. A simple test is to place a sharp chisel exactly on the glueline, and strike it with a mallet.
This provides instant wetting of both surfaces without relying on pressure and surface flatness to transfer the glue from one surface to the other. The time varies from species to species, with woods that have an even density across the growth rings, such as maple, requiring less time.
Allow to soak in 'till dry to the touch and then apply the glue full strength to both surfaces and clamp. So I think the take-home lesson is that it's POSSIBLE to create a strong, long-lived glue joint with hide glue, but without knowing how many glue joints were made that didn't survive through the years, we can't even say that it's PROBABLE that a joint made with hide glue will last a long time. Since hopefully you're not spending a great deal of time gluing rotten wood with yellow glue, it'll matter most to you on end-grain joints. As you know, end-grain gluelines are far weaker than edge and face joints, but sometimes you will find that full dowels, splines, shaped joints, biscuits, or various other methods are too time consuming for a quick project requiring little joint strength. Titebond I yellow glue has been around for about 55 years, II for less than 20 years, and waterproof Titebond III for 6 years.
It is amazing how many of the really old pieces don't have glue failure no matter how hard you try to break them at the glueline.
Check in every weekday for news, information, projects, and answers to questions from Fine Woodworking readers everywhere. Woodworkers have access to all sorts of modern and traditional adhesives, but for the purpose of this article I'll deal with the most common glue, polyvinyl acetate (PVA) , also known as aliphatic resin, which comes in white and yellow forms. On a prefinished surface, most glue squeeze-out can be scrubbed off with a toothbrush and water, and the surface wiped clean with a damp cloth. If you wait any longer, the beads get too hard, and you risk pulling off hunks of wood as you scrape. All you want to do is eliminate the air pockets and the full covering of glue does this along with moderate to light clamping pressure. First, it overcomes the viscous resistance of the glue and forces it into a thin, continuous film in contact with the wood, which is necessary for a strong joint. Conversely, extreme pressure can produce weaker joints, however, this is unlikely with common woodworking clamps.


The wood-failure percentage starts to diminish as clamping pressure is increased beyond a certain point, because excessive pressure begins to starve the joint of glue and also to compress the wood and reduce its ability to absorb the glue. On hardwoods, glue joints between radial or quartersawn faces require half the pressure of tangential or flatsawn face joints.
But joints clamped at the recommended levels will be quite strong enough, with the glueline being stronger than the wood itself.
You will, however, have to work fast as the open time for yellow glue can be around five minutes at a temperature of 70O F (21O C) and relative air humidity of 50%.
But in general, the glueline reaches around 80% of its ultimate strength after 60 minutes of clamping.
Indeed, even with the first layer of prep glue on an end-grain joint, I RUB small amounts of glue onto the surface, wiping off any excess to avoid obscuring the actual joint surface, leaving the joint with no glue lying on or above the wood.
As the two sections are brought together, the excess glue is pushed up the tenon, but the bevel prevents it from riding up onto the mortise and instead rolls it over the glue-free section of the tenon. As the tenon slides into the mortise, the glue is spread along the tenon by the walls of the mortise. If you miss some of the glue, perhaps because it is under a clamp, let it dry for a few hours, at which point you can practically peel the glue off the finished areas with a chisel.
Second, as the glue releases moisture, causing the wood to swell, clamping overcomes this pressure and prevents the joint from opening up.
Because modern glues are stronger than the wood fibers, a good glue joint should break in the wood, a process known as wood failure, rather than along the glueline. This peak pressure is the point just before the glueline is starved or the wood fibers are crushed.
We did a number of tests gluing with epoxy and the normal boat builders' urea-formaldehyde glues of the day. After this, joints can be released from the clamps, but the full glue strength won’t develop for about 24 hours. If I remember correctly, he feels if several boards are glued, this amount of glue gives longer working time. The first layer of glue tends to soak quickly and deeply into the joint, which can starve the glueline nearly instantly. Keep your hands clean during gluing, and wipe them immediately with water and a clean rag if you get glue on them. The further you get away from the bar with the top, the more the wood will tend to bow away from the bar. On softwoods, the reverse is true, with the quartersawn-face gluelines requiring twice the pressure of the flatsawnface gluelines.
The U-F glues were stronger with a well made joint, but the epoxies were stronger if the joint was not well made, so there was a thicker and irregular glue line.
If so, then that might be pretty significant, especially if it means that the strength of the glue line falls below the strength of the wood itself.
However, the thin first coat that quickly dries will saturate the wood fibers to prevent soaking or wicking, but still provide a bonding surface for your second coat of glue.
There is no air left between the boards and you will never separate them again once the glue has dried.




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Comments

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  4. Elya writes:
    Extra that one of the and inlays.