Gluing Methods and Strength of Dado Joints
From the original questioner:
Unfortunately it doesn't squeeze itself where it needs to be. It spreads along the bottom, but doesn't get up the sides.
From contributor F:
I use your brush method (acid flux brushes). It is the only way I know that you can insure the glue is spread evenly where it needs to be without excessive squeeze-out.
From contributor K:
It's the wood-to-wood contact on the largest edge where the glue is important. If you are using 3/4" material prefinished, unless you are also rabbeting the ends of the panels to go into the dado, you are only talking the glue going up on the sides along 3/16"-1/4" of material. Not as much sheer strength there versus the 3/4" edge. If the glue where the panel meets on the 3/4" dado fails, I doubt very much the 3/16"-1/4" on the sides will go a long way in saving it if the 3/4" edge fails.
If you are using fasteners (which act as clamps as the glue dries) in conjunction with the dado, even more reason not to worry about it or waste time/material/labor on it brushing it in and cleaning it up after (which means more money in your families pocket).
Do a test piece and you'll see what we mean. If you wiggle it back and forth, the small section of glue on the sides would have to give before the large section of the dado does, right? Unless you are trying to account for an unusual amount of movement/stress on the joint, it really isn't necessary.
Think of it this way - the ply you are using is not glued on the sides, just on the largest surface and there are no fasteners involved and yet it stays together. Not to mention that when your piece is assembled it is also strengthened and held together by it being part of a whole unit. Just curious - what is the nature of the boxes you are making?
From contributor V:
Try the Lamello LK glue system from Colonial Saw. They have nozzles to fit all standard dados and the glue gets spread on the sides and bottom.
From contributor A:
The clamped glued butt joint is strong enough for 99% of cabinetry. The melamine boys mostly screw them together with no glue. We use dado/rabbet construction for accuracy/ease of assemble/and clean glue surface. Most of our construction is prefinished ply. No glue will stick to it. The dado/rabbet cuts it off to give a good bondable surface. We use a combination of staples, screws, and clamps to hold the panels together until the glue dries.
Most of our cabinets are tacked together with 18 gauge 1 1/2" staples while the glue dries. As others have noted the edges of the dado are not necessary for strength. If you are really adamant about gluing every surface you simply need to add lots of glue and let it squeeze out, wipe it with a damp cloth.
From contributor F:
One of the main reasons for using dado construction is that it provides a larger glue surface. Mechanical fasteners (screws, staples, KD) provide little strength to a joint. They are just used to hold things in place until the glue sets. Quality construction is never a waste of time.
From contributor K:
I really want to explore this with, as I, and I am sure others, respect your opinion. "Not if you want to provide a good strong joint. One of the main reasons for using dado construction is that it provides a larger glue surface."
The larger glue surface is where the end of the panel meets the dado. A proper dado (one that is not loose) already holds the left and right of 3/16"-1/4" sides. If you doubt that the glued sides do not provide that much in the way of strength versus the flat of the dado, try gluing one with just the sides and assemble (using fasteners as the clamps), and one with the flat of the dado. Once dry, remove the fasteners, and try to remove the panel from the dado. One knock with a mallet will unseat the joint with just the sides that are glued. It will take a lot more effort with the one with the flat glued and will most likely not be possible without tear-out. "Mechanical fasteners (screws, staples, KD) provide little strength to a joint. They are just used to hold things in place until the glue sets."
Agreed about setting the glue. However, using dado and rabbet construction, you could easily assemble a cabinet using nothing but screws and have a completely reliable product. Don't forget the totality of the structure. Think of the framing in your house. It has no glue, is made up mostly of butt-joints using nails and yet supplies the strength for all the materials that hang on it including cabinetry. If you doubt that, assemble a face-frame cabinet, and put the frame on the cabinet with pocket-screws only. Then try and get it off. It's not going anywhere. It can't - it's like a clamp wrapped around the cabinet. That said, I still put glue in the dado on the face-frame. Go figure.
Quality construction is never a waste of time. Thinking that adding glue to the sides of a dado, versus glue on the bottom only of the dado somehow makes it more quality construction (especially if the dado is a proper joint and it gets squeezed out anyway on the sides unlike the bottom of the dado) is in my opinion confusing activity with accomplishment and a waste of time, material and effort for very little return.
From contributor L:
"Mechanical fasteners (screws, staples, KD) provide little strength to a joint." I'll remember that next time we are screwing our cases together with Confirmats to meet AWI premium standards while your dado joint will not meet the standards. I don't see anything wrong with using a dado joint but the main structural ability of that joint is in shear not tension. Once installed there is little tension on most cabinet joints. I'd wager that because many face frame cabinet builders use plywood and dados that the fit of the 3/4"+- plywood isn't real good in the dado anyway. The shear support is still there even with the gaps to the 3/16" sides. Your dado is adequate without the bit of glue surface on the sides. You could add cast iron brackets to the joint but it would not provide any more value to the customer.
From contributor F:
This isn't my opinion, it is something I have been taught and read in every woodworking/cabinet making textbook I have ever owned from my first high school shop class. I believe it to be true.
Joints must first fit properly, not too snug as to squeeze out all the glue and not to loose as to need glue as a filler. Too much glue can make a weaker joint than too little. Glue should be spread evenly to all mating surfaces of the joint and clamped until the glue sets. The greater the glue surface, the stronger the joint. The time it takes to do this properly will not make or break any cabinet shop.
Too many times on these forums, woodworking questions receive business answers. What is good for business usually has nothing to do with good woodworking. Our price driven market today lets us get away with a lot of poor joinery techniques.
From contributor K:
"This isn't my opinion, it is something I have been taught and read in every woodworking/cabinet making textbook I have ever owned from my first high school shop class. I believe it to be true." I used to think that the only way to assemble a strong cabinet was using dowels and mortise and tenons. Unless you can show it has any demonstrable difference in the end product, why do it in the first place? It's one thing to believe something is true, it is another to put it to the test. On your next job, make one cab your way and one the way described above. You will find that there is literally no difference in the end result, which is why discussions like these are important.
"Joints must first fit properly, not too snug as to squeeze out all the glue and not to loose as to need glue as a filler." A snug joint automatically will squeeze out the glue. It has nowhere else to go. The glue follows the path of least resistance, which is ultimately out to the sides of the dado, the smallest glue surface and where it escapes necessitating a wipe-down.
"The time it takes to do this properly will not make or break any cabinet shop." I disagree. You have to apply extra material, take time to brush it into the sides, then clean-up the squeeze-out afterwards, and that's on every joint on every cabinet. Your other option is to simply squeeze the glue on the largest glue surface, and upon clamping (whether through screws or actual clamps), the glue will make it to the sides, but more importantly across the dado bottom where the real strength is.
"Too many times on these forums, woodworking questions receive business answers. What is good for business usually has nothing to do with good woodworking." Unless woodworking is your hobby, I don't see how you can separate the two in the business of woodworking. That said, I'm totally ok if you want to give more of your money to your clients with a method that uses more material and takes longer if that makes you feel you are offering some level of quality versus the method above. In my opinion it is still confusing activity with accomplishment as in the end there is no appreciable or demonstrative difference, but you can only see that for yourself by doing it both ways.
"Our price driven market today lets us get away with a lot of poor joinery techniques." That is not the case, as there is a large market for apartment-grade or starter cabinets. You would not even approach the same level of quality for this market. You also can't sell high-quality cabinetry to the starter market. So you use cheaper materials and quicker assembly methods to push them out the door.
From contributor G:
If you want the plywood to fit in the dadoes nicely then you make the dado for the smallest thickness of plywood you have and then run the thicker pieces through the tablesaw vertically with the fence setup so it leaves .003"-.005" less than the width of the dado and the height of the dado. This way all your joints will fit just right and you can apply the glue so it grabs on the sides to get the extra strength you seem to want.
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