Glue Joint Prep and Bond Performance

More advice on creating good wood surfaces for glue joints. December 21, 2010

As long as glue joints are freshly machined with sharp knives, they should produce nice tight fitting joints that won't fall apart, right? Now, can these edge glue joints be machined as part of an S4S moulder process? We make up thin panels 0.6" to 0.725" and want to get the thickness close as part of the S4S process through the moulder. This way, the widebelt can finish them without a planer pass. Assuming sharp tooling on the side heads, will this work?

Forum Responses
(Adhesive Forum)
From contributor K:
As long as there is no snipe on the ends from the moulder then you can glue up no problem, same as running boards across a jointer before glue up. The only thing is that usually the boards being glued up are different in width according to the door size but if everything you do is the same then yes an S4S machine will do the trick.

From Gene Wengert, forum technical advisor:
You are correct, with the exceptions that some species (foreign) require wiping with a solvent. Further, if the wood has stress in it, the pieces may warp when molded or the surface is prepared. Lastly, if the wood changes moisture after milding but before gluing, it may become uneven, unflat with gaps, etc. With good adhesives, good wood, and sharp knives, prompt processing will give you 100%.

From contributor A:
We glue right off the Uni saw (with feeder). We haven't had any failures in many years. Why send it through a molder?

From the original questioner:
Based on a conversation with Franklin tech support, I have been using TBII in the RF clamp. If I want to stay with water based, is this a good (or is there a better) choice? As for the why of the moulder - we start with H and M 15/16" stock and flatten it before glue-up. Enough of our wood is very stiff, like jatoba that any twist, warp, bow, etc. makes for a poor joint. We have not had much luck with using a table saw, and even the SLR saw won't compensate for a board with twist - the edge will twist right along with it. Because we flatten it anyway, the moulder will hopefully let us flatten, thickness and joint two edges in one pass, letting us move on to glue up immediately.

From Gene Wengert, forum technical advisor:
Indeed, when you rip a warped piece of lumber (including cup), then the edges are likely not to be square with the face. When the individual strips are then flattened with top pressure in the glue-press, this top pressure will actually force the strips to be flat on their face, but the edges will be not at 90 degrees and so will create a gap in the glue joint surface, which leads to glue joint failure. Your idea of milding S4S is certainly a good idea in this case, especially with a wood like jatoba.

From contributor Y:
I have never used a multi head moulder but it has been my experience that even in a production environment you can only push things so far. Even if the wood has sat in your shop to acclimate as soon as you surface it you begin to release any internal stresses that the wood may have. I have always skip-planed and jointed my stock then let it rest for a day or longer if possible. Putting sticks between each layer. If the wood moves a fair amount I will take a little more off, if it hasn't moved much then I will joint an edge and face then plane to a working thickness. I think that if you push too far you are setting yourself up for a joint failure somewhere down the line.

From contributor C:
Most of our work is glued panel, we even do 0.6" (15mm). There are several tricks for best results, first if you do long panel over 70" long, I recommend to use a glue joint cutter for a straight glued panel, second is very important to clamp your panels for at least 12 hours, and wait another 12 hours to machine you panels. That’s because the glue would change your mc in the joint. If you don’t wait you can have a small depression in the joint after you machine. If the panels can fit in your planer I recommend going through your planer first to take about 1/32" inch for a clean panel. That will take a lot of work for your sanders. It would get you a perfect glued panel. We even make 1/8" thick panels. What we do is clamp 2" boards. After we plane the piece we use our re-saw machine and cut 1/8" at a time.