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Door Joint Strength2/12/14
We have been building doors for years. Recently the strength of the joint has been weak. In tests we can pull a joint apart and little or no wood is pulled out. I have tried 2 different glues? the moisture content of the wood is from 6.0 to 6.4%. The wood I have seen problems with is maple. We are running more tests to evaluate other species.
This is the coldest it has been in memory is my wood too dry?
Have you checked your tooling for sharpness?
Could it be that your tongue and grove are glazed from dull tools?
Just a thought.
Buy a brand new bottle of glue. Perhaps both of your bottles got too cold. What's your shop temp?
Thanks for the replies.
Out tooling is insert carbide, we swapped out the knives and we are still having a problem. We have new door glue and just opened a new drum of general assembly glue, both have the same issue. We made dozens of test joints with different species. The problem is one profile, it is worse in harder woods as you would expect. The Joint seems looser than it used to be. I am getting with the tooling manufacturer. They are top notch and I am sure will help me diagnose this.
Sounds like the profile may have too much end grain glue surface, or the door may just be too big for cope and stick joinery alone. I trust that you have checked the fit, and that it isn't too loose. Have you considered adding dowels, dominos, or floating tenons to reinforce it?
You can test the glue surface to see if it is burnished, by seeing if the wood will readily absorb a drop of water.
Any PVA (tightbond, etc...) should result in a bond that is stronger than needed. Same thing can be said about most other adhesives that are recommended for wood. The problem is not the glue, unless it wasn't stored properly, or it past its expiration.
Like Blake mentioned, the water droplet test is a great tool to test for the activation (or glueability) of a surface. The drop should not stand up after several minutes, but should disperse and be absorbed.
So what causes a surface to be inactive for gluing? One cause is that the wood is drier than normal which means machining is more difficult--dry wood is stronger, especially under 6.0% MC--so the tools, especially carbide, will burnish the surface. Test this by taking a small piece of sandpaper in your hand and swiping across the surface before applying the glue...swipe it maybe two times, but do not remove a lot of material...just remove the burnished fibers. Also, you want to remember that the outside fibers of a piece can be much drier than the core or the average MC. It is these outer dry fibers in which the machine tool is working.
Are both the wood temperature and glue temperature around 70 to 75 F? At cool temperatures, many adhesives do not develop a strong joint. Some adhesives are designed for cooler temperatures.
Gene our shop is around 68 when in operation and 62 when not. The material stays in the shop so it should be the same. We use National Casein, I spoke to them and they did not mention temperature. I think we have one profile that is not as tight as it used to be. I am getting with the tooling manufacturer to diagnose. We have quality insert tools and should not be having this problem.
Another thought is that you have clamps at the ends of a piece. When you tighten the clamps, you will apply too much pressure to much of the joint. Why? Because in dry weather, the individual staves will dry out just a bit, making the ends a bit narrower than most of the piece. So, when you squeeze the ends to get the ends closed up enough to glue properly, the center sections are too tight with most of the adhesive squeezed out. Too little pressure will mean the ends will not be glued well and will split. The cure is to cut the staves to size and glue within a few minutes...before the ends can lose moisture and shrink in width.
I see that you use casein glue and the operation temperature is at 68 oF ( 20 oC). You should check the glue wetting and drying time of open glued maple surface before you do any next action of gluing. Time of clamping will be 02 times longer than drying time. If the glue shows poor drying time at such operation temperature, you can add more 5-8% pure PU hardener ( non solvent added) to get its cured. Please be noted that glue is just one factor to keep your door structure stable. The additional tenon, dowel at joints help 03-04 times stronger the joint structure.