Getting a Good Glue Joint with Cherry
Hard woods may burnish on the planer, interfering with the wood's ability to take glue for good adhesion. Here's advice on dealing with the issue. May 11, 2008
Question
We are having trouble gluing 8/4 cherry for newel posts. We rip and joint the faces of blanks and glue in a press, with clamps alternating side to side every 4". As many as 25% of the glue lines fail (I mean completely fail - clean split right on glue line). We have tried Titebond and Elmer's wood glues, we have dampened surfaces prior to glue up, and we have tried acetone to neutralize tannic acid. I am at a loss here. Temperature and humidity are not a problem, as the shop is climate controlled. And we always bring stock into the shop a week ahead of time. Only cherry seems to have this problem; we do the same process for hundreds of other projects.
Forum Responses
(Architectural Woodworking Forum)
From contributor L:
Since you are jointing them, you may have a burnishing problem. Try lightly scuffing the glue surfaces before gluing (120 grit). If you run the wood over the jointer too slow, you can glaze the surface and the glue has a hard time penetrating the wood fibers.
From contributor V:
I have had the same problem. I think contributor L is exactly right. The only wood in my shop I have that problem with is mesquite. Like cherry, it is a very hard wood. So in the future I am going to very lightly sand the edge. I had a panel I shipped to be CNCed last week show up with a clean separation right on the glue line. It was not damaged in shipping, though that has happened. That's why I ship in plywood boxes. So thanks, I think this will be the solution at least for me.
From contributor L:
After I went to bed I had this thought: How much clamping pressure are you using? If you squeeze too hard, you can squeeze the glue right out of the joint. Maybe if you lightened up a bit on the pressure, it may do the same thing for you. Just a thought.
From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
Are you familiar with the water drop test? You can tell if the surface is okay for gluing by putting a drop of water on it. If the droplet is absorbed in a minute or two, then the wood is okay. If not, then it may be burnished, coated with resin, etc. Indeed, lightly sanding will restore the gluability. As mentioned, excessive pressure can be a problem. Do you see adhesive on the broken joint? Another possibility is that the surfaces are not flat. After jointing (if there is a delay until you glue), do you check for flatness?
From contributor V:
Doc, on mine, yes there was glue, and yes, they did check for flat (or better have after I told them to). We used Weldwood as the glue. Thanks for the advice on the test.
From contributor C:
Definitely one or more of the stated comments. We routinely run a light pass in the wide belt to get an abraded surface rather than a shiny cut surface. Similar to how a sawn glue line comes out better, tighter and more invisible than a jointed glued edge. There is almost always a microscopic washboard surface from the jointer or planer. If they fit flat, and are glue absorbent, the only thing that should give out is the wood next to the glue line. Change the knives in the jointer first. Dull knives will cause all these problems. Not flat, too shiny, burnished, or beaten smooth.
From the original questioner:
Thanks! I will try the water test. I don't believe the surfaces were burnished, but can't completely rule it out. I keep fresh knives on jointer to avoid such problems. I will try putting them through the wide belt. We always check for flat surfaces and try to glue the same day we machine the wood. Has anyone tried urethane glue for this application? Seems like a lot more work.
From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
There are many expensive adhesives, but the one you are using will already give you a glue line stronger than the wood itself (if done properly).
From contributor T:
We run glued up stock everyday, all day. Everything 5/4 to 10/4 goes through the widebelt, use only contact drum, no platen, and canvas backed 50 grit. No problems - almost invisible glue line. Watch for end clips. Even on wide belt, it can happen. Titebond Type II. Clamping pressure max. You can't squeeze out all the adhesive.
From contributor B:
I would glue right from the straight line saw. My guess is that your problems will be eliminated. The other issue is to coat both surfaces, and a little open time doesn't hurt. Gives the glue some time to soak in.
From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
Many people glue off the sawn edge, but it is best in this case if the saw has been properly side dressed, as it is the sides of the teeth that are preparing and touching the surfaces that you will be gluing. With cherry, the saw must be extremely sharp or you will burnish and then be unable to glue.
From contributor I:
Did you say Weldbond? I had major problems with that glue. It turns out that it washes out of the joint under the pressure required to bring the mating edges together. In other words, it is useless to me.
