Segmented Bowl Glue Line Creep

What we call "glue creep" is really a couple different things that are happening.

It doesn't matter what type of glue it is - white, yellow, hide, epoxy, plastic resin, or whatever, ALL useable wood glues have some degree of elasticity and plastic properties. If it didn't the glue joint would be brittle and break apart with the seasonal wood movement. An example of a brittle glue would be CA, and for that reason it is not a good adhesive for a segmented turning. At the other end of the spectrun we will find the Elmers "White Glue" which retains a considerable "rubbery" consistency after it has dried. The Titebonds are somewhere in the middle, making them a more suitable adhesive for our work.

There are two different things happening. One is that the plasticity of the glue in the joint allows the two pieces of wood to move relative to each other. This is what happens when we use woods with different expansion and hardness characteristics on opposite sides of a joint.

The other is a property called "hysteresis", a common property of all elastomers. This word describes the fact that when an elastomer is stretched, it will not retract totally to its original form. When a segmented bowl expands to a larger size, it pulls the glue in the joint along with it. When the wood retracts back to its original size, the glue does not return all the way. This leaves the joint with a little ridge of glue sticking out from it.

If there is a considerable difference in the characteristics of the wood, a step or offset can develop between the pieces across the joint.

These things are a fact of life when we glue two pieces of wood together, and nowhere are they more obvious than in a segmented turning. How much is acceptable is a matter of definition. Many woodturners and the public that looks at our work have seen so many bad joints that they are becoming acceptable as the "norm" for segmented turnings.

However, there are some things that we can do to all but totally eliminate these results of the wood movements. We can make sure that the joint is tight, and that the glue is totally cured. Then we can a few things to keep wood movement to a minimum. A tight joint will have a minimum of glue in the joint. The glue cure and wood movement can be handled with care and time.

I use what I call the "30-30-30 Day Rule" as the minimum time between the major steps in the process.

All wood is allowed to sit in the shop for 30 days before it is sawn. This allows the moisture to stabilize to equilibrium with the atmosphere in my shop.

The bowl is allowed to sit for another 30 days after it is assembled. We have added a lot of moisture into the wood around the glue joints. This wait allows the assembly to achieve a uniform moisture content and for the adhesive to TOTALLY cure. Yes, glues develop enough strength that it can be handled in a short period and we hear folks talking about such things as waiting 5, 10, 20, or 60 minutes. But, a 100% cure of the adhesive will take up to 30 days or more.

The last 30-day wait comes after the bowl is finished and rough sanded. Turning and sanding heat the wood and lower its moisture content. This wait allows the wood to stabilize with the atmosphere around it. It also allows the wood to get used to its new shape and relieve any internal stresses that were released by the turning, and some additional time for the glue in the joints to cure.

Now, the piece can be finish sanded and finished. Most of the joint problems will be sanded away.

We can also do a lot by choosing woods that behave the same way with temperature and humidity changes. It isn't by accident that most of my segmented turnings are made from Honduras Mahogany, Walnut, Birch, and Holly.

As for all of this "waiting". Is it overkill? Maybe. But, my most stable bowls are those that have had a year between their assembly and turning. I have been able to move several of these several times between the humidity of FL and Western WA, and the dry CA desert, without seeing any changes in the smoothness of the wood across their joints.

Others may disagree with my methods, but this is what works for me.

Russ Fairfield Post Falls, Idaho http:/

Russ, Good description and good solution it appears. I'm glad you put the phrase "glue creep" in parenthesis, since this may not be the culprit. According to mechanical engineers (I'm not one) creep is the permanent distortion of a material under a compression force that is less than its compression strength. I'm not sure that creep is at work since the large forces necessary to make a plastic creep would also crush wood.

Plastics engineers know that a plastic part will creep away from the pressure of an assembly screw given time. That's the reason many assembly screws have a built-in washer, to lower the compression force and eliminate creep.

I've tested some materials on an ASTM device that measures creep. This included polyester resin loaded to 2000psi for two days. Except for an initial creep when the resin lost the last remaining amounts of sytrene solvent, it did not creep any more than aluminum, which is to say almost none at all. Unfortunately, I didn't test epoxy resin or resorcinol, which I think wouldn't creep much. I suspect the water-borne adhesives like PVA and aliphatic resin would exhibit creep. But again, it needs to be a large force. The question is, how large and is that force present?

Also, creep ocurrs under compression. Does compression exist in a glued object? I'd hazard a guess that once the clamps are removed the forces on the glue joints move from being under compression to either shear or tension. All in all, I doubt if creep is the culprit. More likely something related to moisture.