Posted by ken on October 06, 2003 at 18:35:02:
In Reply to: Oak or Mahogany for frames? posted by Scott on October 06, 2003 at 17:50:58:
I already been there, and asked all those questions. I been at DAP/Weldwood tech support, Glen-L , and West systems support. Nobody has a answer that they will sign their name to. If Oak was all that bad it wouldn't have lasted as long as it has. Now, here is what I have gathered so far, and this comes as looking at the common demoninator from all the sources I have mentioned, and applying my brand of common sense.
There is a section similar to this in a webletter that I got started some time ago, re: testing your glue joints. I am building in white oak.
Oak does swell more than many other woods when it is wet and the glue line doesn't swell, causing stress in the joint.
Encapsulating is supposed to keep the wood dry so this aint supposed to matter, but the subject keeps coming up.
Oak is a dense wood, thus it does not absorb glue into the grain very well as compared to mahogany, ash or others. Applying light heat to the wood prior to glueing does help thin the glue and get it deeper into the grain, do not heat the glue though.
Because of the glueing properties of oak I now believe that Oak is a good choice for structural members that are fastened together with physical hardware such as bolts/screws. That way as the wood swells, the joint is a little free to move a little as the joint swells.
In the modern era of epoxy joints, the properties of Oak are not as ideal as other materials.
The gallic acid is something not to worry too much about. I had to talk to a chemical engineer that specializes in white oak to find out that the gallic acid is what gives whiskey its caramel color. My brother-in-law works for a distillary and they age whiskey in white oak barrels because it swells up nicely to hold the liquid, and the gallic acid gives it a nice color.
I did some testing myself with oak-to-fir plywood epoxy joints. Every time, the fir ply failed first. I even carried home buckets of seawater to soak samples in. The oak swelled enough that it could visibly be seen, and the oak got soft seemingly pliable, again, the fir failed first. (the soak test did turn the wood a nice bourbon color)
Comparing glue lines of oak and mahogany, under magnification it can be seen that the glue does penetrate the softer mahogany deeper than hard oak.
Bottom line is, the oak is so dense, that it is difficult to get a deep penetration of the glue.
I scuff the oak with sharp 60 grit, warm it with a heat lamp, glue it up with a well fitted joint, and it holds for me. If the designers specify marine ply for the gussets, and the final frame is as strong as the gussets, then I quit worrying about the issue.
It is more work to build in oak though, more care in the joints, harder to shape, and I certainly will not entertain the idea of fairing my Double Eagle in oak.
White Oak does however bend very well, it bends considerable before it gives way, as compared to yellow pine or mahogany.
Mahogany is popular because it does so many attributes well, but specializes in none.
That is, other woods are stronger, or hold better fastenings, or rot resistant, or lighter, or glue better. Mahogany does all these things well, and other woods specialize in a single attribute.
Examples:
Spruce is light but rots easy.
Oak hold fastenings well, but hard to glue.
Ash bends well, but not rot resistant
Good luck.