On wood joints: Long grain to long grain joints (as seen in the bead & cove)
are strongest. Cross grain joints are the second strongest. Both joints are
stronger than the wood itself, even if the glue used is not as strong as the
wood (eg. Elmer's white glue). End grain to end grain are the weakest and
should be avoided, even if the glue is stronger than the wood. I have seen a
guitar neck to body, end grain to end grain joint with aerospace epoxy work,
but I still am not comfortable with the idea.
Most cabinet making/woodworking and jointery books have really great info on
glue joints.
Chris Rudesill
Robert Harper <rharper@xxxxxxxxxxxx> wrote:
On Wed, 06 Feb 2008 11:17:45 -0700, Tom Stearns
wrote:
> Gentlemen:
>
> Mr. Harper said in his response to the Soldiers, Bulkheads and Transoms
> thread that butt joints are the weakest you can make, and that joints
> with
> greater surface area (cove & bead, for example or long scarfs) are
> stronger.
>
> This may be true, but have you tested the theory?
No I haven't. Others have so why take the time to recreate the whole
process especially when I don't have the proper equipment to do it so that
others will accept my results. I can also attest to this from personal
experience. I used epoxy to glue my strips on the canoe This did not help
the butt joints too much but did increase the weight of the finished boat
by quite a bit. On the up side, it is very strong.
>
> Conventional wood glues are weaker than wood hence greater joint area
> results in stronger joints. With filled epoxy adhesive, which is stronger
> than most woods, a wider joint can be stronger (because more
> compliant)
> and greater surface area doesn't increase the strength. Thus an
> assembly
> with simple rectangular strips with the adhesive forced into the gap
> after
> the strips are positioned is an assembly method which allows maximum time
> for adjusting the position and fairness and can result in a structure
> which
> is as strong as the wood everywhere while avoiding the waste of
> expensive
> wood and time spent milling bead-and-cove.
The problem with a butt joint is the contact surface area. Butt joints
minimize surface area and that is one of the biggest reasons they are
weak. I read in a book on boat building, sorry I don't remember the exact
one I've read so many over the years, that some school had run several
tests on joints that maximized strength and minimized waste. They found
that the 12:1 scarf joint was the strongest. With a cove and bead joint
having about 1/3 more surface, it should gain strength both from a
mechanical and surface adhesion aspect.
>
> Bead and cove maintains fairness since the strips lock into each other
> between formers. However, unless the strips are exactly the same
> thickness,
> and unless the milling cutters reference from the same surface when the
> beads and coves are cut, "locking together" can result in unfairness.
> Try fitting your strips together first one way around, then the other.
> Often
> you will find that tolerance stackup in the strip thickness and location
> of
> the bead and cove on the edge of the strip results in a fairer surface
> when
> the strips are aligned in one direction than the other.
This is why it is important to do a good job milling your strips. I went
one step further than most and ran all my strips through a thickness
planer to make them very uniform. Another thing you can do to minimize the
angles of the joints is to use thinner strips. This does, however,
increase the amount of glue and number of staples. You can also get a much
more uniform curve over small radiuses.
>
> Tom Stearns
> 3 Brander Court
> Nashua, NH
> 03063
>
>
--
Live large, love lots, and sail fast!
The mark of who you are is determined by what you do when you don't have
to do it.
USA 050 Fugu
77959 Wasabi
Robert Harper
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