2. Mitred joints - opening Pandora's Box
The
cases are simply mitred to avoid anything distracting the visual flow
of the wood grain and figure.
Now
I chose to glue these up without any reinforcements using Old Brown
hide glue. I could have used splines, biscuits or dominos. Instead,
the panels were taped together ...
and
glued with a sizing technique ...
I
came in for a great deal of stick for this at Sawmill Creek. "You
can't just glue mitres together - they will come apart!".
"Mitres are weak. They need to be reinforced (with dominos,
biscuits, etc, etc)". Or, "eventually the joints will fail
owing to movement", and "what it they are dropped?".
I
think some are a little surprised not to see dovetails, but I did
mention that I have made many boxes or cases like this over the
years, and they have proved to be strong construction. A few examples
...
I have used this pencil box every day for the past
12 years …
This
is a box made for the Veritas Combination Plane. It has been bumped
around for the past 5 years …
I
lug this tool box to demonstrations. I am not especially gentle with
it …
And
more recently I built a new plinth and isolation table for a
turntable. The isolation table has compound mitres …
I
would not have done this if I did not believe in its ability to stay
together.
There was a recent video by Patrick Sullivan, which
was an assessment of the strength of end grain glueing
...
https://www.youtube.com/watch?v=m7HxBa9WVis&ab_channel=PatrickSullivan
Frankly,
while Patrick did produce a good video, his conclusion that "end
grain-to-end grain is twice as strong as side grain-to-side train "
is incorrect, but not because the glue joint is not as strong as he
depicted. I must admit that I was taken in by this at first, mainly
because when all these criticisms were levelled at the joinery, I
wanted to avoid further discussion, and just pointed to this.
However, there are two take aways from this research project (which
would have been better had Patrick included these points):
1.
Glue is stronger than wood. None of
the joints tested broke at the glue line. This includes end
grain-to-end grain. It remained intact under greater forces than side
grain-to-side grain. But this is where Patrick gets off track.
2.
What Patrick did not account for was that the strength of end grain
glue up lay with the grain direction of the boards. The side grain
boards broke earlier simply because they were stressed along the
grain. It is extremely difficult to break boards across the
grain. So, the results of the test really demonstrated that glue
joints are stronger than the wood itself, only.
But this is
misleading and potentially dangerous information, which is the reason
I am writing this post.
Then Rob Cosman, following release of
Patrick's video, did his own testing
...
https://www.youtube.com/watch?v=l_w_or3KhH4&ab_channel=RobCosman.com
Go
to the 25-minute mark where a 200 lb Rob Cosman jumps on a
dovetailed-and-glued corner and a end grain-glued corner. Both
survive. Again, glue wins.
I feel now that it is my duty to
clear up any misconceptions about the joinery here:
What
Patrick's video demonstrated was that glue is stronger than wood.
Most of us have known this for many years. But that does not mean
that it is a satisfactory substitute for joinery. At the same time,
one does not need to use extreme joinery for everything. It is
relevant to differentiate "stressed" from "non-stressed"
joints. Panels (as in table tops and frame-and-panels) being glued up
are non-stressed. If someone adds biscuits or dominos, it is not for
strength; it is for alignment. The glue alone should suffice.
If
you plan to hammer on or lever a glued mitre joint verses a
reinforced mitre joint, all you are demonstrating is the strength in
a non-stressed verses a stressed situation. A small box may have thin
sides and thin mitres, but there is relatively less chance of the
panels flexing than something structural.
The same situation
is present in the mitres for these cases. The only possible stress is
from the atmosphere, and I believe that this is negated by the fact
that the wood is consistent and the grain of the boards joined are,
essentially, coplanar. Will the glue breakdown? Sure, eventually ...
in a 100 or so years. Look at vintage furniture using hide glue.
These pieces are not intended to last 100 or more years. Fashion will
see to that. So ... okay for this situation, not-okay for stressed
joinery.
With regard to mitres and biscuits et al, a big
reason they are used is to prevent movement when glueing up. Glue is
slippery and a misaligned mitre is ugly. Glueing a mitred joint is
tricky. The tape method I used here is fantastic for alignment (and
it also prevents glue leaking out on bench tops), better than
anything else out there (such as other tape, biscuits, dominos, etc).
And it is dead easy.
It is important to emphasise that I
am not recommending
that one use un-reinforced mitres indiscriminately; assess the
demands and the determine the risk.
This is a potentially
great discussion topic. I would like to hear the views of
others.
Regards from Perth
Derek
May 2023