
Wood Joint Strength Testing - UkiahSmith
https://woodgears.ca/joint_strength/
======
jimnotgym
There is another important factor. Wood expands and contracts according to the
moisture content of its environment and screws expand and contract according
to temperature. The combination of these means that screws work themselves
loose over time, often wearing the threads out of the wood.

In the UK, stair handrails (for instance) used to be mortice and tennoned with
a drawbored peg. This is where an offset holde is drilled through the tennon
and a peg driven in to pull it tight. A fashion for cheap construction has
seen this replaced by screws in many cases. The problem is that uk houses tend
to have wet plaster (or at least a skim on drywall) and high moisture levels
at the time of installation. This means that the wood will shrink once
installed... and your handrails come loose, the screws are hidden by this
time!

In a very damp situation, say a wooden gate, this gets a cyclical drying and
wetting, yet properly made oak gates can last many years due to their pegged
tennon joints. Edit: clarity

~~~
exDM69
In addition to wood movement, there's another important factor when it comes
to hobbyist woodworkers: accuracy.

With modern adhesives and precision machining like Mathias' work will make
mighty strong joints even out of the simplest joinery method. A precision made
box joint glued with wood glue is stronger than a finely made dovetail joint.

But when it comes to me bodging away in my garage with hand tools, that kind
of precision is not going to happen. But historical "strong" joints like
dovetails and mortises and tenons with wedges and drawbores are very forgiving
joints and are quite strong even when they're not made to sub-millimeter
accuracy. I can join a decent box with dovetails in about 30 minutes, put it
together with a few drops hide glue and fill the gaps with sawdust and it'll
work great.

But someone with a table saw and a simple box joint jig could join 10 boxes in
the same 30 minutes it took me to make one.

~~~
jdietrich
Modern hand tool woodworking can match or exceed the accuracy of machine
woodworking, albeit with a higher skill threshold. You can achieve very fine
results with very basic tools. Machine woodworking is undoubtedly faster,
however.

Tight tolerances are often completely counterproductive because of wood
movement. Even with an impermeable polyurethane finish, you'll get significant
seasonal movement. Effective fine woodworking is about designing to account
for that movement - building drawer runners slightly undersized so they don't
bind up, allowing door panels to float in the groove so they don't bow or
split etc. One of the key skills of cabinet-making is developing an intuition
for wood movement.

[https://www.canadianwoodworking.com/tipstechniques/dealing-w...](https://www.canadianwoodworking.com/tipstechniques/dealing-
wood-movement)

~~~
nemo44x
What about torrefying the wood first? Wouldn’t this reduce the amount of
movement in the wood?

~~~
exDM69
You can't really stop wood movement or even reduce the amount it moves. It can
be slowed down a little with some finishes (then you have a "wood-plastic
composite"). Kiln drying (do you mean this by "torrefying"?) will make the
wood dry for a while, but when the temperature and (absolute) humidity will
rise on the summer, the wood will absorb moisture from the air and expand and
shrink again next winter.

~~~
jdietrich
Torrefaction (more properly, thermal modification) involves higher
temperatures than kiln drying, causing chemical changes to the wood that
result in permanently lower equilibrium moisture content and hygroscopicity.
It improves the stability of wood and drastically improves the resistance of
softwoods to decay, but it isn't a complete solution to wood movement. There
is a significant tradeoff between stability and strength.

[https://en.wikipedia.org/wiki/Thermally_modified_wood](https://en.wikipedia.org/wiki/Thermally_modified_wood)

~~~
abakker
It also changes the color, which in many cases is inappropriate for the
project. However, if it were being painted I would definitely consider it. I
bought some Torrefied ash recently - it's weird stuff. Kind of like working
with toast instead of bread. the sawdust smells odd, too.

~~~
nemo44x
In the acoustic guitar world, torrefication has become popular for some
guitars. The main idea is that the end result of the spruce (commonly used for
top wood on an acoustic guitar) is closer to a piece of spruce that is many
years old which affects the tone and "improves" it to some ears. The other
benefit is the wood is less likely to crack which is an issue with acoustic
guitars, especially in dry climates where humidification is an issue. Some say
the wood is more resistant to cracking but more brittle in other ways which
aren't too important to how an acoustic guitar applies tension. the main idea
is the wood won't change with respect to humidity as much as untreated wood.

But yes, especially when luthiers started using this technique there were all
sorts of issues with the end result in terms of superficial results/coloring.
However, they've gotten better at this and now we see the wood being a bit
darker but without all the other cosmetic issues from past years.

~~~
jdietrich
_> The main idea is that the end result of the spruce (commonly used for top
wood on an acoustic guitar) is closer to a piece of spruce that is many years
old which affects the tone and "improves" it to some ears._

Torrefied tops can definitely make an instrument sound better. With the right
thermal treatment, you get a slight increase in stiffness and a fairly
significant reduction in damping due to the lower equilibrium moisture content
and the depolymerisation of hemicelluloses. If the top is properly braced to
account for the different properties, you get a more open-sounding instrument
with more volume and/or sustain. Torrefied necks are substantially more
stable, particularly for the flatsawn maple necks on most Fender-style
electric guitars. The durability of tops is very much swings and roundabouts -
you gain a fair bit of stability with respect to atmospheric changes, but the
top becomes weaker and more brittle, so more prone to impact cracking.

 _> But yes, especially when luthiers started using this technique there were
all sorts of issues with the end result in terms of superficial
results/coloring. However, they've gotten better at this and now we see the
wood being a bit darker but without all the other cosmetic issues from past
years._

If anything, the caramel colour of torrefied maple or spruce has become a
status symbol. We're starting to see a lot of roasted ash bodies on electric
guitars and basses, which doesn't really do anything tonally but looks cool.

~~~
nemo44x
I agree, the slightly darker top is nice to my eye and gives it a fine head-
start to the natural aging process of yellowing the guitar.

I think some of the issues that were out there with regards to coloring were
things like "racing stripes" where there would be significantly darker stripes
of darkened color an inch or more wide that would run up the tops, which many
people disliked. It never really bothered me too much, personally.

I agree on the improved tonal characteristics - the reason I say "improves" in
quotes is that this is entirely subjective. Some people are very sensitive
about these types of proclamations for some reason. I believe it does make for
better tone, which is why I have a guitar with torrification (red spruce top)
being built currently. To my ear it is louder and more articulate than the
same species of spruce that hasn't been torrified. As you've said, it's opened
up right from the start rather than waiting years for the wood to change
enough to create that open sound.

Of course when it comes to the tone of the top wood, besides the bracing
(which is obviously unbelievable important), I believe that the finish matters
a lot and as thin a nitro finish as possible is ideal. I've drank the kool-aid
with regards to hide glue as well. At the very least, it makes a neck reset
(dove tail, glued joint neck) or any other type of surgery easier in the
future.

------
VBprogrammer
I'd recommend NOT starting to watch Matthias Wandel's videos. I must have lost
years of my life watching woodworking videos on youtube after he got me
hooked. It also cost me several thousand pounds in tools and culminated in me
buying a home requiring complete refurbishment (not quite finished).

Save yourself! (mostly in jest).

~~~
jdietrich
At the very least, watch some Paul Sellers videos to inoculate yourself
against the belief that you absolutely _must_ give all your disposable income
to Festool and Lie Nielsen.

[https://www.youtube.com/user/PaulSellersWoodwork](https://www.youtube.com/user/PaulSellersWoodwork)

~~~
Loughla
Honestly, if you're interested in woodworking, Paul Seller's original
'workbench' video set is how to get started.

Cheap materials, can be done with cheap tools, and he's pretty straight
forward about what's important to worry about and what isn't.

You'll learn whether or not woodworking is for you, and you'll have a handy
workbench.

~~~
teacpde
His workbench tutorial is excellent, but for me, an absolute noob, it was
still daunting to build a workbench as the very first project. What I ended up
is building the Paul seller's Trestle [1] as my Hello World project, and it
has been super handy. Even after that, I still think his workbench is too much
work for me, and I eventually decided to build Christopher Schwarz's knockdown
workbench [2].

[1] [https://paulsellers.com/trestles-drawings-and-cutting-
list-d...](https://paulsellers.com/trestles-drawings-and-cutting-list-
download/) [2] [https://blog.lostartpress.com/2014/09/18/materials-tools-
for...](https://blog.lostartpress.com/2014/09/18/materials-tools-for-the-
knockdown-nicholson-workbench/)

------
Jedd
As someone who dabbles in woodwork, I can highly recommend it as a hobby.

Exercising the brain in any new problem domain can often be rewarding in
itself, but it's a nice opportunity to blend engineering, practicality,
creativity, usefulness.

From TFA, I'm (also) surprised that end-grain screws worked at all well. I
appreciate the author was trying to minimise variation between the components,
but for end-grain I'd use a much deeper thread, as screwing into end grain is
often compared to joining into the end of a bunch of drinking straws. Not that
I'd ever screw into end-grain. ;)

Similarly, the makeshift pocket hole jointer isn't clear from the pictures how
they got a recessed hole (commercial pocket hole devices use a t-shaped drill
bit, and do not drill all the way through the piece). The recommended bits for
pocket holes also have a threadless shaft near the head, so the two pieces
pull tightly together.

Which leads to the lack of glue in many of those photos. I grew up hearing
(but not really believing) phrases like 'the nails are only there to hold it
until the glue dries'. Wood glue technology is amazing -- when they say on the
bottle 'stronger than wood' they aren't joking. I'd expect to use a _lot_ more
glue than shown, and have it significantly affect the results.

~~~
cmroanirgo
> the nails are only there to hold it until the glue dries

This is a phrase my brother, a cabinet maker/ jointer, uses... And tests
occasionally: Glue 2 pieces of timber together at 90 degrees and clamp it
normally. When dry, break the join (by bashing it with a hammer is his general
way) to see if it's the glue or the timber. Nearly all of the glue he tests
are stronger than the timber (Oz, US and Asian hardwoods), as it's the timber
that is ripped apart, and not the glue join.

A bottle of standard PVA-B (/ exterior grade) from the local hardware store is
pretty excellent.

When you think about glue, it's _everywhere_ these days. All timber laminates
(eg kitchen bench tops, plywood, particle board, MDF, and finger
jointed/laminated timber) are just 'glued'. A lot of this is structural timber
(ie designed to last decades).

From the article:

> This joint failed by the wood in the post shearing along the growth rings

This is the most common place of failure when witnessing my brother's "tests",
and makes a lot of sense it happens that way too. The glue should have been
the strongest part of ALL of his tests. It looks like he 'over clamped' the
timber (ie squeezed out all the glue) in few of his photos [0]: which is
something my brother has warned me about, as it makes the join weak.

[0]
[https://woodgears.ca/joint_strength/failures.html](https://woodgears.ca/joint_strength/failures.html)

Edit: clarification

~~~
bradstewart
Indeed. Glue-laminate beams are a wonderful building material: cheap, strong
as hell, and friendly to forests since they are made from wood scraps.

~~~
maxerickson
The timber extraction industry cares not which tree the fiber comes from and
sells it to the highest bidder.

(Actually, they discard pieces that are too small to handle profitably, so
they do care a little bit)

------
MatthiasWandel
Hey, cool to see my article on the front page of hacker news. Never expected
THAT article to hit hacker news. Got a question, I could answer.

~~~
longb4
Hey there! Big fan of your articles and videos. I've been building box joints
recently and even though the forces they're subjected to are not nearly as
pathological as what you tested, I enjoyed your box joints and dovetails
strength testing article:
[https://woodgears.ca/dovetail/strength.html](https://woodgears.ca/dovetail/strength.html)

Quick question: do you recall what wood you used for the box/dovetail article?
It looks similar to the spruce from the earlier joint strength article. I
suppose using a stronger wood would just get you closer to where the glue is
failing more than the wood, but I'm just curious.

Less-quick question, feel free to punt/ignore: any thoughts on deliberately
building in a very, very slight gap between box joint fingers? I would expect
this to reduce joint strength (thanks to reduced force between fingers and
surface area in contact) but potentially also mitigate wood stress from
movement in service due to humidity. Though I imagine if a work piece is big
enough for me to be concerned about movement in service, I probably shouldn't
be using box joints!

~~~
MatthiasWandel
I'm pretty sure that wood was birch.

As for gaps in joints, if its a small gap, it doesn't hurt. I actually did
some tests in that regard, linked from the article.
[http://woodgears.ca/joint_strength/glue_methods.html](http://woodgears.ca/joint_strength/glue_methods.html)

A small gap actually helped. Probably because the glue soaks into the wood, so
a joint without any gap sort of runs out of glue too quick (my theory). But it
suggests that putting glue on and leaving the joint open for a minute may be
beneficial.

------
woofie11
A problem is that "strength" isn't a scientific term. You have tensile
strength, shear strength, torsion strength, and compression strength.

Screws have incredible tensile strength, which this article was testing. Shear
strength? Not so much. Hit the joint with a hammer from above, and most screws
will just snap. Nails are the opposite -- great shear strength, and no
tensile.

Glue is stronger than wood, as several comments indicated, but for how long?
I've had many glued pieces of furniture fail after a decade of use.

In a lot of woodworking, I use multiple types of fasteners. It's
overengineering, but it doesn't fail.

~~~
jkoudys
Most of the glued pieces I'll see fail after a decade were using your standard
yellow carpenter's glue. It's fine, strong stuff that lasts a while, but it
will still soften if it absorbs some water. Some older glues will also brittle
over time. There's been a big push recently to waterproof glues that fully
cure and can't be re-wet. While it's disheartening to throw out a bottle of
clumpy glue (when I've had bottles of the yellow stuff sit around for over a
decade and are still good), glues like titebond 2 or 3 should last as long as
the wood itself.

I'll still use the yellow glue for anything that doesn't hold a load, like
thin panelling. These days my yellow glue mostly gets used to repair pop up
books my toddler tore apart.

~~~
exDM69
On the other hand, if you build heirloom pieces with nice joinery, you want
the glue to fail before the wood does. When that happens, it's an easy repair.
If there's damage to the wood, it's a much more complex affair.

I use hide glue liquid, which is not very strong and not moisture resistant at
all. But it's a pleasure to work with and I know that if it fails, I can put
it back together again (doesn't even need the old glue to be cleaned up).

If anyone wants to geek out about historical adhesives for woodworking and
other craft, I'll gladly share my experiences about DIY glue cooking at home.

~~~
bluGill
Hide glue is also stronger than the wood joint.

~~~
exDM69
It usually is, but that depends on the additives used and the gram strength of
the flakes used to cook the glue and the quality of the glue job.

E.g. I cooked a batch of 192 gram strength glue with 50% table salt additive
to keep it liquid at room temperature, and I had half of the failures at glue
seams in destructive testing. Made an excellent glue for cold winter days,
though.

In glue strength tests (like James Wright on YouTube and FWW magazine both),
hide glue compares favorably to modern glues.

But all it takes is a few drops of additives to drastically change the
qualities of the glue.

------
teacpde
It is amazing to find woodworking is such well received topic on HN. I don’t
remember how I stumbled on a woodworking video on YouTube about a year ago,
and last week I just finished building the Christopher Schwarz knockdown
workbench. This hobby is truly addictive, to me, it resembles a lot aspects of
software engineering, but in a more vivid and interactive fashion.

~~~
websterisk
I think this is particularly true in the sense that woodworkers (like software
people) can make much of their own tooling in their native media. Nothing I've
seen on youtube is a better example of that than Matthias Wandel. Need to make
a bandsaw out of wood? Here, hold my beer. The Pantorouter... really? Even
dust collection! This makes wooden stuff out of wooden stuff out of wooden
stuff. Three levels deep, inception style.

------
exabrial
Why didn't he join the screwed pieces with glue? I was always under the
impression the whole point point of screwed joints is to stress the glue, not
to actually bear load (Because of the small surface area and hardness
difference of the materials)

~~~
btbuildem
I've never used both screws and glue on a joint.

It's either a quick and lame job and I'll use screws, or I'm making something
nice and I'll go to the trouble of cutting and gluing joints.

------
dlbucci
Cool article! I've made a few very simple pieces of furniture and have always
used screws, so I was surprised to see they were the weakest joint. Guess I'll
need to learn how to mortise and tenon!

~~~
newnewpdro
Screws can be fine if used appropriately. If they're loaded in shear rather
than tension, there's no issue of the head pulling through for instance.

~~~
MatthiasWandel
Yes, screws are fine for most applications, but at a right angle, if strength
is critical, not the best option.

~~~
newnewpdro
Your failures mostly amounted to the joint pulling apart along the glue
interface rather than breaking, I suspect adding a transverse retaining pin
will make the failures more interesting with significantly higher loads.

Did you consider doing any tests with a retaining dowel or some nails/screws
through the mortice and tenon joint?

~~~
MatthiasWandel
The transverse pin weakens the tenon and the piece of wood it goes into. If
the joint is well designed (so that the glue strength and wood strength are
balanced), the tranverse pin only weakens the joint.

~~~
newnewpdro
Does that hold true for all thicknesses?

------
yumraj
Can anyone recommend any good woodworking book _s_ , preferably one that is
friendly to beginners.

~~~
takk309
Check out Lost Art Press
([https://lostartpress.com/](https://lostartpress.com/)) They focus mostly on
traditional hand tool wood working. The Anarchist's Tool Chest
([https://lostartpress.com/collections/getting-
started/product...](https://lostartpress.com/collections/getting-
started/products/the-anarchists-tool-chest)) is a great book to help you get
started with what makes a good hand tool and what you need to start doing
small projects.

------
djbaskin
Yeah, I really need to move away from screws.

I recently made a Dialup line if anyone wants to talk about their projects:
[https://dialup.com/woodworking](https://dialup.com/woodworking)

------
jugg1es
Results are not at all surprising, but it's really great to see it done in a
formal way.

------
zaphod4prez
Matthias Wandel is just the coolest. Highly highly recommend watching all of
his videos

~~~
MatthiasWandel
That's right :)

------
dekhn
woodgears.ca is one of my favorite sites on the internet. really inspiring,
although it's unlikely I'll ever build a pantograph router.

------
Taniwha
woo hoo! science!

