When comparing equally thick birch plywood and edge glued birch wood panels, what should I take into consideration in terms of their different properties?
The major consideration is wood movement.
A plywood panel is dimensionally stable and will respond minimally to changes in relative humidity, so fixings can be rigid and don't have to take any movement into account. With a solid-wood panel on the other hand movement &#; expansion and contraction across the grain only &#; must be borne in mind, especially on wider panels where it can be considerable.
Any decent furniture plans will naturally account for this in the design. Where it's a potential complication is with anything originally intended to be made from plywood but solid wood or a glued-up panel is substituted for the ply.
Other factors relate to strength and stiffness. Outright strength shouldn't be a big issue, few designs push materials beyond reasonable limits and in terms of how they're put together most are over-engineered so there's a good margin of safety. Stiffness on the other hand is something that often needs to be considered, solid wood being generally much stiffer than plywood, and the stronger the wood the more pronounced the difference (a brief look at the values in the Sagulator will give some idea). But where the material is used edge-up (not flat and spanning a gap, as in a shelf) this is far less of a concern.
While some like the look of plywood, I was considering some options closer to solid wood.
Just in case it's the visible plies and glue lines along the edges of plywood that's a major aesthetic issue for you it is common to edge plywood with solid wood where seeing the plies would be undesirable. This is very commonly done with shelves for example.
There's a little on this in this previous Answer.
In traditional woodworking most of the glue bonds we do will have some mechanical strength, and a well cut joint will hold together without any glue at all &#; it&#;s just there as a bonus.
But when we&#;re edge jointing long boards, such as when we glue up a wide panel or top, it can be an exception. For these joints we&#;ll often look to the glue alone to hold the two pieces of wood together.
Glue is one of those miracles, like sending an , that does its job without leaving any hint of how it happens. For my simple woodworking mind, that disturbs me.
For small glue ups, like on our side table&#;s top, I&#;m happy to appreciate this miracle.
But as the joint gets longer and thicker, I start to lose trust.
For these joints, I always like to add reinforcement for peace of mind.
There&#;s several ways to do this, and on a table I&#;m building this week I&#;ve opted for loose tenons, draw bored in to each section of the top. This physically holds the two pieces together, so I can see how it works, and that makes me happy.
With the modern wood glues that we have available this may seem like over kill.
Modern glues have exceptional strength and there&#;s a lot of work in adding these extra tenons. I posted a picture of my reinforced top on social media, and a good friend brought up this exact point &#; that my joints are outdated due to modern glues.
Link to ymtc
I was going to explain myself in a comment back, but it got very rambly, so I decided to write this post instead.
Modern glues will often say they are &#;stronger than the wood itself&#;, but I have two problems with this.
Firstly, strength is important, but when we build furniture we&#;re not normally looking for the strength to withstand an attacking Northern Yob. Instead we want a balance between strength and flexibility. I look for methods that hold wood together in a manner that sympathises with the nature of the wood, its elasticity and it&#;s will to move.
That&#;s why joints are so important in woodworking, we don&#;t simply stick the end grain of an apron on to the side of a leg with super glue, we use a mortice and tenon.
I don&#;t consider it over kill to take the same approach when edge jointing long boards, in fact I find it quite strange that it&#;s so uncommon to take this approach today.
The second problem I have with putting all my faith in a glue is consistency of application.
Good glue lines are unlikely to have a weakness that would lead to a crack or failure, but it&#;s difficult to be assured that they&#;re good.
Building workbenches for a living gave me more reason to develop an obsession with glue lines than most. And it really did become an obsession (ask Helen).
To create a level of consistency and stability within our benches we took the modern approach of creating the tops with face to face laminations. This is more problematic than edge jointing, and it troubled my mind to simply trust the glue, so the early days of development were filled with research and glue experiments.
At first this just confirmed my worries, as seemingly consistent test glue ups gave completely varied results when attacked by hammer and chisel. Good joints would not give no matter what. Bad joints plopped themselves apart with a mere touch.
The reason for this is because gluing is very scientific. It&#;s affected by humidity and temperature, the level of surface prep, and whether you&#;ve applied too little or too much clamping force.
We learnt that some glues will not be able to reach their stated strength if there&#;s any more than 2% difference in the moisture content between the pieces of wood being joined.
This is usually easy to avoid using kiln dried timbers, but large sections can have more variation than that just within themselves.
We developed our system to perfection, with an exhaustive process of precautions and steps for the environment, the timber, the glue, and the surface. Then we added reinforcing tongues between each join.
That all became very scientific, but now I&#;ve gone back to building furniture and want to leave my glue-line obsession behind.
A Quick Word On The Hazards.
A big part of hand tool woodworking for me is being in control of the pace, the details and the processes. This is helped by not having to wear goggles or ear muffs, let alone a hazmat suite for chemical protection.
Health and safety is bloody boring, but some of the more sophisticated glues are pretty nasty, and not just whilst applying them, cleaning up and sanding can expose the toxins.
If you do your tarting up with a beautiful Festool (or such), orbital sander, complete with dust collection, then you&#;re going to have a mask on anyway. But a sweaty half hour of deep breathing as you flatten your top with a hand plane, and you&#;re going to find a fair old bit of that in your lung. And some very strong glues will easily damage your plane iron too, and this shows in your finish.
The suitability really depends on application, and approach.
So yes, modern glues are great.
But I&#;ll stick with the glues that are simple to apply and are considered non-toxic. For the likes of large edge jointing I&#;ll keep my trust in reinforcements I can see.
I accept that I probably am out dated, but then we are all here to talk about hand tools&#;
This isn&#;t a post to scaremonger you against simple glue lines. There&#;s some world-class furniture out there that&#;s held together with the stuff.
It all depends on your application, and since my methods are traditional, I find it suits best to stick with what&#;s always worked; just as you wouldn&#;t introduce concrete to a mud and stud home.
If you are feeling dubious about your glue lines when edge jointing long boards, just think of the extra practise you&#;ll get chopping out for these loose tenons. Or for the faster approach, have a look at the &#;planked top&#; construction which depends on the simple flexibility of the humble nail.
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