Intro: Benches and Tables Built As Stressed Skin Panels
The 5-foot bench panel pictured above, with over a thousand pounds of gravel on it, uses 1/4 as much tree to create, yet has the comparable strength of a 2 inch thick solid wood slab. The panel itself weighs less than12lb and is a structural beast!
The stressed skin panel is an incredibly efficient engineered scheme, in which the structural material is only placed where it can do the most good.
I have made benches, tables, a motor home interior, and a large building, from stressed skin panels.
To date, I have used plywood, door skin, frp, osb, and reinforced concrete for the skin. And, and for the innards, I've used eps foam, ribs cut from plywood, and steel open web trusses, all with good result.
In this instructable I will discuss in detail, how I construct the panels for the upholstered benches that we sell at our website, SpiralConeLegs.com, and also some table panels that I am working on as well. It's really about the panels themselves and not so much about general shop skills or the upholstery etc, I will include a video on the making of the legs.
I think these panels are well suited for furniture construction. I'm going to make all sorts of stuff from them.
Step 1: Bench and Table Panel Sandwiches
A stressed skin panel consists of a structural skin and a lightweight rigid core, like a surf board, hollow core door, or airplane wing.
I make the bench panels with 1/4" baltic birch plywood, 2lb density eps foam and wood glue, I use Titebond 2 or 3.
We make the benches in lengths from 3-feet to 8-feet. I use the same 1/4" ply for each but a different thickness foam, this gives me a thicker stronger panel for each progressive bench length, just by adding more foam.
I use a notched trowel, which is fast and that's important as you need to get the glue spread and the panel assembled before the glue dries.
You can pile a bunch of flat heavy stuff on top in order to clamp it while the glue dries, or make some sort of clamping fixture, my latest is 2-feet x 8-feet, made of 6-inch thick ssp's and utilizes pipe clamps.
I have also built table tops of similar construction but with a 1" deep solid perimeter of the same birch plywood. This gives the table a very robust edge of very nice looking plywood edge grain.
I have also used strips of the same 1/4" plywood used for the skin, as ribs glued in place instead of the foam core, to excellent effect. This eliminates the expense of the foam and makes use of what would other wise be scrap pieces of wood, and requires a small fraction of the wood glue compared to the foam version. Though until I figure some kind of jig, it is a lot more time and labor intensive than the foam version.
As to proportions, you can certainly make very useful panels with less wood than described above. I am not aiming for the lightest, most efficient panels here, I want very stout tough pieces of furniture, I just don't want to wastefully fill the center of the beam with structural material where it is of no structural value. the 4x5' dining table tops weigh over 40lb, which is about 80lb less than they would weigh if made of solid plywood.
Step 2: Cut It Into Any Shape You Want
These panels cut quite well with all manner of woodworking tools. I cut the final shape with a router, I'm not going to teach you how to router, you'd probably just learn bad habits.
I will say that I really like this urethane router guide, you just bend it to the shape you want, and screw it into place, I use a print from a cad drawing.
I cut a template from a piece of plywood. I then use that template to cut the benches of a given shape and length. The template is also a drill pattern for the 12 leg mounting points. If it's a one off bench then I use that nifty urethane guide directly on the panel.
The longest thickest benches are too thick for the tallest router bits I have found so far, so I have to cut from one side then move the template to the other side and then cut that side. I do this by using the holes cut for the legs to register the template from either side.
We have two shapes that we make and sell, we call them Blood Cell and Dog bone. Use your imagination and make any shape/length you want.
As for the table panels, the shape is determined by the perimeter frame of solid plywood, so I just trim the skins down to the frame with a router and pattern bit or with a circular saw.
Step 3: Hardware
I mulled over the hardware question for years before I came up with the following solution and could finally proceed with the benches.
I ultimately came up with something that, uses off the shelf components, allows me freedom of placement, meaning I can place the hardware anywhere I want after the panel is already made, and leaves me with a 1/4-20 steel female machine thread.
I drill 5/16" holes through the panel. I hammer a wide flange t-nut into each of these from the top side, and into each of these from the bottom side, I install a threaded stud and weld nut, previously welded together just so. I glue these in place and tighten them with a screw and screw driver until I can feel the panel start to distort inward around the weld nut, then I remove the screw and I am left with a 1/2" deep blind threaded steel female thread, which is of course wonderful.
This of course only works if you can live with these t-nuts exposed on the top side of the panel, as is the case here with the benches where they will be hidden underneath the upholstery.
This is not necessarily so good for a table and it's exposed top side, so I've been doing those with a solid wood insert, and then I use a threaded insert or you can simply screw directly into the wood.
Step 4: Upholstery
My better half is in charge of the upholstery, sorry I don't have process pictures here, but again this instructable is more about the panels.
I can tell you that the upholstery utilizes 1" hi density hi resiliency polyurethane upholstery foam, polyester batting and rugged upholstery fabric that we usually buy as remnants.
They are boxed without welting and stapled using standard techniques.
We use the thin but firm foam because we want a streamlined not so thick look. I think it's just right even for my skinny butt.
Step 5: What About Those Legs
Of course we use our own design spiral cone legs that we make and sell, but you can use or make anything you like with these panels.
This instructable isn't so much about the legs, but we do have this video that shows how I make the legs, so I'll include it here.
You can see more of the legs and benches at our website, SpiralConeLegs.com
Step 6: More Photos and Some Proselytism
When I first learned of stressed skin panels, I was immediately struck by their lack of broader use in our built environment.
We live in a world of flat panels, a plurality of which would be better served if built as stressed skin panels. The walls of our homes for instance, would be of vastly higher quality in every way, if they were made in such a manner. The stressed skin panel is a marvel of efficiency , and should be all over our manufactured landscape!