Bent Ply is beautiful, strong, and flexible in ways that make its appeal to designers obvious, but making bent-ply furniture can be a real challenge. While many pieces simply cannot be made without gigantic industrial molds (which apply heat and steam to make the wood flex during bending), and use special glues that can only be cured in the machines, smaller Bent Ply furniture can be made in a regular wood shop without much more than a band saw, table saw, and a bunch of good clamps.
I made the single-curve nightstands in the photos above, but the same steps I've used here, including many rules of thumb and limitations I discovered along the way, could easily be applied to a wide range of bent ply pieces and parts. Once you've made the molds the nightstands are easy, so this is a great project for designs with lots of self-similar parts, too.
You can check out more and larger images of the finished product on my website. http://www.phil-seaton.com/ Click "NightStand" once you get there.
Step 1: Tools and Materials
In addition to some everyday items, such as a paint roller & tray, quart-size mixing container, sandpaper, etc, a few special items are needed:
Glue: I recommend DAP Weldwood Plastic Resin Glue. It comes in a powder form and mixes easily with water; it cures chemically to an ultra-hard brittle glue with absolutely no give. This is important, as the layers must not be allowed to flex next to each other to maintain the curves. Also, the chemical cure helps to make sure the glue in the middle, far from any air, still cures completely.
Particle Board / MDF: Your mold will need to be made solid, so be sure to purchase enough for both sides of your mold, including at least 2-3" beyond your actual piece. I highly recommend assuming that the two halves of the mold will come out of a rectangular area, as the parallel outside edges will need to be clamped to each other. For this project, I used exactly a full sheet of 3/4" Particle Board.
Veneers: There are several factors to consider when choosing veneer for your project. 1) The look you want on your final piece 2) The tightest curve you're planning to bend (I recommend bending a small piece around the tightest curve in your mold to make sure it doesn't snap) 3) The number of layers you'll need given the veneer's thickness and 4) The type and thickness of your interior veneers (which aren't as visible as the surface veneers).
Veneer Tape & Roller (+ a spare sponge): Veneer tape is special stuff that shrinks as it dries, closing gaps that may have been present in your veneers before taping the edges together. Chances are very slim you'll have veneers the right size & shape for your project, so you'll need this to make large enough continuous sheets. You apply water to one side (like a stamp), and then roll it on with a hard wood roller.
Dowels & matching drill bit: You'll need these to align the slices of the mold as they're cut, and also to stack them all once they're done. You'll need at least 3 dowels per side of your mold, and it helps if they're all in once piece. So if your piece is 2' wide and you can only get 3' dowels, I'd recommend buying at least six. Get a spade bit to match; 3/4" or 7/8" for dowels and spade.
Flush-trim Bit: Once you've got one slice of your mold perfect, you'll be using a flush-trim router bit to make the others exactly the same. Get a good beefy one, but make sure it fits your router. I found a bottom-bearing bit to be easiest for the setup, but you should be able to make it work with a top-bearing bit also if that's all you can find.
2x4's (qty 2 or 3) for clamping cauls (if you make these curved cauls they can save you some clamps, but they're not totally necessary)
Stop and reconsider if you don't have, or don't want to buy, the following tools which can be found in most well-equipped shops:
- Table saw. A nice heavy one is always best, but even a contractor's saw should do for this
- Band Saw. For rough-cutting mold slices
- Router. For flush-trimming mold slices
- Heavy clamps (you'll always need more than you think. Make sure you have some long enough to go around the longest end of your mold.
- Drill press (it's critical that the holes you drill for registration of mold slices be drilled straight!
Step 2: Design Your Profile
Explore possible designs for your bent plywood piece. Keep in mind that, except in exceptional circumstances, bent ply is only curved in one direction. That is, the shape will be a simple extrusion of one profile curve you draw.
As you explore the design, try to come up with ways to employ the special properties of bent ply. In this design, the uniform strength in all directions throughout the curve prevents the design from having weak "break points" which would require much thicker wood if separate pieces or solid wood were used.
Step 3: Iterate & Select Final Design
It can be helpful to explore the 3-dimensional implications of the design, and refine its scale and details. Consider issues such as the tightest corner that your veneer can bend, the difference between inside and outside edges of the profile, and ergonomics of the finished piece. For my nightstands, I had to make sure it was possible for a human hand to comfortably fit in between the top and bottom layers; otherwise, the "private" space inside the nightstand would be useless!
It's helpful to do simple studies in CAD software. I use Rhino, but almost any software package would help to explore these sorts of implications.
Step 4: Build the Molds
Once you've selected your profile, carefully trace its shape onto a piece of particle board you've cut to fit it. It's very helpful at this stage to print the profile if you have a large enough printer or plotter available. You can spray-mount the profile to the first piece of particle board to make sure your cuts are precise. The two halves of the mold must have exactly the right tolerance between them, or the molds will not apply even pressure to your bent ply piece!
Once you've cut one carefully on the bandsaw, sand down using a (1" belt sander if available) to the exact edge of your profile curve.
Step 5: Cutting Mold Parts / Mold Assembly
Using the drill press, drill 3 or 4 holes for the registration dowels. Make sure they're located well to hold the parts together.
Mark these first two pieces as your template. To use the template to generate an identical slice:
1) Drill the registration holes through first.
2) Drive short dowel segments through the template and the new slice
3) Rough cut the new slice on the bandsaw. Make sure not to gouge the template, as it will affect all future slices!
4) Clamp the template piece and new slice to the workbench, and use the router flush-trim bit to cut both pieces identically.
5) Remove the new slice and repeat until you've built up the full thickness of the mold. Apply wood glue between each layer as you stack them together on the longer dowel sections. When they're all in, Clamp the mold together while the glue dries.
6) Scrape any excess glue from the mold surface. It must be completely smooth!
Step 6: Verify That the Mold Halves Fit
Once you're set, assemble the mold halves with spacers to verify that the space between them is consistent and exactly the thickness you've planned on. If They don't look like this image (where you can see an even gap all the way through), something's gone wrong and you'll need to fix it. Note that the perspective in this image means you can't see all the way through from any one eyepoint, but the gap is consistent no matter where you stand.
In this image, you can see the positions of all the dowels, and the measurements I've used on my night stands.
Step 7: Prepare Your Veneers
1) Figure out the dimensions of the rectangular veneers you'll need. This will be the total length of the profile you've drawn plus about 2" for trimming on each end.
2) Cut the veneer edges straight if necessary, and tape strips together using the veneer tape and roller into full rectangular sheets. For the outermost layers, put the veneer tape on the outside (it can be sanded off at the end).
3) Alternate grain directions with each layer (the crossgraining is very important for strength). You can likely use thicker veneers for the cross-grain sheets, as the wood will be much more flexible in that direction.
4) Make sure to use an ODD NUMBER of veneers.
5) Make sure your stack is symmetrical around the center! You shouldn't have different wood on the different faces, or a different pattern of veneers going out from the center one.
Step 8: Glue Up
As with all glue-ups, you'll have to work very quickly. The glue I've recommended has a "pot life" of about a half hour. That means you have a half hour from the moment you mix it to the moment you insert the veneers into the molds. In that time:
1) Use a paint roller to coat BOTH SIDES of each veneer sheet in glue. DO NOT coat the outside faces of course! MOVE FAST. You need to make sure you cover everything, but by coating both sides you should be saved from any quick work missed spots.
2) Place a layer of wax paper on each side of the stack to keep it from gluing to the mold with any excess glue.
3) Insert veneer stack into the mold
4) Apply clamps
1) If you're doing this in your living room, as I was unfortunate enough to do, don't tell your landlord about the project!.
2) Be careful not to breathe the dust form of the glue. It's nasty stuff. Mix it outside. Once mixed, it's mostly odorless and can be used indoors.
3) Don't be shy about how much glue you mix. If you have a little extra, that's fine. If you run out, it's game over for the project, and you've just ruined a whole bunch of expensive veneer.
Step 9: Wait
The glue takes 24 hours to cure at room temperature. You'll want to pull it out sooner, but don't.
Step 10: Pull the Nightstand Out of the Mold
Removing the clamps will be easy, getting the mold apart again may take a little elbow grease if glue has seeped out or the wax paper has torn.
Don't be disappointed if the piece is very rough looking around the edges. It'll clean right up!
Step 11: Clean Up the Final Piece
Depending on the size of your mold, you may be able to run the entire workpiece + mold combination through your table saw to clean up the edges in one go. Since the overall mold is rectangular, you can use it to guide any sort of tool for the cleanup. You can rough-cut on the bandsaw and then flush-trim using a router if you want. But use the mold's rectilinear edges if it's helpful!
If everything has gone to plan, you'll see the beautiful edges of a new bent-ply piece of furniture! Congratulations!