Introduction: Better Bent Lamination Forms
There are only a few ways to get large curves in furniture that are structurally sound. If you tried to saw a large curve from one board you would have a huge amount of waste and it would surely break as soon as a toddler leans on it.
Chairs are an extreme example, where people want delicate, flowing curves, but they have to be able to withstand Uncle Ted throwing all his (not insignificant) weight onto it and leaning back precariously.
One way of getting curves is to use bent lamination, where several layers of wood are glued together around a form- essentially creating plywood.
In this instructable, I will share some hard earned wisdom and best practices when it comes to building forms for bent lamination. The better the form, the better the results. There will be a few tips and some "don't make this mistake..." along the way.
Plywood (I used 3/4" and 1/4")
Pine scraps (I used 2X4 scraps)
Hardboard or Masonite
Hardwood scraps (I used discarded furniture)
Glue, nails and screws
7-10 large "C" clamps or "F" clamps
HDPE plastic (I used an old cutting board)
Epoxy and Paste Wax
Circular saw, jig saw, table saw, band saw
Spindle sander, sanding blocks
Compressor and nail gun
Step 1: Start With a Good Design
This is where it all starts. About 10 years ago I set out to make a David Haig inspired rocking chair. The problem was I didn't have plans...they don't exist. All I had was a photograph and a pretty good idea of how it went together.
I made the chair and then realized we had a problem...my feet didn't touch the ground when I sat in it. Because I made it from a photograph, I didn't have the critical angle where the arm support/back leg meets the seat. This made the chair sit about 3-4 inches too far back.
No big deal right? Well, those 3-4 inches are enough for your intestines to lurch out of your stomach and your arms to flail out to your sides. I didn't set out to make a "GOTCHA!" rocking chair-I set out to build a "wow, this is amazingly comfortable" rocking chair. So even though it looks great, it doesn't sit right. You won't fall over, but it won't be pleasant.
The chair was made using bent lamination. Each part of this chair (except the seat) is about 11 layers glued together. That's a lot of layers and a lot of glue. The form I'm going to show you how to build will make this process easy.
With my past failure in mind, for an upcoming project I'll be making an Adirondack chair designed by someone with a lifetime more experience designing chairs...Michael Fortune. It was while making the templates for this chair that I realized I needed the "whales" from a previous instructable to draw the curves.
Step 2: Build the Form Structure - Initial Cutting
Bent lamination forms are usually made the same way: you draw out the design onto one board, then cut and smooth that board. Then that board is used to trace the design onto more stacks of plywood. Cut it out, template route- rinse and repeat.
The problem with this method is the form can get really heavy if the piece you need is wider than a couple of inches. It also requires a lot of plywood and can get expensive. "You spent $180 on plywood to make a $60 chair?" It's complicated...
This method creates an exoskeleton that is much lighter and easier to use. I was also able to build the form and the first test piece from scrap plywood and pallet wood. Some pallets have a plywood base, and that is what I used for this form. The only thing I purchased for the form was the 1/4" plywood. It was about $20 for a 4' X 8' sheet.
Once you have the design onto paper, nail two pieces of plywood together and attach the pattern. Cut and sand to the line. Use sanding blocks to get even, smooth lines- this is the most important part! Smooth form = smooth parts.
Step 3: Build the Form - First Assembly
Once the plywood is cut and smooth, separate the two from each other and remove the nails.
The "ribs" are pieces of scrap pine that are the width needed for your form and 7/8" thick, spaced 7/8" apart.
Put glue on the ribs and place them on the form, securing them with a couple of nails. Make sure that on tight curves the corners line up with the edge. Add glue to the top and secure the top piece.
Mark a line about an inch from the edge and add screws into the ribs to reinforce the joint. Give everything a light sanding to ensure the face is smooth.
Cut a piece of 1/4" ply the size of the face and secure it with glue and nails. I used 3 band clamps to keep everything in place. There will be 2 layers of the 1/4" plywood on the face, however the second one will be glued on with no nails so there are no nail holes or clamp impressions to worry about.
Step 4: Build the Form - Second Assembly
The next step is to make the clamping cauls. These will push up against the pieces being glued and distribute the clamping pressure evenly. It will also prevent the clamps from leaving impressions on your final piece.
I cut hardwood blocks 7/8" X 7/8" and nailed them to a piece of 1/4" plywood the width of the form. I also cut out 4 pieces of hardboard and one piece of plexiglass.
I glued sandpaper onto one piece of hardboard to provide better grip for the clamps, this way they won't slide off the form or move around under clamping pressure.
Step 5: Build the Form - Third Assembly
We will now use the clamping cauls to attach the final plywood face to the form.
I used a glue roller to spread glue on the form and on the 1/4" plywood face. I added the clamps making sure they were clamping against the hardwood blocks on the front and the ribs on the back. You want to make sure you have even glue squeeze out along the edges.
Step 6: Build the Form - the Bottom Alignment/support Pieces
One of the most important aspects of a successful bending form for me is that NO GLUE STICKS TO IT!
There is A LOT of glue used in bent lamination, and you don't want to have to scrape dried glue off the form. It would be a waste to use a form only once and then have to throw it out because there is dried glue everywhere that won't scrape off.
With that in mind, the parts of the form that will come in contact with the glue-up will be glue-proof!
These supports work to align the plies while clamping pressure is being applied. With glue on the faces of each ply, the wood is very slippery and if you need a 3" wide piece, you may only get 2" of usable material if the plies slide out of alignment. I used hardwood blocks and cut a notch to accept a piece of old cutting board. This way, the ply's get clamped against the HDPE and no glue sticks to it. I also won't need to wipe the form with paste wax every time I use it.
Step 7: Finishing the Form
I coated the front face of the form with epoxy because it provides a smooth surface and wood glue won't stick to it! Just be sure that the face stays smooth, I had the epoxy pooling on the edge and it needed to be sanded flat.
Once the front face was coated in epoxy, I wiped the rest of the form with paste wax and screwed on the support pieces.
Step 8: Prepare the Stock and Do a "Dry Run"
I used regular pine pallet wood to create my test slats. I cut them about 3/16" and planed them down a little more than 1/8". The curve is relatively shallow so pine will take the curve and not break.
If you are doing a severe bend, you will need to make slats 1/8" or thinner, and pine is...probably not the best choice. But in my case it is free and will work...
You will notice in the second photo the clamp divots that were left behind when gluing on the face board. This is what you do NOT want happening to your final piece. Don't skip this step...make good cauls.
Put all the pieces together and add clamps without any glue, this is good practice to find any problem areas before you are doing the final glue up.
It's also worth noting that I did my glue-up on the light table, which is a glass surface. The excess glue cleans up really easy. If you have a beautiful maple bench top - beware. It is also a bit of a pain to scrape glue off concrete, so do your glue up on a plastic bag or somewhere that is easy to clean up.
Step 9: Final Glue Up and End Product!
The glue up went like you might expect, hurried and frantic...which is why there aren't a lot of process photos from this step.
Once to glue goes on the boards the clock is ticking. I again used the roller to apply glue to both sides of the ply's, except the two outside faces. In my experience, you can use glue on only one side of the ply's if you apply a thick coat with a 1/32" notched spreader. If you are using a roller like mine, it's best to coat both sides.
The cauls lift the clamping pressure away from the boards enough to get a good, even squeeze. The bent lamination sandwich is in this order:
- The curved plywood form.
- The 5 pallet ply's with glue.
- 1 layer of plexiglass (so glue won't stick)
- 3 layers of hardboard/masonite.
- 1 layer of 1/4" ply with hardwood blocks.
- 1 layer of hardboard/masonite with sandpaper facing the clamps.
Let this cure where it is warm (wood glue won't cure if it's cold!!) for at least 24 hours. Woodworking tools hate hardened wood glue, so scrape the excess off the edges before you cut it to final size.
I sandwiched 3 light pine boards with 2 darker cedar boards (all pallets) to create the final piece. My thought on the chair is to use a tough hardwood on the outer layers and pine on the inner layers. The final piece will live outdoors and be painted, so I'm fine using pallet wood- the price is right!
This process requires A LOT of clamps, you basically have to win 2 or 3 instructables contests to afford enough clamps! It is possible to use a combination of through bolts and eye bolts instead of clamps, but I didn't go into detail on that because at the end of the day it's only slightly more cost effective. Just know that it is an option.
The result is a nicely curved board that is strong enough to carry all of Uncle Ted's weight!
Second Prize in the