Spine and rib construction is key to this design.
Thanks to advanced design tools, an entirely new category of modern architectural details, furniture design and art has emerged. Parametric design is its name. It was created using specialized software tools.
Parametric furniture is made up of a series of stacked shapes or rows that are evenly spaced. This creates the illusion of larger three-dimensional forms. Usually, these exacting shapes are cut on a CNC and assembled in several ways. To form large laminations, stacks of parts are glued together. Evenly spaced shapes can be held together by complex internal structures or drew together using threaded rods and spacers.
I am a furniture maker and an artist. I have been researching parametric sculpture and developed new techniques that woodworkers can quickly use to build their parametric furniture. This technique is called spine and rib construction.
As the name implies, the main components are a spine and a lot of ribs. The ribs are positioned so that they fit around the spine’s shape. You can have your spine straight, bent or curled in many different ways. This feature allows the builder to experiment with the overall look by tweaking the spine shape. And, because the plywood parts have large, glued, connecting surfaces there is no traditional joinery. The bench in this article is based on the shape of classic Cabriolet furniture legs.
Rib DesignThe bench consists of 48 ribs plus a spine for an overall length of about 91 cm was made from two sheets of 3/10 cm Baltic Birch plywood. You can also make a shorter version with 24 ribs. This makes it a great side table. Certainly, make a longer version or change the rib shape as you prefer, but keep in mind the weight and the details used to produce 24 Cabriolet leg-shaped ribs per sheet.
I developed a variety of rib shapes during project development and made a few full-size and scaled models. Besides shape and efficiency, Ive discovered that its important to consider the shape or symmetry of the top corners and the size and position of the slot in the middle. If you create a new rib design, these factors will affect the overall look.
The ribs stack around the spine. The spine’s shape is followed by the bench as you push the ribs against the spine.
Spine Design
Because of the slot, the ribs are pushed against the spine to collectively form the overall bench shape. This makes the spine design important. For a Cabriolet-like look, I designed spines with sweeping curves.
The spine shape determines the overall shape of the bench.
Spine designs carry both limitations and possibilities. If the spine curves too short, the ribs will protrude more than progressively moving in and outside. I suggest gradual changes over at least 46 cm or so. Bonus: If the sides of your spine are parallel, curved or straight, you may choose to have a curved rather than flat seating area.
Using the same ribs with different spine shapes, you can create several different benches or side table designs.
Construction Technique
There are several ways to build this project. There are several ways to build this project. You can use a CNC machine to cut your parts, or you can use a smaller CNC to create the pattern and sample.
It is possible to quickly make large quantities of parts with a CNC that can machine large pieces of plywood. If you dont have one, Ive come up with a couple of new techniques for imitating the accuracy and repeatability of a CNC using a router.
Get started with your patterns
This project required me to make the following:
The Shaper Origin is a handheld CNC router. It registers itself using marker tape and auto-adjusts to the programmed cut.
The full-size sample is used to lay out the parts youll cut from 3/10 cm plywood. The second is a special offset design. It is 1/20 cm smaller around, which allows for parts to be shaped with a router and a 3/20 cm router bit. After drilling the holes at the ends of each rib as shown on the drawing, and having completed both patterns, you can use a marker for the smaller offset shaping pattern to ensure it is not confused with the sample.
Materials
Most parametric designs feature visible and touchable plywood edges. This is furniture and not the type of project that would suit low-quality plywood from home centers. They have thin top layers, few layers, and voids in their core. Just like carefully selecting hardwood for fine furniture, your material choice is important. Baltic Birch plywood is the best material for parametric furniture due to its many solid core layers, and 0.005 thick top layer. There are many grades available, from the most basic to the finest Eurocore plywood. This project was made with standard Baltic Birch plywood.
Lay Out the Parts
It is difficult to handle heavy plywood sheets 4×8 inches in size. If you don’t have a large CNC, it might be worth cutting sheets into thirds. (Ive included a layout drawing for efficiently yielding eight parts out of 35 cm x 420 cm pieces.) Next, position your ribs on the plywood. Layout the full-size sample Rib, not the smaller offset pattern. Start at the lower left-hand corner of the plywood and trace around the pattern. Continue tracing the layout using a small ruler.
Cutting Blank Parts
Allow an additional 1/20 cm to be added when rough-cutting blanks. You can use a bandsaw, but I have found that a good jigsaw works best for this job. It gets into the tight corners and doesn’t wrestle with a sheet plywood.
Think Like A CNC
For most projects that require shaping, I use a router table or a shaper. My adjustable shaping jig is used to hold the parts. But, the parts in this project are small, narrow and shaped on all sides which makes holding the pattern and blank safely particularly challenging. You are now ready to try some CNC thinking.
Operators often use what is known as a spoil, or wasteboard, when cutting plywood parts with a CNC. Typically, its a piece of MDF that sits underneath the plywood to be cut. Its function in life is to be sacrificed. The cutter can use a spoil board to cut through plywood and into the CNC bed.
Spoil boards are an essential tool for machining plywood with a CNC. If you use one with your handheld router, you can do your shaping upside down.
We will be doing our shaping differently for this project. Turn the work upside down using a router with a spoilboard. The spoil board can be used for 48 or more parts. For your sacrifice, I suggest a piece or 3/10 cm MDF thats at least 2 x 3 to 4 in size. Plywood works, too, but the layers chip off easily. Use a few clamps to secure the spoil board corners onto your workbench before you start.
No CNC? No Problem!
Begin by creating the shape pattern. Next, make a sample using full-size paper drawings and spray glue it onto.
The full-size sample is used to lay out the parts youll cut from 3/10 cm plywood. The second is a special offset pattern. Its 1/20 cm smaller all around to allow for shaping parts with a router, 3/20 cm router bit and 5/20 cm bushing. Once youve drilled the two 1/20 cm holes near the ends of the ribs as shown on the drawing and completed the two patterns, use a marker to identify the smaller, offset shaping pattern so you dont confuse it with the sample part.
Special Parts Cut First
With a few exceptions, all the ribs in the bench are the same. The standard ribs have two holes near the ends for alignment and holding during shaping that ultimately will be hidden. The exceptions are the visible outside ribs and second ribs where we dont want to see holes. Each of these pieces should be cut and shaped first. Instead of using screws to hold the parts together, you can use two-sided tape to attach the blank to the spoilboard and the smaller offset pattern the blank.
Use a plunge router with a 3/20 cm spiral upcut or compression bit and a low profile 5/20 cm pattern bushing. Because the parts are small and narrow, its all too easy to tip a heavy router slightly as you go and spoil the visible edges of your plywood parts. To help stabilize your router, I suggest gluing up several small support blocks out of scrap the height of the combined 3/10 cm plus
A plunge router, a 3/20 cm bit and a 5/20 cm pattern bushing are used to make the bench ribs.
Begin with the most important parts. Double-sided tape can be used to attach the blank to your spoil board and the pattern onto the blank.
With your pattern, rough-cut parts and routing station all set, its time to mill your pieces.
Shaping the Ribs
To make a smooth plywood cut, adjust your router’s depth to penetrate the spoilboard. After your part, pattern and router are complete, you can begin to cut a 1/20cm section at a time. Next, work your way around the rib shape counterclockwise. Step down in increments until youve cut all the way through and just into the spoil board. For the two special outside ribs, rather than cut a through slot, cut the slot just 5/20 cm deep. Cut the slot through for the second ribs. Repeat until you have two of each of these special ribs.
It is much easier to produce large quantities. The parts and the pattern will be assembled right on the spoilboard. Place your blank piece on the spoilboard and place the middle of the pattern on it. Then, hold the two pieces together and drill holes of 1/20 cm through the pattern holes. Use a deck screw to attach the pattern and the 3/10 cm blank stack to the spoil board. Once you have your support blocks and plunge router in place, start shaping the special ribs. Each time you complete a rib, move your setup to a new location around the spoil board so youre not using the same holes. After a couple of dozen parts, the spoil board will be pretty beat up. Just flip it over and use the other side.
With the ribs cut out, I care-fully sand to 220 grit with random orbit and pad sanders.
Then add a small chamfer with a trim route, before softening the edges with 320 grit by hand.
Set aside the end and second-from-end ribs and the ribs modified to accept furniture glides.
Make the Spine Once you have assembled your parts, it is time to create the spine that will define the shape of your bench. The bowed design is what I prefer, but you can use any shape. Make sure the spine is at least 1/10cm wider than the slot. For length, measure your stack of all your parts and add an extra couple of inches to make assembly easier. As suggested, keep the spine shape simple and curves gradual and consistent. With no reason to use fancy plywood, my spines are made from scraps from other projects.
Test fit your spine to see if it moves easily into the slot. I sized the slot drawings to just match the default thickness of 3/10 cm plywood so chances are it will be too snug. Use a belt sander, to gently and evenly sand one side of the plywood spine until it fits into the rib slots. You can soften the edges of your spine to make glue-up and dry fitting easier.
Time for a Dry Fit
Dry fitting is one of the most important steps in this project-it also happens to be the most fun. Putting it all together allows you to see and adjust what the overall bench shape will be. You can easily create a new spine to change the shape and width of the bench that you have created.
Dry fitting is one of the best parts of the projects. You get to try out different shapes of the spine and change the look of your bench.
When dry, place the bench upside down on its bottom. Alternate ribs back and forth until your bench is assembled. You can skip the second and special ends for now. Carefully rotate the now heavy bench right side up. To create the bench shape, lift each leg forward until the inside bumps into your spine.
Leg Spacers Stiffen Bench
Once you have a shape you like, carefully clamp the set together. Youll notice that the heavy top is rigid but the legs wiggle a bit at the bottom. Whats needed are spacers between each leg. There are a variety of ways to do this, but for the cabriolet shape of the legs, I chose to compliment the rounded shape with round spacers. I made mine from commonly available
For spacers, I used a closet rod. After smoothening the rod, I made test cuts to match my plywood thickness. Then, I batch produced a few with 1/20 cm alignment holes.
To avoid confusion with the dozens of parts youll deal with during assembly, set aside the two sets of special outside and second rib parts. We will modify four main ribs to make them more compatible with furniture glides. Set up a table saw and fence to make a small flat spot by trimming off just 1/41 cm off the feet of the four ribs. Slide the flat part of the rib along the fence. Set these four ribs aside and add four more regular rigs to your collection of special ribs. There should be two ends ribs that have no through holes or slots, two second ribs without holes, four regular and four flattened ribs for attaching furniture glides.
Final Preparation
With a random orbital sander, sand the ribs with 220 grit sandpaper. Remember, the top layer is only 0.005 thick so use a light touch. It is harder to smoothen the edges which can be seen and touched. As a furniture maker, I have both spindle and edge sanders. However, if you don’t have either, a pad sander with a controlled speed or a block sander that uses the flat and curved sides of the block sanders should be sufficient. After sanding, I use 1/41 cm to chamfer all edges. Then, I soften the edges slightly with hand sanding using 320 grit.
Assembly in 2 Stages
Excessive glue can be difficult to remove from this project so make sure you apply it carefully. Note that the parts that touch each other during dry fitting are located near the end and not at the shoulder of the leg. As the spine shape is applied and the parts move into position, the glue area varies. The area closest to the end, above and below the slot is where you should apply glue. To keep it from seeping out, I only apply glue to the area around the spine (see image 20). Also, I keep glue about 3/20 cm from all outside edges. Unlike regular furniture joinery, theres a massive amount of glue surface area to hold the bench together.
Apply glue to the two areas shown in white. To minimize squeeze out, keep it away from the edges.
First Backstop
With a lot of parts to handle, to make final assembly easier, Ive found that it helps to first vertically build a backstop. The backstop is made up of the first six ribs. Start with the end rib, with the slot facing up. Like any mortise and tenon, apply glue to the slot and end of the spine.
Assemble the backstop vertically first. Be mindful of alignment and glue squeeze out-its very hard to clean up afterward.
Next, carefully add glue to the top of one of your prepared spacers. Position the second rib as it will be once added to the stack. Using a nail just big enough to pass through the 1/20 cm hole in the special second rib, align the spacer underneath. To support the rib, hold the two pieces on your bench with 3/10 cm scrap. Then use a brad nailer to attach the brad nailer with 11/10 cm brads. Now, apply glue to the other side of the second rib and the spacer, slide the assembly down the spine and align the part. Make sure to dry the assembly. Always push the ribs all the way into the spine until they touch. Use the same process of adding spacers and glue for the next two regular ribs. Add the two flattened, final ribs. With a square, align the short stack of six ribs and spine and clamp or hold down the backstop to your bench and let dry before proceeding to the main assembly.
After the backstop has dried, turn the bench upside down so it rests on its back. Assembling the bench this way uses gravity to your benefit and is the only way to align the parts and keep the bench flat. Now it’s time to assemble the bench.
For the main assembly, push the legs outward as you go and use a clamp to hold the stack of parts together.
Continue to add ribs, spacers and glue, pressing them against the spine. As I go, I use ratcheting bar clamps to pull and hold the horizontal stack together. After you have used up all the regular ribs in your kitchen, add the final two flattened ribs to the stack, then the next two regular ribs, and then the second rib.
Your last step is to trim off the excess spine thats leftover. To do that, use a piece of
Trim off the excess spine with a hand saw.
Use glue to attach the bench assembly to the mortise at the end of the spine and last outside rib.
Dont forget to add furniture glides to the four flattened legs. Being a heavy bench, use small HDPE glides so it can easily be pushed or pulled around.
Be aware that plywood can have a high grain with a water-based coating and may blotch easily with certain colors. If you have a spray system, the bench should be ideally coated with lacquer. Watco oil, which can be applied by hand, is another option. Remember that it can be difficult to get into the little spaces with any hand-applied finish.
Wanting to add a little warmth, I chose a long-time personal favorite: Osmo Polyx Oil. The Ger-man floor finish is made of various vegetable oils and waxes. The very thick Osmo can be applied by hand in the same way as a varnish or rub-on oil. Wipe on. Wipe off. It will give you a rich, durable finish.
The Beginning
Parametric plywood furniture is a great area for woodworking hobbyists to explore. And, using Spine and Rib construction and upside down shaping you dont even have to have a CNC to do it. I am looking forward to seeing your creations.
Head to popularwoodworking.com for more on parametric furniture. Downloadable PDF project files are available to assist you in making your patterns. You will also find details on how to CNC parts or use Shaper Origin to build the project bench. You will also see how the design was made, as well as explore other designs and finishes and tricks for making the most of cheaper plywood.
- Modern Parametric Tips and Details
