Introduction: Strata Bench / Fabrication / Part 3 of 3
This project is an exploration into form and workflows - a combination
of using manual hand skills, computer aided design (CAD), and digital fabrication techniques - to create a sculptural and functional bench made from stacked and carved plywood.
In Part 3 - Fabrication, I'll cover how to cut out, assemble, and carve the full scale bench.
Once the design was modified and finalized, I scaled the mesh model to 1:1, figured out the material thickness, and sliced the parts in 123D Make. I sourced high quality Baltic Birch plywood that is 3/4" thick. The final bench will be 8 feet long and after nesting the parts, it will take 9 sheets of plywood to fabricate. The steps for exporting the 2D layouts is identical to Part 2 above.
- Water jet cut the plywood parts.
- Use a vacuum bag to glue them into sections
- Assemble smaller parts into a single solid form.
- Use handheld grinders to carve it into a smooth surface.
Step 1: Hello OMAX, Trusty Water Jet Cutter
Here at Pier 9, we've got an OMAX 5-axis waterjet. She's a beast - clean cuts through metal, wood, glass, tile, felt, you name it, with a very small kerf, and none of the work-holding problems that come with a rotary cutter - the water jet is by far the most popular tool in the workshop.
I repeated the steps from slicing and exporting 2D layouts from Part 2, except at full scale, and ended up with 9 full 4' x 8' sheets of parts, which I cut out on the OMAX water jet cutter. These layouts first need to be brought into the OMAX Layout software to create toolpaths in order to run the machine. One must take the 4 hour OMAX orientation class with Martin at Pier 9 to run the machine. For detailed instructions about creating tool paths and running the OMAX, check out Paige's lovely Instructable about how to set up and run the OMAX. Here are a couple tips:
- OMAX Layout software doesn't understand splines, so the 2D layout has to be saved in the earliest version of Illustrator to be imported into OMAX Layout.
- Use Quality setting 5 - faster cuts means your plywood sits in water for less time. The edges don't need to be super clean anyways.
- The Score setting still cuts really deep in plywood, so I deleted all the red labels from the Make layouts, left only the CUT outlines, and just wrote part numbers directly on the parts after they were cut out.
Attachments
Step 2: Get Your Parts in Order
The bench is made up of 61 layers, a total of 90 plywood parts.
Because I didn't use dowels (I didn't want to risk grinding through and exposing a dowel), I had my 123D Make model open and used the Assembly step to help layout the parts for gluing into stacks. I decided to glue up the parts into smaller stacks that were more manageable using the vacuum bag, then glue the stacks into a whole body with clamps. With each stack, I was thinking strategically about in what order I would glue them up in order to get the whole bench together at the end. It made sense to get the bottom half together and then add the top half at the end, once it was sitting flat on the feet.
Step 3: Vacuum Bag Glue Ups
A vacuum bag is great for getting even pressure for large scale woodworking glue ups. The bag can't take anything too big or thick, so I maxed out at 6 stacks (4" tall). I used the following supplies for this step:
Tools and Supplies
- A large vacuum bag set up.
- Titebond 3 (I used about 1.5 gallons).
- 8" paint roller and tray.
- Damp rag for cleaning up squeeze out.
- Lots of scrap cardboard.
- Pneumatic nailer and 1 1/4" nails.
I started by using the paint roller to lay an even layer of wood glue on both sides of each part. I used the pneumatic nailer to tack each layer in place so they wouldn't shift once they were in the bag. Once the stack was glued and nailed, I carefully slid it into one end of the bag, put layers of loose cardboard sheet over the form (to buffer the bag from glue and sharp edges), close up the end of the bag with the air tight seal, and turn on the vacuum. Make sure you wait until the bag gets super tight before walking away. Leave in the bag at least 45 minutes for the glue to set. I used the vacuum bag to glue up about 10 sections, and used clamps for the rest.
Step 4: Assembly Ton's O' Clamps
Most of the glue ups will have to be done with standard clamps - I used various kinds - bar clamps, pipe clamps, and cabinet clamps. I was able to assemble the bottom half of the bench form, flip it over, on the feet, and attach the last couple top sections as the last step. I used a straight edge to approximate the flat for the stepped feet bottom.
- The clamps must stay on for at least an hour (I left most clamp set-ups over night).
- Put glue on both surfaces that you will be clamping together.
- Use as many clamps as it takes to get a good glue seam with even pressure on all sides.
- Use enough glue to have squeeze out, but not too much that it becomes a gluey mess.
Step 5: Grinding and Making Dust
This is a repeat of making the scale model, but in full body! I was all over the bench. Much of the time I was up on the table, above, below, around, under, holding the grinder at all different angles. I max out at about 4 hours of grinding at a time, to
TOOLS FOR SHAPING:
- 4 1/2" Angle Grinder with variable speed.
- "Holey Galahad" abrasive discs for the angle grinder (made by King's Arthur's Tools).
- Pneumatic Die Grinder.
- Double cut carbide bits with 1/4" shank for the die grinder.
- Sand paper.
- Rubber mat and clamps for work-holding.
SAFETY EQUIPMENT:
- Face Shield
- Ear protection
- Respirator, snug fit with particulate cartridges (see my Instructable about How to Clean your Respirator Mask).
- Gloves are optional - just be real careful! The can easily get caught in the spinning tools.
Step 6: Making the Feet Flat
This part was a little tricky. The feet were way out of alignment when I finally got it upright. I started with a rough grind just to make the steps on the bottom of the feet flat. Looking at the largest gap, I scribed a line all the way around the foot, using a block of wood roughly the same dimension as the gap. Then I flipped it back on its side, carefully ground away material until I hit the line. It actually worked pretty well!
Step 7: Finished!
Order of operation is always rough to fine. After angle grinding, the form is ready to be sanded. An orbital sander with a 5" disk was small enough to get most of the bench sanded to 220. All the parts that the sander couldn't get to, I sanded by hand.
FINISHING SUPPLIES:
- Tac cloth.
- Respirator with chemical cartridges.
- Gloves.
- Blue shop towels (lint free).
- Oil or varnish of your choice. I used Rubio Monocoat - its actually a floor finish, but works great for furniture. It comes in many different tints, has a satin sheen when dry, and you only have to put on one coat. Its worth the steep price.
Once everything is sanded, I wipe the dust off with the tac cloth. To apply finish, you need to be in a clean, dry, dust-free, well lit, and well ventilated area. My shop has a spray booth and drying room shown in the photos. Start at one and and I worked my way across, saturating the wood, and wiping it back with a clean cloth about 10 minutes after applying. Leaving it to dry for at least a few days is a good idea. I was able to move it two days later, and it will be fully cured in 5-7 days. Done! For good measure, I promptly dusted and swept my space to get rid of lingering dust, which had covered everything. Now, I'm ready to start the next project.....
Thanks for reading!
