Furniture Making with Digital Fabrication (2 of 4) Seat and Back

The Seat and the back of the chair are grouped together in this blog post because they are both processed from the same material. Instead of using solid wood we are using a 1” thick Baltic Birch Plywood as the core material. This makes processing multiple chairs very simple using sheet stock on a 4’x8’ ShopBot.

The seat of the chair is set at 2” thick, both for appearance and for strength, so we need to laminate two sheets of 1” inch Baltic Birch Plywood together to make the 2” sheets.

Nesting 2D:

The Seat and the Back are the simplest to nest simply because they are cut directly from sheet stock and because they are both completely 2D cuts. To start with we extracted curves from the edges of the parts and from the areas that are accepting the legs and arms.  These curves are what we will use to create the toolpaths.

The seats of the chairs fit nicely together simply by rotating them and placing them next to each other. (Note: Do not “mirror” the seats because if you are going to accommodate arms you will need to make sure and mill the slots for the arms in the tops of the seats, if you mirror the parts you will then be milling these arm slots on the bottoms or the parts.) The backs do not fit together as perfectly as the seats but still nest well enough to get a good yield from a sheet. The back also cannot be “mirrored” and must be placed face down because you will need to cut a step into the inner circle to support the center decorative piece.

Now that both sheets are nested, the next step will be how we choose to hold down the parts while they are being cut. Both the seats and back are two small and awkwardly shaped to be held down by vacuum pressure alone and we do not want to screw down our parts because of the holes it would leave. Our solution is to use tabbing and to hold down the remaining parts with screws. Since the seats are so closely nested together we will need to add the tabs to the inner circles and screw down the circle pieces to the table.  (The inner circle cut outs are useful in creating a hold down system, but mostly they remove excess weight in the chair that would be a real inconvenience in the final product.) In the backs we can add tabs to the outer perimeter since we can screw down the excess pieces that are left around the parts.

Process of Nesting:

  1. Nest seat with top facing up
    1. Make a closed line where you want to pocket out the slot for the arm connections.
    2. Add tabs to the inner circle for table hold down (when you are screwing down parts make sure that the vacuum system is also running to hold the spoilboard down, or that something is keeping the spoilboard secure and level during cutting.)
    3. Nest the backs facing down, so the part that your back would rest on is against the table.
      1. Make a closed line around the perimeter of the piece making sure to include a slot where the upper part of the arm meets the chair.
      2. Create a line that will generate a toolpath for a step in the inner circle.
      3. Add tabs to the outer edges of the piece since you cannot add them to the inner circle because of the step that will be cut.

We will use a Compression Bit for all of the toolpaths on these parts because it will not tear out the top or bottom layers of the plywood during cutting. We are using two different sizes of Compression Bits, one for 2” sheets and one for 1” sheets.

Steps to cutting the seats:

  1. Make pocketing cuts for the arms in the Seats.
  2. Make the profile cuts around the inside circles including the tabs.
  3. Make the profile cuts around the outside of the parts.

Steps to cutting the backs:

  1. Make the inside step profile cut (this cut only goes 7/8” through the material)
  2. Make the interior circle profile cut.
  3. Make the profile cuts around the outside of the parts including the tabs.

Before we make the Motise holes for the legs we need to attach the seat to the back.

  1. Use a table saw to cut the complementary angles along the bottom of the Chair Back and the rear of the Chair Seat.
  2. Glue the two pieces together and let it set. (Make sure it is really set because we will be milling the Mortise holes into the glue joint.)

Now we need to set up a jig on the backside of the CNC table to cut the Mortise holes in the bottom of the assembled parts.

  1. Fasten the chairs upside-down on the back of the table using blocks and other extra scrap that will stop the part from moving side to side and from lifting.
  2. Create the pocketing toolpaths based on the model that was developed.
  3. Cut the pockets out making sure to leave a little excess space for the legs to fit in smoothly
    1. We used epoxy to join the acrylic legs to the wood seat so leaving a healthy gap in the Mortise was desirable since the epoxy would fill in any extra space.


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