Octagonal Bowl (Box Joints)

Box joints are traditionally used to join two pieces of wood at a right angle, with the tabs sanded flush with the surface of each board.  Using extended length box joints on an octagonal bowl, with its 45 degree angles, creates an interesting effect on both the inside and outside of the bowl.

This instructable shows how to design, cut out, and assemble the pieces used to make such an octagonal bowl.

I made this at TechShop www.techshop.ws

Vector drawing software (I used CorelDRAW)
Laser cutter (my TechShop has a Trotec Speedy 300)
150 grit sandpaper
5 minute epoxy
Clear polyurethane
2-1/2" x 15" poplar (1/4" thick)
5" x 5" poplar (1/4" thick)

This instructable assumes some basic familiarity with CorelDRAW and a laser cutter.  If you don't have access to a laser cutter, you can still cut out the box using other tools such as a scroll saw or coping saw.

Each finger of the box joint will be 0.2" tall, so start by drawing a 0.2" long vertical line.  Create five copies of the line with a vertical gap of 0.2" (either drawn manually or with the step and repeat function).

Make sure that you set the line width to hairline so that the laser cutter knows that you want to cut these lines and not just engrave them.

Now draw a 0.75" long horizontal line starting from the bottom of the top-most vertical line.  This establishes the length of the finger joint.  Using the same technique as the vertical lines, create 9 copies of the horizontal line spaced vertically 0.2" apart.  Then close the open ends of the box joints with 0.2" tall vertical lines.  This completes the first box joint.

Make a copy of the first joint and mirror it around the vertical axis.  Move it to the right so that the main body of the segment will be 1" wide (for a total segment width of 2.5").  Keep copying and alternately mirroring joints until you have 9 total joints.  Draw two long lines creating the tops and bottoms of the segments.  This completes the basic design for the sides of the bowl.  Feel free to add clip art to each segment.  You will see later that I added winter themed images to my bowl.

When the sides are assembled, they will form an octagon with a width of approximately 4.25" (as measured from flat side to opposite flat side).  In a new project, draw a 4.25" wide octagon for the base of the bowl.

Now cut out the eight sides and the base on the laser cutter.  The poplar cut well at 100% power and 0.7% speed, and engraved at 100% power and 45% speed. 

If you don't have access to a laser cutter, print out the patterns on standard paper.  Tape the paper directly on to the wood and cut out the pieces with a scroll saw or similar.  Simply follow the lines with the saw blade as closely as possible.  You will end up with somewhat rounded corners for the end of each box joint finger for a slightly different appearance.

Sand all of the pieces with the 150 grit sandpaper (I happened to use a sanding pad here).  Be careful around any engraved areas so that you don't sand off the fine details.  Apply the first coat of polyurethane, let dry, and sand again.  Waiting to apply the first coat after assembly will make it very hard to sand all of the fingers smooth.

It is pretty challenging to epoxy all of the pieces on to the base at the same time and keep them properly aligned.  To keep the spacing as even as possible, I started by epoxying down two pieces directly opposite each other.  Once the epoxy set, I dry fit the next three pieces on one of the open sides.  Their interlocking nature holds them pretty closely in place, but you can slightly lift them from the base and spread some epoxy under their edges.  Then carefully align them to the edge of the base as the epoxy sets.  Repeat the same process on the remaining side of the box.

Finally, we need to keep the top of the bowl from spreading apart.  Add a small drop of epoxy to the inside of the third finger on each of the eight joints.

Once the bowl is fully assembled, apply the final coat of polyurethane to the bowl.  Be particularly careful of drips around the extended fingers of the box joints, as they tend to collect polyurethane.

Enjoy your bowl!