Flat-pack Interlocking Table

The group to which I belong gets wooden shipping crates from a local business. We re-purpose them into various projects, such as planter boxes, Christmas and Halloween decorations, and other practical items used in our workspace. This project was inspired by a picture of a similar table sold commercially so I wanted to see if I could do something like that. The main idea was to design it so that it could be easily replicated, used re-purposed materials and would be easy to store when not in use. This table's legs interlock using slots and the top is fastened to the legs using three removable woodscrews.

I especially liked this project because it was challenging to figure out the angles that would allow the pieces to fit together. Included are short explanations of geometry involved (optional).

Materials:

1. (3) 24" x 48" x 3/8" (610mm x 1220mm x 9.5mm) plywood sheets (thicker plywood okay)
2. (1) 10" x 10" x 3/4" (254mm x 254mm x 19mm) plywood piece (optional)
3. (3) 1.25" (32mm) woodscrews

Tools:

1. Saw (handsaw, jigsaw, circular saw, bandsaw, table saw)
2. Drill
3. Measuring tape
4. Protractor
5. Assorted sandpaper
6. Rasp or file or rotary tool (ex. Dremel)
7. Screwdriver (head to match woodscrews)

The leg pieces are measured and marked according to the plans. It is helpful to mark the first piece, cut it, then use it to mark the remaining leg pieces since they are identical. The table top is marked according to the plans. Since I used 3/8" plywood, I cut two identical pieces and glued them together for extra strength.

Note: Using power tools can be hazardous. Use at your own risk. Safety first.

Optional reading: Bottom (short side) leg angles are 30 degrees from the perpendicular to the bottom edge of the plywood. Top (wide side) leg angles are 60 degrees from the perpendicular to the top edge of the plywood. This is seen by drawing the perpendicular line from the bottom edge to the top edge to form a right triangle. The sum of the 3 angles is 180 degrees (right angle 90 + 30 + 60).

The tabletop shape is optional. You can use whatever shape you want (circle, square, oval). The polygon shape I used matched the angular theme. Measure and mark the points around the edges and draw diagonal lines as indicated.

The slots are also based on 30 degrees. It is helpful to cut out a template from cardboard or paper to keep the lengths of the slots consistent. This will help to mate the slots properly.

The marks on the template are set at 3.5 (89mm) and 7.0 (178mm) inches. With the template aligned at the top edge of the leg, mark the piece at 3.5 and 7.0 inches. Diagonal lines are drawn to indicate where the slots are to be cut (see photo). For the first slot, draw a line from the first mark to the second. The width of the slot equals the thickness of the boards used for the legs (in this case 3/8 inch). Draw a parallel line 3/8" above the first line. Complete the slot marking with a perpendicular line from the 3.5 inch mark to the parallel line.

Please note the position of the slots for the three leg pieces in the photo from Step 1. On one leg, both slots point downwards. On another leg, both slots point upwards. On the third, one slot points upward and the other downward. This allows the pieces to mate properly. Continue marking the slots on all three leg pieces using the photo as a guide.

Using the saw of your choice, cut and remove the slots. Label each piece as follows: A (two slots facing up), B (one slot up and one down), C (two slots down). Accuracy is helpful for a snug fit, but some leeway is okay if the slots are slightly wider than the thickness of the wood.

*The following is the most challenging thing to understand for this project to work.

Since the slots meet at an angle, the slots need to be beveled. Please note the top view photo showing the intersection of the leg pieces. If the joints were at 90 degrees, no bevel would be needed since the slot is the thickness of the wood.

Another consideration is that the bevels need to be on opposite sides of the wood for any given slot. The drawing shows why this is so. Mark each leg piece on each side either "in" or "out", with "out" being the visible side of the leg after assembly. On the next drawing, the side of the slot for the bevels are indicated. If you notice a pattern, on the "out" side of the boards, the bevels are on the sides of each slot closer to the vertical edges of the legs, and vice versa for the "in" side.

Using a rasp (or file or coarse sandpaper or rotary tool), cut the bevel about 60 degrees for the length of the slot on the designated side. It may be necessary to adjust the bevel using trial and error for the slots to fit in the next step.

Optional: The bevel angle of 60 degrees was derived from the geometry of the angles where the legs meet. The last drawing shows that the three legs form an equilateral triangle along the inner edges with each angle being 60 degrees. The inner and outer faces of the boards are assumed to be parallel. The bevel angle is the same as the equilateral triangle angles since the line of one side of the equilateral intersects a parallel line of the other.

Keeping the "out" sides of the legs facing you, start with pieces A (two slots up) and B (one slot up and one down) and fit them together using the slots. Then fit piece C using the remaining slots. Ensure that the top edges are relatively even. If not, the slots may not be fitted properly or they may need adjusting.

Note: photos are from a smaller mock-up and not the actual pieces.

Place the leg assembly upside-down on the bottom side of the table top facing up. Center the leg assembly by measuring from the top of each leg (wide edge) to the wide edges of the table top (if you are using another shape for the top, then center appropriately). With the leg assembly in place, draw lines along the inside edges to form a triangle. This marks where the connector will join the top to the leg assembly. Marking both the leg assembly and table top at one corner may be helpful for final assembly.

The connector is 3/4" (19mm) thick, using either 3/4" board or two pieces of the 3/8" board glued together. Measure the length of the sides of the triangle drawn on table top. Since the leg assembly fans outward, the connector will be a triangle beveled outward. With a 3/4" thick connector board, the triangle for the larger face is 0.20 inch (5mm) larger on all sides. To get the larger triangle of the connector, draw a line parallel to each of the the original triangle's sides that is 0.20 inch away. To be clear, this is NOT simply another triangle with sides that are 0.20 inch longer. New triangle edge is 0.20 inch away from original edge. I cut off the corners of the triangle to match the shape of the table top, but this is optional.

After cutting the connector, place leg assembly on the bottom of the table top to ensure a snug fit for the connector. A good fit is needed to hold the top to the legs.

Extra credit: Calculate the length to be added on to all sides of the connector to get the proper bevel of the connector. See drawing to get started. Helpful link: calculator.net/right-triangle-calculator.html

Ideally, the holes should be equidistant within the triangle drawn in the table top. This is so that the connector holes will line up after disassembly and reassembly, regardless of how the piece is oriented. Alternatively, you can make a mark on the connector and a mark on a specific leg piece for alignment.

Mark spots for the three holes in the middle of the shorter segments (pictured above) and the same distance from each edge. Leave yourself some room to turn the woodscrews. Drill as marked through the connector with a bit that is a little smaller than the shaft of the screws. With the leg assembly upside-down on the bottom of the table top in the centered position, place the connector inside the leg assembly with the small side against the underside of the table top. Fasten the connector to the table top with the screws through the holes drilled earlier.

Turn the table assembly right-side up. Check to see if the table top is secure to the leg assembly. If it is too loose, it could come off unexpectedly. You may need to make another connector to fit properly.

Congratulations! You have completed your table. Sand and finish with paint or varnish as desired.