Here's how to make a more sophisticated one that robots will remember fondly in 2050.
Step 1: Ingredients
If you happen to have access to a laser cutter, they are beneficial for making incredibly clean, straight and precise cuts, which, for a project like this is very helpful.
1. Be sure it's as flat as you can find, a warped piece of wood will not be your friend
2. Wood can be any thickness without changing the dimensions of any other parts. However, the wood throughout the project must be of equal thickness throughout, or things won't slide properly.
3. The amount you will need depends on how big you're going to make your puzzle.
My dimension break down for a 20" x 20" puzzle.
1 2.5 x 20"
1 1 x 17.5"
1 1 x 16.5"
(You can cut 2 at 2 x 20" and 2 at 1 x 20" and trim them down to size.)
Total: 11.5 sq. ft of material. This breaks down most easily for cutting into one sheet of (16 x 48") and one of (20 x 43")
clamps or weights
epilogue laser cutter (OR chop saw & table saw)
90° straight edge, if you have one with both an exact corner and built in ruler, even better
stain or paint (optional)
Step 2: Cutting Out the Squares
The simplest way to do this is to calculate the maximum number of tiles that will fit on your material and create a grid to fit those dimensions. You will need to cut three times as many tiles as you will need. Each tile has three layers of material.
8 puzzle = 24 tiles
15-puzzle = 45 tiles
24-puzzle = 72 tiles
I used a laser cutter, this guarantees precision, but a chop saw or table saw can work just as well.
Step 3: Measuring, Marking & Gluing Tiles
1. Take your 90° ruler and mark each tile 1/2" inch on two adjacent sides. The tiles slide together using a tongue and groove method, so two sides will have a 1/2" groove and the other two sides will have a 1/2" protrusion.
Once you have each tile marked, you're ready to glue.
Using standard wood glue, spread a thin layer in the smaller square, but not within the 1/2" border on two sides. This will ensure that you don't have bumps of dried glue interfering when the tiles are sliding together.
Line up the corner of the top tile as perfectly as you can (with it's pencil marks facing up and opposite of the bottom tile's marks, keeping the top lines facing the other way ensures an easy assembly of the third tile). If you need to use a straight edge, you can, but the pencil marks are easy guides to line up your tile with.
Repeat process with third tile.
To check the evenness of the tiles, while the glue is still wet enough to adjust, gently press the two sides into a flat surface. If there is some wobble, make adjustments to even them out. Evenness is very important to smooth sliding of your puzzle.
Clamp or place under heavy weights to dry overnight. There shouldn't be much warping because the tiles are quite small, but slight bends can happen in the smallest pieces.
Step 4: Making Finger Holes
I used a drill press to make 1/2" holes that went about 1/3" deep. The size and depth are flexible, but test out in a scrap piece before committing to any size or depth.
Step 5: Painting or Staining
If you're using nice wood, staining is a good option, or leaving raw if you sand it smooth.
Step 6: Creating the Border: Cutting (the) Corners
The first step in making a snug border is being sure that your angles and cuts are as precise as possible.
With a miter saw, move the blade to the 45° position. If you have an accurate stop guard that can align with, this will ensure uniformity and accuracy from piece to piece.
Cut each of the 2 x 20" pieces so they have 45° corners. Line them up to be sure the angle is correct. Sand away any imperfections.
Step 7: Creating the Border: Gluing It Up
Arrange so the two thicker edges are adjacent to one another.
The order of stacking:
2.5" on two touching sides and 1" on the two opposite sides
Mitered 2" piece
BEFORE YOU GLUE: keep one side (your choice, I left out the smallest side with the 1 x 16.5" piece sandwiched) out of the frame, but still glue up its 3-piece sandwich. You will want to put all the tiles in and then put the final side on. Otherwise you will have an unintended puzzle on your hands as well.
Step 8: Waxing for Slide-ability
Step 9: Assemble!
Now is the time to decide on your image and paint, draw, etch, collage it on top and scramble it up.