I've had this idea for a jewelry box with a secret compartment rolling around in my head for a few months. There are no metal mechanisms, all movement is caused by wooden springs activated by an unassuming key.
Step 1: Selecting Your Wood
I used a Walnut for the sides and Curly Maple for the back, front, top and bottom. The key is made of Ebony
The amount of lumber needed will depend on the size of box you intend to build. You will need two blocks, roughly the same size, for the sides and thin stock for the back, front, top and bottom.
Step 2: Cutting the Secret Compartment Housing
Using a bandsaw, slice off the two sides of block you intend to have the secret compartment in. Mark them with pencil so you don't mix them up. I use a the triangle method. With the two large sides cut off, cut the other short side to the same thickness. Mark them with a pencil as well and lay them all aside. The remaining block you have will make up the top of the block and the secret compartment itself. Cut the top of it off and lay it aside, and cut one extra strip of material off the top roughly 3/4" (depends on the size of your box). The extra cut off will give you room for the pop out spring mechanism
Step 3: Glue the Housing Together
Using a wood glue, glue the pieces back together lining up the woodgrain and your marks as best as you can. Clamp it together and set it aside.
Step 4: Prepping the Secret Compartment
Using a router or a chisel, cut a groove in to the side of the compartment, this will hold the spring to lock the compartment in place. Cut a small piece of wood along the grain and sand a slight taper on one side. Glue the taper in to the groove you cut to make a wooden spring. Once its fully dry, drill and chisel a hole into the block. This will become the hole for your secret stuff to be hidden. I offset my hole away from the wooden spring so i had room for the pop out spring to push up against it.
Step 5: Making a Pop Out Spring and a Groove
This is very similar to the locking spring, but it will have to be sized to fit in to the top of the compartment housing. Measure the depth of the cut out and subtract your hidden compartment, this is your window to fit your mechanism. Cut a small strip of wood along the grain, sand or cut a taper on one side, glue it to a block of wood that will fit in the gap you have at the top of the housing. Once you have it cut to size, glue it into the top of the housing.
Now use a small chisel or carving tool to cut a lip on the inside of the housing. When the secret compartment is pushed up into the housing the locking spring should click into the groove, keeping it in plane and not letting the pop out spring push it down. This may take a few tries to get right but take your time.
Step 6: Drill a Hole
Drill a 1/16" hole on the inside of the housing where the locking spring will rest in the closed position. Don't make a key yet.
Step 7: Assembly Your Box.
Because of the cutting done on the housing, your two end pieces will be different sizes. Cut the two blocks so the are the same size at this time. I used a router to cut a small grove in the end blocks and glued and clamped the front, back, and bottom in place. Any method you like can be used to assembly the box, it spend on your application. Mount the lid to the box with hinges.
Step 8: Make a Key
Cut the tip off a small 1/16" nail and insert it in to the hole you drilled in the housing. This should be on the inside of your box, near the bottom. Using a small piece of wood, or other object should you so desire, line it up against the nail and mark it so you know the location of the nail. Drill a 1/16" hole in the block of wood and glue the cut nail into the key. This should like up exactly with your hole in the housing. Mine registers against the bottom and front of the box so its very easy to line up.
I've burnt my initials into the bottom of the box and the key to give a slight clue on how to reveal the secret.
Step 9: Finishing Your Box.
Once your box is complete and you've made any adjustments that need to, you can finish the wood and bring it back to life. Sand your box with 220, or higher, grit sand paper and apply stain of finish. This box was finished with Beeswax and Mineral Oil