Introduction: Twin Lock Box

About: My name is Erhan. I am a mechanical/manufacturing engineer. I am an amateur puzzle collector, craftsman and designer. I like mechanical puzzles, board games and wood works. I write also a small non commercial…
“Twin Lock Box” is a wooden combination lock puzzle box that I designed. I applied the mechanic of my “Tricky opening bottle” to this box.

This puzzle box has five keys on both side. You must push all wooden keys to correct position for opening the box. After opening the box you can change order of keys and you can make your own box configuration.

I designed an other variant of this box with hidden keys. This variation has a fixed key configuration. You can see my other puzzle box and puzzle designs from my blog “Diypuzzles” and you can find a lot of free plans.

Step 1: Box Mechanism

On both side of the box there are 5 wooden sliding key. Each key has two position and can move until the borders of the box. After opening the box you can remove all keys and you can make your own configuration by changing their order. If your friend have the same box, he can't open yours.

This puzzle box has three main section
1. Wooden keys
2. Box
3. Box cover

I started to make this puzzle by making wooden keys. After I made box and box cover.

The overall dimensions of this box is about 70x82x150 mm. But I advise understanding the design and making your box according to your materials an the dimensions. This is a simple design for converting dimensions.

Step 2: Materials and Tools

Warning: Wood working and power tools are dangerous. I share in this document my personnal experience. If you don't have enough woodworking experience or if you think something wrong or dangerous don't do it. I am not responsible for any demage. Everything is your own responsability.

Wood choice:
For this project I prefer hard woods. When you change the key configuration, you must memorize it for opening your box later. For this reason you must know front and back side of the puzzle. But I don't put any mark or ornament on the box. Instead of this for making the box I used a kind of olive wood. This wood has a rich texture and it's easy to memorize the front side of the box. For inside construction I prefer ready beech stocks from a big hardware store.

But if you like make a cheap prototype you can start with plywood. The wood choice isn't critical. But I advise using hard woods especially for keys and frames

For making this box I used a hand mitre saw, a jewelery hand saw, a small desktop table saw and a drill press. Acoording to your woodworking experience you can fnd your best methods.

Box Material List

All dimensionsare in mm

Wooden keys 10x10x58   10 pcs
Slider support columns 14x10x70  4 pcs
Frame base and top 10x10x70  4 pcs
Left and right panel 6x70x70  2 pcs
Front and back panel 6x70x118  2 pcs
Bottom panel 6x70x150   1 pcs

Cover Material List

Slider body 30x10x70   2 pcs
Cover plate 6x70x150   1 pcs
Wooden pins D3x10    10 pcs
Wedges for corners 20x20x30  2 pcs

Step 3: How Wooden Keys Works

There are 10 keys. Some of them are left hand and some of them are right hand. Left hand keys are in open position when they are pushed to the left and right hand  keys are in open position when they are pushed to the right

I made 5 right hand key and 5 left hand key. You can make any combination that you like.

The 3D picture show only left and right keys. NOT IN SCALE. See drawing for key dimensions.
Total dimension of each key 10 x 10 x 58 mm

Step 4: Making Wooden Keys

On the photo below you can see two method for making the keys. The keys at the bottom of the photo is made with my first method. ( with clear color made of beech) For making them after cutting the slot with a jewelery hand saw, I drilled two holes and connected them with a knife. This method is difficult for making the keys

I developed an alternative method. The keys on the top of the photo (made from mahogany and with dark color) are easy to make. You can cut the slots easily with any method that you like. I cut them by using a hand mitre saw and a jewelery saw.

Step 5: An Alternative Key Design for a More Complex Box

In this box design keys with two position works fine and I think enough tricky for fun and keeping content from curious eyes. But if you like something moere tricky you can make and add some keys with a third position. You can open this box only when this key is at the center position

Step 6: Making the Box Frame

The picture show the box construction before mounting front and back panels.
Prepare the material given by the material list. Make a grove 5x5x10 mm to the middle of the “frame top” pieces.

When making assembly I used small paper pieces for giving necessary clearances.

Step 7: Left and Right Side Assembly

I assembled first left and right sides. 

Step 8: Box Construction

After I mounted them on base plate. When you fix the panels check them with a machinist square or carpenter square.

Step 9: Fixing Front and Back Panels

For front and back panels there are a planned dimension. But that change related to material thickness. For a best fit, I prefer cut them after mounting left and right panels by measuring real dimension. But this is your choice according to your wood working experience.

Step 10:

Now your box and keys are ready, you can make the box cover

Step 11: Making Box Cover

The box cover is a flat wood pieces as seen on the photo. Two cover sliders are glued on this wood pieces. For fortify the construction two triangular wood pieces glued to the botom of that sliders as seen on the photo.

Step 12: Making Sliders

Wooden pins are cut to 10 mm length. Drill holes 3mm diameter and 6 mm depth. After fixing wooden pins, you will have 4 mm length wooden pins. Distance between pin centers is 10 mm and very precise. For easy marking pin locations you can use a compass. Instead of 3mm wooden pins you can use 3 mm brass pins. You can cut them easily from a brass welding rod.

Step 13: Fixing Cover Sliders to Correct Position

Gluing sliders to correct position on cover precisely is tricky. I put small cardboard pieces under both slider. They became higher than the box height. And it's became easy to fix them in correct position. To fortify sliders I used two small triangular wooden block. But according to your material and wood working experience, you can mount them with wooden dowels.

Step 14: Finish Your Box

Now your box is finished. Close the cover and sand it. I used 80, 120, 180 and 300 grit sandpapers. For finish I prefer an oil finish. Boiled linseed oil, teak oil or danish oil are alternatives. But this is your choice. 

If you like puzzle boxes and puzzles, you can visit my small non commercial puzzle blog and you can find more woodworking projects.