I've wanted to make a sneaky magnetic geocache for a while now, but until recently I didn't know how I wanted to do it. I was inspired by this instructable to make a container out of a bolt. My idea was to hollow out a bolt, and to use a magnet to attach it to a metal object, where it should look like an ordinary bolt.

After a bit of messing about, I decided to mount a magnet on one side of the nut, and to use the threaded section of the bolt as the container. When assembled and in position, this should simply look like an ordinary nut and bolt. Here's how I made it.

Step 1: Materials and Tools

Materials:

A large bolt, preferably 12mm in diameter or larger
A rare earth magnet, I used one 6mm in diameter and 1.5mm thick
Super Glue
A rubber O ring

Tools:

Electric drill (a drill press would have been great, but I gotta work with what I have available...)
Hacksaw
Metal ruler
Centre punch
A file
Drill bits in a range of sizes from 3mm up to desired size (I ended up with an 8mm diameter hole)
Vernier calipers (not really necessary, but handy)
A pencil
Scrap plywood to make a jig to find the centre of the bolt

Step 2: Prepare the Bolt

The first thing to do is to cut the head of the bolt off, as it is not needed. I initially started to do this with my Dremel, but I'd not charged the battery, so I had to switch to a hacksaw. In the end the hacksaw was quicker anyway.

I needed to cut the head off leaving as much thread as possible, as the bolt was quite short. I filed down the cut end to remove the sharp bits left from cutting (I found out the hard way that the thread is quite sharp where it was cut, ouch...). Having done that I then cut a 3mm long length of the bolt off to become the bottom part of the container that held the magnet.

Step 3: Making the Magnetic Part

The first step to make the magnetic part was to drill a 6mm hole in the short section of bot cut off in the previous step. My apologies, somehow I forgot to take photos of this step. I wasn't too fussed if the hole for the magnet was centred or not, and in the end it actually helped to have it off centre. I was careful not to drill all the way through the short section of bolt, and ended up leaving the magnet sitting slightly high.

I then screwed the short section of bolt into the nut, making sure that the hole for the magnet was on the outside. I ended up using a nail in the hole for the magnet to get the short piece screwed in. I test fitted the magnet to make sure the nut sat flush when placed on a flat surface. I then used super glue to hold the magnet and short section in place.

Next I needed to make sure the container was waterproof. I found the smallest O ring I could, which turned out to be too big to fit inside the nut. I cut a small section out of the O ring, which allowed me to fit it inside the nut, hard up against the bottom. I hoped that once the long section of bolt was screwed in that this would make the whole thing waterproof.

Step 4: Hollowing Out the Bolt

The next step was to hollow out the long length of bolt. After cutting off the head and the base of the nut section, I was left with an 18mm length of bolt. I decided to drill down 15mm, so that I didn't drill right through the full length (which would have ruined the container). I used a piece of electrical tape to mark a length of 15mm on my drill bits as a quick and easy depth gauge.

In order to get the largest hole possible it was going to be necessary to find the center of the bolt. After a bit of head scratching I ended up making a quick jig using a scrap of plywood. I first marked a square, and draw in the diagonals. I then drilled a hole the same diameter as my bolt at the point where the diagonals crossed. I was then able to screw the bolt into the hole, and scribe the diagonals onto the bolt, which ended up being very nearly perfectly in the centre.

I then used a centre punch to make a small dent at the point I marked, to ensure that when I started drilling that the drill bit didn't wander. I started out by making a 3mm diameter pilot hole to the correct depth. This was by far the most time consuming part of the build. It's important to resist the urge to press down hard on the drill, it won't drill any faster, and doing that is a good way to destroy the drill bit.

Having drilled the pilot hole, I then drilled the hole larger in increments, using 4mm, 5mm, 6mm, 7mm and finally 8mm drill bits. Enlarging the hole was far quicker than drilling the pilot hole, and I soon had the hole the right diameter. I checked the depth of my hole, and opted to drill it fractionally longer than my initial 15mm, being careful not to drill too far (a mistake I made on my first attempt at this...).

Step 5: Putting It All Together

With the thread section drilled out it was time to make sure everything fit together, and was waterproof. I screwed the threaded section into the nut, and stuck it to a metal filing cabinet to make sure it looked like a real bolt. It looked fairly convincing stuck to a metal filing cabinet, so I'm happy that it will look okay when placed in its final location.

I also needed to test if it was waterproof, so I put a scrap piece of paper inside and screwed it closed. I filled the kitchen sink with water, chucked it in and left it for 30 minutes. After waiting for 30 minutes, I fished it out of the sink, dried it off and opened it up. The paper inside was bone dry, so I felt confident that this will keep out any water that it would encounter in use.

As a final test I put an actual log sheet inside and repeated the waterproof test. I was glad I did, because I found that my smallest log sheet protruded out of the bolt by a fraction of a millimeter. I tightened it up gently, and chucked it back in the sink for another 30 minutes. When I got it out, I was very happy to discover that my cut down O ring had done its job perfectly, and my log sheet was dry.