Make a Split Bell From an Old Oxygen Tank

I like bells. We visit Bok Tower in Lake Wales to hear the beautiful carillon and enjoy the scenery. It's amazing how much is involved in making a bell. There's a wax model that is cast in sand and replaced with molten metal. When the metal cools, the bell is turned in a large lath. The bell gets a complex outer profile and a smoothed inner surface. Next, it's carefully tuned by removing material until it rings with the right pitch and timbre. Finally, it's finely polished, which brings it into its final tuning and gives it a bright surface.

This split bell is much less precise, but it sounds really awesome for something that can be made in an afternoon from trash. Let's make some noise.

Step 1: Cutting the Slits

The key to the split bell is the two tongues. When the bell is struck the tongues vibrate independently, sounding two separate notes. Also, when a cylinder is struck the sound waves spread out from the striking point. When the waves meet up on the opposite side, they cancel each other out, preventing it from having the long sustain associated with a bell. When the tongue is struck, the waves remain to bounce around the tongue, making the note ring and decaying slowly.

Normally the bottom of the tank would need to be removed. I had already removed the bottom half of this tank for another project, so it was ready to have the slits cut that separate the two tongues. To cut the slits, first I marked them out with a marker and straight edge. I used an angle grinder with a metal cut-off disk to cut a slit on each side of the tank, from the open bottom to the shoulder of the tank where it starts to curve. I used a grinding wheel in the angle grinder to smooth out the outside edge of the slits to prevent cutting my fingers as I made the bell.

Step 2: Testing and Tuning the Bell

With the slits cut, there are two options for finishing the bell. It can be tuned to sound specific notes in order to achieve a particular note interval or harmonic relation. In an installation with multiple bells or an instrument made of bells, precise tuning is very important. When tuning a split bell it's important to work on one tongue at a time. You can use a spring clamp to deaden the tongue you are not trying to tune. I use a digital tuner and a microphone to read the pitch of the tongue. The pitch can be raised by shortening the tongue. By alternately shortening it and reading the pitch, you can gradually reach the desired note.

The other option is to cut the tongues to two different lengths and shorten the shorter tongue until the bell sounds nice. For a non-musical bell tuning is less critical. Just shorten the tongue until it has a nice sound and be done.

Since this bell will hang on the porch as a doorbell I decided to skip the precision tuning and just make it sound nice. I used a short length of rope knotted in a loop and threaded through the eye of a fender washer to hang the bell while I worked on it.

Step 3: Shaping the Tongues

I started by shaping the long tongue by rounding the edges with the grinder. I cut the other tongue to approximately 4/5 of the length of the long tongue. I rounded the corners, then used the angle grinder with the cut-off disk to clean up the edges and round the curves better. I finally used files to smooth all the edges and round the corners.

Step 4: Preparing the Bell for Paint

Before I could paint the bell I had to do some prep work. First of all the bare metal inside the tank was pretty rusty. I used a wire wheel mounted on a rod to clean up the inside of the tank. I used a 4" wire wheel to finish off the inside of the tank. I also used it to clean all the loose paint and rust from the outside of the tank, making sure to remove all the decals and adhesive. I sanded the outside of the tank with course sandpaper to roughen the surface to make the paint stick better. Finally, I wiped the bell down with an alcohol soaked rag to remove any remaining dirt and grease.

Step 5: Painting the Bell

Next I painted the bell. First I gave all the bare metal a thin coat of primer. When that dried I coated the whole bell with marine grade motor paint. Finally, I wrapped the bell with lace and shot a coat of light blue. when the lace was removed, it left the lace pattern on the bell.

Step 6: Hanging the Bell

Since the original hanging rope was covered in paint, I made a new one with a long tail to attach the bell clanger. The clanger was a round disk of plywood cut out with a hole saw. It was painted around the rim with silicone caulk to soften its impact with the bell. I tied a hard drive platted to the rope tail so the wind would catch it and make the bell ring. It also serves as a handle to ring the bell. I hung it on my front porch as my new doorbell.

Unfortunately, I'm reduced to shooting video on my phone since my camera died. Here's the bell in action-