Make a Torus Out of PVC Tubing

A torus is a circular tube. You might want to make one as part of a toy or ornament, for a school science project, or, as in my case, as part of a prototype to test an invention. Bending metal tubing requires a completely different process to bending PVC (plastic) tubing. There are at least three reasons why you would prefer PVC over metal.

1. PVC is lighter
2. PVC is cheaper
3. PVC does not interfere with magnets

There might be easier methods, but the method i'm going to describe worked the best for me. I am using a tube of 25mm outer diameter with 'walls' 2mm thick. The diameter of the torus in relation to the tube diameter is rather small. That is, i need to bend the tube a lot. This caused problems which i had to solve...

Printed on the tube is "25mm".

To make a torus of diameter D, the tube at the end of the whole process will be D times pi. For instance, i want a torus with average diameter 230mm, so the length of the tube (circumference of the torus) will be 230 times 3.1416 = 723mm. So you need a piece of wood (the "mould") at least as thick as your tube diameter (e.g., d = 25mm) and big enough for you to make a hole in it, where the hole is D + d.

The problem is that if you try to bend the whole length of the tube (e.g., L = 723mm) to form the torus in one go, the ends of the tube will probably deform due to the heat (see later), and if you use a piece of tube even slightly too long, you'll have overlap in the mould and the torus will be deformed. The solution: make two crescents. The length of tubing to make the crescents is L/2 + 100mm. L/2 is half the circumference and 100mm is an extra piece which you'll cut off later to get rid of the deformed ends.

You will need

• thick gloves to prevent you burning your hands,
• electrical/insulation tape to seal the tube ends,
• sea sand or fine salt to heat up and pour into the tubes,
• a funnel (i made one out of two pieces of A4 size paper; make the top opening nice and big and the bottom opening the right size for your tube diam.),
• a pan to put the sand/salt in to put in the oven,
• a spring which can fit snugly inside the tube and which is at least as long as a crescent (L/2 + 100mm) and
• an oven.

Tips:

• I wore two pairs of gloves, one over the other.
• It takes two people to do this, one to hold the tube and one to pour the sand in.

These are the steps:

1. Cut two equal lengths of tubing for the two crescents (L/2 + 100mm, e.g., 723/2 + 100 = 461.5mm).
2. Seal one end of each tube with insulation tape. Have some tape ready to close the other ends later on.
3. Put the pan containing the sand\salt in the oven.
4. Set the oven heat to 230 degrees Celsius / 446 degrees Fahrenheit.
5. Let the sand heat up for 30 minutes.
6. Put the spring into the first (second) piece of tube.
7. Put on your gloves.
8. Take out the pan from the oven. Careful; it is VERY hot.
9. Spoon some sand into the tube thru the funnel. My 461mm tube took only 3 and a half scoops (the spring takes some space).
10. The tube will become limp in about 10 seconds.
11. Seal the other end of the tube.
12. Guide the now flexible tube into the wooden mould.
13. Make sure the tube is pressing neatly against the sides of the wood in the hole.
14. Hold the tube in place for 2 or 3 minutes.
15. Now leave the tube there. Don't remove it from the mould for approx. 10 minutes, else the spring will want to straighten the tube.
16. Place the pan in the oven again.
17. Come back after 10 minutes, remove the crescent and repeat from point 6 with the second piece of tube.
18. Switch off the oven.

You'll find that the ends of the crescents are deformed due to handling. From each end, cut off half of what was added (i.e., 100mm/2 = 50mm). Now you should have two half circles. Glue them together with "PVC cement" to get your torus!

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