Monday, 4 June 2012

Balloon balls

I've seen fabric covers for balloons on a few different websites now, and I thought this would be a fun thing to make for my son, and more to have in our present box for other small people.

Some of the blogs I looked at which have free patterns for balloon cover include:

[Update January 2013: i have since made balls to each of these patterns. See my post about it here.]

Then I googled "how many hexagons to make a sphere", learnt that you can't make a sphere-like form with only hexagons, and ended up browsing polyhedra on wikipedia; in particular the images labelled "net". They lay out all the individual pieces as you would need to make a full solid form. (Look up polyhedron and dodecahedron on Wikipedia, you can see some funky shapes!)

I decided to give pentagons a go, making a "dodecahedron". That's 12 pentagons sewed together, with 30 edges. If I've understood the mathematical formulas on Wikipedia correctly, the approximate radius of your sphere will be 1.3 times the length on the side of your pentagon.If you figure out how wide you want the ball to end up (maybe you want to put 25cm balloons inside, like me, or make one about 18cm for 20cm ballooons), take that diameter and divide it by 2.6. So for a sphere approximately 22cm wide, I should need pentagons with sides roughly 8.5 cm, plus seam allowance. I did a Google image search for Pentagon and found this.

And more Wikipedia research tells me that I need the diagonals, (aka the width of the image) to be about 13.75 cm. If you want to do this with different figures, you can take the length of your pentagon sides and multiply by 1.6.

I've inserted my pentagon image into a Word document, adjusted it so each pentagon (the largest, outside one) is 13.75 cm wide, and copied it 12 times. Printed out, they are about 13.75 cm across, with sides of about 8.5 cm. Now to cut them out, use them as paper patterns, cut out fabric including seam allowance outside that, and sew them all together.

An observance at this point, they seem HUGE! Wonder if my maths is wrong, or my perception.

Anyway, now I have (another) nice little TV project for a few evenings. Or many evenings. I'll let you know how it goes...

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