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Mon 30th Sep 2013

Here's the first draft of a shaped elephant trunk. The trunk starts of thick at the top, tapers down, levels off then flares out a little at the end of the trunk. I wanted to move the narrowest point one step downwards but was a daunted by the prospect of having to work out all the dimensions again. This looks like the perfect job for the computer!

I've worked out the size of each cone section (each frustrum) Now I need a way of quickly calculating the dimensions for the unwrapped paper. Starting with the bottom radius r1, the top radius r2 and the height of the section h I've put together a spreadsheet to calculate the inner and outer diameter as well as the angle of sweep of the piece. You can download the spreadsheet here. The spreadsheet is rough and ready but perfectlyserviceable. This is no error checking so you have to make sure you put in the correct numbers. Do that and it'll work just fine! If you don't have a copy of the spreadsheet program Excel (like me), try the excellent free-to-download Open Office. Once you have secured a spreadsheet program, simply download the spreadsheet, open it up and plug in the numbers for r1, r2 and h

The Results are:
fr1 the radius of the larger circle (fd1 is its diameter)
fr2 the radius of the smaller circle (again fd2 is the diameter.
ang shows the angle  of sweep of the final shape.

Plug in the numbers, construct the shape, print out, cut out and make!

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Wed 13th Mar 2013

Friend of the website Edgar pointed out this fun design on the Instructables website. The download with the article was the template of a pentagonal spring. In the pictures, other designs of spring were shown but no templates were provided. Below, I have presented the techniques for creating your own spring with any number of sides.

The template for the five sided spring looks like this. The spring is divided into five vertical sections, the sixth, grey, area is where the sides overlap to make a tube.

Each unit of the spring is made from a parallelogram divided diagonally with a crease line.
The sides of the parallelogram are all the same length. In this case, 38mm.

To calculate the angle of skew of the parallelogram it is simply a matter for dividing 360° by the number of spring side - in this case 5 giving an angle of 72°

360/6 = 60° for the six sided spring.

and for the nine sided spring. 360/9 = 40°

I'll be uploading templates and instructions shortly for those of you who want to try your own.

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Wed 11th Jan 2012

The dome on the recent Flying Saucer project was a rough representation of half a sphere. I could have used Blender to create the net as I did in the banana but decided to use maths. Sometimes its quicker to walk than take the bus.

The original design was based on a dome with a radius of 50mm. I decided to divide it into seven sections round by three sections high. You could change this number if you wanted by changing the numbers in the equations that follow.

To find the length of each side I've used a bit of basic geometry. For this I divide each section into two to make two right angled triangles. I know the length of the long side (the hypotenuse, r) and the angle  in the corner so I can find out the length of side a, then double it to find out the length of each side of the seven sided shape.

The corner angle is 25.7° (360/14) To calculate the length 'a' multiply the length of the long side 'r' by the sine of the corner angle. Google is your friend here. You can find out the value simply by entering "sin 25.7 degrees" as a search. Make sure you use the word 'degrees' in your search otherwise it will give you the answer in radians as I found out. Eventually.

Multiply the answer by 50 to find out the length 'a'. You can use Google again can help you with your calculation. Simply type 50*0.433 into the search box. (The asterisk sign is used as the multiplication sign in computer programming, and web searches) Double it to find the length of each side. It turns out is is 43.3mm. Okay - one down, what's next.

This is a side view of the dome. Each of the three horizontal strips is 30° wide. To work out how high each strip is ('d' x 2) we can use the same technique as with the previous example but this time using sine of 15°.  Google says it is 0.259 with a bit of rounding. Double it and multiply by 50 to get a strip with of close enough to 26mm.

Finally, the values of 'b' and 'c' show how much the side tapers. We don't need an actual length, just a ratio compared to the radius. This time the angles we know are adjacent to the length we are looking for so we'll use cosines. b = cos 30° and c = cos 60°. Back to Google

b= 0.8660 (that's one I always remember!)

c= 0.5 (I remember that too :-)

So the final step...

This is the shape of each segment plotted onto a computer. We calculated the width 2a (43mm)  first then the length 2d which turned out to be 26mm. Starting with the four sided shape on the left, the left hand edge is 43mm, the right hand edge of that shape needs to he tapered down to the length 'e'. Multiply 43mm by 0.8660 (remember?) resulting in a length for 'e' of approx 37mm

The final length is 'f' This is 43mm multiplied by 0.5, the value of 'c', which is 21.5mm.

Plot it all out as above to make one segment.

Copy seven segments, rotating each one by 45° and join 'em up!

And there it is.

To make the seven sided hole in the saucer section I used the same technique I'd used on the Christmas Tree here.

Glue it all together.

"You have the Bridge Mr Worf."

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