Frustums and Elephant Trunks

Mon, 30 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 frustum) 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 perfectly serviceable. There 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 Excel, 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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Comments (4)

  • umehta September 30, 2013 at 2:24 pm

    Yes, it looks realistic. I

    Yes, it looks realistic. I bet this tapering shape will enhance Mr Cool's snake as well. 

    Mathematics in elephant's trunk? What a fine work.

  • drinkumbrella October 12, 2013 at 1:00 am

    These also look like they

    These also look like they would be good octopus arms.

  • motleyjust March 5, 2014 at 3:35 am

    Sorry for the dumb question.

    Sorry for the dumb question.
    I can plug the numbers into the spreadsheet and get the results. Do I then need to draw out the shape manually on paper with compass, or is there someway to get the computer to do it so I can just print it out?

    My apologies for the lack of clarity. The numbers are your starting point. You’ll need to construct the appropriate shapes your self using either a compass and ruler or a suitable drawing program. It might be an interesting project to write software that outputs the finished pdf file – when I have time (!) I’ll have a go :)- RI

    • cool022883 March 6, 2014 at 2:29 am

      rob originaly made a flexarm
      rob originaly made a flexarm which you can get here
      http://www.robives.com/blog/mk_ii_flex_arm_download_and_make
      the post you were looking at is explaining how he changed the arm to look more like an elephants trunk for his flexiphant project
      http://www.robives.com/blog/flexelephant
      those projects you can download and print out if your a paid member then it would be free if your a non member you will have to pay the small fee for the project.

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