# Calculating frog: make your own mechanical calculator

6 Steps
In grade school kids are taught about math "triangles." 3+4=7 or 7-4=3 or 7-3=4 for example. The same principle works with multiplication and division (3x4=12 or 12/3=4 or 12/4=3) By putting each number on the angle of a triangular flash card and by hiding one of the three corners, not only are children practicing memorization, they are grasping the concept that addition and subtraction or multiplication and division are like the flip side of the same coin. They can literally see the relationship.

My mechanical calculator is based on this triangle, but by using simple mechanics the triangle formed by the frog's feet and his clasped hands can move. A single card will show the whole multiplication table, and watching it move around is a whole lot more fun than flashcards.

I should say that I did NOT come up with the concept: William Robertson, in 1916, designed a tin toy he named the "Educated Monkey." When I fist saw it a few months ago I thought it would be fun and easy to reproduce the idea on paper -- I was dead wrong.

My desk has been strewn with pins and frog body parts for weeks as I've been trying to fine-tune the template. Changing the monkey into a frog was easy and natural; the way the creature moves is much more frog-like than monkeyish. Getting everything to line up properly was a bit tricky, but manageable. I was able to improve Mr. Robertson's design by figuring out a way to display when a number is multiplied or added to itself (i.e. 5+5) He just drew a little square which represents the number, but my template shows the actual number (you can't see me right now, but I just patted myself on the back). I found tiny aluminum eyelets at a paper supply store, but during the design process, for expediency, I used pins sticking up and tried not to injure myself as I banged my desk in frustration... Because after getting everything to work perfectly, when I finally printed my frog out in color, cut the pieces for what I thought would be the last time and used the cute little eyelets --- it didn't work. The pins I had been using held the paper body parts in place while allowing them to pivot freely. The eyelets, on the other hand, held the paper together too tightly, so in a few critical places instead of pivoting the paper stuck and buckled instead. The d... thing needs to be made with a stiffer material after all.

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## Step 1: Materials and tools

You will need:

the PDF template

A scalpel knife (also known as an Xacto knife) with a cutting mat to protect your table

An empty tea box

A hammer (with a nail or screw)

A dremel tool or any other rotary tool which will cut through thin sheet metal

A file

Glue

Small aluminum eyelets and a matching setting tool (I got mine from the Paper Source)
HollyMann says: Apr 28, 2012. 7:15 AM
Wow this is amazing and my son would also love it! :)
biomech75 says: Mar 22, 2012. 7:38 AM
NICE!!! Good Job!!!
nead says: Feb 28, 2012. 6:26 PM
Wonderful project! My young son and I really like. We will try to make one using card stock.

Reminds me of the pantograph I had when I was very little.

Wonder what else this could be used for.

Thank you for sharing.
We made it using card stock and 1/8" eyelet. It works but we have to "assist" it or the arms bend. It's not going to last very long.

We will try again using a donor cookie tin.
Laminating the card stock should help too, and it's definitely easier than cutting the tea or cookie tin.
mcraghead says: Feb 28, 2012. 7:37 PM
Great work!

Here's "Consul the Educated Monkey," the little dude in the red suit from 1916: http://www.officemuseum.com/kids.htm.
My kids had a modern spinoff, here's one (of many) stores selling it:
http://www.huntarcompany.com/mm5/merchant.mvc?Screen=PROD&Store_Code=&Product_Code=50201&Category_Code=MKL
...I was always impressed with the engineering of the thing. I wonder if it would be easier or harder to make with the arms over his head than under?
Anyway, well done!

I hadn't seen the modern version... Thanks for sharing that. I imagine making it with the arms over his head would be pretty much the same level of difficulty to make.
mikeasaurus says: Feb 26, 2012. 8:02 PM
Great project, and I love the way you made this frog.

Though, is it weird my first thought after seeing the video was this classic cartoon?
belsey (author) in reply to mikeasaurusFeb 27, 2012. 3:27 PM
Thanks for posting that, maybe I'll change the music on my video to this one.... The music I have now is more toad-like than jumpy, happy frog-like...
GASSYPOOTS says: Feb 27, 2012. 2:07 PM
math triangles?
zomfibame says: Feb 27, 2012. 1:52 PM
cool project
AngryRedhead says: Feb 27, 2012. 1:31 PM
So cute!! Instant love!
halmars says: Feb 27, 2012. 9:22 AM
Great instructable. Have you thought about "encapsulating" the printout with one of those machines that stich a plastic pocket to a sheet of paper?
I have read the end part where you say paper and bent paper clips will do, so once I get a printer up and running I'llhave a go at this. Thanks
belsey (author) in reply to halmarsFeb 27, 2012. 10:13 AM
Yes laminating! That's a great idea!
Alberta Leong says: Feb 26, 2012. 11:49 PM
Very well done! Looks quite intricate to diy though.
belsey (author) in reply to Alberta LeongFeb 27, 2012. 6:28 AM
Actually it's not so bad at all if you skip the metal part and just make it with heavy card stock (that's what the video prototype is made of).
rimar2000 says: Feb 27, 2012. 5:16 AM
YOU are awesome! Thanks for sharing this.
sunshiine says: Feb 26, 2012. 8:21 PM
Nice job! Thanks for sharing and have a super day!
Sunshiine
paganwonder says: Feb 26, 2012. 7:55 PM
This looks really fun! Thanks for sharing.