Find the Center of a Circle

This instructable shows how to find the center of a circle. Many, many projects involve circular things and a need to easily find the middle. For various reasons pre-determining the center of a cut circle can be difficult, particularly when it will affect balance in motion.

I was working on the Cardboard Wind Turbine project and the most convenient large diameter template wasn't conveniently sized for pre-measuring and cutting out a square and then matching the template to it seemed tedious and error prone. Also I've never been very good at drawing on the lines. So here we are.

I'd like to thank and acknowledge the folks at Math Open Reference for their contribution.

So what we're going to do is work with a cardboard circle I've cut out, find the center point and then create a "crosshairs" so that work on various projects can proceed smoothly.

Step 1 Bill of Materials

Here's a list of items we're going to be using.

Carpenters Square or other right angle equal to or larger than the diameter (size) of the circle. I've got a fancy, schmansy carpenters square that I finally found a use for but any reasonably stable right angle will. I've used both the carpenters square and a large manila envelope to demonstrate the utility of the technique.

A compass - I show a basic protractor and compass set available at any drugstore, supermarket or department store. The protractor (the half circle thing) isn't actually used. It is possible to substitute the traditional pencil and string compass but I do not demonstrate that technique. Save yourself a great deal of trouble and spend the two dollars, over time you will thank me.

The box cutter and cardboard box isn't required unless you don't have a circle handy to practice on.

I show two cardboard circles of approximately equal size that I need to find the centers of so I place them on an axle and have them rotate at fairly high speeds so the better the balance the more stable my contraption. Any circle may be substituted.

Comments

[Blofish]

I just use a v block and a straight edge. Maybe I should do an insturt on it....
But thanks for this. I was looking for it cause some of the stuff I work on don't fit in a v block.... OMG! I just thought of another idea.egbertfitzwilly you always seem to inspire me!

[egbertfitzwilly]

Thank you for your kind words.

I encourage everyone to write an instructable and disseminate their information. Its impossible to tell how it will it cascade out.

[conrad2468]

my uncle is a geometry teacher and he taught me this.....im goin to geometry next year! oh yeah! head start!

[egbertfitzwilly]

Math is divided into two great fields, arithmetic (the study of numbers) and geometry (the study of shapes). While all that is living derives from the forms of physics bound by arithmetic all that is beautiful to eye or ear (art and music) derives from geometry. And I will leave you one other tip that will serve you well as you go forward: In your conversations with others remember that little people talk about people, mediocre people talk about things, great people talk about ideas. But don't be a jerk about it....it costs nothing to be gracious

[conrad2468]

I was kind of being sarcastic about the "head start" thing.......It wasnt my intention to be a jerk.......I went through alebrag 1 and it was alot less simple than i thought it was.....so assuming that geometry is anything close to algebra 1 i think im in for alot more than finding the center of a circle. Also, I dont like talking about people unless i know them personally and know what they go through.

[egbertfitzwilly]

You weren't being a jerk, that was just an observation in general. I'm confident that you'll do well.

[conrad2468]

just something you felt like saying.....something esoteric? Thanks.

[egbertfitzwilly]

Exactly, while not quite a geezer I'm certainly of an age where I feel completely confident giving unsolicited advice to young people. Having people listen is one of the two greatest things about being grown up. The other is the ability to walk go out for dinner and order dessert first. Here is something I've tried to live by: A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects. -Robert A. Heinlein

[conrad2468]

Im not much of a dessert person but i like the first one! Id say im smarter than the average bear...not a genius (not even close) and every time i have an idea i share it....NOBODY listens! then when im right i get to rub it in...thats the only good thing about having no one listen to my advice. Also I like that poem..or quote thingy 12 out of 21 aint bad. Well it depends on the definition of "program" is. I like your advice. P.S. I finally figured out how to pronounce your name!

[jackm7127]

Wait...no. Draw to lines of equal lengths (and are shorter than the diameter of the c

[egbertfitzwilly]

This seems to be cut off, did you change your mind about commenting?

[rimar2000]

Fantastic! Now it is necessary to design a small tool that makes it, quickly and reliable.

[WirelessMonk]

Like one of these? http://tinyurl.com/dzy8bb

[rimar2000]

Thanks, very much! That gadget is CEFU, all that I like: Cheap (I can make it), Easy, Fast and Useful. If you trace with that device three or more lines that form a small polygon in the center, you do not need to worry about the thickness of the pencil.

[chichimus]

... or one of these (slightly different):

http://www.leevalley.com/wood/page.aspx?c=2&p=43205&cat=1,330,49237

[AndyGadget]

Thank you very much. I have wondered how to do that, but never got around to looking it up. BTW, the tool actually does it in a different way by drawing the two tangents to the circle from a point, then bisecting the (90 degree) angle.