I'm working on a project, making a large emblem that is circular. It is about the size of a large pizza and I need to find the exact center point. With just simple tools what is the best way to do this?

Practice and a 7.62.

If you know the diameter, make a stick with a couple holes in it the EXACT radius of the circle. Pin one end to a point in the circumferance, put a pencil in the other hole and make a mark....dot that 3-4-5 places around the circumferance, and where the lines intersect is the center.

get a straightedge
draw a line which cuts across the circle
draw another line which cuts across the circle
get a compass
bisect each of the 2 lines within the circle
draw a line perpendicular to each of the 2 lines
where these last 2 lines intersect is the center

[This message has been edited by lurker (edited 05-25-2012).]

Cut out a piece of paper the same size,fold in half, and half again. Open it flat, Where the folds cross is the centre point.

Start with a string, pin, pen and paper, tie the string to the pin and the pen with the distance between the two half the width of the circle you want to end up with. Stick the pin in the center of the paper, stretch out the string and draw your circle. You've already marked the center.

KInd of depends on where you're at in the project, if you've already got the circle, then use one of the other methods.

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Ron
The key thing is to wake up breathing! All the rest can be fixed. (Except Stupid - You can't fix that)
Always remember these words of wisdom.

"The Lord must truly love fools, for he made them in abundance."

Send the whole thing to Marvin McMinnis with your requirements. It will be perfect when you get it back. Don't be surprised when you get a request for a large check.

If you want to be exact, then use a compass.

1. Measure the diameter of the circle and divide by 2 to get the radius.
2. Set the compass to the radius.
3. Set point of the compass on the edge of the circle and put about a 2" curve mark where you think the center is with the compass' pencil side.
4. Repeat #3 two more times at three different points on the circle at what you approximate to be thirds of the circle.

If #4 intersects at a single point, then that is the exact center. If they do not, then your measurement of the diameter was off. Remeasure the diameter and repeat until all three lines connect at a single point.

As they say...
Measure it with a micrometer. Mark it with chalk. Cut it with an axe.

quote Originally posted by fierofetish: Cut out a piece of paper the same size,fold in half, and half again. Open it flat, Where the folds cross is the centre point.

Problem is, if you cut the paper as a circle, folding it in half is about as difficult as finding the center (because you would need to devide the circle in two perfect halves). And the only way to get a rectangle with sides the same size as the diameter of the circle, is by by knowing the diameter and that again is what the problem is with a circle.

But here's a 100% accurate method I just thought up myself.

Draw a straight line anywhere on the circle from one side to the other side. It doesn't matter where. For example:



Now measure the line you have just drawn and mark the exact center of that line:



Now using a straight angle, draw a line from that point towards the other side of the circle (I drew these without measuring anything so the lines are perhaps not very accurate, but you get the point):



Repeat, but in another position, again, doesn't matter where:



Where the two lines cross will be the exact center of the circle.

I found the fastest and simplest is the method mentioned already where you just make a straight line at any 2 places on the outside of the circle. Then I use a right-triangle to make a line at 90* to those two lines. Where they cross is the middle.

I just did a quick "googly" and apparently, this is indeed one of a few quick and easy ways to find the center of a circle.

quote Originally posted by Cliff Pennock: But here's a 100% accurate method I just thought up myself.

this sounds strangely familiar

pizza sized? just how exact does it need to be?

[This message has been edited by lurker (edited 05-25-2012).]

quote Originally posted by lurker: this sounds strangely familiar

Yeah, I thought I had come up with something original but that was of course pretty stupid to think. As if millions of mathematicians before me wouldn't have found that already. Sometimes I worry myself.

I think I just found the easiest way to find the center. This can be done with just a carpenter's square and a pencil:

http://www.mathopenref.com/constcirclecenter2.html

quote Originally posted by Doug85GT: If you want to be exact, then use a compass.

I know which way is north, Doug.

quote Originally posted by Raydar: As they say... Measure it with a micrometer. Mark it with chalk. Cut it with an axe.

Who says this? GM engineers? No wonder we are in trouble.

quote Originally posted by heybjorn:

EVEN I avoided THAT one...You've sunk to a new low...

quote Originally posted by Cliff Pennock: Sometimes I worry myself.

well, you did use pictures. i was trying to imagine how to explain some of the steps in words, and it wasn't going to be pretty.

quote Originally posted by Doug85GT: I think I just found the easiest way to find the center. This can be done with just a carpenter's square and a pencil: http://www.mathopenref.com/constcirclecenter2.html

That would have been my suggestion...... Lacking a square, I've used pieces of paper.......

quote Originally posted by lurker: get a straightedge draw a line which cuts across the circle draw another line which cuts across the circle get a compass bisect each of the 2 lines within the circle draw a line perpendicular to each of the 2 lines where these last 2 lines intersect is the center

x2

quote Originally posted by lurker: well, you did use pictures. i was trying to imagine how to explain some of the steps in words, and it wasn't going to be pretty.

Oh wait - I just now realized you posted the same thing.

Like I said - I sometimes worry myself.

quote Originally posted by fierofetish: Cut out a piece of paper the same size,fold in half, and half again. Open it flat, Where the folds cross is the centre point.

x2

Unbelievably complicated answers here
Draw ANOTHER circle the same diameter on a piece of paper with a compass, and place it over the one you want to find the centre of...and you have the hole from the compass which will be the centre!! Poke the compass point through, and you have your mark
Cliff...difficult to fold a circle in half?? And then quarters?
Blimey...
See...ask a 'tech' and they will produce the most complicated system you ever heard of . Ask a FABRICATOR, and you will be amazed how SIMPLE it really is

quote Originally posted by fierofetish: Draw ANOTHER circle the same diameter on a piece of paper with a compass

he said "exact", but he didn't say how exact. this will work, as will the paper fold technique, but is not very precise. how good is good enough?

[This message has been edited by lurker (edited 05-25-2012).]

quote Originally posted by fierofetish: Unbelievably complicated answers here Draw ANOTHER circle the same diameter on a piece of paper with a compass

Meaning you would have to be able to measure the diameter of the original circle. And if you could do that, you would automatically be able to find the center. So finding the solution with the solution is hardly... ehm... a solution.

quote Originally posted by fierofetish: Cliff...difficult to fold a circle in half?? And then quarters?

Unless you can cut perfect circles - yes.

Besides, why is it more difficult to draw 4 lines on a circle than it is to first copy the circle on a piece of paper, then cut the paper, then fold it? Drawing those 4 lines takes perhaps 10 seconds. Hardly "the most complicated system you ever heard of".

See what I mean?? 'Unless you can cut perfect circles...' Why do you have to cut it??? Draw it on a piece of paper. Hold it up to the light...and fold the bottom of the curve up to the top!!
But again...if you know the original diameter (which I assume the OP does ), draw another circle with the same diameter on a piece of paper with a compass...and you ALREADY have the centre point of BOTH! Simply place the second over the first, and push a pin through.

[This message has been edited by fierofetish (edited 05-25-2012).]

quote Originally posted by fierofetish: See what I mean?? 'Unless you can cut perfect circles...' Why do you have to cut it???

I agree. Cutting a circle is stupid. But didn't you say:

quote Originally posted by fierofetish: Cut out a piece of paper the same size,fold in half, and half again. Open it flat, Where the folds cross is the centre point.

quote Draw it on a piece of paper. Hold it up to the light...and fold the bottom of the curve up to the top!!

And that's easier and more accurate than drawing four simple lines?

quote But again...if you know the original diameter (which I assume the OP does ),,

Again, if he knew that, I'm pretty sure he wouldn't be asking here for a way to find the center of the circle.

quote draw another circle with the same diameter on a piece of paper with a compass...and you ALREADY have the centre point of BOTH! Simply place the second over the first, and push a pin through.

Which is probably the most ineffective way of finding a circle's centre if you know it's diameter.

quote Originally posted by fierofetish: if you know the original diameter

yes, he said it was "pizza sized". large, i assume. pepperoni, mushrooms, no anchovies, please.

[This message has been edited by lurker (edited 05-25-2012).]

Does anybody else really want to know what Rotrexfiero is working on, and if these solutions helped?

I read each post til the end, just hoping to see his thank you, and an explanation of how awesome his project turned out because of PFF.

Hahahaha!!!! lurker

quote Originally posted by lurker: yes, he said it was "pizza sized".

Oh, then it's even easier to find the center. It's where the Pizza saver is.

quote Originally posted by Cliff Pennock: Which is probably the most ineffective way of finding a circle's centre if you know it's diameter.

quote Originally posted by 1988holleyformula: Does anybody else really want to know what Rotrexfiero is working on,

I think we've established he's making pizza's. Large ones.

since we're talking pizza, the answer probably has pi in it somewhere. see, i didn't sleep through all of geometry class!

quote Originally posted by fierofetish:

Instead of throwing those rolling eye smilies around, please explain why it's easier to draw anything on a piece of paper first, then cutting it out - or not, folding it - or not, then overlaying it on the original circle than it is to draw 4 straight lines?

quote Originally posted by heybjorn: Send the whole thing to Marvin McMinnis with your requirements. It will be perfect when you get it back. Don't be surprised when you get a request for a large check.

Or a large Pizza!

Kevin

quote Originally posted by Cliff Pennock: Instead of throwing those rolling eye smilies around, please explain why it's easier to draw anything on a piece of paper first, then cutting it out - or not, folding it - or not, then overlaying it on the original circle than it is to draw 4 straight lines?

then there's the ugly question of what to do if the pizza is 7&3/32nds feet across.

Measure the circle at its largest from one side to the other with a yard stick, draw a line threw it. Then turn the yardstick to what you think is about the half way mark and draw another line threw it. And so on and so forth. The center will be where all the lines cross in the middle.

And I ain’t no engineer.

Steve

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Technology is great when it works,
and one big pain in the ass when it doesn't
Detroit iron rules all the rest are just toys.

quote Originally posted by Cliff Pennock: Instead of throwing those rolling eye smilies around, please explain why it's easier to draw anything on a piece of paper first, then cutting it out - or not, folding it - or not, then overlaying it on the original circle than it is to draw 4 straight lines?

Because I neither need a RULER, nor a SET-SQUARE to do it...whereas...you do

[This message has been edited by fierofetish (edited 05-25-2012).]

quote Originally posted by fierofetish: Because I neither need a RULER, nor a SET-SQUARE to do it...whereas...you do

Yes you do and yes you do. But I think I get the feeling where this is going to so... you are right. Your methods are much better/easier/quicker/time efficient than drawing 4 simple lines. What was I thinking.