centerpoint of a circle

[req 5,000+ posts]

anyone know how to find the centerpoint of a circle when you only have the circumfrence? i know there is an easy way using chords and intersecting lines, but i cant remember it.

anyone remember how to do this? //content.invisioncic.com/y282845/emoticons/blush.gif.99bc659ee2012b7d826165e26fb5eebe.gif

Click to read more...

 

[Rattlebrain 10+ year member]

Crap, Now I gotta think. I cant remeber either. Give me a few, Figure it out.

 

[FoxPro5 5,000+ posts]

Circumference= pi * r^2...I think

Diameter = Circumference/pi...for sure

 

[JimJ 5,000+ posts]

Pi*r^2 is the equation for the area...circumference is Pi*d

 

[Chevillac 5,000+ posts]

Circumference divided by pie would give you the diameter, IIRC. middle point of that. I guess im missing something in the question.

I was late....

 

[req 5,000+ posts]

no i need to be able to physically draw on a surface. i have to find the centerpoint of this circle so i can use my router to cut it out on the jig, but i had to trace the circles from an already-cut piece and i cant just draw a diamater on there and it will magically be perfect.

i need to know how to find the centerpoint without any refrences on a physical surface - ie, the radius\diamater

 

[Rattlebrain 10+ year member]

http://www.woodworkingtips.com/etips/2004/11/05/wb/

http://www.mdk12.org/mspp/high_school/look_like/geometry/guide/g31.html

http://www.math.uakron.edu/amc/ReviseGeoLesS02/HSpdf/5.1CircleConjectures.pdf#search='finding%20the%20center%20of%20a%20circle'

 

[bonesninja 5,000+ posts]

put the circle on a piece of paper---cut it out. fold it in half then in half again the corner is your center.

 

[req 5,000+ posts]

i think im going to draw two chords that are parallel using two sides of a ruler to make sure they are parallel. then divide the lengths of the parallel lines in half to find the centerpoints, then draw a bisecting array across the two centerpoints to form the diamater of the circle and measure from one side of the circle out to the center of the diamater with my already know radius of 6 and 15\16ths inches and mark the centerpoint.

sounds good? good.

 

[noob with an RS 10+ year member]

draw a circle. now using a ruler, draw two straight lines anywhere inside the circle. then using a right angle ruler draw lines perpendicular to the first set of lines. the second set of lines will meet at the exact center of the circle. neat, huh?

EDIT: i think if you use more than two lines you can be more precise. but two should be enough considering theyre perfectly str8 and the perpendicular line is exactly a 90 degree angle.

 

[saywhat? 5,000+ posts]

can you not measure the radius from the edges, draw some lines, and that could show u the midpoint?

 

[ngsm13 5,000+ posts]

Well if you know the actual physical circumference...what is so hard.

Divide by Pi...that is your diameter. Divide by two, that is your radius.

You don't need to draw it out physically...those numbers are your measurements.

NG

 

[CBFryman2 5,000+ posts]

i dont see what is so hard either...i can draw a perfect or near perfect diameterdraw 3 near perfect diameters to get a vert VERY small triangle and you have an almost exact location...kinda how Navigation used to work before GPS //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

 

[Rattlebrain 10+ year member]

Well if you know the actual physical circumference...what is so hard.
Divide by Pi...that is your diameter. Divide by two, that is your radius.

You don't need to draw it out physically...those numbers are your measurements.

NG

He doesn't need the measurement, he needs to know where the middle of an existing circle is physically.

(not being snippy just saying) //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif

 

[ngsm13 5,000+ posts]

He doesn't need the measurement, he needs to know where the middle of an existing circle is physically.
(not being snippy just saying) //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif

You can still do this with said measurements. //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif

NG