How do you find the center of a circle?

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[ARCHIVED THREAD] - How do you find the center of a circle?

Posted: 1/15/2006 11:22:41 AM EDT

Assuming you don't have any machinist's tools, what is the easiest way to find the exact center of a circle if you need to drill a hole in the center of an object?

Posted: 1/15/2006 11:23:40 AM EDT

[#1]

Measure diameter. Divide this by 2 ... This is the distance from any edge of the circle to the center (the radius).

Posted: 1/15/2006 11:27:00 AM EDT

[#2]

Have a compass? Set it for the radius of the circle. Stick the pointy end on any part of the circumference, draw a small arc near the center of the circle. Move the pointy end to a different part of the circumference, draw another small arc near the center. Where the two arcs you drew intersect is your center.

Posted: 1/15/2006 11:28:10 AM EDT

[#3]

Isn't there some rule that you draw a triangle inside the circle (with the corners touching the circle), and then draw perpendicular lines from the center of each side of the triangle - and where they cross is the center of the circle?

Or did I just dream that - it's probably been over 30 years.

Posted: 1/15/2006 11:28:50 AM EDT

[#4]

Can't you just draw a line in one direction across the middle and then another at 90 degrees from it and where they intersect is the center? Like a scope's cross-hairs?

G

Posted: 1/15/2006 11:29:04 AM EDT

[#5]

use a compass set at about center. put the needle on the circle and make an arc neer center .repeat this several times around the circle and the center will become apperant

Posted: 1/15/2006 11:29:16 AM EDT

[#6]

Quoted:

Isn't there some rule that you draw a triangle inside the circle (with the corners touching the circle), and then draw perpendicular lines from the center of each side of the triangle - and where they cross is the center of the circle?

Or did I just dream that - it's probably been over 30 years.

I just tried it on paper and it worked for me

Posted: 1/15/2006 11:29:24 AM EDT

[#7]

Quoted:

Isn't there some rule that you draw a triangle inside the circle (with the corners touching the circle), and then draw perpendicular lines from the center of each side of the triangle - and where they cross is the center of the circle?

Or did I just dream that - it's probably been over 30 years.

Oops, that's it.

G

Posted: 1/15/2006 11:30:43 AM EDT

[#8]

Quoted:
Can't you just draw a line in one direction across the middle and then another at 90 degrees from it and where they intersect is the center? Like a scope's cross-hairs?

G

Yeah, but how do you find the half way point to draw the line on? This is the question.

Posted: 1/15/2006 11:36:24 AM EDT

[#9]

Alternately a perpendicular from a chord will cross center. (an equilateral triangle inside a circle is 3 chords)

If using barstock, set on barstock on flat surface and lay pencil on same flat surface and draw a line along the end of the barstock. Do this at least two more times rolling the barstock so it's sitting on a different point. Then take a small square and draw perpendicular lines from the 3 chords you drew. The center of the barstock should be at the intersection or inside the resulting triangle of error.

Posted: 1/15/2006 11:41:27 AM EDT

[#10]

In CyberChase, the kids cut a piece of cloth the same size as the circle and folded it in half twice to find the center.

Posted: 1/15/2006 11:43:34 AM EDT

[#11]

Fold it in half....and then fold it in half again. X marks the spot.

ETA: KISS

Posted: 1/15/2006 4:39:59 PM EDT

[#12]

Compass, two arcs and a line through the points where the arcs cross.Then divide the straight line in two.

Posted: 1/15/2006 4:44:30 PM EDT

[#13]

Quoted:

Isn't there some rule that you draw a triangle inside the circle (with the corners touching the circle), and then draw perpendicular lines from the center of each side of the triangle - and where they cross is the center of the circle?

Or did I just dream that - it's probably been over 30 years.

Don't listen to him! It's pure sorcery!

Posted: 1/15/2006 4:54:42 PM EDT

[#14]

It's where the crosshairs intersect...

Posted: 1/15/2006 5:00:03 PM EDT

[#15]

Bisect it with a compas. Perfect center every time.

Posted: 1/15/2006 5:08:13 PM EDT

[#16]

Draw two chords on the circle. They can be any length and the ends don't need to touch, or they can overlap.

Construct a line perpendicular to each chord. Their intersection is the center of the circle.

Here is a link to show you how to construct perpendiculars -

www.math.csusb.edu/courses/m129/perp.html
regentsprep.org/Regents/math/construc/perp.htm

How else are you going to reconstruct a broken gear when you only have a piece? You can't draw a triangle inside a circle, and besides, that is a redundant solution.

Posted: 1/15/2006 5:52:41 PM EDT

[#17]

To find the middle of a line with a compass and ruler, set the compass to any distance shorter than the line.

Place the point on one end of the line, and draw and arc that crosses the line.

Without changing the compass adjustment, place the point on the other end of the line and draw another arc that crosses the first arc.

The two arcs intersect at two places. Place a ruler across those two intersections, and draw a line.

Where that line crosses the first line will cut the first line in half, and will be perpendicular.

Posted: 1/15/2006 8:34:22 PM EDT

[#18]

Simply amazing. All of you and your genious.

Posted: 1/15/2006 9:54:22 PM EDT

[#19]

Drill the hole first. What size circle do you want?

Posted: 1/15/2006 10:08:45 PM EDT

[#20]

Let me take one shot at it from 300 yds..........

There's your answer.

Sorry, jus' felt like a little

ing was in order tonight.

Posted: 1/15/2006 10:09:19 PM EDT

[#21]

"Right about....there"

If its paper in the shape of a cirlce, fold it over in halfs.... Bingo.... Why make things so hard?

Posted: 1/16/2006 12:34:31 AM EDT

[#22]

Quoted:
"Right about....there"

If its paper in the shape of a cirlce, fold it over in halfs.... Bingo.... Why make things so hard?

I'll assume you came up with this idea on your own even though I posted the same thing half way up the page.

However since I am assuming you came up with this idea on your own...you and I are the only ones on this board with any common sense.

Posted: 1/16/2006 12:50:24 AM EDT

[#23]

Quoted:
Drill the hole first. What size circle do you want?

Best answer by far

Posted: 1/16/2006 1:38:05 AM EDT

[#24]

Quoted:
Quoted:
"Right about....there"

If its paper in the shape of a cirlce, fold it over in halfs.... Bingo.... Why make things so hard?

I'll assume you came up with this idea on your own even though I posted the same thing half way up the page.

However since I am assuming you came up with this idea on your own...you and I are the only ones on this board with any common sense.

Oh yeah. just saw that.