How to find the center point of a circle

[shmokes]

On a pretty regular basis during the construction of my cab, I've found myself needing to find the exact center point of a circle, for example when I've drawn a circle I need to drill out with a hole saw, but need to position the drill exactly in the center to make sure it gets cut out in the right location.  I often just eyeball it, or make a bunch of rough measurements.  But I just stumbled across this simple method of finding the exact center point with just a pencil, straight-edge (ruler) and compass. 

Just thought others who don't already know the trick might find it useful.  After you do it a couple of times you get very fast at it.  You don't actually need to draw complete circles because you can estimate approximately where they will meet and overlap.

[surface tension]

Shmokes, were you not paying attention in school? 

[jardine]

Shmokes, were you not paying attention in school? 

I think i was always off chasing girls....great link shmokes..

[shmokes]

Shmokes, were you not paying attention in school? 

Heh . . . I barely even went to school.     As graduation was coming up my school offered after school detentions that students could attend to bring their citizenship grade up.  I had to attend more detentions to qualify for graduation than any other kid in my high school.  I skipped at least one class almost every single day.  Too bad.  I could have done high school in my sleep if I had cared.  Years later I ended up having to work full-time all through college.  I learned discipline way too late in life (and I still only have a modicum of it  ).

[surface tension]

Discipline to build yourself a machine is discipline enough. Though maybe you ended up building it at night to make up for shirking in the day!

[TheShanMan]

Nice find! Seems like that will eventually come in handy for me.

[ultrageek]

Ah... flashbacks to geometry class...

Now, if someone could just tell me how to divide an angle into 3rds....  I'd be able to take over the world.

[fatfingers]

Ah... flashbacks to geometry class...

Now, if someone could just tell me how to divide an angle into 3rds....  I'd be able to take over the world.

Here you go! 

[Qesmann]

That method seems overly complicated. If you have a compass, set it up to draw circles with the same radius as the original circle (which is trivial to measure). Then draw two circles centered anywhere on the original circle's circumference. The two new circles will intersect at the center point of the original circle.

[shmokes]

. . . draw circles with the same radius as the original circle (which is trivial to measure).

If you are measuring the radius, you already have the center point. 

[FrizzleFried]

If you have a compass,  simply set the compass to draw the same sized circle you have already drawn...take a couple of test swipes...yup...same size...good...the pin----tallywhacker--- in the middle where you set your compass down to pivot is the middle...sheesh.

[ivwshane]

And if you don't have a compass then draw a straight line any where in the circle, find the middle of that line and draw a perpendicular line, then measure and find the middle of that line, done , thats your center, no compass needed.

[miles2912]

Draw a line square to the side of your cp where you want buttons.  Measure out a distance on the line how far apart you want your buttons (this will be the center of the button-circle).  Now with a small drill, drill out a hole.  Now use your hole saw or better yet forstner bit to drill the hole.

[Casey120]

Cut out a paper circle the correct size , fold it in half and then in half again and maybe once more .
Now cut off just the small tip and unfold , voila your center spot is ready .
Stick that on your original circle and mark the middle hole with a pen marker .

A minute work .

[Ed_McCarron]

Dunno, I just start by drawing the circle in AutoCad and dimensioning with a center mark...

[Luigi]

hehe.....I only thought "Why would someone use a compass to draw a circle??"

I didn't know that it's also the word for this:



(Picture lent from www.werbekonzepte.com)


I only thought of this:



(Picture lent from www.ruhr-uni-bochum.de)


 

[pmc]

I usually draw a bunch of lines right thru the circle. As long as you are getting the full diameter of the circle for each line, they should all cross at the center. No compass needed, but I guess it's not perfect. It always works OK for me.

You could draw a square around the circle and then draw lines between the opposite corners. That would locate the center of the circle. Kinda like you do when you set up a lathe.

-pmc

[Ummon]

I'm wondering where you're getting the circle in the first place, if you haven't drawn it. If you have, don't you already know its dimensions and center point?

[pmc]

I'm wondering where you're getting the circle in the first place, if you haven't drawn it. If you have, don't you already know its dimensions and center point?

I sometimes use a coffee can or whatever other container is laying around that's about the right size. It's fast and easy to just position and then trace. I can get the job done well before I could locate a compass.

Point taken that if you use a compass, by definition you have the exact center point already.

-pmc

[mcdubbin]

hehe.....I only thought "Why would someone use a compass to draw a circle??"

I didn't know that it's also the word for this:



(Picture lent from www.werbekonzepte.com)
I only thought of this:

(Picture lent from www.ruhr-uni-bochum.de)

thought the samething

[massive88]

You can easily find the exact center with just a ruler.

Pick a length of a chord that makes sense for the size of the circle that you are dealing with that makes an even measurement, say 1" for example. 

Move the ruler along the radius till two points of the circle match your dimension (1" here). Draw two chords that are 1" making an extra tick at the half way mark, 1/2" in this example.

If your ruler has a square bottom, make a perpendicular line from your half way mark on each line toward the center of the circle.  The two perp lines will cross, and X marks the spot.

Heres a quick sketch of this method in Autocad, on a 2" diameter circle using the example numbers above.

[ivwshane]

You can easily find the exact center with just a ruler.

Pick a length of a chord that makes sense for the size of the circle that you are dealing with that makes an even measurement, say 1" for example. 

Move the ruler along the radius till two points of the circle match your dimension (1" here). Draw two chords that are 1" making an extra tick at the half way mark, 1/2" in this example.

If your ruler has a square bottom, make a perpendicular line from your half way mark on each line toward the center of the circle.  The two perp lines will cross, and X marks the spot.

Heres a quick sketch of this method in Autocad, on a 2" diameter circle using the example numbers above.

That's the same thing as my suggestion but with two extra lines:p

[massive88]

That's the same thing as my suggestion but with two extra lines:p

Yes but less math and measuring  ;p

Both will work splendidly without the aid of a compass   

[Paulson]

 What about if you slide a rule up & down your circle so you can find
 the widest part. Then mark the centre?


 Sounds easy to me...
 
 Just a thought...

 Paulson  (uk)

[pmc]

What about if you slide a rule up & down your circle so you can find
 the widest part. Then mark the centre?

Yah. And if you do it twice (once up and down and once left and right), you can reduce any measuring error by using the cross-point. That's kinda the way my mind works, but maybe there's no value in the logic.

-pmc

[Ummon]

What about if you slide a rule up & down your circle so you can find
 the widest part. Then mark the centre?

Yah. And if you do it twice (once up and down and once left and right), you can reduce any measuring error by using the cross-point. That's kinda the way my mind works, but maybe there's no value in the logic.

-pmc

This is what I've done. I'd think accurate enough for general needs.