# Centre of a circle



## Giff (9 Aug 2010)

This maybe an obvious question to someone but is there a quick way of marking the centre of a circle similar to drawing diagonals on a square or rectangle. Giff


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## cambournepete (9 Aug 2010)

Yes.

Draw 2 lines across the circle at roughly 90 degrees to each other.

Mark the point on each line half way between where the lines intersect with the circle.

At both of these half way points draw a line at 90 degrees to the line.

Where those 90 degree lines intersect is the centre of the circle.

Here's a link to a picture.


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## Giff (9 Aug 2010)

Thanks Pete

I thought there maybe a way but couldn't work it out...really helpfull. I feel a bit like an apprentice on this site always asking basic questions! should have paid more attention in geometry...Giff


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## cambournepete (9 Aug 2010)

here's another link showing you how to do it with just straight edge, compass and pencil.


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## Giff (9 Aug 2010)

Cheers Pete. The first one is great for me and works on any size. I have been drawing circles all afternoon to check...Giff


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## Sportique (10 Aug 2010)

Giff,

here's a small tool you can make which will find the centre of a circular item: (it works on a similar principle to Pete's above)







Cut an obtuse angle out of the "base" (I don't think this angle is critical), fit the straight marker so that it bisects this angle - this needs to be accurate.

You can make the tool as large or small as your needs dictate.

To use it - place on the outside of the circular workpiece, ensure the "base" is touching the workpiece in two places, mark a line, rotate it around the item and mark another line - where they cross is the centre of the circle.

HTH

Dave


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## Giff (10 Aug 2010)

Thanks Dave that's a really useful tool...great idea.I think I will make a couple of different sizes as you suggest. Cheers Giff


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## devonwoody (11 Aug 2010)

Thanks sportique, I shall bookmark that one, also Pete of course.

Moderators. Perhaps it could also go into the "How-to" ?


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