Find the centre of any circle. (Grrrrr. Found this 1 hour after I needed it.)

Skill Builder: How to Find the Center of Any Circle


A comment of the save article I how I did it:

poppy ann • a year ago
it's just as easy to use a ruler just measure across the circle moving the ruler until you get the maximum distance between the two places the ruler crosses the circle and draw a line across (or draw a small line around the centre position) move approximately 90 degrees (does not matter if it is not exactly 90 deg) and use the rule to get the maximum size again and draw a line and where the two lines cross that is the centre."

I made a jig to find the centres of log blanks for turning, which is used pretty much in the manner you describe with a bit more convenience.

It works just as well for finding the centres of circles marked on flatwork. :)

Quote:

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Originally Posted by barrysumpter

Skill Builder: How to Find the Center of Any Circle


A comment of the save article I how I did it:

poppy ann • a year ago
it's just as easy to use a ruler just measure across the circle moving the ruler until you get the maximum distance between the two places the ruler crosses the circle and draw a line across (or draw a small line around the centre position) move approximately 90 degrees (does not matter if it is not exactly 90 deg) and use the rule to get the maximum size again and draw a line and where the two lines cross that is the centre."

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bah humbug

if it's a true circle, draw two chords approximately 90 degrees to each other, bisect both cords, the exact center is where the bisection lines cross.

Alternatively, draw two pairs of tangents where each pair cross each other. Bisect the included angle of each pair and where these lines cross is the centre.

This is the principle behind Skew's jig; in his case it can also be used as a square but as a centre finder the 90° is unnecessary; so long as the middle bit is exactly at half the angle of the sides it will work.

Hi All,
I do mine a little differently as I have made 3 out of Clear Acrylic in a circle.
Found the approx. centre, drilled a small hole, took to it with the Dividers, & scratched circles aprox. 20mm. apart. Then used a Black White Board Marker, drew on the lines, one at a time then wiped the excess off, & now have a clear black line.
Centre hole made a touch bigger, lay it over your wood to the best circle & punch the centre.

The "easy" method of the OP's link is actually harder than needed. The second step doesn't need to align with one of the first edges. The two positions of the framing square can be placed almost anywhere.

In any right triangle, the hypotenuse lies on a diameter of the circumscribed circle. With the corner of the framing square on the perimeter, scribe the edges and connect the two crossings to form a hypotenuse. Do it again someplace else. Where the hypotenuses (hypoteni?) intersect is the center of the circle.

Cheers,
Joe