How to find the center of a dowel or circle

[Ted Crocker]

We start with a given circle.
1.  Place the right-angle corner of any object at any point on the circle. Any point will do.
2.  Make a mark where the two sides of the right-angle cross the circle.
3.  Draw a line between these two marks. Because of Thales Theorem, this is a diameter of the circle.
4.  Place the right-angle corner of the object at any other point on the circle. Any point will do, but for greatest accuracy, make it about a quarter the way round the circle from the first point.
5.  Make a mark where the two sides of the right-angle cross the circle.
6.  Connect these two points with a straight line. This is the second diameter.
7.  Done. The point where the two diameters intersect is the center of the circle.

Why it works

This method works as a result of Thales Theorem. The diameter of a circle subtends a right angle to any point on the circle. The converse is also true: A right angle on the circle must cut off a diameter. By finding two diameters, we find the center where they intersect. 

Visit Thales Theorem for an animated description of how this works.

[Wade]

...Or you could make a paper pattern and fold in half!!!! But that would be no fun lol!

[Scott Winburn]

Awesome!  Thanks Ted

[Ted Crocker]

[Ted Crocker]

Ted Crocker said:

Fold it in quarters you mean to get center...  

Wade said:  

...Or you could make a paper pattern and fold in half!!!! But that would be no fun lol!

 

[Alan Roberts]

Or you can just use a centering jig...

[Alan Roberts]

centering jig, seen from the top.same jig seen from below.

I'm making a few dozen spool clamps, so having a way to quickly and accurately find the center of a dowel is a big plus.

[billy freedom]

what if the dowel is oblong and not perfectly round?

[Ottie(Sky Dog)Noble]

anybody know how find the center of an egg shaped dowel?

[billy freedom]

i take a pencil between my finger and thumb, point close to the fingers. then use my other fingers as an edge guide and twist the dowel till i get a very small circle like this ( o ) then center punch it. really quick really easy and works on irregular dowels.

[Mortimer Mudbutt]

Here is another way to find center with a cumpus. Just click on the video at this link. This is the way our shop teacher taught us many years ago.

http://www.mathopenref.com/constcirclecenter.html