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Straight Talk About Circles

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Finding the Center

Finding the Center

Say you have a circle, and need to find the center

Easy-to-use center-finding tools, such as the one shown in the opening photo or a centering head for a combination square, are readily available from woodworking-supply dealers. The devices work well on rounds up to 7" or 8" in diameter.

But you'll need to rely on layout methods to find the center on larger discs. The easiest way to do it (though a method prone to some error) is to stretch your tape measure across the diameter of the circle, and make a mark at the middle, as shown above. Then, move 90 around the edge, and repeat. Extend the marks until they cross, pinpointing the center.

If you prefer greater accuracy, try this method. First, draw a chord on the circle, shown as line AB in the illustration below. (A chord is a straight line that extends from one point on a circle to another, but doesn't pass through the center.) Then, draw a perpendicular chord at each end of the first one, shown as lines AC and BD in the drawing below.

Next, draw diagonal lines between the two perpendiculars, shown by the broken lines. The point where the diagonals cross marks the center of the circle.

Straight Talk About Circles

Continued on page 3:  Anatomy of A Circle


Comments (5)
booker brooks wrote:

A paint can - you have many sizes in your shop - set on the wood until each side is the same distance from the corner, will have you going is super fast time. No paint cans, use water glasses or anything that is round in your view. Nuff said.

2/26/2010 12:10:54 PM Report Abuse
2ward2 wrote:

The beauty of the demonstrated method, based on high school geometry, is that any size circular arc can be generated. The other suggestions either limit the arc to a "popular" size or require a separate template for each size. Using a compass can get you close enough to use a jig saw etc., then plane/sand off the remainder.

2/26/2010 10:24:28 AM Report Abuse
southernwayok wrote:

they also have a thing called a circle template that you set over the corner and draw the line.

2/26/2010 09:25:52 AM Report Abuse
acf3838 wrote:

to cut radius with a router I made conner pieces wiht radius cut on one end only an a stop clip on the other conner,1/2",3/4",1", 1-1/2" are the most popular size,just clamp over the conner use a router bit with a top bearing works great every time an fast.Fred

2/25/2010 03:46:45 PM Report Abuse
mrtilly wrote:

For a 1" radius corner you could run a 1" mark parallel to each edge and put the piont of the compass where the lines cross to swing the corner arc.

2/25/2010 10:24:03 AM Report Abuse

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