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What's wrong with a pin, piece of string and a pencil. The pin is tapped into the centre point and the string wrapped around the pencil at the desired radious.
 
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What's wrong with a pin, piece of string and a pencil. The pin is tapped into the centre point and the string wrapped around the pencil at the desired radious.
If the center point of the radius lies outside the walls of your shop or if there are obstructions in the way, you have to resort to something like what Scott Grove showed.
 

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Very cool, good trick. I have often done circles needing a large radius and fortunately I have always had the room, but this could be easier. As I think about it you could probably even jig a router onto it.
The other advantage is if you know width and height you want the arch, you don't have to know the radius. I have always used a formula to get the radius, but this don't need it.(y)
 

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If you know the radius that you want and the distance between the outside points on the arc, here is how you calculate the middle point location.

C = radius
L = length between end points on the arc

Run a line from the center point of the circle through a point that bisects L, That line will intersect the third point on the arc.
To simplify the math, use A as 1/2 L, C as the radius and B as the line from the center to the cord line.

A² + B² = C ²
B² = C² - A²

B = The distance from the line to the arc is C - B

Example:
Radius = 20
Length between points = 6

From center point of circle to line is:

20² - 6² = 400 - 36 = 364

B =
398114

= 19.078

Distance from center point of line to arc is 20 - 19.078 = .922 Ft = 11.064 inches
 

·
Premium Member
Retired since June 2000
Joined
·
15,065 Posts
If you know the radius that you want and the distance between the outside points on the arc, here is how you calculate the middle point location.

C = radius
L = length between end points on the arc

Run a line from the center point of the circle through a point that bisects L, That line will intersect the third point on the arc.
To simplify the math, use A as 1/2 L, C as the radius and B as the line from the center to the cord line.

A² + B² = C ²
B² = C² - A²

B = The distance from the line to the arc is C - B

Example:
Radius = 20
Length between points = 6

From center point of circle to line is:

20² - 6² = 400 - 36 = 364

B =
View attachment 398114
= 19.078

Distance from center point of line to arc is 20 - 19.078 = .922 Ft = 11.064 inches
WOW!
 
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