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Thread: How do i match a radius?

  1. #31
    Join Date
    Feb 2003
    Lafayette, IN
    Quote Originally Posted by Keith Westfall View Post
    Cut a 45* and then cut those 2 points again. You'll be getting pretty close - nothing a disc sander couldn't fix...
    Exactly what I did on mine, minus the disc sander--the board is under a tonneau cover.

    "Don't get stuck on stupid." --Lt. Gen. Russel Honore

  2. #32
    Measure the approximate radius with a tape measure, cut that radius in a scrap piece of wood and test fit. You should be able to sneak up on a perfect fit with a couple of tries.

  3. #33
    Join Date
    Feb 2016
    NE Iowa

    Practically speaking, any of the methods suggested work. Personally, I'd go with Jim's cardboard and scissors.

    But, for the measurement-minded, you can determine the radius directly. Given the length of any chord (line between two points on a circle) and the distance of the chord's midpoint from the circle, there is a simple formula to get the radius of the circle. Call one half of the chord length a, and the distance from the center of the chord to circle, b. Then the radius of the circle, r is:

    r = (a^2 + b^2) / 2b

    So, if you've got a part of a circle and want to know the radius, you can:

    Make a straight, square stick to a known length
    mark the centerpoint on the stick
    place the stick across the inside of the arc you have to work with
    use a combination square to measure the square distance from that center point to the arc.

    That gives you a and b. r follows from the formula.

    The big challenge with this method is that if b is very small compared to a, then any error in b is magnified by the formula. For example, if you have an 8" stick (a = 4) and measure a .25" distance to the circle for b, the radius of the circle is 30.13" - 30 1/8" to a woodworker. But if b should have been .24" inches, the circle is really 33.45" radius. A 1/100", .4% error in measurement becomes a 3", 10% miss on the radius.

    So, it works best if you've got a quarter circle or so to start with.
    Last edited by Steve Demuth; Yesterday at 11:44 AM.

  4. #34
    Join Date
    Dec 2015
    Boone, NC
    Wow, you guys really know how to be anal woodworkers

    Just clip the offending corners off at a 45 and call it a day. This is some 2x framing lumber going into the back of a truck, not a piece of furniture.

    You could scribe it with a compass or figure it out with geometry equations, but why bother if you don't need to for the application??

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