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Thread: radius question, math guru's

  1. #1
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    radius question, math guru's

    ok how do you folks determine your radius's for a preselected arch,,

    example need a arch that is 13.375 long and 1.75 high.. i am sure there is a formula to do it out there but this old noggen has forgotten it and the string trick is getting old.. help also need the radius for a 15.375 arch as well
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  2. #2
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    http://www.handymath.com/cgi-bin/rad2.cgi?submit=Entry

    Why do math when someone else will do it for you? Sigh. I swear the internet will rot your brain (I to used to know how to do this.. at one time..).

    Practically speaking I probably wouldn't measure the arc at all, but would just lay it out with a flexible stick pinned at the middle and edges and mark to the curve.

  3. #3
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    Do you know the arc angle? I'm not sure that table will solve the problem unless another variable is known, i.e. the arc angle. In Larry's example I think he's referring to the arc length rather than the chord length, but.......?
    Last edited by Al Launier; 12-10-2012 at 11:22 PM.
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    Quote Originally Posted by Al Launier View Post
    Do you know the arc angle? I'm not sure that table will solve the problem unless another variable is known, i.e. the arc angle. In Larry's example I think he's referring to the arc length rather than the chord length, but.......?
    Yeah good point, a more complete calculator that tries to solve all of the cases is at: http://www.handymath.com/cgi-bin/arc18.cgi?submit=Entry
    it will solve either the arc length or the chord length given one or the other and the height.

  5. #5
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    thanks guys after i saw the arc swing marks my dusty brain woke up will make me a example and hang it up for later dates as for laying the spring stick down aqnd tracing it ryan my spring stick doesnt flex equally along its length so that didnt work for me i tried that first.. so that stick can become kindling..
    If in Doubt, Build it Stout!
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  6. #6
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    Quote Originally Posted by larry merlau View Post
    thanks guys after i saw the arc swing marks my dusty brain woke up will make me a example and hang it up for later dates as for laying the spring stick down aqnd tracing it ryan my spring stick doesnt flex equally along its length so that didnt work for me i tried that first.. so that stick can become kindling..

    Try the following. I have not tested it out.

    H = height of arch

    L = length of arch at the base, not arc length

    radius = (H² + 1/4L²)/2H
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  7. #7
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    Larry,

    When I get home, I will check to see if I have a better geometric solution.
    “When love and skill work together, expect a masterpiece.” - John Ruskin
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  8. #8
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    Or:

    L=Arc Length = 13.375
    A=Arc Angle = 12.47481
    P=Pi=3.1416
    R=Radius

    L=(R*A*P)/180 or R=(180*L)/A*P

    R=(180*13.375)/(61.43030*3.1416)=12.475
    Thoughts entering one's mind need not exit one's mouth!
    As I age my memory fades .... and that's a load off my mind!

    "We Live In The Land Of The Free, Only Because Of The Brave"
    “The problems we face today are there because the people who work for a living are outnumbered by those who vote for a living."
    "
    Socialism is a philosophy of failure, the creed of ignorance, and the gospel of envy, its inherent virtue is the equal sharing of misery." Winston Churchill

  9. #9
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    Quote Originally Posted by Bill Satko View Post
    Larry,

    When I get home, I will check to see if I have a better geometric solution.
    bill, swinging a couple arch's and using the intersection is fine with me.. i got a outfeed table that works well for large arch's just needed to see the picture for me to wake up
    If in Doubt, Build it Stout!
    One hand washes the other!
    Don't put off today till tomorrow!

  10. #10
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    Ryan, thanks for the handy tables. Didn't know they existed. I saved as a favorite place.

    Any others that are as useful? I had to did into my old Machinist Handbook that I bought new in 1960 (16th Edition) to find the formula.

    Al
    Last edited by Al Launier; 12-10-2012 at 11:55 PM.
    Thoughts entering one's mind need not exit one's mouth!
    As I age my memory fades .... and that's a load off my mind!

    "We Live In The Land Of The Free, Only Because Of The Brave"
    “The problems we face today are there because the people who work for a living are outnumbered by those who vote for a living."
    "
    Socialism is a philosophy of failure, the creed of ignorance, and the gospel of envy, its inherent virtue is the equal sharing of misery." Winston Churchill

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