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Thread: 8 sided pyramid math problem

  1. #1
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    8 sided pyramid math problem

    It's too much for my meager math skills.

    I'm trying to build a birdfeeder for my wife for Christmas that replicates the gazebo-shaped addition off the back of our house.

    The addition has an eight-sided roof that comes to a point in the center, kind of like a squat steeple.

    I can calculate the pitch of the roof easily enough, but it's going to be made out of eight pie-wedge shaped pieces of 1/2" ply, and the sides (interior edges) of those pieces need to be beveled to fit tightly. I don't have a clue as to how to calculate the angle of that bevel.

    A friend sent me a solution that might be perfect, except I don't understand trigonometry, so I have no idea what he's talking about.

    Something tells me there's an easy way to do this. I would bet that millions of such shapes have been created by craftsmen with no knowledge of trig whatsoever.

    Here's the basics: The roof of the feeder will be 14" across at the base. It will be 3.5 inches high at the center. At what angle do I need to bevel the pieces so they'll fit together at that pitch?

    Thanks!

  2. #2
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    Be excellent to each other. - Rufus

    "You can get so exact that you immobilize yourself with accuracy" - James Krenov


  3. #3
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    That's some spiffy math you got there Glenn
    If it was me I would just set the table saw blade at some random angle and rip out some test pieces. If they worked great if they didn't I would adjust according.
    Way less stressful than doing all that math just to find out that the angle it came up with is wrong and you need to adjust the angle just a bit anyways.
    It could be worse You could be on fire.
    Stupid hurts.

  4. #4
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    I did it graphically in SketchUp in a minute or so. It says that the tablesaw blade needs to be tilted over at 9.854°. If you want the illustrations, I can post them.
    Irony: The opposite of Wrinkly

  5. #5
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    And I was guessing some where between 5 and 15
    It could be worse You could be on fire.
    Stupid hurts.

  6. #6
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    Yeah, but you were using your fingers and toes.
    Irony: The opposite of Wrinkly

  7. #7
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    Quote Originally Posted by Dave Richards View Post
    Yeah, but you were using your fingers and toes.
    Nope no math just dead reckoning.
    To be honest I prolly would have put a 5 or 6 on the first time. When that didn't pan most likely would have jumped to 10. Than had to back it off just a smidgen to make your 9.854 on the third try.
    Just like field artillery Shoot one low. shoot one high. Than shoot one right down the middle
    It could be worse You could be on fire.
    Stupid hurts.

  8. #8
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    That's prolly the best way, too.

    Here's what I got.


    Irony: The opposite of Wrinkly

  9. #9
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    or set the angle for 10 degrees and adjust the side the slight bit to accommodate, so what if it a 1/3 of and inch or so higher or wide ?
    "There’s a lot of work being done today that doesn’t have any soul in it. The technique may be the utmost perfection, yet it is lifeless. It doesn’t have a soul. I hope my furniture has a soul to it." - Sam Maloof
    The Pessimist complains about the wind; The Optimist expects it to change;The Realist adjusts the sails.~ William Arthur Ward

  10. #10
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    That's what wood filler is for anyway.
    Irony: The opposite of Wrinkly

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