Radius Dish Build

Discussion in 'Guitar Building' started by telebomb, Jan 1, 2019.

  1. telebomb

    telebomb New Member

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    I’ve seen several methods of making my own radius dishes , but not real accurately done in my observation. I’ve looked all over the web . I’m semiretired, built several acoustic guitars from Compianos book on building with good results not using a radius dish for the top and backs . Buying these dishes can add up to a lot of money!!! Any help you all may offer would be really appreciated Thanks !!
     
  2. surveyor

    surveyor New Member

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    I built the rails/sled like the Chis Paulick video and used a double layer of 3/4" MDF. You might want to do this outdoors.
     
  3. telebomb

    telebomb New Member

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    G
    Yes , l have seen this video before , but how did Chris determine the arc that he arrived at
     
  4. surveyor

    surveyor New Member

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    The "long compass" he refers to is actually two sticks fastened together and set with the pencil lying at the sagitta of the desired arc. Here's the article I copied: ( the sagitta of a 2' arc with a 25' radius is 0.020feet (1/4") & a 2' arc with 15' radius is 0.0334' (3/8")/// Hope this helps--

    The Long Compass


    The long compass is a really simple and really useful device for accurately drawing a portion of a large arc (a short segment of a circle of large radius, to be precise). From what I've heard, it's an old boatbulider's technique. It's used to draw a portion of a circular arc given 3 points on the arc - generally only useful for small portions of arcs of large radius, like a 15-foot radius arc for which you need a 2-foot-long segment of the circle (e.g., for the back braces on a guitar).


    The procedure for making and using a long compass is as follows:

    • Mark the 3 points of the arc on the piece of stock to be cut into an arc; you'll have 2 outer points and one inner point between and slightly above the others:
    *
    inner point
    * *
    outer outer

    (I usually put the inner point equidistant from the two outer points, but that isn't necessary for the method to work.)
    • Drive a small brad into the positions of the two outer points; the long compass assembly will "ride" along these as you draw the arc
    • Take two straight sticks, and place them along the two lines connecting an outer point with the inner point, forming a very shallow upside-down "V". (Put the sticks above the brads, so that the "crotch" of the "V" is right at the inner point - see the picture below.) The sticks should overlap a bit in the region of the inner point, so they can be fastened together, and each should be a little longer than the distance between the two outer points. Fasten the two sticks together so they will maintain the angle they make, i.e., so the angle of the "V" becomes fixed.
    • The stick assembly is the "long compass"; the arc is drawn by putting a pencil into the vertex ("crotch") of the "V", and then sliding the "V" along the brads, always keeping it in contact with them. As the pencil travels in the crotch as the sticks are slid, it will trace out a perfect circular arc through the 3 points! Truly magic!

    The following diagram illustrates the long compass in action:




    [​IMG]




    The compass thus made is only good for the one radius. You can of course make a general-purpose compass that's just two sticks with a hinged joint that permits the angle to be adjusted, so you can use it for multiple-radius circles.

    Hope this helps to "round out" your luthierie! ;-)
     
    mistermikev likes this.
  5. difalkner

    difalkner Member

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    A couple of years ago I made two radius dishes, 15' and 28', to build my first acoustic guitar. I used a piece of 16 AWG copper wire and measured out the 15' and 28' to scribe the arc. And as was said, definitely do this outdoors! I used some low friction furniture moving pads - from Lowe's, I think - the plastic ones that let you set a piece of furniture on them to glide across the floor, just a few dollars, and set these onto a base so I could rotate my dish. In the middle I bored a 1/2" hole part way through the MDF and mounted it like a turntable so the dish could spin.

    I later built a CNC router and recut both dishes and shot a video of the CNC cutting but at the beginning you can see how we cut them outside with the setup I described above.


    David
     
  6. mistermikev

    mistermikev Member

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    heresy! burn the which! jk. that is some neato stuff. no interest in building a dish for a guitar (nor skill) but that is fascinating and I am very thankful to you for posting it.
     

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