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Thread: deflection jig

  1. #1
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    Default deflection jig

    i had a friend sort of help me build one, but it is a bit crude, however , works, If any of you use one , could you post a pic of your jig? Thanks
    Mike Marrs

  2. #2
    Mando-Accumulator Jim Garber's Avatar
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    Default Re: deflection jig

    Hah! I thought this was an Irish tune. Maybe one of you builders can write it.
    Jim

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  4. #3
    Registered User crooksj's Avatar
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    Default Re: deflection jig

    I copied a design I found on the Cafe...

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  5. #4
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    Default Re: deflection jig

    Quote Originally Posted by crooksj View Post
    I copied a design I found on the Cafe...

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    thank you
    Mike Marrs

  6. #5
    Adrian Minarovic
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    Default Re: deflection jig

    I think here are very few builders who use this here on MC. I once was considering botching up something like this from all tha scrap laying around but never got beyond thinking.
    How are you holding the plate against the jig? I don't know much about others (I think I've seen Don M's jig in the SM DVD few years ago) but I believe you would get vastly different readings when the edges of plate are secured against expansion in width and length with some wedges against free laying plate. I would probably go for secured plate for better repeatability (thickness of edges would not contrbute) but I would try to shape the edges of plates as uniformly (among the plates) as possible to exclude this variable.

    My idea was building it upside down, with large hole "underneath" the plate so you can scrape the plate for final very precise adjustments without need to remove the plate at all (think of tiolet seat with plate mounted arch facing down and the dummy bridge inside the tiolet, it would be possible to build this mounted on the wall and foldable when not in use...).
    First do just graduations and then with f holes and later with tone bars...
    Adrian

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  8. #6
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    Default Re: deflection jig

    Quote Originally Posted by HoGo View Post
    I think here are very few builders who use this here on MC. I once was considering botching up something like this from all tha scrap laying around but never got beyond thinking.
    How are you holding the plate against the jig? I don't know much about others (I think I've seen Don M's jig in the SM DVD few years ago) but I believe you would get vastly different readings when the edges of plate are secured against expansion in width and length with some wedges against free laying plate. I would probably go for secured plate for better repeatability (thickness of edges would not contrbute) but I would try to shape the edges of plates as uniformly (among the plates) as possible to exclude this variable.

    My idea was building it upside down, with large hole "underneath" the plate so you can scrape the plate for final very precise adjustments without need to remove the plate at all (think of tiolet seat with plate mounted arch facing down and the dummy bridge inside the tiolet, it would be possible to build this mounted on the wall and foldable when not in use...).
    First do just graduations and then with f holes and later with tone bars...
    I do have wedges with slots in them to slide and lock plate in place. I do mine with f holes cut, and bars in , but still heavy. I have a tee above the jig and I had put a digital scale on the jig, and put a water jug out on the end of the T handle with a clamp, so the jug always sat at the same spot on the handle., and filled jug until I got 37 lbs on scale and have slots cut in jig floor for bars
    Mike Marrs

  9. #7

    Default Re: deflection jig

    Quote Originally Posted by HoGo View Post
    My idea was building it upside down, with large hole "underneath" the plate so you can scrape the plate for final very precise adjustments without need to remove the plate at all (think of tiolet seat with plate mounted arch facing down and the dummy bridge inside the tiolet, it would be possible to build this mounted on the wall and foldable when not in use...).
    First do just graduations and then with f holes and later with tone bars...
    What about building it vertically? A lever and a spring of a known value could load the top. That could be quite lightweight, and easy to use in a variety of orientations. Digital dial indicators would allow you to locate the displays above the table, so you could see what you're doing as you carve.

    I have plans to use my CNC machine and a probe, basically a CMM, to get a more complete map of the plate deflection. Probably not necessary, but it'll be fun once I get the probe made and installed. Will also be useful for my carbon fiber instrument development.

  10. #8

    Default Re: deflection jig

    I build one jig recently for an instrument I discover (octave mandolin first build ) , as others here , not sure it's an help , but here is the model :
    it works without back installed , the strings are in place and tuned so the top is under tension like in the real life .

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  12. #9
    Adrian Minarovic
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    Default Re: deflection jig

    Quote Originally Posted by french guy View Post
    I build one jig recently for an instrument I discover (octave mandolin first build ) , as others here , not sure it's an help , but here is the model :
    it works without back installed , the strings are in place and tuned so the top is under tension like in the real life .

    Click image for larger version. 

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    Click image for larger version. 

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    Nice jig Jean! Is the "tailpiece" block of wood fixed to the jig or to the body? I believe the force of tailpiece anchored at the edge of top adds to the top resistance against bridge pressure. What's your experience?
    Adrian

  13. #10

    Default Re: deflection jig

    Quote Originally Posted by HoGo View Post
    Nice jig Jean! Is the "tailpiece" block of wood fixed to the jig or to the body? I believe the force of tailpiece anchored at the edge of top adds to the top resistance against bridge pressure. What's your experience?
    Hi Adrian
    The strings are attached on a block of wood , himself attached to the jig , not to the side of the instrument
    the block of wood is high enough to simulate the real angle of the strings
    Sorry I have no closer pictures
    however I have a short video of the jig in action https://www.youtube.com/watch?v=tB63...ature=youtu.be

    For this instrument I notice that the deflection is more done by the recurve all around the top
    I remove wood on the tonebar during the test , but nothing really happened and I decide by myself to stop removing wood just by "feeling"

  14. #11
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    Default Re: deflection jig

    Quote Originally Posted by french guy View Post
    Hi Adrian
    The strings are attached on a block of wood , himself attached to the jig , not to the side of the instrument
    the block of wood is high enough to simulate the real angle of the strings
    Sorry I have no closer pictures
    however I have a short video of the jig in action https://www.youtube.com/watch?v=tB63...ature=youtu.be

    For this instrument I notice that the deflection is more done by the recurve all around the top
    I remove wood on the tonebar during the test , but nothing really happened and I decide by myself to stop removing wood just by "feeling"
    wow , very nice
    Mike Marrs

  15. #12

    Default Re: deflection jig

    Genius, really. Make a sturdy rim with a coordinator rod like a banjo. Then you could screw an oversized back on a la Condino and have a playable back and top voicing rig!

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  17. #13
    Registered User fscotte's Avatar
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    Default Re: deflection jig

    I use one but I'm not happy with the typical two foot bridge.

    I'd like to figure out a way to use a gel pack, large enough to cover a large part of the plate. The gel would come to equilibrium, and the pack it's in would allow the entire plate to be measured, since the downward force is equalized over the entire plate.

    Finding a way to build it is a different story.

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