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Thread: Recurve too thin?

  1. #1
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    Default Recurve too thin?

    Hello.
    I’m graduating the spruce top on a stewmac f5 mandolin kit; my first build. I had no problem creating the recurve on the outside, but have shaved an area too thin when graduating the underside of the recurve (inside of the top). Beginning about 2” along the edge from the center of the tailpiece on the bass side, and extending for about 5” I’ve thinned it to between .0092” and .0096”. Specs say it should be .011”. I’ve kept the recurve along the tailpiece at .012”. The rest of the top is still within specs. Is this a fatal flaw or is it likely to hold up as long as the rest of the top is graduated properly?
    Thanks for your help.

  2. #2

    Default Re: Recurve too thin?

    Should be fine. My #1 was at .08 most of the way around on the recurve. It's still around almost 20 years later.

    Thanks,

    Magnus

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  4. #3
    Adrian Minarovic
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    Default Re: Recurve too thin?

    0.009" is almost paper thin, I guess you meant 0.09"... That's around 2.3mm and IMO close to dangerous zone. I wouldn't go under 2.5 mm unless the piece of spruce is extremely stiff. But if you left the centerline along strings strong enough and the piece of wood has reasonable shape of arch and stiffness you may be fine.
    Adrian

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    Default Re: Recurve too thin?

    Quote Originally Posted by HoGo View Post
    0.009" is almost paper thin, I guess you meant 0.09"... That's around 2.3mm and IMO close to dangerous zone. I wouldn't go under 2.5 mm unless the piece of spruce is extremely stiff. But if you left the centerline along strings strong enough and the piece of wood has reasonable shape of arch and stiffness you may be fine.
    Yes, got the decimal point wrong. It’s between .092” and .096”(2.34 to 2.44mm). Center is at .25”. Remeasured the thin area; it’s about 3” of the recurve. Would it be useful to build up the area with glue or epoxy?

  7. #5

    Default Re: Recurve too thin?

    Structurally, it will probably hold up. Soundwise, you'll be losing volume, particularly on the trebles. My experience agrees with HoGo, 0.110" finished thickness is the minimum. When carving, allow for losing another 0.010" or so in the final sanding/shaping.

    You can always take more off later after stringing it up in the white. But you can't put it back.

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  9. #6
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    Default Re: Recurve too thin?

    Quote Originally Posted by kymando View Post
    ...it’s about 3” of the recurve. Would it be useful to build up the area with glue or epoxy?
    No need to build up with glue or epoxy. It would only add mass without adding stiffness of structure.
    I agree with Adrian, it's "on the edge" at that thickness, but if the 3" area is far enough from the string line (center of the top from neck to tailpiece) it will probably be fine as long as the piece of wood is stiff and strong. I probably would not reject the top because of it.
    (FWIW, I used to carve top thinner than 3mm near the edge but now I try to stay right at 3mm and if I need to thin a stiff heavy piece of wood I do so over the entire piece while keeping the 3mm around the edge, 4mm at the string line.)

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  11. #7
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    Default Re: Recurve too thin?

    FYI: Looking for more info on recurve thickness I came across this interview with Michael Heiden. In response to Question #7 he says that with Adirondack spruce he shoots for .075” at the recurve and .190” in the middle. Seems awfully thin.

    http://mandozine.com/media/CGOW/heiden.html

  12. #8

    Default Re: Recurve too thin?

    Just be careful under the tailpiece. There's a lot of stress from the strings there.
    If this is your first build and you don't plan to sell it there's no shame in laminating a patch in a thin spot.

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  14. #9

    Default Re: Recurve too thin?

    Quote Originally Posted by Jim Hilburn View Post
    Just be careful under the tailpiece. There's a lot of stress from the strings there.
    If this is your first build and you don't plan to sell it there's no shame in laminating a patch in a thin spot.
    Yup, I got that wrong on my #1 but after the box was closed. I extended it's life with a cocobolo arm rest lamination. I've since re-topped it and a mouse took up residence and ate the f-hole. I hate to let it go so I plan to finish up some day anyway. I ruined a back also by sanding too much after the fact. Maybe I'm just dumb but s&^t happens. The next 2 came out nice. Live and learn.
    Richard Hutchings

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    Default Re: Recurve too thin?

    Agree with previous from John and Hogo. You're on the borderline. But that centerline from the bridge to the tailpiece needs to be strong. If in doubt, reinforce it or make a new top. Better now than a year from now.

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  18. #11
    Adrian Minarovic
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    Default Re: Recurve too thin?

    If you are going to reinforce teh area I would suggest against flat patch which is hard to fit properly for beginner and requires plaster cast and way to press it in place evenly. If your top is 2.5mm or so thick under tailpiece you can just add small spruce "popsicle" bar (1/2" wide at most and 4"-5" or so long) across the thinnest area under tailpiece. Perhaps 1/16" thick and tapering towards ends. Try to fit it well to the surface (shouldn't be too hard as that spot is close to flat) and glue well - don't use too much pressure or you crack the plate.
    I've seen such small bar on some mandolins of well known EU builder he uses quite thin tops with cross braces and adds one small brace near end of tailpiece (slightly at an angle like on guitars). Those mandolins sound pretty good.
    Adrian

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  20. #12

    Default Re: Recurve too thin?

    I'd just throw it on the body and see what you get. If it's lousy you can take it off and try again. Either way you'll be able to answer this question with authority for the next builder. Don't do the binding til you find out.
    tom

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  22. #13

    Default Re: Recurve too thin?

    I carved the top plate for a two point mandolin. Measuring around the perimeter the recurve turned out to be between .08 and .09. The spruce for this top has a fairly tight grain. What are the consequences if I decide to use this top? Is the mandolin going to collapse at some point (constructional problem) or does a recurve that is too thin only affect the sound?

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    Default Re: Recurve too thin?

    For a first timer (defined as having built less than 5-10 mandolins), the hardest question is how floppy is too floppy? I use "floppy" because that's what the top feels like when it's getting thin enough. I'm on my fourth mandolin, and it's taken that long to start understanding the remarkable strength of spruce. The first two were A2s that were about 1 mm thicker that described above, and they were loud but brighter than I wanted. The third was an F5 with a thinner top and a warmer sound but not as loud. The fourth is an A5 in progress that I carved about 0.5 mm thinner. However, most of the thinning is between the spine and the sides to maintain the strength of the arch. All that to say you're probably fine, but the only way to know is to finish it with the understanding that it may not be your last.

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