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Thread: Bowlback Mandolin Projection

  1. #1

    Default Bowlback Mandolin Projection

    I'm doing some work on a bowlback mandolin for a friend of mine, and when I got it the transverse brace above the soundhole was partially missing and the instrument had basically folded in on itself. I spent a couple days trying to coax as much of the distortion out of the top before pulling the fingerboard and replacing the transverse brace, I intend to add two more on either side of the soundhole to ensure the top doesn't get any worse.

    Since I couldn't pull most of the distortion out of the top, the neck angle is basically non existent, so I glued a wedge underneath the fingerboard, and I'll taper it toward the nut end to raise the projection, but I have no idea what a bowlback mandolin projection should be. I should add the current bridge has been shaved down a lot and I intend to replace it entirely so I don't want to base the projection off of that.

    tl;dr When placing a straight edge on top of the frets, how much much distance should be between the top of a bowlback mandolin and the bottom of the straight edge?

  2. #2
    Full Grown and Cussin' brunello97's Avatar
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    Default Re: Bowlback Mandolin Projection

    Quote Originally Posted by Sam Brown View Post
    I'm doing some work on a bowlback mandolin for a friend of mine, and when I got it the transverse brace above the soundhole was partially missing and the instrument had basically folded in on itself. I spent a couple days trying to coax as much of the distortion out of the top before pulling the fingerboard and replacing the transverse brace, I intend to add two more on either side of the soundhole to ensure the top doesn't get any worse.

    Since I couldn't pull most of the distortion out of the top, the neck angle is basically non existent, so I glued a wedge underneath the fingerboard, and I'll taper it toward the nut end to raise the projection, but I have no idea what a bowlback mandolin projection should be. I should add the current bridge has been shaved down a lot and I intend to replace it entirely so I don't want to base the projection off of that.

    tl;dr When placing a straight edge on top of the frets, how much much distance should be between the top of a bowlback mandolin and the bottom of the straight edge?
    Great question, Sam, and I hope it scares up some replies in a section mostly devoted to arch-top mandolins.

    I've been down this same road many times and it is a fun, if challenging territory. Sounds like you are intuitively thinking and working smart.

    Unfortunately, I'm out of town and away from my bowls, but Jim Garber or someone can certainly weigh in and take some measurements from their instruments on the 'relief angle' so to speak of the shimmed fretboard. I've done that job on a number of bowlbacks, myself.

    Unfortunately, on the Italian mandolins I've attempted this fix...for the same reason of the top collapsing around the soundhole....the problem was compounded by the fretboards being super thin...with the frets often apparently cut after the fretboard was attached...so they would come apart in pieces. With the US bowls the fretboards are typically much thicker and you can get them off, shimmed and back on easier.

    The mini-braces to the sides of the soundhole is a great idea. Luigi Embergher used these on at least some of his instruments I've seen disassembled. Alex T or someone more knowledgable can confirm if that was a regular practice. I've embraced that and have added those braces on all my bowls. Hasn't effected the tone to my ears..and if it's good enough for LE then it's a good practice, I think.

    What it implies to me is that folks all the way back then were savvy to this potential problem. Both Embergher and Martin in the US added a small stiffener plate between the neck block and the top brace you mentioned to help address this potential problem.
    I've done the same thing for my bowls. I want them to remain playable...

    Good luck with the project. I'm sorry I can't answer your specific question about the projection angle / bridge height, but someone will shortly for sure.

    Why not post a few photos of the project? Not many bowlback repair adventures get covered here and there's a lot of great, affordable instruments out there that need help. And not very many folks--at least here in the States--willing to take on the kind of major triage you're talking about.

    Mick
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  4. #3
    Registered User Bruce Clausen's Avatar
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    Default Re: Bowlback Mandolin Projection

    Quote Originally Posted by Sam Brown View Post
    When placing a straight edge on top of the frets, how much much distance should be between the top of a bowlback mandolin and the bottom of the straight edge?
    Hi, Sam. I've got only one bowlback, an old Italian with no problems. On that instrument, the straight edge stands a little over 2 mm above the soundboard at the bridge; strings are about 6mm above the board. Hope this helps.

    Great post, Mick! Thanks for all that info.

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    Full Grown and Cussin' brunello97's Avatar
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    Mandolin tragic Graham McDonald's Avatar
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    Default Re: Bowlback Mandolin Projection

    When they were made, I think most bowlbacks worked on the idea that the plane of the neck (without fretboard) would follow the centre of the soundboard to the cant, so a straightedge placed on the centre of the neck would be continuously in contact with the neck and the soundboard as far as the cant. With the fretboard installed the straightedge should the thickness of the fretboard and frets above the soundboard. This is the theory and as you have found, a century or more of string tension changes that.

    Cheers

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  9. #6

    Default Re: Bowlback Mandolin Projection

    Quote Originally Posted by Bruce Clausen View Post
    Hi, Sam. I've got only one bowlback, an old Italian with no problems. On that instrument, the straight edge stands a little over 2 mm above the soundboard at the bridge; strings are about 6mm above the board. Hope this helps.

    Great post, Mick! Thanks for all that info.
    Wow, 2mm seems incredibly low, but with what Graham says 2mm sounds about like the height of the fretboard + crown of the frets. Right now, with the shim under the fingerboard I'm getting a projection of 3.5-4mm. I'm considering gluing it like that, because I'm assuming the top will settle a lot lower than it is sitting right now. I can't imagine a bridge being 1-2mm taller would cause any damage especially with classical strings, and I'd rather just have to lower the bridge after it settles than have to re-pull the fingerboard and thin the shim again.

  10. #7
    Registered User John Bertotti's Avatar
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    Default Re: Bowlback Mandolin Projection

    I have a 1911 Vega would measurements from that help?
    My avatar is of my OldWave Oval A

    Creativity is just doing something wierd and finding out others like it.

  11. #8

    Default Re: Bowlback Mandolin Projection

    Quote Originally Posted by John Bertotti View Post
    I have a 1911 Vega would measurements from that help?
    Sure, it would be great to know how widely they range from one maker to another.

  12. #9

    Default Re: Bowlback Mandolin Projection

    Here's some pictures of the offending mandolin. First picture shows it after gluing the fingerboard, I still need to sand the neck to blend it into the wedge and touch up the finish on the soundboard. Also you can notice there's a hole in the fingerboard where someone screwed into the neck, I filled the screw hole in the neck with a dowel, and reglued the neck since it was loose. I ordered a piece of rosewood which I'll fill in the fingerboard with, and make a new bridge out of; I'm gonna use the current bridge and veneers to dial everything in and just copy that out of rosewood.

    The second picture you can see how badly the top was distorted, even though I was able to get a lot out, when I got it the fingerboard was practically able to go into the soundhole. So now I'm very happy with how the projection looks. The neck is beech I think, and I decided to use Mahogany for the shim just to match the pattern on the ribs.

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  14. #10
    Registered User John Bertotti's Avatar
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    Default Re: Bowlback Mandolin Projection

    I tuned the mandolin to GDAE. Here are the pictures of various heights. I noted the fret # on the picture. I had no luck finding my smaller protractors so I used two straight edges and clamped them together along the strings to copy the break angle. I then used a framing protractor, a bit clunky to measure it. I came up with 10.4degrees but in my mind, that seems a bit much. Is that normal for a bowl back with a cant top?

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    The relief at the 16th fret is more like 1.5mm. I was not level with the rule when I took the picture. Sorry, I had a kitten trying to help me out.
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    My avatar is of my OldWave Oval A

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  16. #11
    Registered User John Bertotti's Avatar
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    Default Re: Bowlback Mandolin Projection

    My helper apparently worked too hard and is settling in for a break. If only we could work like that. But I will forgive her because she approves of everything mandolin. Please let me know what break angles are common for bowl backs. Thanks!
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    My avatar is of my OldWave Oval A

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  18. #12
    Mando-Accumulator Jim Garber's Avatar
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    Default Re: Bowlback Mandolin Projection

    Quote Originally Posted by Graham McDonald View Post
    When they were made, I think most bowlbacks worked on the idea that the plane of the neck (without fretboard) would follow the centre of the soundboard to the cant, so a straightedge placed on the centre of the neck would be continuously in contact with the neck and the soundboard as far as the cant. With the fretboard installed the straightedge should the thickness of the fretboard and frets above the soundboard. This is the theory and as you have found, a century or more of string tension changes that.
    I wonder how that changes with Vega's take of having the bridge intonated on the tailpiece side of the cant. Or does it matter?
    Jim

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