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Thread: Sanding the bridge of a flat-top mandolin

  1. #1
    Registered User Sevelos's Avatar
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    Question Sanding the bridge of a flat-top mandolin

    How can I sand the bottom of a flat-top bridge (in order to lower it), and keep it perfectly flat?

    Ok, this will probably sound stupid, but here goes the long story:
    I bought a new bridge for my flat-top. It is a one-piece ebony bridge with a thin white plastic saddle on top.
    I wanted to lower the bridge and so I put some sanding paper on the floor and started sanding the bottom of the bridge, moving it from side-to-side.
    I noticed pretty quickly that for some reason both edges of the bridge got sanded too much and started pointing up, looking something like this: \____/
    I decided that I probably held the bridge incorrectly and tried holding it in the middle, still got the same form. Tried moving it in circular motions instead of side-to-side, sanding on different surfaces, leaning less on the bridge while sanding, still - same problem.
    Tried sanding just the middle, but got an arc in the center and the bridge started losing its form.

    I bet the solution is something simple...

  2. #2
    Registered User Ivan Kelsall's Avatar
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    Default Re: Sanding the bridge of a flat-top mandolin

    Sand the bridge 'along it's length'. Sanding it along it's short axis will round off the front & back edges. Also,if you want it dead flat,use a mirror or some piece of flat glass. Realistically,you should be sanding it on the top of the mandolin itself to get the best fit.
    Here's a clip which might help - zoom along to 5 mins into it,



    One point,from your description,i'm wondering if the piece of wood from which the bridge was made,has been machined in such a way that it has some residual stress in it. This could mean that as you sand it,the stress is being relieved & is deforming the wood. Try sanding it as i suggested & see how it goes. If it doesn't get any better,then maybe a new bridge is required ?,
    Ivan
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    Registered User Tavy's Avatar
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    Default Re: Sanding the bridge of a flat-top mandolin

    In short: practice!

    Now that you know what the issue is you need to keep that in mind and try again.

    Don't press down too hard, use good quality sharp grit and let the paper do the work. Pressing hard is what you do when you want to sand one end more than the other and get the shape you describe

    You will also find that either your thumb or your fingers will press harder than the other and so it ends up leaning one way or the other - keep turning it round to make sure that doesn't happen (so much!).

    Good luck!

  4. #4
    Registered User Freddyfingers's Avatar
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    Default Re: Sanding the bridge of a flat-top mandolin

    Thanks for the video. Quite a bit of work goes into it. But the jam at the end was great!
    Its not a backwards guitar.

  5. #5
    Registered User Sevelos's Avatar
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    Default Re: Sanding the bridge of a flat-top mandolin

    Thanks for the replies guys.
    Ivan - I was sanding the bridge along it's length. As I wrote, I got both the left and the right sides of the bridge sanded too much and "pointing up". The bridge got a shape similar to this :
    -------------------- (saddle)
    |.......................|
    \_____________/ (bottom)
    This mandolin is (almost?) flat, so it wouldn't help much to sand on the top of it. The bad bridge shape is seen when I put it on a straight ruler as well as the mandolin top.

    Tavy - perhaps I am pressing too hard when I sand, I don't know... Most of the time I was holding the bridge in it's middle in order not to push too hard on the sides, but that didn't help. Always got this shape (sometimes the center of the bridge bottom was not very straight, sometimes it was, but the sides remained like this).

    Perhaps I'm making a bigger deal from this issue than it deserves, it's just that we don't have mandolin bridges for sale in Israel (not any that I know of), and it took the bridge 3 weeks to arrive from Elderly (and some shipping costs), so it's not just a matter of going to the shop and getting another (which I don't know how to sand either ).

  6. #6

    Default Re: Sanding the bridge of a flat-top mandolin

    I have used a small scraper to remove wood in areas like that. I check with a straightedge, chalk the high spots, scrape them down and sand just enough to even things out. Lots of checking so it does not get away from me.

    Sometimes a flat metal file works also because it does not flex like sandpaper. Everybody works differently though.

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  8. #7

    Default Re: Sanding the bridge of a flat-top mandolin

    Sanding or filing a surface truly flat is actually extremely difficult. The edges will always be relieved more than the center. In the old days apprentice machinists would have to make a cube of steel exactly flat, square and to dimension on all six surfaces with a file. Might take 6 months. Make a tool, or a jig, to hold the workpiece, use something like a shooting board, etc. One trick is to put the sandpaper on a true surface, hold the bridge as firmly as possible and just push it away from you. Don't move it back and forth, just push it away, pick it up, bring it back, and push it away. Rotate it every two or three strokes.

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  10. #8
    Mando-Accumulator Jim Garber's Avatar
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    Default Re: Sanding the bridge of a flat-top mandolin

    Quote Originally Posted by Sevelos View Post
    I wanted to lower the bridge and so I put some sanding paper on the floor and started sanding the bottom of the bridge, moving it from side-to-side.
    Isn't it generally true that a flat-top mandolin top still has a bit of curvature? If it were truly flat it would probably sink with the downward pressure of the bridge. The OP is sanding on the floor (see above). Shouldn't he be sanding on the actual top of the mandolin. Am I missing something here?
    Jim

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    Registered User tonydxn's Avatar
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    Default Re: Sanding the bridge of a flat-top mandolin

    This won't help Sevelos with his problem, but I thought others might like to see the gadget I concocted for sanding bridges. I made it out of scraps, a discarded ball bearing race and a couple of fat wood screws. It eliminates any rocking and gives a nice true base. I usually fit the bridge blank to the soundboard then shape the bridge from the sanded blank.
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