Drop top on hollow body

Discussion in 'Luthier's Corner' started by Max Bogosity, Sep 28, 2022.

  1. micguy


    May 17, 2011
    It's a thick veneer that's bent and glued onto the face of an instrument. Usually, you have a flat surface, or a simple curve - like an arm relief cut, where the material only has to bend in one direction. If the hollow body is complex curved (like an ES 335, where the thing is contoured in multiple planes), then trying to do a drop top is probably not going to work - wood doesn't bend that way, unless the whole thing is steamed all at once - I don't now how you're gonna do that. I could be wrong, but I don't think I am.
    Max Bogosity likes this.
  2. 2groggy


    Jan 17, 2015
    Toronto, Canada
    I wouldn't do that with with one of your builds :D

    Joking aside, could you steam the top panel off? Once you have the panel off, you can use a vacuum bag with no risk of crushing the body.
  3. I've seen the aftermath of a bag implode into an empty side of a curved form, it was like a bomb went off,
    and shook the dust off the light fixtures, like it was snowing in the shop! :woot:
    I guess nobody was paying attention to the new guy's form lay-up.

    Darn near had a grabber :woot:
    Last edited: Sep 28, 2022
    mikewalker likes this.
  4. No doubt. I kept waiting for the bit to grab his glove and chop off his fingers, but it didn't end up like I thought it might :nailbiting:
    mikewalker likes this.
  5. micguy


    May 17, 2011
    Whatever you do, make sure to make a video, and look into the camera and say "watch this" before every step. That way, when it all goes South, you at least have a funny video.
  6. "Here, hold my beer....." :D
    Beej and 2groggy like this.
  7. So am I right in assuming you are doing a carved top like a violin/cello/hollow jazz guitar etc. going onto side rims with kerfing? If you're not planning on carving the top, then you could work with a thinner top, like acoustic guitar thickness and force/support the arched shape with carved braces'.

    The problem with holding a curved top down around the edges, might go some way to explaining why most arched top instruments usually have a flattened edge all the way round?

    Arch top.jpeg arch top 3.jpg
  8. KidAmnesia


    Jul 13, 2022
    Buenos Aires
    For me a drop top was always a thin flat top (ussually less than 5 mm) with an arm bevel that's bent over the back rather than carved.

    If this is what you mean, how I like to do it, is carving the bevel on the body before hollowing it out and before cuting the final shape; then you put the top (also oversized or even sqare) on top of the back, use dowels or screws to position it and clamps to hold it in place and then bend it. Once you have the top bent, you can hollow out the back and glue the top to the back. And finally you cut the final shape.

    I wouldn't use a vacuum bag and would be very carefull with a curved clamping caul. You want to apply all the clamping force on the sides (and on the center block if you have one) but no force at all over the unsupported parts of the top and back that can break or deform during glue up.
    Max Bogosity likes this.
  9. Something I've done for clamping odd shapes and angles on acoustic instruments:

    Make some angled wedges large enough for the clamp to sit on that result in a flat clamping surface. Then put masking tape on the surface of the instrument, and the face of the caul/wedge that will go on the top. Super glue and accelerator in between. You can then clamp the job and simply peel off the masking tape and caul after.
    Max Bogosity likes this.
  10. Yeah, I did that for my EUB project. I was kind of resisting it here because there will be a greater variety of angles, but it might still be the way to go.

    What I'm actually thinking of is not just bending the forearm contour like a regular drop top, but bending the entire top and back across their width. A simple cylindrical bend, not a dome. (Sorry about the dog hair in the sketch. My whole house is like that.)

    This is trying to achieve a reasonably large interior volume while creating a forearm and belly contour and also avoiding carving the top. But it results in a spectrum of top-to-side angles depending on distance from the centreline, which would need a large number of wedges at different angles. Spool clamps with rounded faces as discussed in a previous post might also work, if the angle isn't high enough to make them slide off or dig in. I need to do a scale drawing to work out how large an angle I'm actually dealing with.

    Maybe the way to go is two large cauls the size of the whole body, each with one flat side and one concave side with a radius matching the top/back. Then I can clamp flat all the way around, and re-use one of them as a fixture to hold the body so it doesn't rock around while doing other work on it. (Like running it through the router fixture to create the radius on the sides before gluing on the top and back.) I am most of the way convinced that this is the thing to do.
    Matt Liebenau and eddybuzzard like this.
  11. KidAmnesia


    Jul 13, 2022
    Buenos Aires
    I like that approach, you can also use those cauls to clamp the bent wood and let it cool in that shape.

    Another option is to use a tire inner tube as a clamp. It can work just as good.
    Max Bogosity likes this.