Drop top on hollow body

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

  1. Planning how to clamp and glue a drop top on a hollow body. I'm sure I've seen somebody here do it, but can't remember where.

    Is it doable in a vacuum bag or will that would result in the unsupported top sinking in? I guess the answer is that it depends on the stiffness of the top, the unsupported span, and how much vacuum is applied.

    The alternative is probably a curved clamping caul.
     
  2. Bruce Johnson

    Bruce Johnson Gold Supporting Member Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    That's a tough one that you'll have to figure out. The important thing is that you really only want to clamp around the perimeter. You don't want to apply pressure in the middle of the top, or it will cave down. I think a vacuum bag would be the wrong approach. I'd go with a bunch of clamps around the outside edge, violin style. You may have to make up some angled cauls.
     
    Aqualung60, eddybuzzard, Beej and 2 others like this.
  3. Agree with @Bruce Johnson: don't use a vacuum bag if you don't want it to end in tears. A strong vacuum on something the size of a bass body will result in about one and a half tons of clamping force!

    Spool clamps are probably the best thing to use.
     
  4. Slidlow

    Slidlow The Human CNC Supporting Member

    Apr 15, 2009
    Oshawa, Canada
    You might want to brace the drop top (depending on how thick it is) to mitigate any possibility of warpage after assembly. Of course I say this without knowing the details of how your hollow lower section is constructed.
     
    Last edited: Sep 28, 2022
    Max Bogosity likes this.
  5. Yeah, that will need some testing. In this case the top probably meets the side at a higher angle than the top of a violin, so I would worry about spool clamps just slipping off. I think you're right about the cauls.
     
    eddybuzzard likes this.
  6. It's kind of tempting to try it just to watch the implosion.

    At this point, I also don't know. :D
     
  7. Bruce Johnson

    Bruce Johnson Gold Supporting Member Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    I picture a simple set of caul blocks with an angle in them, so the clamps stay vertical. And maybe some straps from the caul blocks over to the other side, to keep them from slipping down the slope. Planning and test-fit before the glue.
     
  8. This is a good point, and I have thought about this when planning my own carved top build. I came across someone who did their spool clamps with a roundover to allow for a non-flat top:



    Bonus content: he makes a simple router jig to hold the clamps on the router table, then re-uses the jig to split the clamps in half :)

    Depending on how specific you wanted to be with your spool clamps you could make your own with the correct angle built into the face of the clamps.
     
  9. If it’s entirely hollow under the top then you’ll only want clamping pressure around the perimeter, so spool clamps or similar would seem the way to go. I would leave the top cut ‘square’ to allow for easy clamping, then carve/sand once it’s in situ.
     
    eddybuzzard and Max Bogosity like this.
  10. His router jig is kind of terrifying, but the spools with rounded faces are a neat idea. I don't think he mentions it, but if they're flat on one side and round on the other then you can reverse them depending on what you need for a given job.
     
    Jeff Siddall likes this.
  11. Bruce Johnson

    Bruce Johnson Gold Supporting Member Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    Spool clamps are a cool classic design, if you want to take the time to make them up. But you could also clamp up this job with regular F-style steel clamps. You wouldn't need so many of them if you make up the wood cauls.
     
  12. rudy4444

    rudy4444

    Mar 13, 2012
    Central Illinois
    I like to clamp the perimeter with C clamps and "quick" clamps, and fasten the drop top down in a few spots in the central area of the body. If you're following a plan (which you should be...) then you can use screws at points that will be later machined out or obscured by a pick guard.

    Here's an example of fastening down with screws where pickups or other things will be located:

    [​IMG]
     
    Max Bogosity likes this.
  13. rudy4444

    rudy4444

    Mar 13, 2012
    Central Illinois
    You can never have too many clamps, so it's wise to start buying a few at a time as your budget allows. I have "good" clamps, and also a bunch of cheap Harbour Freight C clamps that you can pick up when you have one of those "96% off total purchase" coupons. :)

    I haven't even broke out the HF clamps here...

    [​IMG]
     
    Crawforde and Gruff like this.
  14. What does “drop top” mean?
     
    Flamingo21, Gruff and westrock like this.
  15. Couldn't you just go with a "Go-Bar" setup and stick the bars around the outer rim?

    4a9662e1528edc12472064ce467fdad3.jpg
     
    lowendrv, Max Bogosity and Beej like this.
  16. My understanding is that a "Drop-Top" is a thin top piece that has been bent to create the shape rather than carved.

    Where-as, a "carved top" would be a thicker piece of wood, carved to create the shape, like a PRS.

    I could be wrong tho.
     
    Max Bogosity likes this.
  17. So, not a hollow-body, but an arch-top, not carved but molded?
     
  18. I use cork pads to help with conformity, and protection;
    Spool Clamps.jpg
     
  19. Beej

    Beej

    Feb 10, 2007
    Vancouver Island
    @TerribleTim68 beat me to it - I think I might use a go-bar deck. If you mount strips on the upper deck or drill/chisel holes, you can set the go-bars at angles as well as vertical. They are great for getting clamping pressure into weird spots and angles. The downside is you can only use so much clamping pressure, and it's not nearly as much as most screw-based clamps.
     
    TerribleTim68 and Max Bogosity like this.
  20. micguy

    micguy

    May 17, 2011
    A vacuum bag would apply (assuming a full vacuum, and guessing a bit at the surface area) about 1.3 tons of pressure on the face (and back) of your instrument. I'm guessing that's more force than you want.
     
    eddybuzzard and Max Bogosity like this.