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I'm just entering the world of gravity clamping via vacuum and need a little help from anyone who uses this system.

 

I'm working on building a guitar that has a very arched secondary back (Scherzer 11 string to be exact). The back is made up of two sheets of veneer (Maple outside/Cherry inside) ca. 1.3 mm thick each. I've made a male "plug/form" the exact shape and arch as the back (See photo).  

 

So, my question is...I'm starting to flush trim the form to the exact size of the second back but stopped until I get an answer. When I go to vacumm clamp the two veneers together do I need to trim the veneers to the exact shape of the finished back or is it ok to leave a little overhang to be trimmed later? I'm wondering if the vacuum bag will press/pull on the overhanging edges and distort the back or not. The other solution, if it is the case that there will be distortion, is to leave the lip on the form as it is now, trim the veneer close to but not overhanging the edges and trim the excess later. Any of this make sense???

 

My reason for all this is that the back is a second back that sits above the main back a few cm and is attached to the main body after the instrument is together. I was hoping to use the form as a solid base to aid in installing the binding and perfling in the secondary back as I suspect it will be most difficult once it is attached to the guitar.

 

Any input would be greatly appreciated and if I'm totally off base doing it this way please let me know. It's my first foray into this guitar and veneering style.

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For what I know of vacuum clamping, it shouldn't : if the vacuum sheet presses both faces of the overhanging veener sheet, pressure will be equal on both faces, so no dsitortion should occur. But I'm not sure if I fully understood what this top is meant to be?

Thanks Pierre. The secondary back is ment to seperate the guitar from the players body. The design was one of many different attempts to boost the volume of the 19th century Early Romantic Guitar. It's a full second back which is attached to the guitar via brass screws and a system of wooden risers.

 

I've attached a couple phots. One of the original instrument owned by the illustrious guitar historian Matanya Ophee, photo supplied with his plans of the guitar and the other borrowed frm Gary Southwells website of a beautiful replication of the same instrument. I hope neither of these fellows mind as the photos are being borrowed for educational purposes.

 

In both photos, it's a little hard to see but, the finished attached back is visible.

 

I could make this back the traditional way of veneering but I'm learning to laminate with vacuum pressing and some of the finer details elude me at this point. The form I've made will go inside a vacuum bag with the pieces to be laminated situated on the top. As the bag empties of air it should close around the form and seal tight against it on all sides. My question concerns any overhang of the laminate I might have, Will the pressure press on that overhang and distort the laminate? Or does the laminate need to be trimmed flush with the sides of the form before going into the vacumm bag? 

 

 

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Thanks for the pics. I don't think it would distort (see my first answer). But I would add a 1cm margin, that would help to cut the laminated back with some tolerances.
I would not make the form the exact size but just a little larger as I would be afraid that the press will roll the eadges over the form. I worked in a vener factroy for 10 years and the way we made panels was first with a sheat of 1\28th venear then the core piece then the finish piece the two outside pieces going in one direction and the core going in the other .good luck Bill.''''''''''''''''''''

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