2mm thick backboard gore/gillet

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2mm thick backboard gore/gillet

Post by TallDad71 » Tue Jan 23, 2018 5:38 am

So I bought the books, read them, rubbed my head hard, read them again and decided to embark on a live Walnut back for my guitars.

I wanted to use Trevor’s equation 4.5-7 to predict the final thickness of each board. So I cut, thicknessed, weighed, tapped and measured my frequencies and entered them on my spreadsheet.

Two of my boards , European Walnut, came out as 2.8mm thick. That’s fine, I would have gone thinner myself.
One panel, American Walnut, came out as 2.4 mm thick, again no problem there, that’s feasible.
The 4th, American Walnut, came out as 1.96mm and that is far too thin for my comfort levels.

All 4 panels are crown cut, not Quartersawn, the two American Walnut sets are very pliable across the grain, but stiff with the grain.

Two questions
Is 2mm too thin to make a live back?
Does the crown cut wood render the calculations null and void?

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Re: 2mm thick backboard gore/gillet

Post by Allen » Tue Jan 23, 2018 6:29 am

I wouldn't even make a uke back 2.0 mm thick.
Allen R. McFarlen
Cairns, Australia

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Re: 2mm thick backboard gore/gillet

Post by Trevor Gore » Tue Jan 23, 2018 9:32 am

If you think about it, the equations have to assume homogeneity through the thickness of the piece, and reasonable uniformity across the width. These are not a bad assumptions for the type of cuts we use in guitars, particularly tops where the tradition has been to use quarter sawn wood that is also pretty uniform across the width. Depending on species, flat sawn wood can be problematic for whole stack of reasons (stability, for example) and also for the equations if there is a significant difference between the properties of early and late wood. If, for example, the late wood is a lot stiffer or denser (or both) and is on the surface (which takes most of the bending stress) the measured properties and acoustical performance will be different than if the same piece of wood had been prepared but was showing the early wood on the surface (same thickness, both cases). So measuring at a high thickness and hoping the same properties will prevail through the changing thickness of flat sawn wood becomes a bit of a stretch.

If you think your flat sawn back wood is giving you spurious results, measure it a couple of times as you come down in thickness and leave it a bit thick. Have a look at the bottom of page 12-11, 2nd edition, which gives a method of tuning which deals with these sorts of situations. As well as brace carving, it is still possible to sand the back thinner (hand sand or ROS ) if you want to.

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