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#1
07-29-2013, 11:12 AM
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Building with expansive backs (zebra wood, etc)...
Hi All,
The recent humidity combined with a different thread about necks has got me thinking. A student guitar (built with a different guitar builder than me) that was brought to my shop for a look over and some advice was made with a zebra wood back that cracked. Now, the reason for the crack was partially due to excessive dryness, but was also linked to "bridges" (arched spans of about an inch - one each side of centre) left in the back braces, and also possibly includes the fact that the back was still fairly thick. (The back split directly under one of the opened "bridges".) Zebra wood (of which I have several full sets that I sawed up myself - smells like zebra dung) is described in the wood database as having a radial expansion (shrinkage) rate of slightly greater than double that of Indian rosewood. So, I recently began to wonder if the guitar mentioned above would have split had it been built with rosewood or another wood with less radial movement across humidity changes. Also, and more importantly, I realize that careful consideration must be given to guitars built with woods that are very active across humidity changes. Before I use my zebra wood, I want to consider the ramifications carefully. Do any of you have any ideas to share, or experience in altering the back design (bracing/thickness/camber) for active woods (which also include several maples)?
__________________
---- Ned Milburn NSDCC Master Artisan Dartmouth, Nova Scotia 
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#2
07-29-2013, 06:12 PM
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Quote:
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#3
07-29-2013, 07:09 PM
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I built one guitar with flatsawn spalted maple back/sides, constructed in 30-35% RH. It has experienced 20% and 70% for prolonged periods, and up to 80% for a few hours. I just checked it out and found one loose back brace end where I screwed up the fitting so it doesn't quite reach its pocket. No cracks though.
So be sure to accurately pocket your back braces if you're going to expose it to 40-50% higher than its natural state. You also might use fatter back braces for more glue area. I need to try building with maple in 35-40% and see if it can still survive down to 20% in the winter here. The mahogany ones I built in 50% have been fine at 20%, although both of their spruce tops cracked behind the bridge as soon as it hit about 30%. So mahogany can take a big RH drop. The only rosewood back I've done was in 30-35% and hasn't cracked, but with how brittle it is, it probably can't take a very big percentage drop even though the dimensional change is small. But because of that, you can build in lower humidity without putting any more strain on the glue joints than more expansive woods. For soundboards, I prefer redwood and cedar over spruce tone wise as well as expansion wise. I'm guessing they'd crack at about the same percentage drop as spruce, but because of the smaller dimensional change you can build in low humidity to counter that, and then they don't poof up too much in the summer either, which is especially important on the top since it affects playability. And non-ebony fingerboards. Ebony is about as bad as it gets for movement. Last edited by dekutree64; 07-29-2013 at 07:14 PM.
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#4
07-31-2013, 08:09 PM
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Hi!
I have used zebrawood to build a classical guitar and the results have been interesting. I feel the wood responds in a similar fashion to rosewood, and as far as humidity goes, there have been no problems... that said I do keep my guitars between 35% and 60% all the time. Cheers, Paul Weaver Lamare Guitars
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Paul Weaver, luthier Valencia, Spain
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#5
08-01-2013, 06:10 AM
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Thank you gentlemen for your thoughts and ideas.
__________________
---- Ned Milburn NSDCC Master Artisan Dartmouth, Nova Scotia
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