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I'd like to know how a braced top changes its radius with changes in humidity. For example, say braces are glued to a top at 45% humidity, with a 28 foot radius. Then humidity is slowly lowered until the top is basically flat in the lateral direction. Does anyone have experience to know what humidity change is needed to do that?

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David Malicky

Is the top glued to a rim or not .. I believe that if you have the braces carved, and its not glued to a rim, that it will warp easier at the edges, as there is less bracing holding it in shape. One of the reasons why I prefer to brace and then glue onto a rim within a couple days at the correct humidity level.

One of my part time students braced a back at 45% (or so he figures) at his dad's shop, 18 foot radius. Braces were carved, and thinned at the edges. He then transported it home in his car, about a 2 hour drive, on a hot day, car was air conditioned. It was brought over to my shop a couple days later, and has warped backwards. My shop is at 45%, and after sitting for a month now, it hasnt come back. Those braces are coming off next month when we start up again.

My guess with regards to your question is it will be flat at about 35% or so .... assuming no other temp changes/extremes.

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Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

Thanks, Tony. For my question, I was thinking of the top unattached to the sides. I'm asking because my process is to glue braces to a lowered-humidity flat top, then let the top assume an arch at normal humidity. This for a simplified guitar project in a woodshop class -- we currently dry the top with a space heater in a box until it has shrunk ~1/16", which works fine, but we could reduce the labor (and improve consistency) by just letting the tops acclimate in a drying cabinet.

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David Malicky

Hi David,

In your case, I would have the braces and top in the cabinet for a couple days, then do the gluing. Put it back in the cabinet after they have set (using titebond, an hour on the go bar deck is plenty). Same when carving the braces - carve them, or have the rim ready to go, then carve and glue it up all the same day. Once its glued on, it doesnt need the cabinet anymore. Do the same for the back (in fact I do the backs first)

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Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"