Top flattening after braces

[Dan Austin]

Thanks very much for all of the great information that I have received from this forum. I am recently registered and this is my first post. I am a beginner working on my first guitars.

I glued my top together (very tight grained Sitka Spruce), installed the rosette, cut out the sound hole, made the braces (Sitka Spruce also), and glued the braces, with the exception of the bridge plate. The primary braces all have a 28 foot radius and after glueing the braces, the top took on and held that radius. After carving the braces however, the top basically wend back to flat. I do not think that I took too much material from the braces but suppose that this is one of the only reasons that this would happen. I am following a plan and have thus far stayed true to it relative to all dimensions. By the way, this is a J-185.

So I was wondering if anyone could give me some feedback on what I observed and if this is normal or not. Any comments would be appreciated.

Thanks,

[Herman]

Hey Dan,
First suspect is humidity. If it lowers, the top dries and shrinks. The braces can't follow that movement, so the top flattens out and can even get a concave shape.
Raise the humidity and see if the radius returns.

Good luck,
Herman

[ken cierp]

Hi Dan,

Welcome!

Are you working in a humidity controlled shop?

[Dan Austin]

Hello gents, thanks for the welcome.

I am not working in a humidity controlled space per se. I have a large area that is fully insulated with central A/C and heat. This time of year the heater does not run ver much as I have the thermostat set to about 67 degrees (F). I live in central TX (Austin) where the average high and low humidity is about 83% (morning) and 53% (afternoon). I need to get a digital humidity gauge for the shop so I can track things in there.

One other thing that I did not mention was that I am building two guitars at the same time. I have carved the braces on one of them (i.e. one I mentioned in my first post). The 2nd one is still un-carved and is still holding its radius nicely.

This brings up a couple of additional questions:
1.) is the top that has flattened still ok to use?
2.) how do y'all control humidity in your shops?

Thanks again,
Dan

[ken cierp]

Its pretty risky building a guitar in high humidity (above 50%) when an instrument gets used or placed in an environment where the HR is lower than where it was constructed bad things will happen -- specifically cracks and maybe action problems too.

Get yourself a big honking dehumidifier and keep your work space at about 45% I know some guys have little isolation spaces with low RH but to me that's a bit of a hassle and certainly not a sure fire solution.

1.) is the top that has flattened still ok to use?

Get the RH down to 45% and see what it looks like if it goes concave I would not use it.

[Jim Ebert]

I was just reading last night on the UNOFFICIAL MARTIN GUITAR FORUM that Martin back in the day would use humidity to radius their tops. Here is a quote from one of the threads.
"I double this basically also but in my opinion Martin gained top arch more in using humidity and moderate heat. While humidity always was (and still is) a big matter especially while bracing a guitar at Martin the tops were stored dry and put next to the heating stove before braced. The top grain shrinks before the braces are glued on first. Now when normal humidity returns the grain swells back somewhat and results in a slight top arch. Additional this treatment prevents from future dryness cracks." I have given a link to the discussion I've been reading. I find it interesting and I think it will help give some good understanding to the importance of climate control in the shop. in fact, one of the biggest impediments to me starting my builds has been the challenge of getting my shop ready. I am happy to say that I have concurred the beast and have finally started building. I am starting by building 3 prewar inspired dreads and am thinking of trying this method on one of them. Has anyone used this method to radius their tops?
Jim
http://theunofficialmartinguitarforum.y ... mkTATZIg5t

[ken cierp]

I have heard this notion before -- my contacts at Martin factory say it is nothing more than a myth. Martin made/makes than and now "flat top guitars" Back bracing 15' contour and top bracing a very slight 52' contour. The bulge seen in the vintage instruments is not from some manipulation of the wood but simply the string tension doming the sound board over time ---- causing play-ability problems I might add, most requiring neck re-sets. Of course not to say that using dry well seasoned wood is not important. Sometimes the most simple things tend to make the most sense.