Maple Top Grain Orientation

[jonnyfontaine]

how important is it that maple top (1/4" or 3/8", flat not carved) on a mahogany (1 3/4") bodied guitar be quarter sawn? i am building a guitar now, and it has a 1 piece mahogany body, and a book matched plain hard maple top which is flatsawn... i ran the mahogany through a wide drum sander, and got it perfectly flat, resawed the maple, ran the pieces through the drum sander and got those flat as well, and after a day, the two pieces were cupped... so joined the two pieces, sanded them flat and after a day cupped again, out of impatience glued it onto the flat mahogany body and it was fine for a day, but then i noticed the whole body was now cupped, and sanded it flat again... so it's ok now, but is this gonna cup again in the future when it's all assembled and painted? and would a quartersawn top (i know it's more stable and less likely to cup or bow) if somehow it did cup, would that affect the body as much? and could the body have cupped from something other than the top? ok, thanks... jonny

[Xanthus]

how important is it that maple top (1/4" or 3/8", flat not carved) on a mahogany (1 3/4") bodied guitar be quarter sawn? i am building a guitar now, and it has a 1 piece mahogany body, and a book matched plain hard maple top which is flatsawn... i ran the mahogany through a wide drum sander, and got it perfectly flat, resawed the maple, ran the pieces through the drum sander and got those flat as well, and after a day, the two pieces were cupped... so joined the two pieces, sanded them flat and after a day cupped again, out of impatience glued it onto the flat mahogany body and it was fine for a day, but then i noticed the whole body was now cupped, and sanded it flat again... so it's ok now, but is this gonna cup again in the future when it's all assembled and painted? and would a quartersawn top (i know it's more stable and less likely to cup or bow) if somehow it did cup, would that affect the body as much? and could the body have cupped from something other than the top? ok, thanks... jonny

I just felt the need to bold it.

Nothing should ever be done out of impatience.

That having been said, it sounds like you have a moisture/humidity issue going on. Is the place where you're doing construction a constant temp/humidity? Do you move the project around a lot, in/outdoors? Do you have to transport the project places to use different equipment? All of these can cause warping in the wood. I've got my first project to prove it.

So the maple was fine when you began, and the issues started after you resawed the blank. My assessment is that the maple wasn't dry all the way through, and when you cut it, the inside edges are now trying to adjust to the humidity of the air. Read this first page, it's very helpful for what you're describing. So I would suggest building a kiln like shown, but if you glued it up already, that might not be so good.

You've sanded this maple at least 3 times, then. To sand a cupped board back to level, I'm assuming the sides are noticeably thinner compared to the middle. How many more times will you need to sand before the block stops cupping? Who knows. Each time you sand it, you're opening a new layer of the wood to the outside atmosphere, and my guess is that this will keep happening until your maple top is reduced to basically nothing.

My suggestion, and I don't know how fesable this is with your current situation, would be to pry the top off and let it sit/clamp it flat/put weight on it/build a kiln, whatever you need to do to the get maple flat, and reduce the risk of doing more damage to the mahogany. I'm sure others will have different opinions, though, and some will probably be more sound than mine.

[Supernova9]

It if managed to cup the mahogany as well, I'd try and flatten the thing as one piece. Rich (fryovanni) put a nice post up about it in the tools forum, have a look. Once you've got it flat though, let it settle and acclimatise. It's often a good plan when resawing wood like you have done, to leave it for at least 2 weeks, so that it has time to move if it's going to, and that way you can adjust for any cupping before you get to this state.

[fryovanni]

The orientation relates more to radial to tangential shrinkage rates, The more they are off perpendicular or parallel to your flat surfaces the more mixing of ratios you get and run the risk of shrinkage or expansion that creates off flat surfaces. Bit of perspective though, this is wood and you are nit likely to be able to find absolutely perfect wood in this sense (which is ok). More importantly you need to be sure the wood you will use if very well dried and has equalized to your humidity level (at least at the moment).

There are two considerations when you are dealing with wood. One is the way it expands and shrinks like a sponge does with moisture levels. The other is tension that builds and is stored in wood due to this dryning and the grain of the wood itself. Think about what happens to a freshly cut log. You will often see the wood split along one side. This happens because the wood that was say 30" around does not shrink down to say 28" all at one time. The shell will dry quickly and shrink quickly, while the core hardly shrinks at all and remains similar in size. This means the outer shell literally rips apart as it shrinks around the core. After the shell has reached a fairly dry point and has shrunk it slows down(in terms of shrinking and drying), yet the core begins to lose moisture faster as the shell is able to carry more water out. Now the core actually starts to shrink faster than the shell and more tension is created due to the imbalance. Eventually they catch up with each other and reach equalibrium moisture. However as the process was imbalanced damage was done and odd imbalances in tension remain due to unequal shrinkage. That is basically what happens.

Now you take a piece of maple, somewhat thick, and has not balanced(higher moisture inside). Then you resaw it, exposing the wet core. You have the stored tension, that may have been able to hold itself in place, but now you change the thickness and ability for it to hold its shape against the stresses. You also start a much more rapid drying of the still wet core wood. Picture what happens to a sponge with one side dry and the other freshly wet(it cups tward the dry side).

By sanding while this wood is moving(shrinking or expanding), is like futile,and it will not stop changing shape till it has equalized. You want to allow wood to reach equalibrium (sticker the wood and allow it to have equal exposure to air on all sides so it dries as equally as possible, the exception being the end grain as it releases and accepts moisture very rapidly compaired to side grain. seal the end grain). When building a neck start with well dried wood, then cut it close to shape, allow any stored tension to equalize based on its new shape, then true it up (removing much less material, thus altering the balances much less than when cutting from a blank to a close dimension).

Another thing that is notable about resawing. Is that the heat of a blade tends to rapidly dry the cut surface. If a piece of wood is perfectly dried it may very well even over dryon that surface and shrink a tiny bit causing mild cupping (but this will not be a deep effect and will balance out in a day or two).

If you run up against a board that distorts a lot from pent up tension, and has dried out equally. You may have to try more extream measures to re-set the wood to a new shape (heating, or fiber softening and introducing the new shape).

I suspect your top wood is going to be a problem for you.

Peace,Rich

[jonnyfontaine]

thanks for all the good advice... yeah i let the wood sit out of a few weeks after it was resawn but it was almost one or two days after the resaw that it cupped.... then i sat it under some weights but, i didn't heat it or wet it at all... now i know better... but yeah once i glued it to the mahog. it was only around 1/16" higher in the middle on the top, and vice versa on the back... but sanded it, and the second time it was even less... and now it's actually staying flat... the cupping wasn't severe but enough to notice with a straight edge... but, now that it's staying level... do you guys think it will cup again after finishing and all that jive? but also i just wondering for the next one, if a quartersawn top would be a lot less likely to cup than a flatsawn top... but now that you guys have informed me about the importance of drying properly it seems this isn't as much an issue as once suspected... and yeah, i live in north carolina which is generally very humid, but it being winter it's not so bad... but actually it was cut in fall and glued up in the fall which can still be pretty humid, but there wasn't any relocating of the work, it was all done indoors at the same location... so thanks to all for the informative advice... oh yeah... off topic but is it worth it to get a quartersawn body or due to thickness does it not matter?