Thread: Question about simple zither design and wood movement

The wood will always move with humidity changes. Firstly, stabilise your wood in your environment where the humidity is 40%-50% for a couple of weeks before starting the build. This will bring the timber to EMC (Environment Moisture Content) Secondly, dont glue up when the humidity in your workshop is over 60% or lower than 40% (Mid 40%'s is the optimum)

I store all my tonewood in the house in a humidity controlled room. I take it out to the workshop.....do what I have to do and return the build back into the room.

Cheers

Daryl

Thank you Hooked.

Yes I always leave the wood indoor for about a month at least before using it. Luckily my workshop is part of my house so the conditions should be suitable. I think I really should get a humidity meter, I guess the idea is to glue up when conditions are at their average, so expansion contraction is ideally equal in both directions, thereby halving the potential stress.

Do you have any opinion about using flatsawn wood for backs? I know most instruments use harder to come by quartersawn wood but for something smaller like a Psaltery cupping is not going to be as severe as on a wider instrument like a guitar.

Its part of the reason these instruments are always braced. The timber moves but is anchored by the braces.

There is no functional sonic difference between flat sawn and quarter sawn for instruments. Quarter sawn is preferred as it is though to be more stable under extreme conditions. Most of the really flashy grain you see on instruments is grain cut closer to flat sawn. It tends to be where figure shows better. A Psalter shouldnt provide stability problems. I know there is a school of thought that they were hewn out of a solid log. There was a lot more old growth timber around at that time

Thanks once again.

I think the worry I had was more based around the thought that if the back or top is fixed in place by the interior frame and reinforced by the opposite plate. Doesn't that mean the wood will split, as wood held in place which contacts will surely split?

Perhaps there is some degree of torque within the sides which would mean the wood isn't infact held in place as solidly as i'm imagining.

The only sure solution I can think of would be to build the entire instruments from a single species, make it quartersawn and also make the frame grain direction be the same as the soundboard and back. That would remove the counter acting forces, but then back wood and soundboard woods of the same species aren't desirable.

The sides on a guitar are usually rigid enough to prevent movement in the back leading to movement of the top via the sides and vice versa. I use solid laminated top linings and my sides have splints to further enhance rigidity. You can go one step further and make your sides laminated.

I don't quite see how the sides would stop the back moving. As far as i'm aware expansion and contraction in wood will either result in free movement or either the wood splitting or the object fixing it in place.

Maybe i'm not quite imagining this right.

Originally Posted by Thunrazia

I don't quite see how the sides would stop the back moving. As far as i'm aware expansion and contraction in wood will either result in free movement or either the wood splitting or the object fixing it in place.

Maybe i'm not quite imagining this right.

In a top or (less commonly) back any movement is going to be primarily due to humidity changes and will be primarily across the grain. Because the sides on a reasonably well built guitar are more rigid than the top rigid any expansion is taken up by the top (or less commonly the back) doming upwards. Conversely contraction results in the doming in the opposite direction. It's one reason alot of guitars are built with a slight doming in the top...to allow for this movement.

Originally Posted by kiwigeo

In a top or (less commonly) back any movement is going to be primarily due to humidity changes and will be primarily across the grain. Because the sides on a reasonably well built guitar are more rigid than the top rigid any expansion is taken up by the top (or less commonly the back) doming upwards. Conversely contraction results in the doming in the opposite direction. It's one reason alot of guitars are built with a slight doming in the top...to allow for this movement.

Right the doming on expansion I understand. I didn't know about guitars (presumably also any other wooden plate secured on its edges) being built with a preexisting dome to allow for contraction.


If the guitar top didn't have any doming built in and then contacted, presumably the only thing that can happen is the sides torque in toward the centre or the top splits. Though I would have thought the top splitting would take a lot more force than to torque the sides.

Originally Posted by Thunrazia

If the guitar top didn't have any doming built in and then contacted, presumably the only thing that can happen is the sides torque in toward the centre or the top splits. Though I would have thought the top splitting would take a lot more force than to torque the sides.

Splitting along the centre seam the most common scenario. You're talking about very small movements on a typical top. In your OP you were worried about movement in a top affecting the back via movement in the sides.....this just doesn't happen.

What plans are you using Thun? The same as these? http://www.apsimplepsaltery.com/build/

Hi Sebastian,

The plan I have is for the plucked type. Although I have a plan i would like to make designs of my own, usually based on an existing model but with alterations.

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