Seasonal movement

[Sgian Dubh]

I mean gluing up with regard to something like an instrument where your worried about wood movement causing damage.

I'm probably not the best person to ask regarding that question. I'm a furniture designer maker rather than a luthier or similar. Naturally, wood changes its form and shape as it adsorbs and desorbs water vapour, with cross grain changes, particularly tangentially, being the greatest (usually about double the movement of radially sawn wood, and longitudinal movement pretty much insignificant in most species).

In trying to picture myself dealing with the issues you raise, I suppose I'd look for the different parts being glued together to be of the same or very similar moisture content, and that moisture content being appropriate for the task. I guess wooden instruments generally should be kept and used in RH conditions of about 50% or 60%, which would result in the wood MC hovering somewhere in the region of about 9% - 11.5%. However, I'm sure many a guitar or violin experiences conditions quite different to this, e.g., playing in the open air on a rainy day.

I suppose if I needed to join different wood species I'd also try to pick two species with similar expansion factors so there's limited disparity in movement which might lead to warping or splitting. Having said that, if the joinery is cross grain, e.g., an M&T type assembly there is inevitably a differential shrinkage to consider, but here I'd try to find a wood species with small shrinkage factors if appropriate and possible: American mahogany falls into this category.

Apart from that I think I'd do as I do in furniture making, i.e., build in the expectation of movement, and build to accommodate it one way or another. Sorry I can't help more, but hopefully the fiddle, lute, guitar, etc makers will chime in with more knowledgeable suggestions. Slainte.

 

[MIGNAL]

Ive been trying to learn about this subject.

One question, if anyone knows the answer. In the context of gluing up.

How long does the wood have to be exposed to a higher humidity for it to br relevant?

As the rule is with air drying they say an inch per year. Is the rate of change much different for fully seasoned wood/does wood take on moisture more rapidly than loses? Would you still follow the year per inch rule, taking it down to fractiona of an inch?

For thin plates (2mm) I believe an hour is enough to take on significant moisture. Once in a lower humidity environment I'd want to leave it at least overnight to be sure it had dried out enough. But mine is an application where getting it wrong can ruin the entire work, so I'm cautious.

Thicker wood will change more slowly, but I have no experience with that and humidity. I believe cheap moisture meters are available, though I've not used them myself.

Thanks. That's faster than I expected. I've been trying to reduce humidity in my workshop recently as it's particularly humid this week, at least 75% outside. I bought a 250w 10litre dehumidifier off ebay but it doesn't seem to be able to bring humidity level under 59%. I might think about using a storage chest with light bulb inside as that's supposed to work well.

I work with instruments too but nothing as too sophisticated.

The larger the room the harder it is to control the humidity. If you can seal it from the external environment a little better you gain better control. The best I can do in my workshop is around 8% reduction running my Ebac full blast. It's not a large workshop either. If I need more than that I place the wood in my UV light cabinet. That gives me around 25% RH reduction. With such a small environment it's much easier to control. I have a baby bottle warmer steaming away when I need to increase the RH. If the external environment is 50% RH and I don't fire up the bottle warmer the humidity can plummet, such that instrument wood will distort in a quite marked and scary manner. You can also buy small in cupboard or closet dehumidifiers instead of using lights/heat.

 

[profchris]

When making something like an instrument, this is how I'd think through the glue-up in relation to humidity.

On this (oddball) ukulele you can see a join down the middle of the instrument's top, running through the middle of the sound hole and under the bridge.

In that joint the grain goes in the same direction in both pieces, so I'm not too worried about humidity. However, I wouldn't join them on a really sticky day because the humidity might persuade them to warp slightly and ruin my joint.

The fretboard is about 2.5mm thick, and is glued on a solid mahogany neck. Again the grain goes in the same direction, so humidity isn't a huge concern. But dissimilar woods expand differently, with the biggest expansion cross-grain, so if my fretboard wood is likely to move a lot then I'd glue it to the neck in low humidity. If it gets a fraction wider the instrument is still playable. If it shrinks, the frets stick out and *** into the player's fingers.

Joints where I'm gluing cross-grain to long grain are the main concern. In this one (even odderball in its finished form) the body is round with two braces as in the picture. The top will go on with its grain at 90 degrees to those braces. If the top shrinks the sides of the body will resist its pull, and the braces will try to stop it shrinking. The top would win, and crack. So the top goes onto this in low humidity.

Because the top was fixed in place in low humidity, if the humidity rises it will want to expand. I cope with this by doming the top. Any expansion forces the dome higher, and that only needs the braces to flex upwards which they are happy to do (spruce). You can see the dome here:

So it's a process of thinking through each joint and how humidity would affect it, and then how to counter it.

I don't have climate control because I'm just an amateur builder, but I do have a very effective humidity meter (two veneers glued together at 90 degrees), so I only glue the difficult joints when the meter curls in the right direction.

 

[Jacob]

I get timber moving in opposite directions:
Summer - the sarking boards in my roof will shrink as although the absolute humidity will be high the relative ditto will be low due to the higher temps of the roof.
But then the boards of the suspended floor over the basement will expand because although they are bathed in the same ambient air the temperature there is much lower and the relative humidity locally higher.
In winter the opposite - tight boards in the roof, gaps in the basement floor.

 

[Tetsuaiga]

Interesting idea to use the veneers as a gauge.

I've found my airing cupboard is 50% which I expect might change. I'm going to use a bit of space in there till I think about some kind of light box.

Mignal is there a reason you use UV light? I think i've seen some people use them for curing varnishes or affecting the colour of wood.

 

[Carl P]

Profchris,

I also have done some oddball instrument making, although not as nice looking as yours, but please explain the round thing - what are the holes for!? etc

Cheerio,

Carl

 

[MIGNAL]

Interesting idea to use the veneers as a gauge.

I've found my airing cupboard is 50% which I expect might change. I'm going to use a bit of space in there till I think about some kind of light box.

Mignal is there a reason you use UV light? I think i've seen some people use them for curing varnishes or affecting the colour of wood.

Yes, for curing varnishes and wood tanning. It's just a small single wardrobe, foil lined, two UV strip lights.
No joke but the cops paid me a visit one day and decided they were going to take it away and use it as evidence. Fortunately I had a workshop that persuaded them it was for legitimate purposes.

 

[profchris]

I also have done some oddball instrument making, although not as nice looking as yours, but please explain the round thing - what are the holes for!? etc

What you see there is a concert scale camp ukulele, but with a banjo-style resonator added to it (the holes are, effectively, the sound hole as there is none on the top). I discovered that Martin built about 30 guitars like this for Wm Lange in the '30s, and had to find out what the effect was - answer: very full sound, lots of mid-range. Back and side views might give you a better idea.

 

[Carl P]

Thanks for posting those photo's, that's very interesting indeed, and very well built of course - I'll have to look up the Martins now!

Cheerio,

Carl

 

[Trafalgar]

Our high today in Memphis will be around 95 degrees with 75% humidity. The forecast for Monday is 100 degrees with about the same humidity. We have a cooling trend for Tues and Wed -- high only 97. Wood moves during these extremes.