Thread: Interpreting Shrinkage Data

Mick - the figures you quote are for green (i.e., just cut) to air dry, which is usually taken as 12%. A piece of wood that is around 12% now isn't going to change by more than a fraction of that in response to typical annual moisture cycles. What you need to calculate dimension changes is the coefficient of expansion relative to ambient humidity changes. Using that, and knowing what your annual RH maxima & minima are in your house, you can figure out what gap to leave.

But that's all far too bleedin' complicated!

If you start with wood that has equilibrated more or less (it's always more or less 'cos RH is never stable for long), then a gap of a few mm either side will allow for typical annual changes. There is no need for extra room for the ends, because expansion/contraction along the grain is negligible. In any case, Toona is a very soft & forgiving wood, so if you under-cook the gap a little, you are unlikely to have your frames fly apart.

If it's any comfort, the Toona panels in our bed are about 18mm thick and the centre panel is a bit wider than yours at ~360mm. I allowed about 3-4mm gap. It's been though 5 years of moisture cylces now, and there've been no dramas: Bed1.jpg

Cheers,

Originally Posted by IanW

our bed ... It's been though 5 years of moisture cycles now, and there've been no dramas

Pretty relieved to hear that!

Originally Posted by FenceFurniture

Pretty relieved to hear that!

Hmm, on reflection, that was a rather poor choice of words on my part, but I have a feeling I'll be on a hiding to nothing however I put it. So let me try to be more specific: So far, I haven't had any shrinking or expansion problems with the panels in the bed head.

I hope that clarifies things....

Originally Posted by IanW

Mick - the figures you quote are for green (i.e., just cut) to air dry, which is usually taken as 12%. A piece of wood that is around 12% now isn't going to change by more than a fraction of that in response to typical annual moisture cycles. What you need to calculate dimension changes is the coefficient of expansion relative to ambient humidity changes. Using that, and knowing what your annual RH maxima & minima are in your house, you can figure out what gap to leave.

Thanks Ian. My fear was sparked by memories of cracked Toona door panels from childhood, no doubt caused by people painting over them and essentially gluing panel to frame. I'll rebate 6 mm/side of the panel inside 10mm rabbets to be on the safe side. A friend suggested inserting bean bag balls into the rabbet to prevent it sliding around which is a great idea.

You're quite right about its complexity because you have to know where the timber is in the shrinkage cycle before you even start. The 400mm 20mm drawer fronts will be planar with the frame so the next little decision is how much gap to leave. I'm hoping <2mm/side will do it.

mick

Mick, given the fact that you can't be sure of exactly what the 'real' MC of your wood is to start with, as you say, calculating gaps with great precision is nigh on impossible. That's why rules of thumb have evolved - they work in 90% of situations, which is good odds.

I'd say 6mm per side would be more than enough for panels of the size you're using. I've seen the styrofoam bead idea advocated, the idea being to stop the panel rattling about when doors are opened/closed and keep the panel centred so that unfinished edges won't show when the panel shrinks. Another 'trick' for keeping panels centred is to put a small dowel through the top or bottom in the centre (from the inside where it won't be seen).

I've seen a few cracked panels in Toona doors, too. The panels in these old doors usually sat in a wide rebate, and were held in place with mouldings that were nailed into the frame. If the blokes fitting the mouldings were careless, they'd drive nails through the edges of the panels, effectively 'glueing' them to the stiles. The damaged ones I've seen were all doors where one side was exposed to weather, and often as not, with little paint remaining on the weather side. Inside furniture isn't going to be exposed to those extremes, one would hope, so that should be another source of comfort.

My preference for fine (inside) work is to use panels with slightly tapered edges. If you keep the taper shallow, but enough to hold the panel firmly in its grooves, there is enough elasticity in any wood you are likely to be using in this context to absorb any forces as the panel expands & contracts a bit. Pre-finish your panels before assembly - it's a lot easier and eliminates the chance that the finish will act like a glue & stick them to the stiles and also prevent unfinished wood showing in the dry season. I wax the edges of the panels before assembly so any errant glue won't attach to them.

Fitted drawers are always a bit of pain to get 'perfect', they do take time & care. If you live in an environment where RH is fairly constant, you can aim for a tiny gap. The long-grain edges can always be very close, but the cross-grain fit needs more elbow-room in places like Brisbane where there is a pretty wide swing throughout the year. The wider the drawer front, the more gap required, obviously, which starts to look a bit 'gappy' on wide drawers.

Lipped, or false-front drawers that overlay the opening solve the problem, but may not fit your design. Another trick, which I picked up from old CODs is to make a small roundover around the drawer front and have the draw 'stick out' by that amount - it gives the front of the drawers a sculpted look, and distracts the eye from any extra gap or slight unevenness of the gap (which tends to be the most obvious of 'flaws', even to an untrained eye!). Drawers fitted edge.jpg Cedar COD.jpg

Again, it may not suit your piece. This set was made to match an old COD in the same room, so that was an obvious way to go...

Cheers,

Re the gap in Red Cedar drawers.
I made a small bedside cabinet in Canberra and the drawers would pump each other in and out.
When it came to Sydney it jammed solid.
H.

Originally Posted by clear out

Re the gap in Red Cedar drawers.
I made a small bedside cabinet in Canberra and the drawers would pump each other in and out.
When it came to Sydney it jammed solid.
H.

Yeah Henry, piston-fit drawers are very professional, until they seize - lucky you didn't move to Cairns, you might never have got them open again!

Ian,

Just a quick note to say I have enjoyed your contributions to this thread immensely, especially the tips and tricks. So many aspects I’d not considered. Thanks for taking the time to write such thaughtful answers.

Lance

Originally Posted by IanW

Mick, given the fact that you can't be sure of exactly what the 'real' MC of your wood is to start with, as you say, calculating gaps with great precision is nigh on impossible. That's why rules of thumb have evolved - they work in 90% of situations, which is good odds.

Thanks Ian for taking the time to pass on some great tips. I prefer to shoot for the moon and make flush fit(with tolerances) drawers or close thereto rather than the easier way out. Gotta keep working on the skills and stretching the limits is the best way to go about it in my mind. Every now and then this means wasting good timber but in I think it's worth it if I end up with a piece which has pleasing proportions and incorporates both good design and execution. I think I'm getting there...

The tapered drawer fronts are a great idea. It means if I fly too close to the sun, I can always re-adjust.

Cheers
mick

As been said you are not going to go through the full range of wood movement as specified in any document. As you mentioned the timber has been laying around in your shop for years. The timber is at equilibrium in your shop's environment. If the final product goes to the middle of the Outback you might lose one or two percent of moisture content. If the final product goes to Cairns, you might gain a percent. That at absolute extremes is 3%.

What I have found with our domestic timbers is allow 1/8 inch per foot. (I apologize for Imperial but I'm a Yank.) This is just a rule of thumb but it works very well. I was building a night stand and coffee table in classes at the Community College. The drawer bottoms were required to be solid wood. (As in "We don't use no stinking plywood in this class.) The solution was three or four panels for the drawer bottoms. Where the panels were next each other, a double rebate was used. Each rebate was about an inch. The rebates overlapped and with about 1/8 inch gap. The end panels were glued into the front and back of the drawer. The "loose" panels were fixed in position with a dab of glue and a pin nail at the center of the panel into the drawer side.

There are several advantages to this pedantic effort. First is the panels will not jam or seize and expand freely. The second is that the panels will not rattle. The third is that the panels are locked into position and the allowances for expansion is halved but the expansion must be accounted for on each edge of the panel.

Some years ago Australian Wood Review published a table showing the percentage radial, tangential and axial expansion of a large number of Australian timbers per 10% change in ambient RH (I think).
I'll see if I can find it and post it here.
Rich, with drawer bottoms, I always run the grain across the drawer. It slots into a groove in the drawer front where it is held by a panel pin from the bottom. At the back, it can expand & contract as much as it likes under the drawer back.

Before our graduation exhibition at Sturt, we had the driest spring & summer on record. On the morning of our exhibition it rained for the first time in months. Everyone was at work with planes (once we'd forced our drawers open) minutes before the opening.

Yes, AlexS. Before I knew . . . LOL