Even professionals get it wrong

imageBefore I had the time and space (and skill and confidence?) to build such a thing my wife and I bought an oak sideboard. It purported to be made in England from domestic oak and I have no reason to doubt that. It’s got big, chunky, machine cut dovetails and the drawer fronts are applied. We still like it a lot. It’s a simple design with little decoration, nice proportions and is solidly constructed. The flat panels and simple, turned wooden pulls might be described as ‘Shaker’ by those who need to put a label on simple vernacular furniture.

At the same time we bought a ‘matching’ blanket box. It has an issue that I should have spotted even then. We were clearly carried away with the sideboard and didn’t pay enough attention. Those furniture salesmen eh? Wiley Devils. Here are the tops of both. Can you see the problem?

On the left is one panel of the top of the sideboard. The panel is fitted into an open backed frame and is rebated so that the surfaces of the panel and frame are flush. As the seasons change the panel expands and contracts and sometimes (as now at the end of a warm, damp summer) protrudes beyond the back of the frame. Because the grain is orientated so that it runs lengthways it will never expand along the length of the piece and so will never distort the frame. Good thinking sideboard-maker.

For whatever reason (lack of longer lengths of timber? ignorance? malice? who knows) the blanket box maker has orientated the boards so that they run front to back putting all of the expansion and contraction along the length of the box. Here is the result:

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By the end of winter that reveal is even. This tells us two things: the box was built in winter when the timber was at its least moist. And it was built by someone who didn’t know about (or care about?) seasonal movement.

Hoadley’s calculation for wood movement tells us that the change in dimension across the grain is equal to the initial dimension of the timber multiplied by the total shrinkage percentage of that species of wood multiplied by the product of the change in moisture content divided by the fibre saturation. There are going to be some estimates in this!

ΔDi=DS(ΔMC/fsp)

ΔDi=(31.5)(0.105)((0.28-0.22)/0.28)

ΔDi=0.95

This would be bad enough if I was working in centimetres but I’m using imperial! Almost an inch of potential movement. I’ve been fairly conservative in my estimates and we still end up with horribly large number. Some cabinet makers use a rule of thumb that wood will move up to 4% during the seasons. That’s 1 1/4 inches.

Note to self: Use narrow panels. Work out where the extra wood is going to go.

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When I made this panel at the start of the summer I broke the rules in the same as the way as the maker of our blanket box. The timber is flat sawn and so has more movement than vertical grain planks and the long grain runs in the shortest dimension.

Aware that I was breaking these rules I cut the groove in the frame deep and let the panel in half way into the groove.

It moves around a lot if you make it but it’s never going to show a gap or prise its frame apart.

 

 

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Maths vs joinery

Gentle arch

In a couple of weeks I’m going to be building a garden box – something to hold the barbeque, hedge trimmers, hammock and the tools it takes to maintain the postage stamp of paving and greenery we laughingly call our back garden. It’s going to be outdoors and I want it to shed the rain so the lid will be arched – coopered. Either I build it or the hedge trimmers live in my workshop.

I planned to build the lid it out of several long planks with bevelled edges. Rather than just have at it I decided to try a little maths to calculate exactly how much I should plane away to get the arch I wanted: two inches of height across a width of about twenty two inches. Or for the imperially challenged 50mm over 550mm.

x=a little bit

I like maths (I hope the National Association of Teachers of English doesn’t read this) but don’t remember much of my trigonometry so I spent some time with a maths app on my iPad.

After a couple of hours of faffing I calculated that x=a little bit. Slightly more than a gnat’s whiskers and quite a bit less than a shed load. So I gave up on the diagrams and went out to the workshop.

Bevelled for a curve or just out of square?

I’ve got some oak left from the peg rail. It’s the scratty bits I’ve scarfed together: knots, reversing grain direction and a bit of bowing. There’s just about enough to put together a mock-up of the lid.  I bevelled one edge of each board, put it all in a couple of panel clamps and fiddled with it enough to get an arch. I liked the curve but didn’t fancy my chances of wrestling the whole lot I to the correct shape when it was covered with glue.

Instead I’m gluing up pairs of planks. Then I’ll put them all together and see what it looks like. Design by trial and error, rather than maths.