Thread: Effects of expansion in the construction of a box lid

Hi Clint,
I actually use loose tenons (dominoes) to join the centre panel to the borders and have no problems with movement. I have done this with several different timbers, including Huon Pine and Red Gum.

Regards

Rob

I have never "joined" the panel in the frame of the box lid. I have always left it floating to allow for expansion. It is essential to allow a little room across the width of the timber but not so important for the length of the timber.The gap only needs to be a mm or so to provide enough room for the expansion and contraction of the floating panel.
The floating panel can be quite thick and solid.
You can see in the sketch I have made the profile of the frame and the floating panel how this can be achieved.
Diagram A shows the profiles and Diagram B shows the floating panel fitted in the frame.

C. shows the top of the floating panel, which can be shaped to suit the box, rounded over for instance.
Attachment 198333

I was just looking at some pics of dovetails which are done with 2 different timbers for the front and sides.
Is expansion in this arrangement not such an issue because any expansion is across the grain and therefore less than lengthwise expansion?

John has the way to build a floating panel but hide the expansion gap

Originally Posted by ClintO

I was just looking at some pics of dovetails which are done with 2 different timbers for the front and sides.
Is expansion in this arrangement not such an issue because any expansion is across the grain and therefore less than lengthwise expansion?

expansion across the grain is always greater than expansion along the grain
the box/drawer sides in different timbers to some extent hold themselves together because of the dovetails, because the width of the sides is not too great and because although the timbers are different its the relative difference that matters.

say the box sides are 100mm high, say the dark timber has a 6% expansion, and the light timber is 8%.
The relative difference is only 2% or 2mm over the width of the sides, and half of this might be taken up in the clearances needed to assemble the DT joint, and the rest in a moix of compression in the 8% (expansion) piece and tension in the 6% piece.

Not sure how I missed this before.
You do need to make allowance for panels to expand, and if you don't want it to be obvious, John's method is the one.
Ian's explanation is correct, but the examples quoted are way greater than experienced in practice. A timber that is subject to a lot of change, like casuarina, will move about 6% tangentially for a 10% change in moisture content, but if the timber has been properly dried, the moisture content won't vary by more than a couple of percent, and relative movement will be tiny indeed.
However, for panels, even though the movement will be small, it will occur, and if you don't allow sufficiently for it, it will destroy the box.

The attached table, from AWR #51, may be of assistance.