Attaching Table top

[Joe Austin]

I usually only make small boxes but am venturing out and am in the process of designing a coffee table. I will be using one piece of Sapele wood 36L x 15D x 1T for the top. I will connect the stringers to the legs with mortise and tenon construction. I want the top to have the appearance of floating so I would like to install cross members between the stringers (mortise and tenon as well) that would be higher throughout a portion of the cross member which the top would attach to. I would attach it with hardwood turn buttons. My question is how much movement a table top of this size and species should I expect and how many cross members would be sufficient? How many turn buttons would be enough and how close to the ends would they need to be? I was hoping 3 cross members, one in the middle at 18" and perhaps 2 - 6" off each end with two turn buttons on each end of the cross member approximately 5" in from the edge. I hope this explanation makes sense and any help is appreciated.

[Jamie Buxton]

.. how much movement a table top of this size and species should I expect ... .

Unfortunately, the answer is that nobody knows. The amount of movement depends on the range of atmospheric humidity encountered by the wood. You might live in a place with rather small humidity swings, and so could expect small movement. But then maybe your furniture moves to some other place with a drier environment, or a wetter one, and there would be larger movement.

An additional issue with lumber is what humidity level the mill and distribution system achieved before you bought it. For instance, sapele could have been dried at the mill to 12%, and then it was packed in a container, shipped to the US, and sat in a warehouse for a month, or maybe sat in a warehouse for three years. Who knows what the resulting humidity level in the wood is?

That said, I'd allow a 15"-wide table top to more +/- 1/8" in the furniture. Of course, some other woodworker could give you another number, with just as much justification.

[Kent A Bathurst]

I use this for guidance:

http://woodbin.com/calcs/shrinkulator.htm