QSWO expansion question

[Mike S.]

I understand that wood expands mainly across the grain. Now the question, I building my daugters some Mission style tables, sofa table, coffee table etc. Since I'm using QSWO for the tops do I not have to worry about expansion? I picked up about 40 bd ft of it saturday and price didn't change since Jan. still $4.21 a bd ft. I cut off a foot or so on the end of one board after planing it and put on a coat of Rockler Mission stain and 2 coats of General top coat. Sure looks pretty.

[Fly-by-Night CC]

The US Forestry Service has a very detailed set of 21 PDF files on all things wood - The Wood Handbook. I don't have a URL handy but it shouldn't be difficult to find.

Anywhozle, what you're looking for begins in chapter 12, page 15. There's a formula plus a chart for various woods on the next page showing the dimensional change coefficient for either radial (quartersawn) cut or tangential (flatsawn)cut. Then there's a chart on page 12-4 of various US cities indicating the avg. equilibrium moisture content of wood for each month of the year.

For example, in Portland, Oregon there's a swing of equilibrium MC from

11.7% to 17.4% through the year. Using a 48" wide quartersawn White Oak tabletop:

Dimensional change= dimension * (coefficient * (MC high-MC low))

D change = 48" x (.0018(17.4-11.7)) D change = 48" x (.01026) D change = .49248"

Another question which I didn't look up is the amount of variance in MC that occurs in today's modern houses. Not sure how much that would affect the overall MC range...

[Andy Dingley]

Rule of thumb - tangent is twice radial. Bit more for some species, bit less for others.

Your other rule of thumb is 10% wet to dry tangential (so 5% radial). This isn't so accurate, so look up the real numbers for your humidity range and your species if you want better.

US Wood Products handbook (paper copies from LV, or download and read on line) or else Hoadley's "Understanding Wood".

You have to worry. There'll be either framed panels or breadboard ends in there, and that means cross-graining. It's an easy style to deal with though - it just means making the right allowances on framed panels. Throw the design and numbers back at us if you want to know more.

[BruceR]

There is an interesting article in the latest P.Woodworking from Flexner about this and how finishing both sides of a board (like a table top) really has little effect on preventing warpage and cupping. With regards to your QSWO, it will expand less than other cuts of the wood and it should expand evenly on both surfaces (won't "cup"), but it'll still expand. Design, build, and attach the top with this in mind.

-BR