Expansion of Different Woods

[Don Wallick]

I'm making a workbench. The first layer of the top will be 1" laminated pine. The top layer will be 3/4" laminated hard maple. I was planning on gluing the maple to the pine. Is this likely to cause potential cracking later due to the expansion of dissimilar woods? Should I just screw down the maple to the pine with no glue?

[Gary Keedwell]

I'm making a workbench. The first layer of the top will be 1" laminated pine. The top layer will be 3/4" laminated hard maple. I was planning on gluing the maple to the pine. Is this likely to cause potential cracking later due to the expansion of dissimilar woods? Should I just screw down the maple to the pine with no glue?
Sounds like trouble to me. Have you considered using a sheet good for the first layer? Mdf or plywood would be more stable in MNSHO. Even then, the maple will still want to move.
I would look for thicker maple or glue up two sheet goods. I think you are looking at problems by glueing up dissimilar woods.
Gary

[Jamie Buxton]

Laminating two different species isn't very likely to get you into trouble.

If you consult a table of wood expansion rates, you see that each species has two rates listed -- the radial expansion rate, and the tangential rate. The tangential expansion is always larger than the radial. A random board will have an expansion rate someplace between the radial and the tangential rate, depending how much the grain looks like pure radial, and how much looks like pure tangential. You can think of the two rates as forming a range of possible expansion rates for a given species. If you look at two species, like your maple and pine, you'll see that the ranges overlap. That is, the difference due to species difference is less than that due to grain orientation.

A terrific book about the physics of wood is Bruce Hoadley's Understanding Wood.

[Eddie Darby]

Try a rest run for yourself and you will see that this is a BIG mistake.

[Jason Beam]

Cracking? probably not. Cupping? Maybe ...

The department of forestry has all the information you'd ever want about any woods you can find in the US and some others. The books you can buy should all be getting their numbers from these guys and the info is free. Lemme see if I can find it ... it's a giant PDF...

Oh - looks like they've actually got it split out, too... Here you go:

http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/fplgtr113.htm

In there, you'll find everything you wanted to know about wood. Including shrinkage rates of various species. I think what you're after will be in chapter three.

[Don Wallick]

Thanks for the info. There's a great variance in opinion here. I might just go for the glue up - just for fun, to see how the wood reacts over a long period of time. It makes sense to me that the variance in a wood's grain pattern could have more influence on expansion than the species difference.