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Softwood and hardwood box

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Forum topic by Nduetime posted 09-23-2016 11:35 PM 230 views 0 times favorited 3 replies Add to Favorites Watch
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Nduetime

11 posts in 500 days


09-23-2016 11:35 PM

I know expansion and contraction rates vary from softwoods and hardwood. Looking for an answer from more experienced woodworkers here.
I’m wanting to build a couple small keepsake boxes for the holidays. I’m looking at making something like this…

found here.
However, I was hoping to build something that has hardwood (walnut like shown or maple) on the exterior and adding softwood burl (likely redwood) on the interior. Will there be a problem mixing and gluing softwood to hardwood as I’m hoping?


3 replies so far

View JBrow's profile

JBrow

818 posts in 385 days


#1 posted 09-26-2016 01:45 AM

Nduetime,

I have no experience mixing soft and hardwoods so I cannot answer your question directly. Based on my understanding of wood movement, I have offered some suggestions that could reduce problems that could result from combining soft and hardwoods in your project. But in the end proceeding with the project ignoring wood movement might work out since the project components are small.

My understanding that is walnut shrinks about twice as much as red wood as it dries from green to a nominal dryness typically used in woodworking. Therefore taking care in the construction of the box using red wood and walnut could avoid future problems. However, from my understanding, the expansion and contraction rates of dried lumber can vary from those given when dried from a green state. I do not know the dried rates of expansion and contraction for walnut and red wood or how these dried rates relate to each other.

The thing going for you with this project is that the components are all relatively small. Therefore the amount of shrinkage or expansion of the woods would be relatively small. The amount of lengthwise expansion and contraction (parallel to the grain) would not worry me much since this is mostly negligible. The wood could expand a measurable amount across its thickness and along its width, but expansion across a thickness of ¾” or ½” would, I should think, be pretty small. Therefore my biggest worry would be the wood expanding across its width.

This all suggests to me that allowing for expansion at least across the width of the red wood and walnut would be a good idea. The bottom of the box could be from the redwood used to line the box. Set in grooves cut into the walnut without gluing in the bottom would allow the bottom to float. The side of the inner redwood box could be glued up to about 1”-2” from the bottom of the walnut. Gluing only along the bottom edge of the redwood would permit the walnut or redwood to expand upward providing some insurance against the inner or outer walls of the box from cracking. Rather than gluing the inner box in place, the inner box could be screwed in place. Slightly enlarged holes in the red wood could allow for enough room for differential wood movement to avoid problems.

Alternatively, the redwood box sides could be cut a little short and, using space balls, the inner red wood box could be friction fit inside the walnut box. Along these lines, the sides of the inner box with grain running parallel to the grain of the bottom could be glued to the bottom (no glue where the inner box sides run across the grain of the bottom) and dabs of glue at the center (centered on the width) of the bottom panel would keep the inner box in place and independent of the outer walnut box without the need for space balls. The two boxes would then move independent of one another.

View jdh122's profile

jdh122

879 posts in 2282 days


#2 posted 09-26-2016 04:55 AM

Go for it. Across 5 inches the difference in expansion and contraction between the two will be about 1/32 inches assuming its home gets quite humid in the summer (http://www.woodweb.com/cgi-bin/calculators/calc.pl?calculator=shrinkage) and wood glue has enough give for that.

-- Jeremy, in the Acadian forests

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cooperw

14 posts in 191 days


#3 posted 09-26-2016 10:50 AM

They are mixing softwood and hardwood in boat building, so it will work fine for your application.

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