All,
Sure, now that I need it, I can’t find it… I’m making a shaker clock with quarter sawn maple and the back panel is 11″ wide. I’m trying to determine how much to leave for expansion in the back panel. Googling, I did find a general coefficient for maple movement of .00353 and applied that to a 7% variance…producing about a 1/4″ move between winter and summer. Does this sound about right?..any other suggestions? thanks
Replies
Hi BG. According to the Lee Valley wood movement reference guide - soft maple [which I am assuming] has a radial movement of 0.0012" [ expansion or contraction of this material with a 1% moisture change on a 1" wide board ] Your wood is 11" wide so - .0012 x 11 x 7= 0.0924". Multiply this by 1.25 for a margin of safety and you get 0.1155" or about 1/8". If it was flatsawn you would need just over a 1/4". These guides are about $6 I think and very handy. You just got to hope they are right! Peter
Peter36,
Thanks, that is helpful. All the wood has come from a 16qtr x 8" kiln dried stock that sat for a couple of years in my basement. I resawed it a few weeks ago...planed it close 2 weeks ago and glued up a week ago...whew!! I can't tell if it's hard or soft maple...I'll leave a 3/32 nd gap on each side in the back....if it expands farther then that I don't think anyone will know....lol
BG,
Being that you are in the Boston area I would definitely calculate wood movement over a 10% range, and that is a safe range to use no matter where the furniture is built, because you never know where a piece will be even five years from now.<!----><!---->
The amount the wood will expand is dependent on two factors, the wood's moisture content now and the probable moisture content, usually around 14% to 15%, that the wood will have during a humid summer. Right now, in New England, in a heated workspace, the wood's current moisture content could be as low as 4%, which is why I calculate for at least a 10% range. If the wood's current moisture content is greater than 4%, then it won't expand as much by mid summer, but the back will need to be set deeply enough into the grooves to allow it to shrink some in addition to expanding. <!---->
Contrary to popular opinion, whether or not the wood was kiln dried and type of finish you plan to use, won't change these calculations
As to your comment that nobody will know if you use grooves that are too shallow, the case of the clock certainly will know it and you will also after it is too late. <!---->
If you don't leave sufficient room, the expanding back panel will crack the joinery of the case by mid summer. The force created by an expanding panel is huge, hundred and hundreds of pounds, if you don't leave room for the panel to move, you are asking for trouble. I see the joints of badly designed furniture cracked for this reason all of the time.
Hope this helps, please ask if you need more information as to how to figure this out.
John W.<!----><!---->
JohnW,
Yes, I was not sure what percentage to use for potential expansion...10% sounds good. My basement workshop is quite dry...never get any rust (lucky me)...but in the summer, as you know, it's another story.
I've calcuated all the maples on 'Shrinkalator' and it appears 3/16" would be safe but maybe I'll give just a tad more. I was not being flipant with my comment about expansion ...what I meant was if it expands more than expected by 1/32"...I doubt anything would be seriously affected. However, if it shrinks too much...that would be a diaster.
To tell the truth, after cutting tiny mortices all day just an 1/8" from the end of the stock..this piece will be damn lucky if it doesn't see the dumpster... ugh!
Go to the Woodbin at http://www.woodbin.com and click on the "Shrinkulator". Input the species, starting width and range of relative humidity it will be subjected to and the output will be the expansion/contraction.
Howie,
That is what I was looking for..thought it was in my bookmarks. Thanks
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