Replies

1. 
   robkelly1 | Oct 09, 2003 03:17am | #1

   3/8" is about right, but I don't think you'll ever see 8% again after it's inside unless you have gas A/C and no humidifier. Most likely the wood goes up to about 10% and fluctuates between 10 and 13% from season to season. It's definitely not going to shrink from the original size, but will expand at first, then vary over the seasons. Sealing the end grain will help.

1. 
   JohnWW | Oct 09, 2003 06:08pm | #3

   Rob,

   A swing between 10 to 13 percent might be the norm where you live, but here in New England cabinetmakers plan on a swing of 10 points in the moisture content of the wood between winter and summer. 

   Sealing the end grain will slow down the loss of moisture from the wood but it has no affect on the final moisture content values, so it doesn't influence the calculations.  Sealed or not, the wood will be very dry after spending a winter in a typical home in the Northeast and by the end of a steamy summer the wood will have a much higher moisture content no matter what the finish was.

   Any freestanding piece of quality furniture should be designed for at least a 10 point moisture content swing, there is no way to foretell where the piece might end up over the course of it's life, which (hopefully) might span a century or more and several owners.

   John W. 

2. 
   ChasStanford | Oct 09, 2003 03:25am | #2

   3/8s is probably about right.  RO is not a particularly stable species.  Take precautions.

3. 
   JohnWW | Oct 09, 2003 08:22pm | #4

   Ron,

   Your calculations, depending on several variables, are in the ballpark.  Plainsawn red oak shrinks .0037 per inch per 1% moisture content change.  As I explained in my response to Rob in the posting above, I always design for a 10 point moisture content change. 

   If the outer ends of your tenons are 20 inches apart the formula would calculate as: 10% MC X .0037 X 20 = .74 inch.  If you used radial sawn oak (.0016 in. change per 1% MC shift) the movement would be far less, just .32 inches and you would also avoid problems with the wood in the headboard cupping. 

   Note that the dimension you use in the formula is for the distance between the outer edges of the tenons, which could be considerably less than the greatest width of the headboard.  If the top of the headboard is arched, for instance, the tenons' outer edges might be only 10 inches apart, which would cut the amount of movement you have to deal with in half.

   The oak was at 8% moisture content only at the moment it came out of the kiln, by now it has almost certainly changed.  If the wood has acclimated to a relatively moist environment since then it will quite possibly shrink over the winter.

   Since the furniture faces an unknown future, you shouldn't design it based on the presumption that it will always be in a controlled environment.  

   Your choice of finish doesn't enter into the calculations at all, since finishes only slow down the moisture content change not prevent it.  With a highly water vapor resistant finish it might take three months for the board to change instead of just a couple of weeks with an unfinished board but, in the end, the movement will be just as great.

   I highly recommend buying the wood movement calculator made by Lee Valley, between it and the booklet that comes with it you will have a sound method of calculating and understanding wood movement.

   Hope this helps, John W.