Replies

1. 
   saschafer | Sep 13, 2007 12:50am | #1

   I haven't done the experiment in terms of measuring wood movement, but I have monitored the humidity in the house over the past two years. I run a humidifier in the winter, and the air conditioner in the summer, and the relative humidity ranges over the seasons from a low of about 35% to a high of about 45%.

   Interestingly, the largest short-term humidity fluctuations are in the spring and fall, when it's not hot enough for the air conditioner and not cold enough for the furnace, so that the indoor humidity pretty much tracks the outdoor humidity. It went up noticeably a few days ago, when we had three solid days of rainy weather.

   -Steve

    

2. 
   SgianDubh | Sep 13, 2007 03:20am | #2

   There is a calculation to find out the range of movement you can expect.

   D X S (MCC/ FSP) =D₁             

   Where:

   •        D₁ = Dimensional Change.
   •        D = Initial Width.
   •        S = Maximum Percentage of Wood Shrinkage possible from 30- 0% MC. (Enter either the tangential movement factors or the radial movement factors found in movement factor charts. For example, when calculating tangentially cut wood movement use the tangential shrinkage factors given.)
   •        MCC = Moisture Content Change. A calculation within the above formula is required—see the alternative expression of the sum below.
   •        FSP = Fibre Saturation Point, i.e., 30%.

   Another way of expressing the formula is:

   (Initial Width of Plank X Maximum Shrinkage Percentage Possible) X ((Highest Wood MC – Lowest Wood MC) / FSP) = Dimensional Change.

   To use the formula remember to divide percentages by 100 and enter the result of this sum at the appropriate place. For example enter 11% as 0.11 after calculating 11/100, and write 7.7% as 0.077.

   So in your case the EMC of wood in 20% RH = approximately 4.5%, and EMC of wood in an RH of 50% = 9%

   Let's assume a panel is made of tangentially sawn white oak and is 24" wide to begin with during the driest period. It will be 4.5% MC in that 20% RH. Using the sum above and the shrinkage factor of 10.5% that represents the shrinkage that will occur between Fibre Saturation Point (FSP) and oven dry. You'll find that during the wettest part of the year- 50%RH = 9% wood EMC, your panel will be approximately 24-3/8" wide. This assumes there is no polish on the part to exaggerate the wood's natural hysteresis. Layers of polish slow down the progress of water vapour in and out of the wood.

   Do the same numbers with mahogany that has a 30% to 0% tangential shrinkage factor of 4.5% and your table top will expand from 24" to roughly 24-3/16" 

   Where you may be getting confused is the charts give the percentage of shrinkage to expect from FSP to 0%. If your wood only changes MC by 5%, eg, from 7% to 12% over the seasons you're only looking at about 17% of the potential range of movement. Slainte.

    

   Richard Jones Furniture

1. 
   Samson | Sep 13, 2007 04:18am | #3

   Well, Mr. Jones, that about nails it, eh!  Thank you for the detailed explanation.

2. 
   BG | Sep 13, 2007 12:38pm | #5

   Sgian,
   Recently I've been trying to get my mind around this wood movement issue and am a bit confused. I bought an inexpensive moisture meter and tested some 5/4 poplar. The surface reading on the poplar was 6-8% and the internal was about 12%. I drove a couple of finishing nails half way through the stock and placed the pins from the moisture meter on the nails to get the internal reading. Am I correct in assuming there is no real 'steady state' for wood...it's almost always either increasing or decreasing in moisture content?
   On the practical side I'm trying to figure out how to apply all these numbers to making better fitting drawers. The poplar will be resawn to 1/2" and finished at about 3/8' thickness for 6" deep drawer sides. In the past your suggestion was to provide 1/4" movement per foot..because we don't know where the piece will end up...wet or dry. So my first question is do you have any suggestions for drawers?The second issue is wood movement while ripping. Almost every piece I rip needs planing at the ends to remove the bow that the saw has caused(a few thousands). (I determined this fact a couple of weeks ago..ran MDF through the saw and the joints were perfect) But if I do that planing, what happens down the road when the wood re-adjusts to it new state..do I then have a convex issue? I have been doing this for about 5 years now without any signs of seams opening up but I'd be curious what your thoughts are.

1. 
   SgianDubh | Sep 13, 2007 02:52pm | #6

   BG, you said, "Am I correct in assuming there is no real 'steady state' for wood...it's almost always either increasing or decreasing in moisture content?"

   Yes you are.There is always a moisture gradient from the outer surface to the core, eg, 10% MC at the core and 12% MC at the surface-- the wood is, or was just recently, going through a drying phase.

   Moisture meters ideally read the MC from a point 1/4 of the way into the thickness of the wood. This way they read the middle of the moisture gradient, eg, 1" thick materials are best read with pins penetrating 1/4" deep, ie, halfway between the outer surface and the middle of the thickness.

   For drawer sides and backs the usual suggestion is use radially sawn stock. It's more stable. However, if you marry this 1/4 sawn stuff with tangentially sawn stuff for the fronts for aesthetic reasons you could run into some mis-matches in movement factors, eg, 1/4 sawn English oak for the internal bits and tangentially sawn English oak for the front. The shrinkadge factors for this material are 5.3% radial and 8.9% tangential.

   On the other hand, if you match 1/4 sawn English oak for the sides and back with tangentially cut American mahogany you get a near perfect shrinkage factor match-- mahogany's tangential shrinkage factor is 5.1% from 30% MC (FSP) to 0% MC, only 0.2% difference.

   The deeper the drawer (the height) the greater the seasonal movement. 'Piston' fit drawers, particularly if deep can be a pain. Perfect fit in the winter, jammed in the summer, or perfect fit in the summer and sloppy in the winter.

   You're right. There's a lot to get your head around regarding wood movement. How wet is it when you start building the furniture? What's the cut-- tangential, radial, rift, figured? What season is it? Is your workshop climate controlled and what's the relative humidity during that building period? Where is the furniture going into service? What about extreme conditions? But you can become over anxious about what could happen, and I see some people number crunching to the n'th degree, very often rather needlessly in my opinion. For the most part as long as you leave a 'reasonable' amount of play for movement you'll generally be alright.

   I'm not sure I fully understand your last part about resawing planks. I suspect you're releasing some minor stress that's causing the bowing or cupping, but without seeing what's happening or some rephrasing on your part I'm at a bit of a loss.

   When I finish my book on timber technology, and if I can find a publisher, and if you decide to buy a copy maybe I'll have laid it all out so that it's all as clear as mud for you and the problems will be solved, ha, ha. Slainte.Richard Jones Furniture

1. 
   KiddervilleAcres | Sep 13, 2007 03:34pm | #7

   Slainte,

   Just remember to keep a bit of humor in that there book.  Can I place my order for a signed copy of the 1st edition?

   Regards,Bob @ Kidderville Acres

    

   A Woodworkers mind should be the sharpest tool in the shop!

1. 
   SgianDubh | Sep 13, 2007 07:56pm | #8

   Hold your horses. I haven't got a definite publisher, although one is interested. Still, signatures don't come cheap......, ha, ha. Slainte.Richard Jones Furniture

2. 
   KiddervilleAcres | Sep 13, 2007 08:53pm | #9

   Richard,

   OK, I'll trade ya one of mine!  :-)

   Or better yet, I'm writing a book called From Mainframes to PCs - The Evolution of Computers, so when I find a publisher we can swap books and signatures maybe.  More grins and chuckles.

   Regards,<!----><!----> <!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----> <!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!----><!---->Bob @ Kidderville Acres

    

   A Woodworkers mind should be the sharpest tool in the shop!

2. 
   BG | Sep 15, 2007 03:06am | #10

   Sgian,
   That's great news about the book, I could use a bit of help. I guess while your dealing with your writers block we'll just slap oak together with mahogany for drawers....sigh! :)

1. 
   SgianDubh | Sep 15, 2007 02:24pm | #11

   Writer's block, BG? I didn't know I had that. It seems more like writer's diarrhoea or verbal flatulance to me. Slainte. Richard Jones Furniture

3. 
   HowardAcheson | Sep 13, 2007 07:37am | #4

   You can answer your own question by going to the Shrinkulator at:
   http://www.woodbin.com/calcs/shrinkulator.htm

   This is a calculator where you can enter the species, range of relative humidity or moisture content and wood width. The outputs are the expansion/contraction.

   Howie.........