Replies

1. 
   HowardAcheson | Jan 09, 2003 08:35pm | #1

   Go to: http://www.woodbin.com and click on the "Shrinkulator". Plug in the species, width and relative humidity range and it will calculate the expansion/contraction.

   Which way will the grain run? If you are going to have a leave in the center, it is normal to run the grain across the shorter dimension of the table. If this is the case, figure 36" as the width.

   Also, you need to figure where to hold the tabletop fixed. Normally, the tabletop is fixed at the center and the ends allowed to move.

2. 
   PlaneWood | Jan 09, 2003 08:46pm | #2

   Figure on about 0.1" for each 1% change in moisture content for 36" wide Black Cherry.  So with the MC changing from 8% to 11%, and with a 36" glue-up, you could expect an expansion of 0.3".  Or, a 0.3" contraction if going from 11% to 8%.

   To be on the safe side, use +/- 1/2".  Here in Houston I assume that MC will decrease when moving a piece from my shop into the house (no way in HE-- it could increase). 

   Tangential Coefficient for Black Cherry is 0.00248.  Radial is 0.00126.

   Delta = Width(Ct(11-8)) = 36(0.00248(3)) = 0.27"

   A common way I use to float a table top is with slots in the inside of the side skirts with a metal clip secured to the top that will move back and forth in the slot.  Wax the slot so the clip will be able to move.  Lightly snug it down.  Remember that the radial contraction will be about one half that of the tangential.

   Also, considering all the common American hardwoods, the highest Ct is about 0.0043 (Beech) and the lowest is about 0.0022 (Butternut).  Cr is always about half of Ct.

   For softwoods, Western Larch has the highest Ct, being 0.00323, and Cedar the lowest coming in at about 0.0017.

   Hope that helps!

   PlaneWood by Mike_in_Katy
   PlaneWood

   
   

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   Edited 1/9/2003 12:53:19 PM ET by PlaneWood

   
   

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   Edited 1/9/2003 1:02:06 PM ET by PlaneWood

1. 
   PlaneWood | Jan 09, 2003 09:12pm | #3

   Expansion table for hardwoods

   PlaneWood by Mike_in_KatyPlaneWood

   
2. 
   JFassino | Jan 10, 2003 10:09pm | #4

   Thank you very much for your help.

   Jim fassino

1. 
   PlaneWood | Jan 11, 2003 06:33am | #5

   Your very welcome. 

   If you use quarter sawn lumber, then use the 'radial' figure instead of 'tangential'.  Half as much expansion/contraction.  When doing the glue up, alternate the growth rings.  i.e. don't have the direction towards where the bark of the tree was the same for any two adjacent boards.

   PlaneWood by Mike_in_KatyPlaneWood