Dimensional Movement

All wood species shrink upon drying and expand when the cells hydrate. Wood does not begin to shrink until all "free water" is removed and "bound water" loss begins. Hardwood lumber must have a moisture content of 6 to 12 percent to meet the standard of the National Hardwood Lumber Association (NHLA) grades. Some woods shrink and expand more than others. However, there is a noticeable variation within any species between its tangential and radial shrinkage. Imagine a small slice of a log. The internal and inconsis­tent stresses in lumber are dependent on how the tree was cut at the mill. Quarter-sawn lumber will expand and contract similarly to plain-sawn lumber, but quarter-sawn lumber is less prone to cupping. As plain-sawn lumber dries, it shrinks much more tangentially, causing it to split more often than quarter-sawn lumber.

It is important that furniture designers and fabricators understand the degree of com­patibility between materials that connect to one another and know what precautions must

Figure 7.17 Today, the MR lounge is manufactured by Knoll, Inc. using a thick cowhide sling with nylon laces for the seat and back, much like the origi­nal. Photography courtesy of Knoll, Inc.

 

Dimensional Movement

be taken to allow for the relative differential in movement by different materials. In Figure 7.18, an end grain butt joint in the structural supports fabricated with two different woods (maple and cherry) necessitated core drilling twice through the entire length of the composite leg assembly. The different woods were then secured in compression using threaded rods and locking nuts. PVA or epoxy glue with dowels would not have secured the connection well enough, and over time, due to the differential in the dimensional changes of the two spe­cies, the joint would have failed.

Подпись: Figure 7.18 Vertical supports of the Ambo are joined using threaded dowels, secured in compression. Photography by Jim Postell, 2000. Wood is not a homogeneous solid material; consequently, movement that results from changes in relative humidity is not equal in all directions. Shrinkage takes place in every dimension of wood except along the length of the grain. Quarter-sawn lumber will expand and contract, as will plain – sawn lumber, but quarter-sawn lumber tends to move and shrink less than plain-sawn lumber and is less prone to cup­ping. Plain-sawn lumber has inherently greater internal forces due to its grain and the way in which the wood was cut at the mill. As a general rule, one should avoid gluing wide and long pieces of wood cross-grain to one another (as in a skirt or apron for a table). When it is unavoidable, consider using a mortise and tenon or a slotted mechanical connection to allow for expansive and contractive movement.

Figure 7.19 Folded copper drawer fronts with integrated copper handles inset in six Baltic birch storage units, designed and fabricated by Jim Postell (2005). Photography by Jim Postell, 2009.

 

Dimensional Movement

Figure 7.21 KorQ—design by Kevin Walz (1996), made with 100 percent cork. Drawing by SAID student Amy Ballman, School of Architecture and Interior Design, University of Cincinnati, courtesy Jim Postell.

 

Dimensional Movement

Figure 7.20 Outside aluminum bench. Photography copyright © William A. Yokel, 2005.

 

Dimensional Movement