This Winter has been very cold in NW Missouri. We have had more snow in the last 3 months than in the last 40 or 41 years.
As a result, I have been running the forced-air furnace in the shop much of the Winter. The following photos show what happens to a bench top when wood moves due to changes in relative humidity: The wood changes dimensions, with the greatest change taking place cross-grain. The high spot on the bench top runs the entire length of the top (the top photo, below); even the end caps have a high spot, as shown in the second photo.
This is unacceptable in a workbench.
Heating with the forced air unit results in loss of moisture from the wood. Due to the length of the boards used to make the bench, the moisture loss is fastest near the ends of the boards, and slowest near the center.
This results in a bench top that is no longer flat. With the precision straight edge flat at the back of the bench, the total gap at the front edge is 1/8-inch. When the straight edge is held horizontally, the gaps are a little over 1/16-inch at both the front and back edges. The gap is smaller closest to the ends of the bench.
As you look at the following photos, keep in mind that the top is 3-1/2 inch thick birch, with the cross grain running in the vertical direction:
Now the critical question is: What to do about this non-flat bench top? I consulted with Christopher Schwarz (Editor, Popular Woodworking Magazine; he also runs the Lost Art Press blog), and with the Sjöbergs Agent in the U.S.
The answer: Wait until the Winter is over and heating is no longer required, and measure the bench top crowning again in late April to early May. If the high spots remain, flatten with a hand plane.
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