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:
The bench top:
The end cap:
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.
Relevant links:
Woodworking Magazine
Lost Art Press blog
Sjobergs workbenches
 |
Sjoberg Nordic Plus Workbenches Here’s the new generation of affordable workbenches from Sjoberg, available in three lengths to meet your shop needs.Enjoy superior versatility of vise location for left- or right-handed use, .. |
The Village Carpenter
on March 5 2008
That’s disappointing, to say the least. I’m very surprised it moved that much given that the boards are on edge. Do you run a humidifier in your shop in the winter? That might help.
Al (Sandal Woods)
on March 5 2008
Thanks, VC. You are right - even 1/16-inch is a lot of movement for 3-1/2 inch wide boards. Just looking at the exposed boards, they seem to be quarter sawn beech. I need to do something, for sure.
In discussing this with various people, the issue of monitoring humidity in the shop came up repeatedly. Great minds thinking alike!
First: Monitoring the humidity. This will lead to some sort of action, depending on how severe the swings are, both Winter and Summer.
So, high on my priority is to get some sort of hygrometer. As inexpensive as they are, I should have bought one a long time ago.
Mark Mazzo
on March 7 2008
Hi Al,
I too am surprised that the bench seemed to move as much as you showed with the straight edge. Especially if it is quarter sawn stock.
I know that when I made my bench I had flat sawn stock that I ripped and then flipped on edge before gluing it into slabs (effectively creating quarter sawn stock) This should cause the movement to be along the thickness of the top and keeping a relative uniform thickness. I guess it’s possible that in your bench a few of the out side boards were not as dry when it was built - so, now they have shrunk a bit more than the others.
I would agree on monitoring the relative humidity in the shop. I keep an eye on mine (basement shop) and I try to keep it at about 40% or so all year long. During the winter this is easier it’s the damp summers that require dehumidification.
Keep us posted on what you find out!
–Mark
The Craftsman’s Path
Note: I made a minor edit, to correct the RH to 40% (from 4%) —— Al
Al (Sandal Woods)
on March 7 2008
Thanks, Mark! You are right on the money.
Indeed, someone I consulted also suggested that the wood “…may have been wetter than it should have when it arrived on our shores…”
In my most recent post I show the hygrometer I ordered and received just today, to monitor relative humidity in the shop.
I will post results in several months. At that time I hope to also have some practical suggestions.
Mark Mazzo
on March 7 2008
Hey Al,
Just saw your reply. Sounds like a good plan to see what the humidity is doing.
Just an FYI…it looks like I made a typo in my previous comment. I try to keep my shop at 40% all year long not 4%. It gets higher in summer and sometimes dips a bit lower in winter, but summertime is the season that’s harder for me to keep consistent.
–Mark
The Craftsman’s Path
Al (Sandal Woods)
on March 8 2008
I corrected the typo, Mark.
I actually read it as 40%, as I *knew* you were right!
Summers can be quite humid here too; but with the A/C running in the shop during the hot/humid months, I expect it won’t be too high. I see an almost constant trickle of condensed water, so the unit is taking out some of the moisture.
The interesting part about the new monitoring system is that it keeps in memory the humidity highs and lows - this will help if I forget to look at it for several days.