Hi,
I am making a dining table, with breadboard ends. The top is 40 inches wide, made of birch. The top is being glued from 4-5 flatsawn boards (that is what I have and can get).
According to this calculator https://www.garagewoodworks.com/moisture.php I can expect around 1 1/2″ seasonal wood movement in extreme years.
If I am understanding this right, will the edge of the top move 3/4″ out of flush of the breadboard end (if this makes sense..)? That seems like a lot.
Have I gotten something wrong or is this what I can expect?
Thank you
Replies
You entered some erroneous information, there is no way it will move that much unless the wood in totally green and you intend to use it in a desert. Expect 1/8 th for kiln dried wood over that width.
I used a difference on 10% since I do not know the start and expected final MC. But that was probably a bit extreme.
I think you've gotten one of the variables wrong. You should be plugging in "birch-yellow" (I assume), "flatsawn", and "40 in" for the total width, with nothing in the "width in CM" box.
If the wood is properly dried when you start the project, I wouldn't think the MC would change over 6% season to season inside a house (in an unconditioned home with no humidity control).
That would equal a 13/16" change total, with 13/32" on each side if you are pinned centered on the breadboard end.
I haven't followed your link to that calculator but 40 years of experience makes me skeptical. Wood movement has too many variables to be reduce to a mathematical formula. Wood species, humidity differential between the shop where it was made and its final place of use, and the humidity fluctuations within the place of use. If the shop and place of use had exactly the same humidity year round there would be no wood movement to speak of. In today's modern climate controlled homes wood movement isn't as much of an issue as it once was. In a reasonably stable environment I would expect around ¼" - ½"of movement for such a table.
You should open the link! It factors in species, RMC to start, total MC% fluctuation, and width into a very reliable mathematically formula. It's really very useful and accurate.
I have a cherry table top that wide. The breadboard ends will either stick out or be recessed about 1/8 on each corner. Which means a total of about 1/2 inch of movement from widest to narrowest.
It's the last time I used breadboard on a tabletop. I've done wider tops since, and they aren't necessary to keep a top flat, if it's fasted to the apron properly. If it's the design feature you want, go for it. If it's just flatness, there are better ways.
On an unsupported panel, like a slant front lid, breadboards help.
I have done a number of small tables, where I attached the top with buttons morticed into the apron. It seems like it keeps the top pretty flat, as long as I attach enough in the right places.
Is that the same method you have used on bigger tops?
Yes, that's what I use, mostly. I've also attached a cross brace to the apron, and screwed the top to that from below, using elongated holes for wood movement.
Do you dovetail the cross brace into the apron then?
I've done a dovetail on a couple, and pocket screws on a couple all experimental, but they all worked just as well.
On a narrow table I used just a single screw into the middle of the top. On wider pieces I used 3, with slots on the outer sides.
Table 13-5 in the USDA Wood Handbook gives a dimensional change coefficient of 0.00338 for yellow birch tangential (use for flat sawn) and 0.00257 radial (for quarter sawn). The units are % dimension change /% MC change.
Assuming a 6% change, like Drew, the width change is 6 x 0.00338 = 0.0203%
Over 40 inches, this is 40 x 0.0203 = 0.81" which is almost exactly what Drew said. Most calculators use values from the USDA book.
I would worry about seasonal cupping in flatsawn boards 10" wide. The tangential/radial ratio is 0.338/0.256 = 1.27 for yellow birch which is not too bad, however. The greater that ratio, the more likely the wood is to cup. At least be sure to alternate the grain curvature during layup.
Alternating growth rings will give a wavy potato chip, not alternating a regular chip.
This is a side note response to your original question and concern of the breadboard end not remaining flush with the side edge of the table.
The following FWW article discusses a dining table with breadboard ends and a center gap for expansions. The breadboard end stays flush with the edge of the table.
https://www.finewoodworking.com/project-guides/tables-and-desks/fresh-take-on-tabletops
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