My new tabletop is made up of 7 red oak boards 42" long by 10" wide; so the
tabletop measures 42" long by 70" wide; well, actually an oval that size.
It is 1" thick, if that matters.
(I had a big pile of 50" long 5/4 oak, and no 70" oak...)
If this is not clear, a picture of it (well of the bottom) is at:
I planned on making the base 60" by 32" and attaching the top with metal
clips set in groves. But I just did the math and it is horrifying. My
reference says that red oak will move 0.31"/ft, so 5' will move 1.6". That
seems rather too much for clips.
How do I deal with this? My only thought is to use clips on the apron along
the movement, and leave the perpendicular aprons free, but that doesn't seem
Any advice would be appreciated.
So don't use commercial clips. I used pieces of oak about 1 1/4"
square by "as needed" and cut a notch in the end. The notch fits over
a piece on the back of the rails; there is a slot in the "as needed"
portion of the oak clips and they are bolted to inserts in the bottom
of the table top. The slot can easily accomodate any movement, make
as long as you need. The oak clips at the end grain ends have no
slots, they just slide on the piece on the back of the rails.
That (1.6") seems like a lot. I built an entertainment center with an
Oak top 50" long and the most seasonal expansion I'm seeing is about
3/8". I used clips in slot all around without a problem
I think it may be variable based on the change in relative humidity in
the final environment. Maybe that figure is a worst case.
It's probably on the high side, but not by a lot.
Note that his table is 70 inches *wide*; length is irrelevant.
FPL's Wood Handbook gives the expansion coefficient of red oak at 0.00369 per
1% change in moisture content (MC). 0.00369 * 70 inches * 4% seasonal
variation in MC = 1.03 inches; with a 6% variation, the expansion would be
No doubt it *is* a worst case, or close to it -- but even a best case figure
(say 2% seasonal change in MC) is still over half an inch of movement in a
panel that wide. Better to plan for the worst case, IMO, and have it move less
than expected, than to wind up with broken joints when the panel expands more
Doug Miller (alphageek at milmac dot com)
Now I'm confused. I always thought (maybe erroneously) that the
primary expansion was in the direction of the grain,not cross grain
and not uniformly in both directions. That would be, on the OP's
top, in the 42" direction, across the table. Have I got it backwards?
Yes, you do. Expansion along the grain is, for all practical purposes,
zero. The principal dimensional change with changing moisture content is
tangent to the growth rings; that is, in the width of a flatsawn board,
or in the thickness of a quartersawn board. Radial dimensional change
(perpendicular to the growth rings) is typically approximately half of
the tangential change.
Using the factors quoted in Doug Miller's post to this thread, it
looks like this is assuming 7% change in moisture content, WAY more
than you would ever get in an air-conditioned home. However, I still
think you are smart to allow for this much movement--who can tell when
a future generation my store it in a garage for a few years before
getting it back out for the grandkid's new house?
I'd use the clips on the side rails. Use one screw in the center of
each rail, so that wood movement is balanced at each side. So now you
are dealing with only .8" at the end rail. Despairing of trying to
explain it, I offer the following ASCII art:
T O P
| A |YYYYY| |XXXXX|
| P |YYYYY| |XXXXX|
| R |YYYYY| |XXXXX|
| O | ______|XXXXX|
| N | |XXXXXXXXXXX|
| | |XXXXXXXXXXX|
X is a block of wood screwed to the top, with a tongue that serves a
purpose similar to the metal clips. Grain should run the direction of
the tongue for strength. Y is a block glued to the inside of the
apron, on which the tongue rides. Alternatively, the tongue could go
into a mortise in the apron, but you might not have enough depth.
Remember the side clips slide along a groove, while this tongue moves
in and out of a mortise or under the block glued to the inside of the
Alex -- Replace "nospam" with "mail" to reply by email. Checked infrequently.
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