Accomodating movement on a 70"w by 42"l table top!?

[Toller]

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:

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 right.

Any advice would be appreciated.

[Doug Miller]

[snipp

May not "seem" right, but it is.

[dadiOH]

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.

[Frank Boettcher]

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.

Frank

[Doug Miller]

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 1.55".

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 than anticipated.

[alexy]

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 apron.

[Frank Boettcher]

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?

Frank

[alexy]

And depending on species, tangential movement due to moisture content changes is anywhere from 50% to 150% greater than radial.

[Doug Miller]

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.

[Frank Boettcher]

Well thanks gentlemen, learn something new every day.

Frank

[Doug Miller]

That's what the wRECk is for. Glad to help.