On Thu, 21 Sep 2006 03:17:46 GMT, "Toller" wrote:

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:
http://www.frontiernet.net/~toller/tabletop.jpg

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
right.

Any advice would be appreciated.




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

In article , Frank Boettcher wrote:

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

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".

I think it may be variable based on the change in relative humidity in
the final environment. Maybe that figure is a worst case.

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.

--
Regards,
Doug Miller (alphageek at milmac dot com)

It's time to throw all their damned tea in the harbor again.

On Thu, 21 Sep 2006 14:58:07 GMT, (Doug Miller)
wrote:

In article , Frank Boettcher wrote:

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

It's probably on the high side, but not by a lot.

Note that his table is 70 inches *wide*; length is irrelevant.

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

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".

I think it may be variable based on the change in relative humidity in
the final environment. Maybe that figure is a worst case.

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.

Frank Boettcher wrote:

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.

And depending on species, tangential movement due to moisture content
changes is anywhere from 50% to 150% greater than radial.
--
Alex -- Replace "nospam" with "mail" to reply by email. Checked infrequently.

In article ,
says...

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.

On Sat, 23 Sep 2006 03:17:53 GMT, Doug Miller
wrote:

In article ,
says...

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.


Well thanks gentlemen, learn something new every day.

Frank

In article , Frank Boettcher wrote:
On Sat, 23 Sep 2006 03:17:53 GMT, Doug Miller
wrote:

In article ,
says...

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.


Well thanks gentlemen, learn something new every day.

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

--
Regards,
Doug Miller (alphageek at milmac dot com)

It's time to throw all their damned tea in the harbor again.