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Warped table top

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Forum topic by David posted 01-02-2018 05:11 PM 1216 views 0 times favorited 46 replies Add to Favorites Watch
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David

42 posts in 572 days


01-02-2018 05:11 PM

Hi,

I built a dining room table from 3/4” cherry wood last summer. The table is about 6 feet by 5 feet (attached image, before warping)

I used pocket screws every 10 inches with glue to make the top. Everything looked good when I finished. I just start noticing that the top is warping downward. There are 2 support on beam on the edges. My questions are:

1- What did you I wrong?
2- How I can fix this, if possible?

Thanks

-- David Tab


46 replies so far

View nkawtg's profile

nkawtg

281 posts in 1450 days


#1 posted 01-02-2018 05:14 PM

Did you apply finish to both sides of the table top?

View Loren's profile

Loren

10477 posts in 3847 days


#2 posted 01-02-2018 05:18 PM

1. It seems likely you ignored wood movement.

2. remove the cross-grain parts and reinstall
if desired with slotted screw holes and leaving
room at the ends for wood movement.

Top should be fastened to the skirts with slotted
screw holes, figure 8 fasteners, or other appropriate
method.

View OSU55's profile

OSU55

1962 posts in 2188 days


#3 posted 01-02-2018 05:35 PM

^ Believe the boys nailed it. Pun intended.

View David's profile

David

42 posts in 572 days


#4 posted 01-02-2018 05:35 PM

I applied finish to one side.

now what?

-- David Tab

View Smitty_Cabinetshop's profile

Smitty_Cabinetshop

15694 posts in 2817 days


#5 posted 01-02-2018 06:26 PM

re: Now What?

—See Loren’s comments.

2. remove the cross-grain parts and reinstall
if desired with slotted screw holes and leaving
room at the ends for wood movement.

Top should be fastened to the skirts with slotted
screw holes, figure 8 fasteners, or other appropriate
method.

- Loren


-- Don't anthropomorphize your handplanes. They hate it when you do that. -- OldTools Archive --

View AlaskaGuy's profile

AlaskaGuy

4772 posts in 2508 days


#6 posted 01-02-2018 06:40 PM

Just to make life more difficult this is what Bob Flexner (finishing guru) has to say about finishing both sides.

https://www.popularwoodworking.com/article/finish_both_sides_not_necessary

-- Alaskan's for Global warming!

View Aj2's profile

Aj2

1872 posts in 1997 days


#7 posted 01-02-2018 08:57 PM

I see the same thing as Loren. The long grain piece going against the expansion direction of the top. I think you should put it in the sun and see what happens. It’s a good lessen

-- Aj

View pintodeluxe's profile

pintodeluxe

5798 posts in 3012 days


#8 posted 01-02-2018 09:06 PM

Yes, Loren has it right. Wood expands mostly across its width. So… when you attach wood with the grain running across the width of a table you have to do so very carefully. It should only be attached with special techniques such as breadboard ends, or screwed through slotted holes (without glue). An example would be a cross-grain cleat on a trestle table.

Putting it out in the sun won’t accomplish anything but a temporary change.

Good luck with it.

-- Willie, Washington "If You Choose Not To Decide, You Still Have Made a Choice" - Rush

View Woodknack's profile

Woodknack

12430 posts in 2579 days


#9 posted 01-03-2018 12:57 AM



I applied finish to one side.

now what?

- David


Finish the other side. After you fix the other problems.

-- Rick M, http://thewoodknack.blogspot.com/

View Smitty_Cabinetshop's profile

Smitty_Cabinetshop

15694 posts in 2817 days


#10 posted 01-03-2018 01:19 AM

Theory:

Biggest waste of time is likely a tie between lapping jack planes and finishing the undersides of table tops.

-- Don't anthropomorphize your handplanes. They hate it when you do that. -- OldTools Archive --

View bbasiaga's profile

bbasiaga

1240 posts in 2194 days


#11 posted 01-03-2018 03:19 AM

You could try adding some cross members under the top, between the aprons of the table. Then screw the top down in the middle to pull the dome out of it. After that, screw it down at the outsides, using slotted holes to allow for expansion. That might pull the table top back to flat.

You’d need to remove those cross grain boards at the end as suggested first. You could add them back after, if you do the same: screw them in the middle only, and through slotted holes at the outsides to allow the top to expand over it.

As far as finishing the bottom, I always finish both sides the same. But there is a school of thought that it isn’t necessary. I guess I can’t really comment since I have only tried it the one way.

The truth probably is that you can make a flat table top, but you can’t keep it flat forever. It will stay flat enough if you use the proper construction techniques.

Brian

-- Part of engineering is to know when to put your calculator down and pick up your tools.

View oldnovice's profile

oldnovice

7325 posts in 3567 days


#12 posted 01-03-2018 05:34 AM

Ditto with Rick_M!
I know that many experts may disagree but I finish both sides and have on every project I have built. After all, it doesn’t take that long to finish both sides.
Over 50 years ago, I worked in a radio & TV repair shop where the owner was also a wood worker nonpareil. He had some beautiful projects, he was a stickler for details both professionally and in his private life.
His key phrase was ”balance”, shelves, cabinet sides, table tops ..... both sides of the wood deserve the same treament!

-- "I never met a board I didn't like!"

View JBrow's profile

JBrow

1366 posts in 1119 days


#13 posted 01-03-2018 05:50 AM

David,

I do not understand. “…the top is warping downward.” and “…2 support on beam on the edges .”. Photos that show these two features would be helpful.

1- What did you I wrong?

It may well be that uneven moisture exchange of the upper and lower surfaces of the top with the environment is a factor. Finishing both faces, and edges and end-grain ends could have reduced the “warping”. Since end grain is more difficult to seal, ensuring the end grain no longer absorbs finish may have helped a little more. Even if this precaution were taken, sunlight striking the top’s upper surface or heat captured under the table from the base board heat could have caused the problem.

While Mr. Flexner believes finishing an entire table top is unnecessary, others disagree:

https://www.finewoodworking.com/media/TabletopsFlat.pdf
https://werefinish.net/sp/warped_wood_on_furniture/

and USDA, although page 16-25 advocates back priming to enhance dimensional stability of wood (especially outdoor siding)
https://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr190/chapter_16.pdf

But there are other factors that could have contributed to the problem such as flattening and finishing the top before the moisture content of the wood reached equilibrium with the environment or the orientation of plain sawn lumber during the glue-up.

Since I am not confident that I understand the support beam comment, I cannot opine whether this may have been a factor. But if the top has a twist, then it could be a factor.

2- How I can fix this, if possible?

There are several approaches that could be considered to fix the top.

If the top is uniformly cupped with a crown running the length of the top at the center, then either adding or removing moisture from the unfinished face could mostly return the top to flat. If the crown (convex) is centered on the unfinished face, then heat could drive out enough moisture. Since you have baseboard heat, the end of the top could be elevated and leaned against the wall so that heat from the baseboard units washes up the unfinished side of the top.

However if the crown is on the finished side (that is the finished side is convex), applying moisture with a misting spray bottle to dampen the entire surface could add moisture. The surface would have to be kept moist with repeated applications as the surface water either is absorbed or evaporates.

In both cases, the top would have to be checked frequently to avoid reversing the cup.

Once it is flat, applying finish to the unfinished surfaces would help equalize the rate at which moisture enters the top. The top should be allowed to rest a day or two before applying finish to ensure it stays flat. It is unclear whether these steps are enough to keep the top flat.

Here is a slightly different approach that equalizes moisture on the two faces of a cupped top…

https://mcglynnonmaking.com/2014/08/16/uncupping-a-cupped-top/

There are more radical, less desirable approaches that could be attempted to affect repair.

View AlaskaGuy's profile

AlaskaGuy

4772 posts in 2508 days


#14 posted 01-03-2018 06:17 AM

”While Mr. Flexner believes finishing an entire table top is unnecessary, others disagree’‘
So, how do we ever know for sure who’s right and who’s wrong if there is in fact right and wrong?

This is like health studies. Some say salt is bad and some say salt is not so bad.

-- Alaskan's for Global warming!

View Woodknack's profile

Woodknack

12430 posts in 2579 days


#15 posted 01-03-2018 06:44 AM

You’re significantly improving the odds of your tabletop not warping by finishing both sides equally. Only finishing one side is half-assing and against the advice of pretty much everyone but it’s your table, build it how you want.

And Flexner’s blog post (it’s not in his book) is rubbish. His reasoning amounts to ‘because some tabletops go concave and some convex, the bottom being unfinished wasn’t a factor.’ That’s dumb, really dumb. I respect his advice on finishing because that’s his career but his knowledge of wood movement is poor.

-- Rick M, http://thewoodknack.blogspot.com/

showing 1 through 15 of 46 replies

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