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Straightening cupped table top

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Forum topic by typing posted 08-02-2019 06:26 PM 437 views 0 times favorited 11 replies Add to Favorites Watch
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typing

5 posts in 66 days


08-02-2019 06:26 PM

I did not do a good job when gluing up 8/4 sapele boards for a table top ( 4 boards 3’ width total ) so the end boards ended up pointing about 1/4” down. I thought is is not a big deal and finished it but now am thinking about straightening it. Since it is across the gran it does not require much strength to bend it back even the wood is 2” thick. But it has to be done before the top installed on the table due to the construction of it.
I wander if someone has tried to bend the wood in the opposite direction overshooting the cupping and fixing it in that position for some time. When the pressure is relieved the wood would spring back but I hope not to where it started from and would end up straightened. What do you think ?


11 replies so far

View Jack Lewis's profile

Jack Lewis

481 posts in 1559 days


#1 posted 08-02-2019 06:40 PM

Is this what you are writing about?

-- "PLUMBER'S BUTT! Get over it, everybody has one"

View Lazyman's profile

Lazyman

3877 posts in 1868 days


#2 posted 08-02-2019 07:30 PM

I think that it will just spring back, especially since it is 2” thick. To relieve the stress, I would think that you would have to heat it while over bending to soften the lignon but I think that the amount of heat required would cause the glue to fail. It is probably possible to use straighten it out by simply attaching it to the aprons. If the table design doesn’t have aprons, it may be better to cut it apart along the joints and take the necessary steps to straighten it out.

Another trick that I have seen done is to rout groves into the bottom to accept 2 or 3 angle irons and screw the angle iron into the grooves to pull it straight.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

View typing's profile

typing

5 posts in 66 days


#3 posted 08-02-2019 08:24 PM



....

Is this what you are writing about?

- Jack Lewis

Its more like this. The apron is too short to be used for straightening.

View Lazyman's profile

Lazyman

3877 posts in 1868 days


#4 posted 08-02-2019 08:32 PM

I think that you may want to cut it apart and remake the top so that it is flat. Steel or angle iron battens might work to pull it back in line but it would bother me to see even a little bit of cupping.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

View Jack Lewis's profile

Jack Lewis

481 posts in 1559 days


#5 posted 08-02-2019 10:06 PM

+1 on Lazyman’s

-- "PLUMBER'S BUTT! Get over it, everybody has one"

View Aj2's profile

Aj2

2430 posts in 2279 days


#6 posted 08-02-2019 11:45 PM

It’s not easy making a top that’s flat without a jointer and planer.
Even with great hand tool skills.
Don’t give up we have all had projects that tested our skills. Sapele Is a not a good wood for bending esp if it’s kiln dried. It’s just it’s nature. :(
Good Luck

-- Aj

View bandit571's profile

bandit571

23602 posts in 3164 days


#7 posted 08-03-2019 02:52 AM

These are called corner blocks. Sometimes can be use inside the aprons. Simple pine boards, cut at a 45 at each end. glued and screwed to the aprons or on a webframe. A slotted hole is made, to allow a screw to hold a top, yet allow for movement. Just use some scraps of the size needed, You may have to allow for the legs’ inside corners, just make the corner blocks a tad longer. Add washers to the screws, so they won’t pull up and through the corner blocks, yet allow the screws to slide a bit, as the top expands, and contracts.

-- A Planer? I'M the planer, this is what I use

View CaptainKlutz's profile

CaptainKlutz

1762 posts in 1975 days


#8 posted 08-03-2019 03:04 AM

IME – 8/4 sapele (especially rift or flat sawn) is not a wood that can be forced into flat shape easily.

If you clamp thick sapele with excessive pressure to make it flat, it warps like a pretzel when clamps are removed. Worse, it keeps moving with every change in moisture level. DAMHIK

Suggest you use heavy steel batons under the table and pray you can keep it flat, or
Better method is to rip it apart and reglue the boards to show zero warp.

The boards must be flat before glue up to get a flat panel. Period.
So remove any twist/bow/warp BEFORE applying glue.

Best Luck.

-- I'm an engineer not a woodworker, but I can randomly find useful tools and furniture inside a pile of lumber!

View typing's profile

typing

5 posts in 66 days


#9 posted 08-20-2019 04:08 PM

Just to explain how I got it cupped. I used some ratchet straps as I did not have enough long clamps. That put much more pressure on one side than on the other and as result bent it. If I have checked the alignment before the glue dries it could be easily fixed. Unfortunately I noticed it when it was too late.
Anyway two pieces of angle iron with two holes near the center on the bottom of the table top fixed the situation. In fact it was very easy as the wood across the grain is not very strong even if it 8/4 sapele. I may try to remove the steel in a year or two and see if the wood holds or just leave it there.

View Lazyman's profile

Lazyman

3877 posts in 1868 days


#10 posted 08-20-2019 08:33 PM

That makes me wonder if the joints might have gaps. It would seem to me that using straps like that for glue up that caused a bow would mean that the joints opened slightly on one side. The wood itself is unlikely to bow if you have an uncured glue joint. My concern is that forcing it straight may eventually cause the joints to fail. Personally, I think that I would have cut it apart and reglued it.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

View hhhopks's profile

hhhopks

659 posts in 2858 days


#11 posted 08-20-2019 10:25 PM

How square is is square?
If it is off a deg or so on each side and if you happen glue the wrong sides, then it would cupped. The more pieces that you have, the greater the opportunity to be off.

Check the fence (jointer) and the blade alignment (saw) too.

-- I'll be a woodworker when I grow up. HHHOPKS

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