Is this what you are writing about?

 

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.

 

....

Is this what you are writing about?

- Jack Lewis

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

 

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.

 

+1 on Lazyman's

 

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

 

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.

 

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.

 

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.

 

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.

 

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.