Dealing with a cupped top?

[®smpr_fi_mac®]

1-3/4" thick.  White oak.  60" diameter round top.

Just realized that it's cupped about 1/4" across its span.  

It never sat on a surface that sealed one side from air/humidity movement and it was flat when it came out of the clamps about three weeks ago. 

I've tried soaking the concave side with water in hopes of flattening it, but only lost about 1/8" of the cup.

My options, as I see it now, are to attack the outer high points on one side and the high center on the other with a hand plane or a belt sander. 

Or to take it to a local shop that has a CNC to flatten it.

What would you do?  Ripping and regluing isn't an option.

[joe1086]

Just a thought....after removing the clamps, did it rest with one face on a workbench or table where air couldn't circulate? If yes, keep it stickered for a few days and see if it changes.

[Willyou]

There is an old trick of placing it out on the lawn, convex side up, in a sunny spot. That sometimes works. Long term, it needs to be mounted on its base whether that be a skirt all around or 2-3 large battens. With a cup of only 1/8", that will likely pull it flat and keep it there. Also, put the same number of coats of finish on both sides. That won't stop the cupping but will slow it down and dimish the effects between temp/humidity changes.

[jteneyck]

If it came out of the clamps flat but now it's not, then there's been moisture gain/loss in the top.  One scenario would be if the boards used to make the top didn't all have the same moisture content when it was glued up.  Another scenario would be that more boards in the top have growth rings in one direction than the other.  In both cases, when the moisture content changed, either from drying or from gaining moisture, that imbalance caused the top to warp.  

If the warping happened because the boards weren't of consistent MC when you glued them up, but now are, then if you flatten it it should stay flat.  But if it warped because the grain orientation wasn't balanced when you glued it up, it's never going to be flat again except at whatever RH it was at that time. 

Trying to hold a 1-3/4" thick oak top against its will is not likely to turn out well.  

John

[Willyou]

If it came out of the clamps flat but now it's not, then there's been moisture gain/loss in the top.  One scenario would be if the boards used to make the top didn't all have the same moisture content when it was glued up.  Another scenario would be that more boards in the top have growth rings in one direction than the other.  In both cases, when the moisture content changed, either from drying or from gaining moisture, that imbalance caused the top to warp.  

If the warping happened because the boards weren't of consistent MC when you glued them up, but now are, then if you flatten it it should stay flat.  But if it warped because the grain orientation wasn't balanced when you glued it up, it's never going to be flat again except at whatever RH it was at that time. 

Trying to hold a 1-3/4" thick oak top against its will is not likely to turn out well.  

John

Not say'n your wrong. But, considering the alternatives, I think I would try it; especially since he said that ripping and re-gluing is not an alternative. He can plane it flat, but I think it is just as likely that when more material is removed it will just warp again. If I was going to plane it flat, I would first store it for about a year on edge in a location with temp/humidity similar to where it will ultimately live.

[jteneyck]

Not say'n your wrong. But, considering the alternatives, I think I would try it; especially since he said that ripping and re-gluing is not an alternative. He can plane it flat, but I think it is just as likely that when more material is removed it will just warp again. If I was going to plane it flat, I would first store it for about a year on edge in a location with temp/humidity similar to where it will ultimately live.

If I wanted to try it, I'd adjust the RH to whatever it takes to get it flat again on its own, and then put dovetailed cleats in the underside.  If it tries to bow again at least it won't warp the base it's on.  

John

[DieselDennis]

Not say'n your wrong. But, considering the alternatives, I think I would try it; especially since he said that ripping and re-gluing is not an alternative. He can plane it flat, but I think it is just as likely that when more material is removed it will just warp again. If I was going to plane it flat, I would first store it for about a year on edge in a location with temp/humidity similar to where it will ultimately live.

No time to wait.  I believe it's a commission he's already got sold.  We need a now solution.  

I'd personally take the 1/8" cup that's currently there and see if I could pull it down a bit.  Or maybe tell the customer it's a feature of yours to make sure spills don't run off the table.

[ianab]

I've seen this happen with a eucalyptus top I made for my son a few years back. Glued up 4 boards to make a top, ~3/4" thick, Left it leaning against the workshop door, and it rained. Didn't get the piece wet, but it had maybe 1" of bow in it next day. Well crap.... I turned it over, and walked away. Next day it was flat enough to mount on top of the frame and fix with sliding buttons to handle it's obvious movement. 

The "lay it on the lawn" idea is actually valid, damp grass on one side, and hot sun on the other will soon force some movement. Just be careful you don't cup it the other way. 

The fact that wetting one side partially corrected the issue suggests that it's a EMC / Humidity issue. Just takes time for water to leave or absorb into wood.

[Gary G™]

1-3/4" thick.  White oak.  60" diameter round top.

Just realized that it's cupped about 1/4" across its span.  

It never sat on a surface that sealed one side from air/humidity movement and it was flat when it came out of the clamps about three weeks ago. 

I've tried soaking the concave side with water in hopes of flattening it, but only lost about 1/8" of the cup.

My options, as I see it now, are to attack the outer high points on one side and the high center on the other with a hand plane or a belt sander. 

Or to take it to a local shop that has a CNC to flatten it.

What would you do?  Ripping and regluing isn't an option.

Hi Brad,

You haven’t responded yet to say how you’re making out so I’m presuming it’s still an issue and I’m really hoping you can successfully resolve this.
If you can’t get the cupping out, you might want to review your order/contract.
What’s the commitment?
If you thin the top, does it still meet their expectations?
Your original post mentioned they wanted a very thick (1-3/4”) oak top.
And, if you thin the top, what’s to say it won’t continue to cup in their home?

Have you given thought to building a new top using QS Oak?

I reviewed your youtube first half tabletop glue up video and wonder about the cathedral grain on the 3rd board (R->L)?
Could this be the culprit?

Let us know how you’re doing.
Wishing you success.

[®smpr_fi_mac®]

Oh, no, not just 1/8" cup *now*.

Still closer to 3/8" cupping.  It lost almost 1/8" of the cup when I wet one side. 

Thanks for the advice, all.  It does need to be done soon; this is a customer's table.  I think I'll build a router sled, shave it down and add two bars of C-channel across the under side.

(Edit)

Taking 3/8" of the thickness won't be a problem for the end product.