I just got into woodworking and have ran into a problem. I recently completed a farmhouse coffee table for a friend and encountered a problem when gluing on the table top. I used 6 dowels for alignment along the frame. For the table I had 6 poplar boards 3"W X 32"L x 3/4" T glued together using loose mortise and tenon joints. After gluing I noticed a good amount of warping, too much to plane out. I counted on the glue holding them in place and it appeared that it was working, but one corner keeps popping up. Is there anyway of fixing this problem? I am using Tightbond 1 for glue.
It seems like you should disassemble the top, and investigate what causes it to warp this much. It could be wood movement of the top -- which could be due to the underside not being finished to the same level as the topside. It could be a problem in the frame too, except that having a single corner popping off makes that more unlikely. Difficult to say.
Attaching tabletops to bases using dowels is unconventional. There's a good article on keeping tabletops flat here, by Christian Becksvoort, which will cover different options for attaching a top to a base. Given that glue used to be good enough to hold it in place, it might just need the help of tabletop metal hardware, such as figure 8 brackets. You could consider that before ripsawing the top and refinishing.
If the top is shaped like a hockey stick when you take it off, you might have to saw in the middle of the warped portion along the grain, correct the edges, and re-glue the edge after correcting. You might have to do that more than once.
On alternate grain directions: After sawing (if you do decide to take that route), you could consider flipping one side around to the alternate orientations. But keep in mind that sometimes, a single bow can be easily fixed with appropriate fasteners. In that same article for instance, on p34, Becksvoort writes: "grain orientation makes little difference". You'll find advocates in both camps, but many woodworkers will favor what looks best over grain orientation when making a panel. For instance, it might bring the sapwood up in half your boards, which can mean more visible knots and "defects".
A top with a single convex bow (like a portion of a very large barrel), can still produce a very flat top once the edges are fastened down. If you're alternating grain, you might give your top additional inflexion points (like the shape of the spanish tilde "~"), and you might find that you need even more fasteners to keep everything flush with the base.
"I counted on the glue holding them in place". Oh my.
Well, welcome to the world of real woodworking - a land of infinite reward and heartbreak.
You've just run across a pretty fundamental truth - you cannot fight wood movement in large pieces. If you successfully immobilize a piece of wood that really wants to move, in the long run it will tear itself apart.
With that said, you might (might! I say) try the following. Figure some way to remove the tabletop. A super-thin saw such as a flush-cut saw might possibly fit between the top and the frame. Redrill your dowel holes. Now reassemble, but this time put spacers around the dowels, each one being about half the thickness of the gap you show. Even better, trim the spacers so that the bottom of the tabletop rests snugly against all four spacers when no glue is applied.
Now glue. Notice that the clamping pressure does not distort the wood, it just applies force to provide clamping between the tabletop and the frame/spacers, and be forewarned that any glue joint between the end-grain of the side supports and the long grain of the spacers will be very weak.
At this point, the dowel joints are not under strain. Although the tabletop will want to move with changes in relative humidity, at least you won't have contact at some intermediate point on the frame which will apply leverage to the dowels. However, the dowels will have a lot of possible shear force on them and they may fail.
"But," you might say, "the top is not flat. I want it to be flat." It's a little late for that, at least for thick solid wood. Oh, you can do it, with bolts replacing your dowels and inset nuts in the side supports, but then you'll need to inset the bolt heads and cover up with plugs. Probably not the look you're going for. And in the long run that too can fail.