How do you glue up boards so they don't cup or bow? I'm trying to get an overall width of 21" and I'm using 3 pieces of 8" cedar. Do I use dowels or glue or a combination? Do I clamp them a certain way to prevent cupping/bowing? Thanks in advance.
The key is in having perfectly square and straight edges. then apply a decent coat of glue covering both surfaces and clamp using just enough pressure to raise beads of glue from the joint, not squeeze it out.
Of course, you have to be sure the boards stay flat in the clamps. I always put just a little tension on the clamps and then go around with a deadblow mallet and make sure everything is donw on the clamp bars before tightening the clamps to raise the glue beads.
Even with the right equipment, a little experience goes a long way to getting flat panels froma glue up.
in addition to the other good advice given, alternate the direction of the end grain on successive boards so that the cupping that each board tries to do is in the opposite direction from the cupping force of the next board. If all boards have the same curve of grain seen from the end, then the cupping of each will add up to a large cup across the wide panel. When alternated, they tend to cancel, with no more than a wavy effect across the width.
Since most boards are not quarter sawn, the end grain curves along the circumference of the tree and this is the factor that produces cupping as humidity levels change.
In addition, you can also alternate the clamps, one on top, one on bottom, so that any tendency to put a cup in the panel in minimized.
You do not need to dowel the pieces together in gluing up a panel, but it may help alignment. If you use cauls or alternate the clamps, as well as beating the boards as flat as possible, you should be golden.
Since we're on the topic of gluing up panels, I'll add this. Use just enough glue to coat each side of the joint. Then, use just enough clamping pressure to squeeze out little pinhead sized droplets of glue. When those droplets have dried to the point of turning a little darker in color and lose their gloss, scrape them off with a scraper or chisel. You might want to put some wax paper across your clamps to keep from discoloring your wood and sticking to your clamps should any amount of glue try to bond the two together.
Have a good time!
"Ever notice how good enough, is usually neither good nor enough?"
One other suggestion is to rip the boards in half before you glue, so instead of clamping three 8" boards, you'll be clamping six 4" (3 7/8", if you want to be technical) boards. This may add a little complexity to your glue-up, but it will help minimize the risk/effects of cupping down the road.
If you are talking about the bowing that can happen as you apply force with the clamps the edges need to be square as stated before. also for a six foot run of planks I like to use 5 clamps starting off with three on the bench dropping the planks into them and two set down from the top So I end up with the clamps staggered one facing up one facing down Etc Then staring with the first up facing clamp applying
Just enough force to start pulling them together then move to the next clamp
Dead blow hammer in hand until itís all flat and tight as they need to be
As for grain cupping that was covered, I use the rip in too smaller strips
Method as well