can't get a straight edge.....frustration

Sounds like you have the making of an unintended Spring Joint…

6ft long is a long piece of stock to joint… not that it can’t be done, but is it just a cumbersome process on a jointer with a bed less than its total width.

When you clamp their pieces, do they come together flush and square, even with the concave?

It could either be your jointer isn’t coplaner or your board is reaction wood, have you ever ripped a board on your tablesaw and the kerf started to close up or, widen.

-- If I cut it too short I can scab a piece on, but if it's too long what do I do?

If the gap is small, it can likely be closed with clamps. Give dry clamping a try and see what it looks like. A gap of 1/8” or less is no big deal. Just leave your top oversized, and trim it after glueup.
Otherwise, try an outfeed roller, set even with the outfeed table of your jointer.

-- Willie, Washington "If You Choose Not To Decide, You Still Have Made a Choice" - Rush

Apply extra pressure to the front of the workpiece at the beginning of the feed and then go light in the middle and finally extra pressure to the rear as you finish the feed.
When I started using my jointer I tried to keep all the pressures and feed rates even as possible and I had the concave problem also. Tried the variable pressure thing and the problem got better.
Also, an out feed roller support set ever so slightly higher than the end of the out feed table might help with the long boards.

-- Michael :-{| Diapers and politicians both need to be changed often; and for the same reason.

Did you check the adjustment of the outfeed table? This will happen if the table is slightly BELOW the level of the blades.
Here’s a link to the correct position of the outfeed table-
http://www.woodworkweb.com/woodwork-topics/work-shop-tips/308-setting-jointer-knives.html

-- Lew- Time traveler. Purveyor of the world's finest custom rolling pins!

I would say your out-feed table is too low. If it is, as well as making the cut concave, you’ll also get a ‘snipe’ at the end of the board as it comes off the in-feed table.

-- Come to the dark side....we have cookies...

I use a Freud glue line rip blade for my glue ups, and the boards are perfectly straight and smooth.

-- Joe

+1 what joe said. Jointer is better suited for squaring. Table saw for strait line, or making parallel.

A lot of good suggestions, thanks.

When using smaller stock on this jointer things come out perfect, no snipe. I painstakingly set the blades, I don’t think the blades are higher than the outfeed table.

My normal method for panel glue-ups is to joint one edge on the jointer. Then rip to parallel on the table saw and glue. Done. It just isn’t working with these big boards. I tried an outfeed roller too, still concave. I must not be able to achive the correct downward pressure when the ends of the board are over the jointer blades. I normally strive for percise edge mating before glueing. I guess his project might need more muscle than finesse. That or a jointer with a extra long bed…I wish.

It’s also possible that your jointer fence isn’t square. Flip one board over and see if the joint changes. If so then the skewed edge might be the culprit. Outside chance. Just another possibility to check.

-- The smell of wood, coffee in the cup, the wife let's me do my thing, the lake is peaceful. http://gagnerwebsite.com/Deceiver/Craftsman_on_the_lake/Craftsman_on_the_lake.html

Similar to what crank49 said. If jointer adjustments are within spec then what usually remains is technique. Here is where class instructions, in my opinion, are most helpful: the instructor noticed that my edge jointing technique of applying pressure to the board as it moved over the knives would produce an out of straight joint. He helped my correct the procedure and I’ve not experienced that problem again.

-- Miles

I try (and normally do) apply pressure to the board on the outfeed bed only, however, with a heavy 6 foot board on a somewhat small jointer table I realized quickly that things can’t be quite so perfect.

I’m used to a no gap glue-up. When those gaps are there it makes me want to whip my hat on the floor, stomp on it, and swear like a 2-bit whore. I can’t help myself.

Another method you might consider is attach the board to a piece of mdf or plywood with a factory edge. Then run it through the table saw with the factory edge against the fence. Or, you could use a router with pattern bit run against the factory edge.

+2 for Joe.
I don’t have a jointer in my home shop. A well tuned TS with a glue line ripper is what I use.
Bill

-- bill@magraphics.us