Aw, c'mon yourself ... or is it that you just want to argue this morning?
You know my point about starting with properly prepared stock is valid probably better than most, if you do indeed teach woodworking as you say you do:
Once again, stock selection and preparation are the two MOST important factors in a successful "flat" glue-up like the OP is talking about.
Start there, then solve any problems that remain ... (my contention is there will likely be NONE in this case, but if there is you will surely be a step ahead by being able to rule out twisted, warped, curved, bowed stock)
Now go ahead and continue to try and argue against those three points, and their order, all you want, but you'll be wasting your time and looking foolish.
I don't think so. Using few biscuits over a length, the glue area is much larger than the combined biscuit area which amounts to the face-areas of the cuts being the "lost" area from both surfaces. Otherwise, think of the two pieces being cemented with glue and no biscuits.
Either way, the pieces are much stronger at the wood/glue surface than wood/wood. The biscuits essentially act only as an alignment guide. A Google will find other sources in agreement with this. The region of the biscuit contact is very small over the length of most pieces being joined this way. The glue over most of the non-bicuited length is what is doing the job. Biscuits stop slipping and sliding in all directions, and make gluing /clamping much easier than otherwise, an they are used for alignment, that's all.
And certainly don't do this experiment... Don't cut biscuit slots into a 2x4 and into a 1x3 and then try to join them together with the biscuits. Some would not want you to see that the top edges should line up nicely even though those two pieces of wood are different dimensions. Think about it - how does your plate joiner index the wood?
Think again. The biscuit is tougher to break across its random compressed self than the wood along the grain just as the glue line is more difficult to break than the wood.
So, though it's acknowledged as a difference which is really pretty meaningless, it does is move the fracture zone a bit further from the glue line.
I agree I'm wasting time trying to get you to think, however, there is a man with a problem who needs some help, which you're not addressing.
The only way you'll find the answer to the problem is by elimination, and I don't mean the crap you're putting out, I mean by eliminating possibilities. Thus, knowing that pieces of unequal thickness, if referenced and assembled on the same faces will be flat eliminates unequal stock thickness as a cause. Thus it's likely he's got bad equipment or technique which is causing him to bore off square to the edge or ream the slots. That being the case, he can get perfectly prepared stock and ruin it again with your advice.
You can reach as far as you care to into your alimentary canal for further straw men, but until you address the proper issue, you'll not arrive at a proper conclusion.
Oh, fer crisssakes ... inexperienced woodworkers are always ready to blame the tool when the problem is that they fail to grasp the BASICS of woodworking.
You "think about it" ... try your little "experiment" with stock when both pieces are warped or bowed, even slightly, and see just how far you get with "lined up nicely".
The OP is admittedly using purchased S2S lumber and can't get a "flat" glue-up ... IF you've done any woodworking whatsoever, experience would have already told you where the FIRST place he should look.
Now go back and read what I said in previous posts instead of trying to find cracks in it ... there aren't any!
And if you still think you can argue with that, you'd do better to go get some more shop time instead.
I appreciate everyone's input, and have a preliminary answer.
Tonight I glued a 3/4" back lip onto a 51" long shelf, with 6 biscuits. I didn't expect it to work very well because the wood was too narrow to really get the guide flat; but since the defect would be on the back, I didn't much care.
It came out perfectly, despite horrible technique; if not for the pencil marks I could skip sanding. The difference was that both surfaces came from my tablesaw rather than the lumberyard's planer; so they were pretty straight.
This settles it for me, my problem is inadequately flat wood.
Your description of "there are a couple inches with a 1/16" difference, and maybe a foot with a 1/32" difference".sounded pretty much like your stock would be the first place to check, IME.
I've got a similar situation/problem coming up with some S2S 3/4 walnut that someone gave me ... much of it is simply not flat enough and there is not enough material to mill flat and still end up with stock a suitable thickness for many of the type of projects I do.
In this case, and because it was free I can spend the time to do it, my solution is going to have to be to laminate pieces, then joint and plane to desired thickness.
Arguments can rage about whether you need a jointer and planer, but they are a tough combination to beat when it comes to making it easy to do the best work of which you are capable.
Fer chissakes my ass Swingman. You have demonstrated a keen ability to completely miss what is being said to you in favor of piping off like some sort of sage. I would not use the term inexperienced to describe myself, but I won't attempt to dissuade you since by now it's pretty clear no one can tell you anything. Believe what you will, it's no sweat to me.
There you go - again... throw a red herring in the mix. Do yourself a favor and go back and re-read what myself and other poster actually wrote, can the straw men, and try to figure out what has been said. Then attempt to discuss within the context of the conversation without throwing in irrelevant artifacts.
The first place he should look is at the way he's trying to do it. I don't know if he has a bad tool or if he's using it wrong or what the problem is. Unlike you, I can't make the ultimate determination from the comfort of my armchair, without seeing the situation. You still fail to explain how a plate joiner is indexed. Then apply that to your ramblings. If you were more of a woodworker than a newsgroup expert, you'd know how this type of joinery works. Have you never joined two dissimilar sizes of wood?
Your over inflated opinion of yourself that couldn't help but put in your closing comments on that last post of yours is what caused me to respond. Go back and take a look at it for yourself.
The great and mighty one has spoken. Maybe a little more shop time and a little less dissertation would help you understand that there is more to woodworking than planing everything to the same size. You might even discover that two differently sized pieces of wood can very nicely be put together. Your jumped to conclusion, while appropriate at a different point in considering the project as a whole, is completely irrelevant to the OP's problem.
You're confusing we folks who consider "flat" as describing the broad face and "straight," the edge. What did you do with your tablesaw?
Flat panels demand square or offsetting angled edges, panels without gaps along the glue lines demand straight ones or some truly bigass clamps.
Not that I would recommend it, but you _could_ take a board bowing east and a board bowing west and make a straight(er) north-south board out of the glueup as long as you had a surface "local" level good enough to reference your joiner. Some people have even clamped such boards to straight ones to obtain a good local level. Next time you leave a couple boards leaning against the wall too long, remember it.
Well in this case, I had a 13.5" wide 5' long panel that I ripped to 12.75". I slotted the top of the panel and the bottom of the scrap and glued them together; so the scrap was a lip for the panel.
The glue line was the lumberyard's planing; probably not too good, but the lip was flexible enough to accomodate it. The edge was the TS rip. Apparently it was straight enough to give perfect results. The lip was pretty bowed, but it was no match for the shelf.
OK, now that you've used the word "rip," I understand. It wasn't flat so much as straight and square which you lacked. Nonetheless, do you understand how a fixed distance between fence and blade is what the biscuit joiner is all about as well?
What thickness stock is below the slot is as unimportant as the width of the offcut when ripping.
Completely irrelevant, as it turns out, to the OP's original question/problem.
You and 'Yabut' both introduced the tool "index"/vast difference in stock thickness issue as justification for missing the point.
The original question, and problem, clearly revolved around a "panel" glue-up. Experience should tell both of you that it would be rare indeed if the stock in a "panel" glue-up would not ideally be the same thickness.
So - you felt so cocky as to post your presumed righteousness immediately after toller posted that he had met with success and that he had ripped an edge with his table saw, even though that edge he created had nothing to do with board thickness, but now it's back to panel thickness. You still haven't explained how panel thickness has anything to do with what toller is explaining that he's doing. That stock in a panel glue up would ideally be the same thickness does not require great genius to figure out so why don't you move past that point and just pretend that you're not the only one who knows that. His points and his descriptions have not been different thickness panels and as has been articulated to you, and even if they were, with his joining process, he should be able to get a good top surface alignment.
As yet, you haven't explained a thing to the guy beyond ranting about selecting materials and plaining down boards. You haven't help him address or resolve his problem yet you mouth off at those who have tried. Maybe such a thing is beneath one so high and mighty as yourself.
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