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Lets say, for argument sake, that I have 2 or 3 10" blades for my table saw. That would give me just under 3/8 inches, when all mounted together, which I could use to make dadoes.
With what I have available this would be easier than constantly stopping my saw to adjust my fence to enlarge my dadoes.
I could easily see this being a bad idea since the blade kerfs could touch while the saw is operating. Also sawdust might get stuck between the blades. Has anyone tried this before?
There's another factor you're not considering: torque requirements. There's a reason the majority of dado sets are 6" or 8" in diameter. Most table saws won't have the power to run a 10" set. A 10" set will require 25% more torque than an 8" set, and 67% more torque than a 6" set.
Not surprisingly, Matthias over at Woodgears has written an article about doing more or less the same thing you're proposing. Except he solves the torque problem by using multiple 7-1/4" blades.
Instead of making a dado blade you can instead use a sled and then move the workpiece side to side on that to enlarge the dado. Make the outer cuts first and then you have a reference for where the blade should go; the middle cuts aren't that important to get accurate.
Or set the fence for the left most cut and clamp a piece of wood the with the same thickness of as the width of the desired dado minus the kerf of the blade.
You make the cut and then remove the block on the fence and cut again. then between those positions you can cut repeatedly and clean up with a chisel.
I know I'm really late to the party on this guys, but since I only heard half of the answer, I want to toss in.
It is dangerous for two reasons: first, as stated, torque. It DOES take significantly more energy to move that many large blades. Especially when they're tearing through a substance as chewy as wood. The other one, as briefly touched upon by the OP, is chip clearance. The more volume of wood being cut, then more waste material/sawdust there is that needs removing, QED. That's why he have chip-breakers in dado stacks: they also serve to even out the cut, sure, but even more critically, they provide a place for all the excess sawdust to GO, and a spinning "arm" to help ensure it DOES.
Same reason rip blades have deep gullets and fewer teeth: fast, hack and slash removal, deep, wide disposal. Try ripping a 2x4 with a plywood blade. It's doable. Just REAAAAAAALY slowly. Go faster? And you just hit on the concern I'd have, after the torque considerations, of course: the blade(s) binding. Nowhere for the material to go. All those middle blades take a bite, and can't shed the waste from any direction but straight OUT vertically (vs vertically and sideways). And with that many teeth taking a deep bite simultaneously, any hiccup is begging for kickback. Like, the "orbital escape velocity" flavor.
Now, don't take this as me preaching doom and gloom. I'm just talking theory (if you wanna call the engineering design considerations "theory") here. Species, moisture content, grain, ambient humidity, lunar cycle, mother's maiden name, along with the ever-critical "is my lumber just going to be a dick about this?" no doubt all play a role. If you're making 1/2" box or finger joint cuts through the face of panel material on a sled? Knock yourself out! As many as the arbor will hold! Maybe 1/4 of the teeth will engage before you're done anyway. But "ripping a 1-1/2-inch dado lengthways in a 16-foot FPS* 2x4"? If you're really lucky you'll only knock yourself out.
If it were me, I'd limit myself to TWO stacked blades (that aren't designed for the purpose), so each has at least one lateral plane available to it for ejecta, unless it's something trivial like a bunch of finger joints. Just stagger the teeth so the blade isn't bent when you crank home your arbor nut. Which, by the by, isn't so the blades "won't spin into each other". If your arbor nut is that loose you have other, much bigger concerns. It's so the tooth profiles don't press on one another, which would bend your blades, make it possible for said nut to loosen and give you a good chance of ballistic carbide, to boot.
*"Fir/Pine/Spruce" (softwoods), not "first person shooter", JIC. And personally, I think because FPS IS soft and gummy/pithy, it'd be MORE dangerous than something with more attitude, lie bubinga, ipe, or rock maple. At least those would yield fine dust, not fibrous chunks.
The other thing you're missing is that dado sets also come with shims to fine tune the width. This is especially important if you're using plywood, as thicknesses of plywood varies greatly.
Also, dado sets have the middle blade with flat cutting edges so that you get flat bottoms. Look at your regular saw blades, the teeth are angled, which would create a very slight "V" shape on the bottom of the dado.
If you're doing so many dados as to warrant using multiple blades, I would encourage you to just spend the couple of extra bucks on a dado set. They're only 100 bucks or so.
Our shop teacher used five separate 7-1/4" blades to make a dado on a 10" table saw in 7th grade wood shop. He did this to show us that you could safely make a dado cut even without the specific blade.
Has anyone tried putting a few small tack welds to hold them together? Well I have, about two years ago. Still use the same stack when I do. And remember I said small tack welds really small. Balance is key.
