Either of those increase the contact and gluing area.
Edge glued plywood has minimal of both.
Also, plywood is not solid (well duh!).
I'm not had a lot of luck edge gluing air to air.
I'd expect the seam to open up from temperature cycling.
Even though the expansion rates of the plywood match, the glue doesn't.
It'll have about half the strength of the same thickness of solid
lumber--essentially forget about the end grain plies and only consider
the plies w/ long grain as being the effective thickness of the material.
That's not exact because there is some plus from the endgrain section
but there's also a counter effect that the small sections of edge grain
aren't connected. I've seen some test data from like Purdue or the
Forest Products Lab or somesuch place but a quick search didn't locate
it this morning...
But, for the OPs purpose it'll work perfectly well...
It does not necessarily have to be "thin". There is generally more
strength in a glued plywood butt joint than most would suspect, even
plywood to plywood, as was under discussion.
The overriding requirement is that there be sufficient strength to stand
the test of time _for the purpose intended_ ... much like using pocket
hole joinery for face frames - just the right amount of strength for the
task at hand
Were there not usable strength to be had from an edge glued plywood
joint because of the orientation of the plys, plywood would be much less
useful for any number of uses that are routinely considered practical.
IOW, joints such as this, or any framing of plywood with other material,
that are considered entirely practical, would not be so:
That said, I would not expect to butt joint/glue two 3/4 x 48 x 48
panels of plywood and expect that join to be in the middle of an
unsupported span to be as strong as a single 4 x 8 sheet.
However, reinforce that same span, with even minnimal support
perpendicular to the plywood to plywood butt jointed join, and for all
practical purposes the fact the plywood is made from two butt jointed
panels becomes of much less concern.
I have saved many a not-wide/long-enough end panel/cabinet part from the
scrap pile by butt joining plywood where any downside from a weaker part
is of absolutely no concern ... just as the OP pondered.
IOW, it's all in the application ... :)
Not so fast...
Assuming the same material in the same orientation so the plies match
up, there is (roughly) half the material that is edge grain to edge
grain and thereby an effective glue surface.
On the other half, you have end-grain facing end grain and it will have
essentially no strength from the glue joint (normal gluing practices,
yellow pva, etc., etc., ...).
So, in essence, if you start w/ a 3/4" ply, your end result is (again
roughly) as if the actual material were more like 3/8" in thickness.
That ain't too bad, and will suffice for many applications as in OP's
and as Swing notes he's done (and I'd think virtually all who have done
any significant amount of cabinet work will have at some point--it's
just too common a problem of having need for that smidge more material
that doesn't justify the cost of a whole sheet for the purpose).
You're right that if you were to do the classic joint test on the result
w/ a test specimen you would find that most of the fractures will be of
the long grain breaking rather than the glue joint itself separating.
But, what you're not accounting for is that there isn't that much long
grain...so even if the joint that is glued is as strong, it's putting
the 90-lb weakling in the same ring as the weight lifter when it gets
down to the actual strength--that smaller glue joint area and material
just can't support what twice the material can/would.
As noted down thread a little earlier (I hadn't seen this part before
that seemed worthy of direct comment), I have seen such test data altho
I don't recall precisely where (and on T-day I'm not spending any more
time looking for it again). In that, study abicr the result was
generally slightly less than that of the equivalent long-grain plies and
was attributed to the two or three plies of solid material not being
physically together as one solid section.
I don't recall it being in that study, but I'd expect w/ proper
preparation such as sizing the end grain plies and using epoxies one
could manage to increase the bond strength a fair amount but I would
still doubt one could achieve the same strength as an equivalent of same
thickness of solid material.
If it's from the same piece and same face up, of course. But, not
_necessarily_ so in general. Some isn't symmetric from face to rear,
for example. And two panels of same species veneer but different
manufacturers might not coincide even if a decent match of the face
grain is possible.
Simply included for preciseness of description...
One other thing...lumber-core ply does act in this regard much like
solid lumber--not surprising since, in fact, it is mostly a solid core.
Of course, the core ply in solid core is rarely glued (if ever; won't
say it hasn't been done but I've never seen a sheet that was nor recall
a spec for it) the strength along the length isn't nearly what a regular
piece of ply of the same thickness will bear...
I'd be _real_ surprised at that indeed...
W/o the continuous surface faces and with the smaller cross-section of
length-grain, it makes no physical sense. Even epoxies don't do _that_
well in butt-grain applications.
Speaking of which...
A while back (as an experiment) I fastened two pieces of 3/4" ply (each
maybe 18" x 6") edge-to-edge along the longer side with three pocket
screws. I managed to get the joint lined up quite nicely and it felt
pretty solid, even without glue. I don't know if I would trust such a
joint for something heavy-duty, but it doesn't sound like the OP needs
that. It was quick too.
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