Edge Joining Plywood

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On 11/22/2012 7:41 AM, Swingman wrote:

Agreed.
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.
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Richard wrote:

Bet you don't think you can edge glue veneer either, huh? Edge glue into one large sheet before laying it, I mean.
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dadiOH
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On 11/22/2012 1:22 PM, dadiOH wrote:

I've never tried it, so you are probably right.
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On 11/22/2012 3:11 AM, Dave wrote:

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...
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Ok, I hadn't read the original message. I'll concede the intended attachment of such a thin piece should be ok, whatever method you use to attach it.
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On 11/22/2012 8:44 AM, Dave wrote:

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:
http://e-woodshop.net/images/StackedTansu4.JPG
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 ... :)
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On 11/21/2012 8:43 PM, -MIKE- wrote: ...

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.
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On 11/22/2012 10:46 AM, dpb wrote:

One would probably presume that you would not change outer veneer grain direction. If the outer veneer grain is run in the same direction the inner ply's should be running in the same direction also.
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On 11/22/2012 10:52 AM, Leon wrote: ...

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...
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On 11/22/2012 10:46 AM, dpb wrote:
Your point is valid and should not be ignored in deciding upon the suitability of this method for an intended purpose.
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On 11/22/12 10:46 AM, dpb wrote:

Like I said.... rocket science. :-p Happy Thanksgiving!
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dpb wrote:

I guarantee you that one can glue two pieces of ply together edge to edge with epoxy and that the ply will break before the glue joint.
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On 11/22/2012 1:26 PM, dadiOH wrote:

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.
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On 11/21/12 4:46 PM, snipped-for-privacy@mail.con.com wrote:

So, you now realize that everything I've been describing does just as well with any and all woods, correct?
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-MIKE- wrote:

Simple and easy way is to clamp thin, narrow boards or ply along one piece along the edge where you are going to put the other, apply glue, slide piece #2 between the clamped on guides.
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On 11/22/2012 10:32 AM, dadiOH wrote: ...

Be sure pieces are either finished or use wax paper or blue tape or some other method to not glue them to the finished work from the squeeze out...
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dpb wrote:

No need, once the joint has been glued and clamped just remove the guide pieces.
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On 11/22/12 10:46 PM, snipped-for-privacy@mail.con.com wrote:

than $5 to go pick up MDF. And I don't use MDF for anything except jigs and flat surfaces.

I understand. The ones around here have bins with smaller pre-cut pieces.
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On 11/22/12 10:32 AM, dadiOH wrote:

Excellent. Hopefully, remove clamped on guides before glue sets is the next step. Ask me how I know. :-)
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On 11/21/2012 5:31 PM, -MIKE- wrote:

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.
Official Disclaimer: The source of the above "advice" is a novice.
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