Plywood is constructed of multiple layers of thin veneers glued up with alternating grain direction. This process results in an extremely stable material that does not experience the same drastic changes in dimension as solid lumber, which in turn, allows for extremely stable assemblies that will last for years.

 

Yes, Stick, I know it does, but my question is why the alternating layers don't work against each other as they do in other joinery?

 

Perhaps one reason is that the veneer is taken off the log rotationally, ie shaved off along the circumference(?). Like string it has strength in tension along its length, but not perpendicular to the fibres.
Just thinkin' out loud here...

 

Also it is compressed with a lot of glue so it is not as affected by moisture like solid wood is.

 

Possibly because the layers are so thin the wood is able to stretch a little bit before it fractures? Also, the individual plies are very dry when glued up. They are unlikely to shrink but might swell a bit.

 

Anybody ever look at a section of wood with and cross grain through a microscope? It looks like you have a handful of straws. Them babies suck up water even as condensation. Seal them off with a finish and you still have off gassing with nowhere to go when the temp rises.

Most plywood except for CDX has a snazzy veneer but the insides are often pressed (often unevenly) shavings flooded with glue. Underlayment is without voids in the layers to prevent high heels from puncturing it and severely denting or ruining vinyl flooring. The tube thingies swell with fluid, overcoming the surface tension of the water molecule. The plys thin 1/16" crossed layers cancel each others expansion out.

Ever notice the rotted bottoms of older sidewall shingles that run up a cheek? Move them a 1/4" above the roof shingles and you can add a decade or more to the shingle.

Through a scope

 

Here is some additional information. My logic is the same, the thinner the layer the more stable the finished product.

Plywood - Wikipedia, the free encyclopedia

 

Very interesting plywood info. including Aircraft plywood, used for British built "Wooden Wonder" Mosquito bomber. My father worked at building these during WW2 in a huge plant, converted from building farm machinery, (Massy Harris, another British firm) at the end of our street.
I can remember many a night the bunch of us kids sat with our mother, in the dark for what seemed hours to a kid, listening to the air raid sirens (practice only because we lived so close). Because of the constant sound it was dubbed "Wailing Willie".
Amazing how one little blurb on plywood can stir all that up.
Thanks again for the Wikipedia info.

 

OK. If thinner = more stable, then maybe when I cut inlays then they should be thin as well. I generally cut them about 3mm to make them less liable to break, and to give some sanding leeway. Maybe I should check up on some old jobs to see if the inlay has pulled away from the substrate.

 

3mm sounds like a good thickness for inlays to me. A lot of cheaper plywood has plies of that thickness and it's OK.

I think fundamentally the reason that you can get away with the cross-grain gluing in plywood is that the thin veneers don't build up sufficient tension when they shrink to break the large glue joints. A thicker cross-section of wood will generate more force. Just like a thin elastic band is easier to stretch out than a thick one.

 

I cheat and use ready made inlays which vary between 0.7 and 1mm in thickness and I've never ever had a problem.

 

+1 for the combination from Dan & Charlie. All wood has the straw like appearance. That is how the tree draws water from the roots to the branches before it is cut and why the moisture has to be reduced in a kiln or through aging of the stack. Turning a piece of "green" wood is akin to standing by a lawn sprinkler.

 

Been awhile since reading it, but an article on glue said that the glues used in ply actually bond with molecules of wood, thus making it one solid piece. Is glue strong enough to resist separation? Just try to break a glued up structure. Never breaks on the glue line. With all the other information supplied, I believe the combination of layering, grain direction and crossing and the binding of the glue all combine to resist separation. But add enough moisture, heat, cold, unprotected ply will eventually disintegrate, separate, rot.

 

excellent write up on plywood construction:

How plywood is made - material, making, used, processing, structure, product, industry, machine, Raw Materials, Plywood Classification and Grading, The Manufacturing Process of plywood

 

I remember the old Cold War nonsense in the schools... something about 'drop roll and cover' or something(?)...OK, when I say "I remember" I use the phrase very loosely. I think the gist of it was to hide under the school desk while everything for 25 miles around you was turned to glass.

 

duck and cover......



I took that as just govt. propaganda.. Remember ' one flash and your ash"....

 

AS Info: The thin Aircraft grade ply is still used on many home built aircraft. Especially on the leading edges of the wing or horizontal stabilizer. Many wood covered wings require a long scarf joint to join multiple pieces. Nice to see not so easy to produce. If you have need for some google Wicks aircraft supply or Aircraft Spruce. Warning: anything stamped aircraft grade is $$$$$$$.

 

Yes, Stick. See the title of my post.

While I was doing NBCD (Nuclear, Biological and Chemical Defense) training in the Navy similar instructions were given.

If you are at sea and see the flash, then you were within the hot zone....

I look at at is as, bend over, put your head between your knees and kiss you butt goodbye...LOL

Thank God, not many had to go through that.

Sorry for the hijack, Darryl.

 

Sorry for the hijack, Darryl.

Don't worry about it James. I think the original question has been answered and I am fascinated at what responses have been triggered. I also think it is Important that we understand our history or we will repeat it. We are now living in times where the possibility of wide spread conflict is close enough that everyone has at least thought about it. Your "one flash and you're gone" stories help us understand the reality of major conflict, particularly because they were triggered by something as mundane and ordinary as plywood.

Darryl

 

James, that reminded me of a Chemical plant I had to work at one day, many years ago. Before you could start working, you had to go through a short safety meeting. And you had to wear two different types of gas mask, on each hip. And during the safety meeting, they had you listen to a sample of 3 different sounding horns. "If you hear this horn, put the mask to your left on. If you hear this horn, put the mask to your right on. If you hear THIS horn, put your head between your legs and kiss your butt goodbye." Talk about my boss get a ear full when I got back!! ;o) Never had to go there again!! I told him if I ever had to go there again, you were going with me. LOL

 

James, that reminded me of a Chemical plant I had to work at one day, many years ago. Before you could start working, you had to go through a short safety meeting. And you had to wear two different types of gas mask, on each hip. And during the safety meeting, they had you listen to a sample of 3 different sounding horns. "If you hear this horn, put the mask to your left on. If you hear this horn, put the mask to your right on. If you hear THIS horn, put your head between your legs and kiss your butt goodbye."

That's pretty hard-core! The site I used to work on many years ago also had 3 alarms, but the safety procedures were a bit more basic. If the fire alarm went off, evacuate your building. If the toxic gas alarm went off, under no circumstances leave the building! (What would they have done if there were toxic gas and a fire? I don't know.) We used some fairly nasty stuff on both accounts - chlorine and phosgene were the main poison gas risks, and nitrotoluene was one of the big fire risks.

The third alarm on the site was a big old WW2 air raid siren that was supposed to warn the residents of the neighbouring town if a cloud of poison gas was heading their way. That alarm was tested once a year. I imagine that test could have caused a few heart attacks over the years, if any local residents hadn't received the letter warning them about it the week before!

 

Sometimes the hijacked threads are the most entertaining. Used to work in a building in which there were many plywood parts that had been made by Hughes as spares for the Spruce Goose. Hatches, in particular since there were so many to allow access to internal parts for inspection. Wish I'd thought to pick out a few to collect, probably worth something now. But even though they were something like 25 years old and were poorly stored, they were intact with no separation whatsoever.

 

I think like anything else new, it took plywood a while to gain acceptance. The earliest stuff I can remember didn't have voids, patches, or delaminations. They would have have had a tough time selling it I think if it had had any of those.

 

I think that change in quality is typical of any product that ends up being a mainstream product. Once it is accepted as a product that everyone wants, the next phase is to make it cheaper than competitors, which often leads to lower quality. In the IT industry, modems are going the same way. Don't know why, but 2 years is about the reliable life of a modem these days, but they are affordable. At least with plywood, it is graded so you have an idea of the quality you are getting.

Darryl