Is there a definitive explanation as to why plywood varies so much in thickness? Is it moisture which causes it to expand? Once, a fellow tried to convince me that the sheets at the bottom of the stack are thinner because it is under compression from the rest of the stack.
Chris @ www.flairwoodwork.spaces.live.com
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Production costs! As in rejects from specifications. Widen them specifications and you have more product at a lower cost?
You're alsolutely right. Production costs. But it is a matter of getting buyers (or building code specifiers) to widen specifications, even though the manufacturing is done to very tight specifications to save material costs. (If if was just wide manufacturing specifications there would be about as many over sized sheets as undersized.) How many oversized sheets of ply has anyone seen in recent years? Any? And how many of those examples come from ply that has first been soaked or otherwise subjected to abnormally high humidity levels.
I don't know the reason behind it, but most "3/4" inch ply is actually either 18 or 19 MM, and quite often once it goes out the factory door, those in charge of distribution pay little attention to which is which. I've had custom orders of sequentially numbered bookmatched ply arrive, randomly laminated on 18 and 19 MM cores. Boy, can that mess up some cabinetwork ! Also some veneered stock is stroke-sanded, and tends to be a little thinner at the edges than the center. Then there's just sloppy work.
Chris ,
It really may depend on exactly what type and grades of plywood you use .
It seems I was told that plywood needs to be not more then the sold as size any thing thicker would in most cases render the panels as rejects , so they error on the thin side as opposed to too thick to fit .
I have noticed in the more expensive plywood the closer to true measurements they tend to be . So most Maple and Birch 3/4" shop plywood I get is on the thin side .
1/2" Apple ply is 1/2" at $75.00 a sheet wholesale it ought to be a full 1/2".
1/4" plywood and veneered panels really vary in thickness . When I need it I ask for true 1/4" as opposed to the normal 5.2mm that is so common with import products as well as domestics .
Not sure about any expansion you speak of . I have toured a Columbia products plywood mill and I will tell you this , each unit is laid up then slid into a press about the size of a large van . The plywood and glue is cured with some heat and a whole lot of pressure applied to the entire unit at the same time so I don't think the sheets on the bottom shrink from the weight of the others .
I suppose it is not un like a 2X4 dosen't measure up any more ?
As always the bottom line is all about dollars 1/32nd of an inch over 32 square feet times thousands of units per month or year adds up to a lot of core dollars saved .
dusty
Edited 10/6/2008 9:43 am ET by oldusty
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