I spend good money buying what I am assured is "good quality plywood ", so why, when I get it home, does it make me want to turn it into firewood?. That was the hard hitting comment from an associate of mine recently. He's not the only one who is disappointed in the quality of plywood ... thousands more just like him have the same comments, I hear it ALL THE TIME!!.
So, lets talk about plywood and why it is the way it is, how can we best use it and what are some alternatives. First of all there are two kinds of plywood, "sliced" and "rotary". These terms are used when talking about the face of the plywood. So if you were to purchase a three-quarter inch 4x8 sheet of "Oak Plywood", you would have the choice of purchasing either "sliced" or "rotary". So ... what's the difference other than price you ask ... whew, a LOT .......
Of the two kinds of plywoods "sliced" is the most expensive and the most attractive as it simulates boards of wood. Rotary plywood manufacture costs less than the sliced method and it looks more like "plywood", but here are the two methods for actually making plywood.
"Sliced" plywood is just as it says, slices of wood are sheared off with a giant knife, just like slicing cheese. These are then assembled on the face of the plywood core where they are glued and then sanded to make the finished sheet.
"Rotary" plywood on the other hand is made quite differently. A select log is mounted on what is essentially just a huge lathe. When the tree is spinning a high rate, a large blade digs into the tree deeper and deeper and at the same time a long flat vaneer is unfurreled off the blade. It is this veneer that is then glued to the plywood core, then sanded to make the finished sheet.
Now ... lets talk about the "core" for a minute. Most hardwood sheets of plywood use less expensive softwood cores on which to build the hardwood face of the plywood. Much of the problems with plywood are a result of these cores, and here's why. Some of the cores are not well made, they may have "voids" in them, or the quality of the wood they are made from may just be of poor quality. To compound these quality issues, some of the wood that is used to create these cores is dryer that other portions. Some of these veneer sheets within the plywood expand or contract depending on how wet they were when they were used to make the "cores", and what the atmospheric conditions are where they are stored.
When you purchase plywood that says its 3/4". According to the Plywood Manufactures Codes, it does not mean the plywood is exactly 3/4" ... it means the plywood will not EXEED 3/4", which means it is always somewhat less than the 3/4" ... sometimes by 1/16th" (or more).
This means that if you are sizing a project or cutting dados, rebates etc. It is more likely than not that the wood is going to be lose in these because if you followed the instructions of the dado blade and made it exactly 3/4" ... it will be too big for the plywood. Also, if your project has one 3/4" piece on either side and those sheets are actually 1/16th shy, your entire project will be 1/8th narrower ... see the problems this can create!!! AND, WHAT'S WORSE ... if you purchase two or three sheets of plywood, there is an excellent chance NONE of them will be exactly the same depth, in fact, there is a good chance none of the sheets will be consistently the same depth along each edge.
Like all other problems that woodworkers encounter, there are solutions.
Solution #1 - don't use plywood - I say this a bit "tongue-in-cheek" because we all know we MUST use plywood for many projects, but now we can be a bit more selective.
Solution #2 - re-manufacture the width of each edge of plywood you will use. Now I know this sounds tedious, but it's not that bad, and considering all the time you save by using plywood, you can afford to use up a bit of that time making it perfect. The re-sizing of an edge can take a few different methods, such as running all your edges vertically through your table saw to have every edge exactly the same.
Solution #3 - Use the Medium Density Fibre (MDF) and similar versions of hardwood sheets. Some of these come with hardwood vaneers laminated to their face sides, others are plain. Generally speaking, these sheets are flat ANS evenly thick because of their method of manufacture. Of course they have other challenges, like heavy weight, poor substance for using screws with and some will absorb water either from the air, or from "puddling", which makes them swell and become weak.
Well, that's the story of plywood ... love it or hate it, it remains a great product for many applications, but not necessarily the only product. Like anything you do in woodworking selecting materials is half the battle and knowing what to look for and what application you are working on will go a long way in helping you select the right materials.
Copyright - Colin Knecht