Perfect Miters?

[Lew Hodgett]

MIne works quite well.

I don't; however, I do trust the automation used to trim 4x8 sheets to size since it is not a manual labor job these days.

Lew

[Ferd Farkel]

As many subfloors as I've run, I can't say I've ever seen gaps at end joints where the plywood was off square.

[Chris Friesen]

The factory edges are not cut by some guy with a circular saw.

The whole process is automated using something called a "DD Saw". Among other reasons, plywood right out of the press is still pretty hot.

Chris

[Leuf]

Oh come on Ed. You make it sound like there's some kid with a skill saw that makes the plywood at the factory. The first time that plywood gets touched by a person is when you buy it. If it's a typical hobbiest shop and their square says the factory corner is off then my money is on the $10 square from the Borg being off not the ply. I do double check when it's something critical like a miter sled though.

-Leuf

[Ed Bennett]

Yes, yes, yes, I know. I appreciate all of the comments about guys with skill saws vs the "perfection" of automation, etc.

I've seen how plywood is made. The sheets are gang ripped to width and then cross cut while in motion. The feed rate of the cross cut saw (traveling at an angle) in conjunction with the feed rate of the board determines the squareness of the cut. The factory is staffed with unskilled workers. The person overseeing the crosscutting may never have even touched a circular saw in their entire life. I'm quite sure that they care more about length and width than they do about squareness. If they track these measurements as process parameters (a big IF), then these are likely to be checked on a random audit basis with simple instruments (i.e. tape measure) by more of the same unskilled minimum wage workers.

A long time ago I heard this same folklore and tried to apply it in my woodworking. "Use a factory edge as a straight edge, use a factory corner as a square, etc." I probably read it in a magazine article written by some journalist who (while pretending to be an expert) had never touched a circular saw in his life. That's when I learned not to trust the factory edges (even on furniture grade plywood). They're good enough for construction purposes (sheathing, flooring, roofing, etc.). And, there are probably a lot of people who will never notice a problem in their own work. But, they are far from "perfect" (what Lew originally said) and don't suit my needs. I also learned not to take technical adivce from journalists - a lesson that has been reinforced countless times over the years.

The original poster has related that his problem was being caused by deflection of the blade when making thin cuts. I could be wrong, but I suspect that the demands of his work exceed the accuracy of factory cut edges on plywood. I'm glad he was able to find his solution in one of my articles.

Ed Bennett [e-mail]

of the TS-Aligner

[Ed Bennett]

Your sled? I'm sure it suits your needs.

That would be another topic for the "Myths and Folklore" page. Just because its automated doesn't mean its perfect or accurate. I run a shop full of automated machines and I assure you that accurate results still depend on the skill and knowledge of the people using them. I've hired people to run the exact same programs on the exact same machines with the exact same fixturing and they still manage to screw up the parts. Heck, I've even screwed up a bunch of parts over the years. The machines used to trim plywood to size may be automated but "perfect" accuracy is not automatic.

Ed Bennett [e-mail]

of the TS-Aligner

[Ed Bennett]

I haven't done so many subfloors (two or three) but I've noticed the gaps in each one. Not that it matters, it's just a subfloor. I thought we were talking about woodworking with slightly higher standards.

Ed Bennett [e-mail]

of the TS-Aligner

[Ellestad]

In about forty years of doing this I don't think I've ever found a sheet of plywood of any kind that was reference square - you're not going to cut good cabinet quality joints basing them on the origingal corners. No supplier that I have ever dealt with would ever even suggest that their plywood of any type was "joint square". I find that often the sawn edges are even wavy. The automated sawing of 4x8's at the factory isn't concerned with making the edges and corners to joint standards - it is to simply get the sawing done very fast and cheap!

I agree that you might have problems with cheap squares and straight-edges. Every time I see a magazine or book showing someone laying out cabinetry cuts with a framing or rafter square I immediately know that the author either didn't know what he was talking about or he was dishonest (most framing squares need some corner peening to reasonably square them up). I have a fair amount of money in my squares and rules.

Tim

[Swingman]

Hmmm ... just walked out to the shop and the first three sheets I checked had all four factory corners that were "Starret square".

In my 45 years of doing this I don't find that unusual/remarkable at all ... but then I build _lots_ of cabinets out of quality hardwood plywood.

Go figure ...

[Peter Huebner]

Lots of 'interesting' forces brought to bear on the wood and the saw(blade) when cutting mitres against a fence. I've seen those slightly rounded cuts, distortion through my RAS climbing on to the wood during the cut or being forced off its intended direction by grain. Similar effect through timber climbing up on a mitred table saw blade. (I don't have a mitre saw myself, but these forces will apply there, too).

Apart from the obvious - adjustment as close as possible - I have developed the technique that if I cut to within 3/4mm of size first and then do a cleanup cut to a knife-marked line I get very good results because these forces are largely reduced or nearly eliminated. I have also found that the cuts made at the >90 degree side in this fashion seem to be slightly superior on my RAS.

Meanwhile, I am avidly reading what others have to say on the subject ;-)

-P.

[Peter Huebner]

Hmmm. We reclad our shearing shed with construction grade ply a couple of years back. Some of those sheets were *considerably* off square. Nearly 1/4", some of them. Didn't matter because we put battens over the joins, but .... ;-)

As an aside: I laid parquet flooring in my old folks' house a few years ago, and found that even a lot of the 12" sq. prefabricated tiles were out of square! Had to recut many on the RAS to eliminate gaps. Yeesh. Not sure to this day how I managed to eliminate pattern creep between the tiles over several runs, I guess it must've evened out.

-P.

[Mike O.]

We find a few sheets of 3/4" hardwood veneer core that are not square on occasion. It does seem to be related to certain vendors which I think is related to the quality of their products. Since most of the time our builders do the purchasing we try to "steer" them to better quality material which normally means a different vendor.

Mike O.

[Ellestad]

It must be my back luck as to typical supplies available in my area then. I know that my experience is common with most of the collegues that I have mentioned this with over the years in my area. I do find that the short base of a machinist square is too small to reveal many of the problem pieces that I've encountered (in fact I had a problem with this on some Baltic Birch two weeks ago - and its getting to be a challenge for my old gray-beard mind). A

24 inch blade helps but . . . I've found sheets with square corners that were'nt square to each other due to swoops in between. Years ago a couple friends who worked for the Forest Products Laboratory told me that there was no particular concern on the part of industry to deliver clean, square-dressed product assuming that typical handling may likely ding the edges a little anyway.

Certainly, though, as per your experience I would presume that some manufacturers, suppliers and yards do better than others. When I bought the Baltic Birch that I mentioned I asked the kid filling the order if we could find a few sheets that were "reasonably flat" with a good straight edge ot two. He said something to the effect of, "We'll see what we've got but it is what it is, you know." This yard supplies a lot of the area cabinet shops that aren't big enough to get supplier-direct purchasing.

I envy your resource.

Tim

[Swingman]

Granted, that's an important point. However, the underlying point that Lew was making with his "factory plywood corner" suggestion is valid, IME.

One of the best "miter jig" concepts for making frames is the one using a factory corner from a sheet of plywood and the concept of complementary angles to alternate the cutting of adjacent parts to insure 90 degree angle joins.

It is not that difficult to find a corner _square enough for woodworking purposes_ on a sheet of quality plywood these days, and for miter jig purposes you generally need a relatively short distance, say 8 - 12 " on each axis, so the square you use on your actual woodworking projects is going to be your limiting factor in any event.

That said, I agree that you certainly want to measure with the best available tool, and not just take it on faith, that a factory plywood corner is square enough for your purposes.

[Ellestad]

Yes, no arguement. I guess that my main point was that I haven't had the good fortune to find all that many good corners on the product that I've had available here - even on supposed high-grade hardwood plywood (although the price would imply - no pun - that the corners should be pretty good). One of my good friends was a furniture maker and he ran a hardwood operation along with his shop. He carried high grade hardwood plywood and Baltic Birch and I purchased wood from him from time to time. He never felt that his supplies would predictably have reliable corners and his stuff was generally better than I would get from the main dealer in my area. I really think from your experience that quality standards may vary depending on product chanels in your area. As I said before, I really do envy anyone who can count on the corners and edges of their raw materials to be square and true.

Tim

[Ed Bennett]

Yes, I'm sure that the quality of the edges and the accuracy of the corners varies considerably. Not just from sheet to sheet but it's not unreasonable to believe that some facilities produce better quality than others. Maybe one factory has an OCD guy like me who checks the machinery at the beginning of every shift. Maybe another factory doesn't bother with it unless it breaks down (or needs a blade change). I strive to make recommendations that will work for anyone who applies them correctly. So, I just can't recommend trusting the accuracy of edges and corners of plywood sheets. If you can verify that the corner is square, then it certainly would work well on a sled.

Ed Bennett [e-mail]

of the TS-Aligner