Perfect Miters?

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wrote:

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
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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
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"Ellestad" wrote in message news:

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 ...
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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.
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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

...
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"Ellestad" wrote in message

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.
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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

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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 snipped-for-privacy@ts-aligner.com
http://www.ts-aligner.com Home of the TS-Aligner
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On numerous occasions I've noticed that my DeWalt 708 (slider) tends to pull the stock ever so slightly as the blade makes it way through the cut. Or, maybe it's due to the Jessada blade tooth configuration (30 deg ATB). It could also be the casings themselves. If they are off-the-rack, store-bought millwork then you can NEVER count on one piece being exactly the same dimension (on any plane) as the next.
--
NuWave Dave in Houston



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Dave in Houston wrote:
<<If they are off-the-rack, store-bought millwork then you can NEVER count on one piece being exactly the same dimension (on any plane) as the next. >>
I bought them at a millwork shop, but yes, I've noticed that different batches seemed slightly "off" from the pieces I had.
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Miters can lose their tight fit if you wait around to glue them.
Most chop saws that I have looked at don't hold "picture frame" miter accuracy when the table is clamped. The near side of the table climbs (squirms up) when the clamp is tightened taking the table surface out of square with the fence. That takes the cut out of glue-up precision. You can still frame a garage with it but . . . My old Porter-Cable stays pretty true and likewise the Delta version. I've seen some awful examples of this, though, with Bosch and DeWalt.
The sliding miter saws son't necessarily cut so true, either. Ther're good for joist cutting, though.
Tim

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Ellestad wrote:
<<Most chop saws that I have looked at don't hold "picture frame" miter accuracy when the table is clamped.>>
With great foresight (i.e., laziness), my saw isn't clamped or bolted. :-)
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Yes, I myself exhibit this kind of sloth quite often. :-)

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Maybe I'm not understanding. Is the 45 accurate and it is just not meeting on the finish surface? Most carpenters set a pencil or nail under the trim so that there is a slight back cut on the miter to allow the front face to meet.
If the 45 is not making, you might glue a strip of sand paper to the back splash and/or table of your miter saw to prevent creep.
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DanG wrote:
<<Maybe I'm not understanding. Is the 45 accurate and it is just not meeting on the finish surface? Most carpenters set a pencil or nail under the trim so that there is a slight back cut on the miter to allow the front face to meet.>>
You got it. But it was only on the thick part of the trim. Now it's perfect since I changed the cutting technique. However, I was wondering how to do the "back cut". Nice to know how professionals are doing it.
Thanks
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snipped-for-privacy@boundvortex.com says...

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.
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