The best way to get a true and square frame is with end-joints like most modern extrusion-frame 3DPs use. Angle bracket are a waste of time; whether plates or machined the simply have no rigidity compared to a properly machined end-joint. They can be helpful in conjunction with; but they are no replacement for a properly made end-joint.
They are not a waste of time if you're not relying on them for rigidity. I never said they were the better solution for a floppy open-frame.
I will of course bow to your deep experience and direct knowledge of the Creality factory and practices.. and still maintain that at this price point expecting a properly square frame every time is optimistic at best.
That said, it's also best not to rely on cheap tools to square a frame. A lot of typical squares in DIY shops are anything but, tapes are easily fooled, and even a rule can rapidly mislead you.
Every time, no. MOST of the time, from CReality themselves, I'll say yes. That is the "minimum standard of quality" I was talking about. I don't think that CReality is GREAT quality; but what I've seen indicates they are better overall than your generally dismissive tone would suggest.
I don't have superior knowledge of their practices; only a lot of digging through numerous usergroups describing the problems CReality had with licensee vendors not maintaining QC and then pulling the plug altogether.
I DO have superior knowledge (compared to most) of the materials used; I know how they're supposed to be cut & assembled, and I know how easy it is for people who don't know/don't care to make a complete dog's breakfast of even a basic aluminum extrusion fab. I also know how easy it can be to do the job right if you spend the money on the right tools and a little time learning how to use them. There's a reason they call it a Build SYSTEM, after all.
That knowledge tells me that the first likeliest cause is as I suggested: either a damaged part, or one with burrs from the machining processes interfering with the proper fitment of the joints. Alternately, if a fabber is careless, they CAN bend the crap out of the extrusion as part of the machining process if they don't take the time to do prep correctly; this doesn't happen often if the correct tools and technique are used.
You're right, I don't know for a fact that not using the right tools is the cause here; but I'm confident enough in my surmise to proffer it as VERY probable fact. Hoofbeats, think horses not zebras, etc. That is my PROFESSIONAL opinion, as someone who has done 8020 for a living, and done it well, and had to clean up other people's 8020 build messes more times than I care to count.
Agreed, a cheap carpenter's square is not going to cut the mustard here. Well, it COULD, but you'd have to validate it against a known GOOD quality square, preferably of the machinist variety.
The plastic rollers these printers use are very forgiving; they can eat more than a mm of runout over the lengths we're dealing with here. That does, however, mean that the finished moving assembly is NOT going to be as precise as a machine that uses the extrusion as a frame for precision movement hardware.
xrunner has the correct attitude; start with the decent foundation the E3 gives you, learn on it, then when you know enough about what you're doing, upgrade to precision linear slides/rods & bearings.
It's too bad so much time has passed; I'm pretty sure that if HobGoblyn had detected the warped parts he's talking about during the original build, the vendor would have made it right. I know CReality would have.
mnem