3d rotate / isomatric doesn' work properly

Thanks. So it's not an error, just approximation matter...

It's a rounding of a value which is similar to an approximation "cheat" which was commonly used in the pre-computer days of iso drawing "on the board," and which was considered rather sub-standard even back then, because dedicated isometric ellipse templates were commonly available.

Back in those days, there was a distinction sometimes made between what was referred to as "isometric drawing" as opposed to "isometric projection." In a nutshell, "isometric drawing" referred to using enlarged ellipse templates (accurate templates labeled isometric) in order to use full-measure (not foreshortened) along the axes. Inversely, "isometric projection" referred to using down-scaled (foreshortened) measure along the axes in order to use ordinary drafting ellipse templates. That practice also was a rather inaccurate approximation; a foreshortening of 75% was commonly used, when the proper scale would actually round to 82%. One of the most common and most egregious error was committed when a beginning illustrator would forget and use ordinary 35° drafting ellipse templates with full measure along axes, resulting in dead-giveaway grossly disproportionate objects.

That kind of "cheat" was arguably forgivable in the days of drafting machines, plastic templates, and Rapidiograph pens. But I see no excuse for it in software.

I've built multiple interactive semi-automated solutions for my own use (Flash based, JavaScript, SVG) which have a stop at the isometric angle between 35 and 36 degrees. I see no reason 3D Effect (since its angle fields do accept values to at least two decimal places) couldn't do likewise. My guess is the assumption was that rounding to 35° is "close enough." But as you've discovered, it often isn't.

...best way to avoid it is just draw isometric (or rather dimetric) grid and align to it. Am I right?

You specified isometric, not dimetric. So I'm not sure why you're mentioning it here. You wouldn't draw a dimetric grid in order to draw in isometric. But, frankly, having done mechanical axonometric drawing since well before personal computers, I've never resorted to grids, and have very seldom seen professional mechanical illustrators use them, and even then only as an underlay when drawing freehand on a pad. I've only used them in software when the grids have some actual functionality. (For examples, see Corel Technical Designer or Serif DrawPlus or to a lesser degree, Serif Affinity.)

The whole point of isometric drawing is to enable the illustrator to efficiently draw "directly into" the mechanically-correct perspective (yes, it's a "perspective" view, just not a converging perspective) without having to resort to things like grids. Grids are fine for trivial things consisting mainly of boxy shapes conveniently aligned parallel to the axes and to each other (like early faux "3D" video games). But in mechanical illustration (parts catalogs, for example), that doesn't get you very far as soon as you encounter object edges which are not so conveniently aligned. That gets you into simple and compound rotations, and a grid would be of little help.

Once an illustrator really understands the principles of axonometric drawing, he doesn't need grids--again, unless they actually add some programmatic advantage.

JET

I see.

If you compare coordinates of the top and bottom points of your sides, you will notice that they are not in a straight vertical line.

The same is true for my method, but at least they have the same distortions.