Brian McK,
I think it is accurate to equate choosing or setting a UCS orientation or position with establishing a construction plane in other software. I had meant to mention this in the video, and was considering a followup. Effectively the xy plane through the 0 z coordinate is the current construction plane, wherever the origin of the ucs is established. Unless a positive or negative z coordinate is specificed, all 1d (lines) and 2d (curves, rectangles, anything 2d defined by at least 3 points) entities will fall in the xy plane. 1d and 2d entities
can be drawn in planes that are parallel to the xy plane without moving the ucs origin point, but the z coordinate must be specified either by keystroke or snapping to a point with a non-0 z coordinate. This is why I'm most often really interested in which way the z axis points, rather than the origin point or specifically which way the x and y axes point.
If you want to draw on a plane that isn't parallel to the current ucs xy plane, then you must re-orient the ucs, or switch to a different saved ucs. You can after all define and save any ucs in any orientation or position you like, and switch to it at will. What's interesting is that most often I really don't care where the ucs origin is, but rather only which way it is oriented. I have a tendency to draw wherever I like, or off of center lines, but if necessary use object snaps to put things where they need to be. Explicit positioning of the ucs origin is only truly relevant, to me anyway, if one prefers to establish points using absolute coordinates. I can't think of the last time I did that, so it isn't terribly relevant to me at all
. In every instance I can imagine where I move, copy, stretch, slice, extrude, rotate, or otherwise manipulate an entity, I am doing so relative to the entity's beginning position, usually by entering a distance or by utilizing object snap points.
One can select faces or edges of a 3d model to establish the ucs. The toolbar is always an option, or it can be done from the command line with UCS-->Face-->select face. And then there is a Dynamic UCS capability wherein when enabled the ucs will automatically temporarily orient to local entities if you hover over them. I don't care for this functionality, as invariably in my experience the ucs changes to something that doesn't suit my purpose. Dynamic UCS can be toggled on and off, but I just turn it off permanently. Your mileage may vary.
Brian oldavendale,
Think of the ucs as a piece of paper on your desktop, on which you draw something like a circle. The desktop is equivalent to the xy plane in a ucs. Now hang a piece of paper on the wall and draw on that. The wall is now the equivalent of the xy plane in the ucs, but it is clearly not the same as the plane that is the desktop. The xy plane in any ucs, no matter it's orientation or origin point, is effectively the "surface" that all 2d entities are created on in autocad (excepting the note above about planes parallel to the xy plane), akin to writing on a tablet on the desktop or painting on a canvas sitting on an easel. There can only be one ucs in effect at any given time, but you can switch between any ucs as needed.
Just to recap, 1d objects like lines are independent of ucs orientation. 2d objects like curves or the base of 3d objects are created in the xy plane or a plane parallel to the xy plane. 3d volumes like boxes or cylinders will have their base geometry in the xy or parallel plane, with a height specified in the z direction. If you want to change the orientation of 2d or 3d objects when they are created, you have to first change the ucs to the proper orientation.
Jim