Kent Elrod has created a visual guide to dimensioning 3D objects in AutoCAD’s model space environment.
"People new to 3D in AutoCAD often have a hard time adding dimensions to the model in model space; the dimensions either end up on the wrong plane or the dimtext is not shown in the right direction. I have developed a PDF cheat sheet that covers the basics for getting the dimensions placed for 3D modeling correctly every time.
"First, set an isometric view and use the UCS (user coordinate system) command with the '3 Point' option (Command line, toolbar, selecting an endpoint on the plane desired at the lower left of the eventual dimension text). The second pick should be in the direction of the dimension line, and the third pick will be aligned with the extension line. The idea is that the pick in the x-direction would underline the dimension text, and the y direction would be up to the top of the dimension text (refer to the PDF file). The UCS will be moved and the z-axis will be pointing toward the user, which means the text will read correctly for this isometric view.
"This method is most useful when you want the dimensions to show in isometric views of the model. If the model is shown in a layout in an ortho view(s), then dimensioning in paper space is usually easier."
Notes from Cadalyst Tip Patrol: We can create models all day long but for the majority of design industries, a 2D document is needed for construction. Dimensions are a very important part of those design documents. Many users dimension their drawings in model space, especially if what they have is a 2D model. That’s easy enough to do. It is much more difficult to dimension a 3D object in a 3D environment; typically, I would recommend that you dimension your 3D objects in paper space. Simply create a viewport that displays the view you want and dimension it in two dimensions. However, if the need to dimension your 3D model in model space arises, this is a great visual guide to assist you.