Using oblique dimensions on isometric drawing with unequal extension line lengths

[millsnA2XPT]

I am having some trouble creating an oblique dimension properly. We are working on Isometric views and place dimensions on the object. However, I cannot figure out how to get an aligned dimension to have uneven or unequal extension lines…  From what I understand, Aligned dimensions need to have equal extension lines – AND – to place a dimension for oblique editing, it needs to be an aligned dimension. What I want, is an aligned dimension that I can edit for oblique but has uneven extension lines…(as we can with linear dimensions). I have attached a picture of the sample and a picture of my drawing. I want there to be an “offset from origin” on the oblique dimension. I have been just working around it by placing another line in it's place and getting the offset 'close'. 

I want there to be an offset from origin at the bottom of the 1.94 dimension without having to add an extra line

[pendean]

Share your actual DWG file please.

[Kent1Cooper]

It still involves adding a Line, but you can have the gap at both extension lines, without needing to suppress one of them, and be precise about the gap size, this way:

Draw your Aligned Dimension [or a Linear/Vertical one in this arrangement, but that won't be an option in other isometric directions] from the bottom to the green dot instead of to the top of the green "N" shape [just as in your image, the Dimension is faked by going not to the bottom of the "N" edge but to a point within the bottom side, so that extension line needs to be suppressed to get the gap at the bottom].  The dot is only a location -- you don't need an actual object such as a Point there, if you can locate it without one.

Draw your added Line [red only for illustration purposes] from the top of the "N" to that dot.  With the OSOPTIONS System Variable set to something less than 4 so Osnap will see the ends of Dimension extension lines, MOVE the red Line from the dot to the near end of the extension line, and the gap at the other end will be exact.  [In the image, the red Line is shown in the Moved, not the initial, location.]

[dany_rochefort]

Personally, i achieve this by using Linear Dimensions rather than Aligned Dimensions. 

You simply need to properly manipulate your UCS orgin and UCS x/y axis' to tell autocad where in 3d space you want your Linear dimension to start. 

Like this :

[Kent1Cooper]

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@dany_rochefort wrote:

Personally, i achieve this by using Linear Dimensions rather than Aligned Dimensions. 

You simply need to properly manipulate your UCS orgin and UCS x/y axis' ....

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... which requires constructing the object in 3D and viewing it from an isometric direction, rather than drawing it in isometric [see the Topic heading].  @millsnA2XPT, is that an option?  Is this, for example, an isometric-drawing class assignment that a 3D construction would not fulfill?

EDIT:  Another difficulty of doing it in 3D is that everything is foreshortened in the viewing of it from an isometric direction, so plotting scale is inaccurate.  Here, the edge is 1 unit long, and the yellow Dimension is in model space with it and is correct.  But this is viewed in a Layout through a Viewport, and while that is at full scale, "on paper" the length of that edge is the red Dimension because of the foreshortening.  To have it be accurate as one would want in an isometric drawing, you would need to increase the scale of the Viewport [by some factor that could be arrived at by trigonometry].

[dany_rochefort]

Drafting an Isometric detail, to scale, or measuring off one requires an Isometric Scale Ruler that takes into account the 30 degree angles of foreshortning.  Although your isometric detail might be ''to scale'', generally a plotting scale is not attrituted to the isometric detail itself because nobody has an isometric scale ruler laying around these days.

Although you could easily make one in Autocad.  

[millsnA2XPT]

Yes, this would be a class assignment that I am teaching Isometric in 2D drafting. I am just having a hard time understanding why AutoCAD doesn't have this more achievable with a dimension line. My background is strongly Architecture and I just simply don't use isometric drawings for anything other than schematic views that don't typically require dimensions. I feel like this is a pretty basic method of drawing for mechanical drafting/engineering that has been around for ages, I remember doing this in hand drafting as a student ages ago. I was just looking for a more efficient way to teach students to accomplish this in Isometric 2D drawings. I certainly can continue with the way I have been doing it, just hoped there was a better way. I have found lots of resources online for the standard dimensions, but no one seems to be talking about the scenario I presented. Just one of the anomalies I suppose. Thank you for your time, it is much appreciated! If you happen to have any other ideas, I am always open to new ways of doing things.

[leeminardi]

@millsnA2XPT 

You state that you are “teaching Isometric in 2D drafting.”   What’s not clear is the goal of an isometric assignment for your students and where your need is for creating a dimensioning isometric drawing with AutoCAD. Will the students being using pencil and paper or CAD for the assignment?  If the former and you are using AutoCAD to create good looking dimensioned isometric drawings for an assignment book then I would suggest you create the objects in 3D and dimension them in 3D then use an isometric view to create the illustrations for your assignment book.  I do not think you should have a goal for your students to learn how to create 2D isometric drawings with dimension lines?   

As you note, isometric drawings have been around for ages.  Their construction follows a few simple-to-follow rules and were therefore popular for communicating the shape of mechanical parts and assemblies often in consort with orthogonal views. With the advent of CAD over 50 years ago and its 3D modeling capabilities the need to directly create 2D isometric drawings with a CAD system has become almost obsolete. Most drafters and engineers confronted with the need to create an isometric drawing would probably create a 3D CAD model and derive an isometric drawing from it.  It can be a real hassle to create an isometric drawing in 2D when dealing with issues such as that dimensions can only be measured in the direction of principal axes and the difficulty of creating an appropriate ellipse to represent a circle on a non-principal plane.

If you want to have your students learn isometric drawing principles I suggest having them use pencil and paper with the option of using isometric grid paper. If you really believe there is the need to learn how to use AutoCAD in a 2D mode to create isometric drawings I would not include adding dimension lines as part of the assignment. Personally, I do not think a first course with AutoCAD should include anything to to do with isometric drawing. I think its best to wait until you get to the 3D modeling portion of the course before introducing the creation of isometric drawings with AutoCAD.

A side note, to expand upon @Kent1Cooper comment, an isometric projection of a 3D model viewed at an isometric orientation will be scaled by about 0.816 (sqrt(2)/sqrt(3)) relative to the size of an isometric drawing.