Dimensions randomly changing

[bruce13557]

When I am dimensioning, primarily in paper space, I select the 2 end points of the item  to be dimensioned, the dimension text shows the correct dimension. 
Often when I place the dimension line one of the end points will jump to some other position. Often it is to the other end point showing a dimension of zero although it frequently jumps to some random spot which does not even have any geometry associated with it.
It has even happened when I have saved and reopened a drawing resulting in incorrect dimensioning on a fabrication drawing.
Has anyone else had this happen?
Does anyone know how to fix it. 

[ReMark]

Are you using OSNAPS?

[bruce13557]

Yes I am. Mostly endpoint, midpoint, and centre.

[CyberAngel]

Check to see if you're using Associative Dimensioning. In the Options window, it's under the User Preferences tab. This setting ties the dimension ends to the entity(s) it's measuring, so if they move, the dimension moves with it. Sometimes the dimension doesn't get associated with the entity you wanted or expected, which would explain your issue.

 

If you depend on associative dimensioning, that is, you adjust your dimensions by adjusting your entity(s), you may need to find another solution.  Take a look at this page: Bad Dimension.

[ammobake]

If you're dimensioning something that isn't "flat" in the autocad sense, and you're doing it in a plan view, The dimension will appear snapped to the ends of your object when it is actually perpendicular to the desired snap point in the Z dimension (because endpoint and perpendicular snaps look basically identical). 

 

In this case the dimensions won't become associative with the object because they aren't actually attached in any way.  This tends to give dimensions a mind of their own.

Normally for fabrication drawings I would draw the object in 3D and set up paperspace with the viewports needed to show a true isometric projection for the part.

Personally, from there I would just put the dimensions, text and callouts in paperspace to keep things as simple as possible.

 

At that point all your linear dimensions will snap to the endpoints with no problem since everything on the viewport is on the same plane at 0 elevation.

Basically, the model would be the only thing in model space with all annotations and everything else in paperspace, if that makes sense.

 

Probably the easiest way to do it.

 

Since the object is the only object in model space, this has the added benefit of being able to use that data and export it to a different format if needed for like CNC machining - which I am unfamiliar with.

But I did work in a metal shop for a while that used DXF shape files for the plasma cutter.  But that's just all 2D data.

 

ChriS

Edited June 13, 2019 by ammobake

[bruce13557]

I have attached a series of screen shots of the issue.
First is choosing the first dimension point
Second is choosing the second dimension point.
Third is positioning the dimension line.
Fourth is the dimension when I select a point for the dimension line.

[SLW210]

Looks to me you are using PERPINDICULAR Snap. You need to stick to snapping dimensions to real locations, i.e. END, MID, CENTER

Can you post the drawing?

[BIGAL]

Using Dim Ver will do what you want as it implys pick two vertical points.

[bruce13557]

SLW210 That may be the problem I will check it out next time I am dimensioning. I had a quick look and it may work.

BIGAL I am using Dim Ver 

Thanks

[ammobake]

Linear dimensions always draw "FLAT" in the sense that if the original snap point is 0,0,20' the next available snap point is perpendicular to that but won't give you the option for an endpoint if there isn't one at 20' in the Z dimension.  Perpendicular would show up as the only option.

 

So another possible solution is to add linear dimensions with a 3D perspective so you know the first snap point for the linear dimension is exactly where you intend.

 

There are some pretty handy LISP routines for snapping which we use here at my work.

So instead of using OSNAP and keeping it on we strictly keep it off and use shortcuts for snapping instead.

 

So for like endpoints is F2, intersection, is F3, etc..

That way the snap point is always the intended target point.

 

-ChriS