I have noticed on many occasions that when using the include command while ordered modeling in part or assembly mode (ex. Sketching for frames or sketching in part) that the included geometry (single wireframe-wireframe chain-tangent wireframe etc..) result is often underconstrained. (maintain assocativity checked) especially concerning end points on line and arcs face edges ETC.
I have to attempt to finish constraining the desired included geometry with sketch relations.... anything that will work basically. There are occasions when I am unable to fully constrain the included geometry making that geometry and anything that is built from it parametrically unstable down the line.
Why does the include command yield so many unconstrained results? Can it be improved thru a fix?
2) What are the best practices to insure that down line updates don't break when using include geometry?
Please see attached screen shot
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When geometry is included, that geometry is associatively linked such that arcs are concentric and of equal radii, and lines are colinear. As you have discovered, end points are not constrained and the overall length of the element can easily change.
Use included geometry to locate edges only and then constrain it as usual to make it robust. Also be aware that when including geometry, the selection options can impact how the included geometry is associated to the parent geometry. For instance,
Single Wireframe - easy enought to understand
Wireframe chain - will try and maintain a chain
Tangent Wirefram - will try and maintain a tangent chain
Single face - will maintain all edges on a face
Tangent Face Cain - will maintain tangent connected edges on a face
Loop - not sure about this one but I think it is similar to the Single Face except you pick the loop you want
To try and give you a physical example, think about if you put a part down on a table, and then butted a 123 block up against its side. Looking top down at the part, I'm calling the side of the 123 block touching the part your included edge. This locks movement in the direction perpendicular to the 123 block edge. To lock movement in the other direction, (keep the part from sliding left and right along the block), we must physically constrain the part/relate it to a second edge, which is perpendicular to the first. (Corner the part, essentially.) In CAD world, this means you must also include enough geometry for a corner, or 'hard' point somewhere, if you want to control movement of included geometry in all directions, not just perpendicular to the line.
You're seeing problems with included elements shifting on geometry that only locks one axis, degree of freedom..... however you want to term this. This is easier to understand if you've got some inspection background and have worked with Datums before and understand what geometric elements constrain what degrees of freedom.
Hope that helps.