In the last post we discussed sketched features that rely on projected geometry from features that are not orthogonal to the sketch plane.  Similar errors occur when the face of an object’s angular relationship merger with another.  Kind of like collapsing bleachers.  The results vary depending on how clean the application, and how careful the planning.

What I was doing

The intent was to create a pocket whose angular relationship to the surrounding features is parametrically controlled.  The remaining control is left to dimensional and geometric constraints tied to projected geometry, in order to keep the pocket location consistent in all cases.  This will allow different variations of the same geometry to be planned for production.

The Problem

When the feature merges a face into another, the projected geometry loses touch.  Rather than have 2 lines of projection overlapping, we get 1 line.  The projection that was being constrained is no longer there.

 image

In the example above, you may notice the the pink error at right and its respective origin as shown at left.  That face merged into the bottom projection.  inventor has no idea where the line went.  Fortunately, nothing was tied to this projection, so the model responds as intended.  The key here is that nothing was tied to the failure.

Here is another example:

image Notice in this scenario the dimensional constraints were accomplished, but the geometric constraint at the top was lost.

Once the projections are lost, the following error occurs:

image

When the face emerges as the parameters change, the application knows how many features were projected, the application tries to hold the projected geometry to it’s origins.  This is evident in the example below.

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In This scenario, the circled point passes off the end of the part, and loses sight of its projection.  The application ties it to the projected intersection at the bottom of the part.  As the face emerges again in the example, note how the yellow projection is re-created.  The origin at the top remains intact, but the projection has lost sight of it’s origin (the circled point) and has rooted itself at the bottom of the part.  Again here, nothing was tied to the failure, so the design emerges properly.

image

Here is the result of poor planning and improper use of projected geometry, and the result at the right.  The entire feature fails.

The Solution

As in the last article, the solutions are the same:

  • Cut Edges
  • Surfaces
  • Projecting Sketch Geometry
  • Work Features

While good planning in the first examples allowed the feature to emerge properly, there were so many instances that failed at different points, depending on how the feature relationships were approached.  Eventually, these failures will get you.

Something else worth mentioning is keeping your sketches as simple as possible.  It’s nice to get a small handful of complete sketches to share throughout the design, but selecting those profiles gets to be counter productive.  When altering parameters during the design process, the possibilities of feature failure rises. When the feature is created from profiles in a very complex sketch, then as parameters change, Inventor often loses sight of which curves comprise the correct profile.  You can’t keep redirecting after a parameter change.  You’ll be looking for another job.

I ended up tying the outside corner to a cut edge of an exterior surface.  This worked flawlessly.  The bottom edge was tied to the projected Work Axis that started it all in the first place. Any further projections were completed from the sketched geometry to other Sketch Planes. The vertical flute wall was then tied to the projected sketch geometry, and moves with the adjusting pocket.

While these recommendations seem to disregard the intended workflow of Inventor, the fact is that when features converge, the results are lost loops. 

Plan your designs to avoid this issue, not suffer through them.