Lofting error rails cannot be tangent to profiles

[spamtemp22]

I need to create a complex wedge shape that looks something like this:

 The problem appears to be that Rail 2 pushes the shape through Profile 2

To keep things simple Rail 2 has had a number of details removed. It was through the process of trouble shooting I created this drawing.  Regardless of the level of detail the problem remains the same. In the larger part Profiles 1 and 2 interface with other parts of the drawing and can't be changed. The loft needs to use rails to connect them in a smooth, organic way.  I'm stuck, what is the best work flow to get the shape I need? 

 

[JeanFlower]

Hi spamtemp22,

It's possible to build the loft you want by helping Fusion along with some additional human inputs. 

First I suggest you use the "Keep" option in the loft dialog, to "keep tangent edges".  In this video I select the two profiles, ensure "Keep" is checked, and select the two rails.  I can see that the loft is somehow "folded over" near the shorter rail and I can "unfold" it by sliding a blue dot along the profile.  This kind of control over the loft shape is powerful when the loft tool makes a poor choice of how to "map" the shape from one profile curve to the next.

By using the "Keep" option when your profiles have sequences of tangent curves, you are able to slide these endpoints along the profiles to adjust the loft shape.  I hope that helps you get to the loft you want,

Jean


Jean

[JeanFlower]

It's unusual for the loft tool to make bad decisions about how to connect up profiles, so I took a closer look at what might be the underlying issue making things difficult.

There's a small gap between the end of the sorter rail and one of the vertices of Sketch15.  It doesn't look as though the gap is intentional, and if the points were lined up exactly the loft would be more symmetrical and balanced.

So I tried editing Sketch15 and constraining the point that is close to the YZ plane, so that it lies exactly on that plane.  It's a small nudge to the sketch geometry but after that change, the loft completes without any extra adjustment to the "mapping" between the profiles.

Making the sketches constrained, and snapping together geometry that is nearly in the same place, will make the downstream geometry tools work more robustly with better quality results,

Jean

[JeanFlower]

Some additional tools to help with debugging tricky lofts - there's an Analysis pane in the loft dialog which can be helpful when lofts seem twisted or folded. 

Starting with your file with the misaligned vertex positions, so the loft needs mapping adjustment.  Make sure the Analysis visibility is on in the browser, then select the "isocurves" option in the Loft Analysis dialog.  These isocurves help to visualise the effect of dragging mapping points along profiles.