Loft preview does not match final loft.

[jason.znack]

Can anyone explain why the loft preview looks one way, but the final loft looks completely different? I know someone will tell me to use rails to define the shape. Ok, but why does the preview look one way, and the result another? What's the point of the preview then? Why should I use rails when the preview does what I want already? Just give me that.

[jason.znack]

https://knowledge.autodesk.com/community/screencast/812fbc04-a937-4551-a3eb-1b33785ded38

It wouldn't let me embed it in the first post for some reason.

Here's a link to a screen cast showing what I mean.

[jhackney1972]

You might get better results if you use the Section options.  Attach your model and let the Forum users take a look.  If you do not know how to attach your Fusion 360 model follow these easy steps. Open the model in Fusion 360, select the File menu, then Export and save as a F3D or F3Z file to your hard drive. Then use the Attachments section, of a forum post, to attach it.

[jason.znack]

Sure, here's the file. Thanks for your time.

[TheCADWhisperer]

I think that I would model very differently from earlier on.

Notice in Loft1 that your map points are not symmetrical.

... same issue with Loft2.

Loft3 (and several other features) I would probably have done with Shell (at the appropriate time).

I would probably have 8 divisions on this circle (and only one circle - see Shell comment).

[TrippyLighting]

Something that would help not only in tis design, but would help in a lot of other designs is better defaults for the loft.

Many users are not aware of what the purpose of the "keep tangent edges" option is, and unfortunately that is NOT  the default.

I changed that in this design and as expected the sketch projected from the top face went invalid.

Then fixing it with "Manage lost projections" fails to work, because the original edge was one continuous spline, but now it's 8 segments, but you can only pick one.

I re-projected and re-created the loft, also with "Tangent edges" set to "Keep" and then the loft completes without a problem. 

[jeff_strater]

@jason.znack - definitely listen to all the good advice here, from @TrippyLighting, @TheCADWhisperer, and  @jhackney1972 .  That said, though, your original issue is, indeed a bug.  The preview should always match the final result.  The details are related to "point mapping".  When you have profiles with different numbers of segments, Loft has to figure out how to map from points on each profile.  Somehow, in the commit phase, this point mapping is getting lost.  I will create a bug for this, so we can fix it.  Thank you for sharing the model, that is always very appreciated, and saves us lots of time to investigate issues.

[edit] the Fusion bug is FUS-106173

[jason.znack]

Thanks for all the helpful tips and suggestions. When I go in to do final modeling of this I will take them into account. The model as it currently stands is just pre-prototype playing around. I was mainly just struck by how the preview and the final result were so different. Glad to hear it will be looked into.