Complex loft with flat surface in the middle

[mattiasAYUHJ]

I'm trying to create a smooth soft transition (loft) between a 52 mm circle in bottom and a more complex shape at the top (marked light blue), 10 mm above. In the middle (5 mm up) there needs to be a completely flat circular surface 14 mm in diameter (marked darker blue). I have tried using 3 guide rails at the outer edges and a fourth guide rail tracing the needed outline shape from bottom, to flat surface and to top but it doesn't create a completely flat surface as seen in the pictures and there are strange curves all over making it not a very smooth transition. What to do?

Side viewTop viewNOT flat surface createdVery strange curvature

[davebYYPCU]

Try a surface Patch / Mirror on the yellow outline, but the geometry will more difficult with the flat disc, as you know.

Depends on how smooth, 

Might help....

[laughingcreek]

if all the transitions need to be smooth, you will need to do this i multiple loft operations.

creating a loft that has flat sections in it, it is frequently best to define the flat sections first and loft to it.

is it necessary for transitions coming off the flat area to be smooth? can you just extrude a flat into it after lofting? 

attaching a file (export as .f3d) will probably yield more help from forum users.

[TrippyLighting]

The problem here is not necessarily the loft.

Can you sketch with pencil/paper  what the resulting object is supposed to look like? 

Also, please attach your file so we can have a go without having to make undue assumptions.

[mattiasAYUHJ]

Thanks, this does actually work, BUT instead of a smooth circular shape where the two "mirrors" meet in the middle I get a more pointed shape and it would also be nice with a more smoother transition to the circular flat surface. Is there any way to accomplish that?

[davebYYPCU]

Yes, for the two centre lines, create a small surface extrude, parallel to the disc, patch half of the disc.  In the patching for the surface you need, use these body edges, and not any sketch articles.  You will be able to call tangent to these bodies.

Might help...

[TrippyLighting]

Is there a reason you cannot share your model?

[mattiasAYUHJ]

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@davebYYPCU wrote:

Yes, for the two centre lines, create a small surface extrude, parallel to the disc, patch half of the disc.  In the patching for the surface you need, use these body edges, and not any sketch articles.  You will be able to call tangent to these bodies.

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I don't really understand the process you describe. Can you please show and describe in more detail? I have attached a f3d with the sketches used.

[mattiasAYUHJ]

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@TrippyLighting wrote:

The problem here is not necessarily the loft.

Can you sketch with pencil/paper  what the resulting object is supposed to look like? 

Also, please attach your file so we can have a go without having to make undue assumptions.

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It should look like this image below BUT the whole area inside the blue circle should be completely flat (the edges are not flat right now) and there should be a soft smooth transition to the flat surface. I have attached the f3d with sketches used.

[mattiasAYUHJ]

Can you please decribe in more detail exactly what you mean? I tried dividing this in two separate loft operations, creating two bodies but the resulting bodies didn't align/combine very well. Yes all transitions needs to be smooth. I have attached the f3d with sketches.

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@laughingcreek wrote:

if all the transitions need to be smooth, you will need to do this i multiple loft operations.

creating a loft that has flat sections in it, it is frequently best to define the flat sections first and loft to it.

is it necessary for transitions coming off the flat area to be smooth? can you just extrude a flat into it after lofting? 

attaching a file (export as .f3d) will probably yield more help from forum users.

---

[davebYYPCU]

Direct modelling - can't show how, there are new sketches and bodies, you should be able to work it out.

Change the shape of this curve, for a better result, but you will have to redo the Patch if you do make changes, (benefits of direct modelling.)

Tangent on all edges of the patch.

Might help....

[TrippyLighting]

Can you explain in technical terms what you mean with smooth ?

Are you trying to achieve Tangency (G1) ? IN that case @davebYYPCU method might work out.

Or are you trying to achieve curvature continuity (G2), Then you'll likely have to follow @laughingcreek advice.

With a large irregular shaped patch, the quality of the surface is going to be questionable.

The 2 lofts in the file you share both have tangent only surface transitions. The are at least 2 reasons for that.

The sketches you use  only have tangent constraints and and that is all you can achieve using circular curves (Arcs) with constant curvature.

In general, in order to achieve good surface transitions, you want to have control over the surface and braking the problem into smaller pieces might make sense.

[TrippyLighting · Accepted solution]

Here's one go at it using a mixture of Dave's and Alex's approaches with reasonable results (IMHO).

A 4-sided loft will always create better surfaces than a patch, but with a 5-sided area to cover it would take a bit of a curve network to loft this properly. A patch is a good compromise in this case.  Go through the timeline and pay particular attention to the loft settings.

Ask questions if something isn't clear.

[davebYYPCU]

I nearly.... added that rail you have in the bug report, but the front curve mod is what I think will help....

[TrippyLighting]

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@davebYYPCU wrote:

I nearly.... added that rail you have in the bug report, but the front curve mod is what I think will help....

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I have a hard time understanding your cryptic post 😉

It really strikes me how often I came across these "small" bugs and misbehaviors of Fusion 360 when working with surfaces and splines. 

[mattiasAYUHJ]

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@TrippyLighting wrote:

Here's one go at it using a mixture of Dave's and Alex's approaches with reasonable results (IMHO).

A 4-sided loft will always create better surfaces than a patch, but with a 5-sided area to cover it would take a bit of a curve network to loft this properly. A patch is a good compromise in this case.  Go through the timeline and pay particular attention to the loft settings.

Ask questions if something isn't clear.

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Thank you very very much! The processes you described in detail by looking at the design history actually made a really good end result.

But when I played around in Surface mode with patches and stiches (to convert from surfaces to body) I sometimes ended up with uneven top and bottom surface that wasn't completely flat. Trying to fix this by extruding 1 mm and then cutting away 1 mm made things even worse, ending up with unvisible gaps in the edges that made it impossible to use fillet or other edge based modifiers without error messages. Do you know the reason for that? Perhaps some settings that need special attention or some "best practices" I don't know about?

[TrippyLighting]

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@mattiasAYUHJ wrote:

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Perhaps some settings that need special attention or some "best practices" I don't know about?

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That is hard to answer. There are some best practices, but of course I don't really know what you tried.

You should be able to see all those little ribbon or helper surfaces that help define surface transitions. Lofting to an edge of a surface allows you to set the loft condition to tangent (G1) or continuous (G2). You don't get these options if you loft to a curve.

Using those helper surfaces is a best practice.