topic Re: Unable to loft a 3D sketch with rails and then use circular pattern on it in Inventor Forum

Unable to loft a 3D sketch with rails and then use circular pattern on it

angela.peralta — Sun, 07 Nov 2021 23:51:54 GMT

As the title says, I am having a frustrating time trying to create a loft with 2D sketches as rails. After giving up on the planned loft and making a normal loft, I am unable to create a circular pattern.

When I create the loft I use the base of the surface projected ellipse to the point where the lines intersect successfully. I am not able to add the rails of the 2D sketches afterwards. Sometimes it works for 1 of the 4 sides, but it doesn't do what I want it to do.

I was able to loft close to the rails by changing the point condition to "Tangent" and changing the weight to 1.7. I would like to be able to get the exact shape I want but I'm just trying to get this prototype printed at this point. The problem after this loft is that I am unable to do a circular pattern with that loft.

Re: Unable to loft a 3D sketch with rails and then use circular pattern on it

peterKMFDM — Mon, 08 Nov 2021 04:28:29 GMT

This is the "exact loft" - and it will pattern.

Re: Unable to loft a 3D sketch with rails and then use circular pattern on it

SharkDesign — Mon, 08 Nov 2021 06:27:45 GMT

Do you need to loft, or can it be done with a revolve?

Does your last message mean you've solved this?

Re: Unable to loft a 3D sketch with rails and then use circular pattern on it

peterKMFDM — Mon, 08 Nov 2021 18:50:30 GMT

The last message includes the part with the correctly lofted 'lump' which can be patterned. The loft can't be constructed as a revolved feature because it is elliptical in two planes. I ran out of time to write more than a line of explanation 😁.

Re: Unable to loft a 3D sketch with rails and then use circular pattern on it

JDMather — Mon, 08 Nov 2021 19:10:35 GMT

Sketch7 is not fully defined?

Extrude43 does not look "right" to me?

Re: Unable to loft a 3D sketch with rails and then use circular pattern on it

angela.peralta — Wed, 10 Nov 2021 06:38:07 GMT

Works like a charm! Many thanks. I will have to learn more about how you did it.

Re: Unable to loft a 3D sketch with rails and then use circular pattern on it

angela.peralta — Wed, 10 Nov 2021 06:39:20 GMT

I would like to be able to loft it. For the real life function, revolve won't do it.

Re: Unable to loft a 3D sketch with rails and then use circular pattern on it

angela.peralta — Wed, 10 Nov 2021 06:39:57 GMT

Hmm interesting.. I guess I have to learn more about how to loft properly

Re: Unable to loft a 3D sketch with rails and then use circular pattern on it

peterKMFDM — Wed, 10 Nov 2021 07:30:50 GMT

Hmmm - @angela.peralta - if you can define "properly" then we might be able to help. In my book, if you model the shape we need to make, then it you've done it "properly". Anything more becomes cad-snobbery, and usually takes too long 😉

There are probably easier methods, but I like this approach because you have complete control over "everything". Lofting this shape isn't easy because the rails and sections all meet at three points - it's almost a revolve, but not quite. This is how I tackled it:

Sketch the profiles

Note the use of construction lines for the ellipse axis and 'normal' lines for the curves. When you extrude the curves as a surface, you don't want the axis to be included.

Extrude each curve as a surface - AWAY from the finished shape

By default, the sketches are consumed by the extrusions. This is good because you don't want to accidentally select the sketches for the loft - you want to select the "Edge" of the extruded surface. If the loft dialog box shows any "Curve" selections, go back and hide your sketches:

Select any two sections, and one rail.

The reason for using extruded surfaces as your inputs for the loft is so you can use them to control the tangency of the loft:

Set tangency condition for all edges.

That's it. From here, make the input surfaces not visible, mirror the lofted surface, and use the "Sculpt" tool if you want the resulting shape filled with solid.

If you missed the tangency step, then the resulting surface will look wrong because the curvature drops away too quickly from the input surfaces. If you turn on Zebra analysis, you can see the zebra lines do not "flow" over the surface. The step in the zebra line is where it is missing tangency:

The cad-snob in me says it should look like this:

Hope it helps!

Re: Unable to loft a 3D sketch with rails and then use circular pattern on it

SharkDesign — Wed, 10 Nov 2021 07:36:46 GMT

'CAD snobbery' = less errors 😉