Approximating a Complex Lofted Flange Profile [Sheet Metal]

[Anonymous]

I have a bin with a cone with has a round to stadium (oval) transition. The outer edge of this cone welds to the inner wall of the bin shell.

Ideal shapeCross Section

I'm trying to determine how to best draw a profile for use with the Lofted Flange sheet metal feature.

The dimensions of the bin are driven by the inside dimensions, so I start by laying out the governing profiles in both directions.

Inside dimesnions

Then flesh out the plate profiles that will make up the construction using the inside profile as a guide.

Upper ConnectionLower Connection

Once I know where the plate lies I can determine where the the outside edge of the cone meets the inside face of the shell by making two surfaces and using a split to illustrate the contacting edge profile.

Determine Edge of Loft ProfileLoft Profiles

A Surface Split reveals the contact edge

Now that I know what the perfect edge should look like I can aim to create a lofted flange profile that closely approximates that. 

I start by projecting the known inside edge points to a new sketch, this allows me to draw a line that is very close to the slope of the ideal edge.

Approximation of the contact edge

I can now make a workplane from that line in order to make a lofted flange profile. The profile uses the known edge points and a section of an ellipse for the profile.

Lofted Flange Profile

If I do a thicken of the cone surface, I can see that is results in the exact geometry I'm trying to achieve.

Thickened cone surface matches perfectly

However, if I do a 'project sliced geometry' and compare it to my approximated profile I'm a little bit out. So I know that my ellipse profile isn't quite exact.

Profile Comparison

I know that I'm likely splitting hairs at this point, but I want to see if I can come up with a better approximation without too much math.

Any Ideas?

[Anonymous]

Nevermind. 

I assumed the two inside profiles shared the same intersection workpoint elevations.

They are offset to account for a horizontal contact edge.

That makes things a lot simpler.

LMAO!

[Anonymous]

Dang,

Looks like it is to be modeled the complicated way.

Anyone have ideas for what I might try?

[johnsonshiue]

Hi Andrew,

I was trying to help. I took a look but I don't quite understand where the problem is. Is the issue about the sketch profile not being precise enough? Or it is about the body geometry is not close enough. Please bear with me. Instead of drawing the sketch manually, couldn't you use Project Cut Edge to get the exact intersection sketch?

Many thanks!

[andrewdroth]

Thanks to looking at this Johnson,

It's about getting a profile at a close approximation to the exact interface between the hopper outer surface and the shell inner surface.

The issue is this profile is curved in two directions, and 3D sketches can't be used for Lofted Flange profiles.

I think I was making this more difficult than necessary by trying to reduce the feature count.

I'm not attempting something closer to your Project Cut Edges approach. But instead of trying to get the profile on a 2D plane I've opted to make the lofted flange larger than necessary and trim to suit.

I can make a surface that represents the ID of the shell and use that to split the Lofted Flange. I split the outer (interfacing) surface of the lofted flange using that surface.

Now I can unfold the lofted flange, make a sketch and project those split edges. This gives me a tool for the Split Body command.

I refold the Lofted Flange and I have the perfect fit with the shell ID.

My concern is that this workflow breaks some of my modeling rules. One being using feature faces to define sketch geometry, and the other using cut edges to define features.

See Screencast attached.

[Anonymous]

This screencast was causing the above post to fail, so I've added it here.

[johnsonshiue]

Hi Andrew,

Many thanks for sharing the video and detail information! I think I got it now. This is about geometry shifting after Unfold/Refold, right? Could you share the file here? I have seen similar behavior before. I vaguely remember we did fix it. I want to know if the fix applies to your exact case also.

Thanks again!

[Anonymous]

No, the geometry is not shifting during the fold unfold.

Initially I was trying to figure out how to make a lofted flange that starts and ends at an irregular interface.

Now I'm trying to trim the lofted flange at that interface.

I think it will work like this, I'm just not fond of using sketches defined on features, and projected cut edges in a production model.

See screencast and model attached.

kelly.young has embedded your screencast for clarity

[johnsonshiue]

Hi Andrew,

Loft is kind of like Freeform. It is under-constrained. As long as the surface pass through the sections and it is smooth. The result is legitimate. How it intersects with the cylinder is not a requirement to the Loft when the Loft was created.

I am wondering, instead of splitting the Loft using the cylinder vertically, why don't you split the cylinder and then use the circular profile to Loft? Does it make sense?

Many thanks!

[andrewdroth]

You can't create a lofted flange with a 3D profile.

At Least I can't seam to.

[johnsonshiue]

Hi Andrew,

Indeed, you cannot pick a 3D loop to create Lofted Flange. But, the intersection cut of a cylinder should be a circle, right? It can be a 2D sketch, I think.

Many thanks!

[andrewdroth]

Because my loft is going from an oblong shape to a circular shape it would be a circle right were the Inside faces meet, but because I'm projecting the outside face of the hopper I have to take the plate thickness into account, and because the two slopes of the hopper, this intersection varies by about 2", hence the 3D profile. 

[andrewdroth]

I just realized that inventor has the perfect solution for this problem built in.

A sheet metal cut can trim the plate to be cut normal to the thickness while maintaining the desired profile.

Unfortunately, it's too complex in this case.

[andrewdroth]

So this is the work around for now. 

unfold-> Project 3 points-> make an arc-> split body with arc-> re-fold.