Just checking - lofted bends

Well that sucks big time. We requested lofted bend development because we use it all the time in the aerospace industry, there aren't many straight lines in an aircraft.

Unfortunately most parts in a wing have 2 lofted bends ie an upper and lower wing surface to mate up with.

Looks like we will be moving over to Catia a lot quicker than envisaged.

But you are just now becoming aware of this limitation?

It would be cool to see the flat pattern process for a one piece rib, joggles fore and aft. I've always wondered how the real people do it.

What about the F-117? ;-)

No they're not. I've been using the lofted bend in a quite a few sheet metal parts and it's worked out perfect everytime for me.

I've ehard htat lofted bends weren't that accurate, but everything I was going to do was flats and cones so it should unfold withotu having to do any apporximations. I might try to loft it and then add bends (old sheetmetal), but as you know it can take quite a bit of setup for a loft to make a true cone - you can't be off the slightest bit (or take the slightest shortcut) or the deals off.

The amount of error depends on the amount of non-Gaussian (compound) curvature in the bend. If your bends do not have a lot of non-gaussian curvature, then the amount of introduced error will be less, perhaps not enough to matter.

We haven't upgraded yet due to being involved in a large design program. We dont update in the middle of a program.

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You've heard all the words and are getting close. Gaussian curvature greater than 0 indicates compound curvature. The analysis necessary to predict the exact behavior of the material is outside the scope of a mechanical CAD software.

What is that supposed to mean? What's the process and application? Is it going to bother you if the length of a contoured flange on a bag pressed rib or frame isn't created to machine part tolerances? How far off will a SW or ProE flat pattern be for that part?

I have been using splined curve flange development for 25 years in various CAD packages. Cadam which is only a 2 1/2D system had the capability in 1980, it would also develope various formed angles around splined flange as well. Granted, the development of the flange became precarious at best when extreme curvature was involved, but, as is good practise these areas were cutout of the flange to assist manufacturing. Flutes are also introduced into the flanged area to relieve the problem of local buckling during pressing. High pressure Hydro pressing is also relieveing some of the flange instability problems. Achieving a tolerance of +/- .030 on sheet metal parts is not hard using this methodology. Some people get too caught up in the minuta detail of the mathematics of developing curved flanges.

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Is creating the part outside the sheet metal environment then converting to sm an option?

Yes - that's my next avenue. But as I understand it you have to be very careful on how the lofts are set up so the resultant model is all cones, cylinders, or planes. The way to check if you actually made a cone or cylinder (vs an algorithmic face) is to select the face as a reference for an axis. To check if a face is planar, click on it and see if you the sketch icon is greyed out or if it is available.

Is that the criteria or can it be expanded to include all developable surfaces; i.e. an extruded conic arc or spline?

Is a Gaussian curvature analysis available to help determine if a surface is developable if the criteria can be expanded?

I'm not a SW user, or a brandX shill. Just a curious net nerd, so please excuse me if the questions are irrelevent to the topic.

SolidWorks does not have a check for Gaussian curvature. Unigraphics has had Gaussian curvature analysis since at least 1996. This was very useful, as I used to have the task of putting decals on odd surfaces.

As far as I know, the criteria for unfolding a model to which sheetmetal is applied are as I posted.

Thanks for being frank - that's appreciated. I don't think the question is irrelevant because it speaks to the wider need to flatten complex shapes. Swx is still not able to do any of the things youb rought up (at least as far as I know - I try not to say SWx can't do soemthing because it will suprise you at times. I just doubt that it will surpise us in this case)

Ed

Thanks for the info, folks.

We do a lot of conical sheet metal and use the usual sheet metal tools to generate the flat patterns and this works just fine. Occasionally we have to generate a flat pattern for an offset cone and need to use the lofted bend feature for this. We found the flat patterns for the lofted bend cones were not accurate at all so we tried a little test. We made a standard sheet metal cone and the same part with a lofted bend. The two flat patterns are not even close! What we found is that the "k" value for lofted bends are not used. If you model the lofted bend with the outer surface the flat pattern reflects the geometry of the outer surface rather than the neutral axis (i.e. k=0.5). Same thing if you create your model using the inner surface, the flat pattern reflects the geometry of the inner surface, not the neutral axis. The work around for us was to model the part using the neutral axis and this gave the correct flat pattern. Unfortunetly the part itself is not correct as you can not do a "mid plane" lofted bend so we ended up with two configurations of the part, one for the correct part geometry and one for the correct flat pattern. Our parts use relatively thick steel plate such as 3/8" or 1/2" which can create a considerable error in the lofted bend flat pattern unless we create the geometry on the neutral axis. Try a flat pattern of a simple sheet metal cone with both tools and a relatively thick wall thickness and you will see the problem. Has any of this changed in SW2007, we are still on SW2005 SP5.

Ed

curious outsider wrote: