Flat Patterns in Rhino

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Monty

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Jul 15, 2010
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1,294
Location
Fayetteville, AR / USA
I said I would do this a while back and got distracted. Anyway, here is my method for creating surfaces for sheet metal fabrication in Rhino. I don't count on this being accurate enough for matched hole type construction. Therefore, I only lay out the rivet pattern on the outer sheet. It is easy to put a couple holes on the centerline fore and aft to get the parts aligned. Then you can proceed to match drill and cleco all the way along the rivet line.

I'm using a generic tailcone here to get the point across. First you start with a profile view. I am so used to working in 3D that I don't bother with the planform view. I just work out from the side view. Then I used the ellipse command to make the top of the tailcone. Trim to the centerline. Then I used the conic tangent at both ends command to create the bottom curves. Mirror them and join them.
cone1.jpgcone3.jpgcone2.jpg

Now the fun part. Use the sweep two rails command. Select the two half ellipse curves as guide curves. Select the three lines as cross section curves. When the sweep two curves dialog pops up, select the simple sweep box. Repeat for the bottom conic curves. You should get something like this:
cone4.jpgcone5.jpg
 

Monty

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Jul 15, 2010
Messages
1,294
Location
Fayetteville, AR / USA
The next step is to handle the bend allowance. Typically since the bend radius is gradual the bend line will be the center of the sheet. So you use the offset surface command to offset the surface half the sheet thickness. Which direction depends on what mold line you are working on. I like to work on the line between the bulkheads and the skin. Some people work on the outer mold line. Since I work on the interface between the bulkheads and the outer sheet, I offset to the outside. Then you use the unroll developable surface command to unroll the offset sheet. Once you have this sheet you can extract edge curves and offset them to produce rivet lines. Then use the divide curve by number of segments to place points along the curve for the proper number of rivets. As I stated I only plan to use a couple alignment holes on the center of the sheets that are pre punched in both sheets. Only the outer sheet will have actual pre punched holes. Then I will match drill through the inner sheet, bulkhead, or stringer. For bulkhead rivet lines you can split the surface at the bulkhead and unroll it, then position it over the original sheet. Extract the edge curve and use that for your rivet line (offset for the flange of course).

cone6.jpgcone7.jpg
 

Tom Kay

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Aug 10, 2007
Messages
398
Location
Ottawa Canada
Monty;

This is extremely interesting to me, because I really enjoy Rhino, and because I was under the impression that you couldn't make flat panels out of curved panels in Rhino. I had just assumed that Solid Works was the program for that type of work.

Your skills are way ahead of mine, but I'm going to try to mimic what you've done. I might have a few "How did you do that" questions for you.

Thanks for posting this. Tom.
 

Monty

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Joined
Jul 15, 2010
Messages
1,294
Location
Fayetteville, AR / USA
Tom,

Rhino will do simple developable surfaces. These need to be simple curvature in one direction only. Compound curvature surfaces will have large errors. You also don't want twist in the surface.

Solid Works and other packages that have a sheet metal module have a lot more capabilities that are geared towads stampings. The problem is, even with these packages you need to know what your bend allowances will be. Bend allowance varies depending on a lot of factors. The size of the bend radius vs the thickness of the metal is the big factor. Since the parts we are interested in are much thinner than the bend radus, the bend line is the center of sheet. This makes it relatively easy to come up with flat patterns.

For design of sheet metal airplanes the following commands are your friends:

Conic curve (tangent at both ends and tangent at start)
Surface:sweep two rails (use the conics as guide curves and straight lines as cross sections)
Offset surface
Unroll developable surface
 
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