Create flat pattern DXF minimum distortion for sheet metal

[John_Draper]

This is a follow-up to my previous posting. I have used Fusion to create a sculpture with several 4-sided hollow bodies. The sculpture will be made with thin sheet metal. I need help creating DXF files of the unrolled/flat templates for laser cutting. The curved surfaces include twists that are not compatible with Fusion Sheet Metal features. I want to use Rhino 8 on a Mac to flatten the 16 surfaces with minimum deformation to make DXF files for laser cutting by SendCutSend. I have used the following process:

Fusion:

Created shell bodies of each surface and confirmed with curvature analysis in Fusion that they appear to be “developable”

Saved each surface as a STEP file in Fusion

Rhino:

Imported the STEP file into Rhino

Used “ExplodeBlock” to explode the one block

Used “CurvatureAnalysis” to see if surface is “developable”.
Used “UnrollSrf” command to create flat template geometry. Selected all surface (selection filter disabled). Relative Tolerance at 0.01.

Properties (right side panel) and deselect “Show surface isocurve” before exporting to DXF

Exported Unrolled surface as DXF: Selected surface. “Export selected”:

The above procedure worked well for the attached STEP files H3 and H4. I checked edge line distances on unrolled H3 and H4 and found perfect matches to the original surfaces. However, I could not get it to work for H1 and H2. H1 and H2 may have very subtle double curvature. I played with increasing the Relative Tolerance up to 8 and created Unrolled surfaces. However the surfaces were distorted.

I have 14 more surfaces to unroll so I need help to figure this out. I will tweak them in Fusion to get as close to “developable” as possible.

Any assistance in creating flat patterns for H1 and H2 with minimum distortion would be greatly appreciated.

H1.3dm (108.6 KB)

H2.3dm (260.4 KB)

H3no tips.3dm (245.6 KB)

H4 w tips.3dm (230.8 KB)

[JimCarruthers]

I’m not sure how you are expecting to handle this double curvature in reality…

So you can use DevLoft to make a technically developable surface between your problematic surface edges.

I mean it looks messy but it’s developable.

Mind you though, I compared this flattened to just YOLOing your original model flat using Smash,and the 2D results were virtually identical, 0.010" max difference in the edges.

[davidcockey]

@John_Draper

H1 and H2 may have very subtle double curvature.

H1 and H2 have double curvature. Use the Curvature (not CurvatureAnalysis) to see it.
Start Curvature.
Select a surface to analyze.
Click on the surface and move the cursor around the surface while observing the two curvature circles. These circles show the principle curvatures at that point. An exact developable surface has one principle curvature equal to zero everywhere. If a surface is exactly developable at a point then
one of those circles will be a straight line (infinite radius/zero curvature).

@JimCarruthers

So you can use DevLoft to make a technically developable surface between your problematic surface edges.

There is an region of the curves where an exactly developable surface between the curves is not possible based on geometry. DevLoft and similar may find a surface which is close enough to developable.