Whether you are working with paper or sheet metal, unfolding is a crucial step in the manufacturing process when you are starting off with a digital model in SketchUp. If you want to build something exactly as designed on-screen, then you will need to unfold complex shapes and lay them out flat for cutting. Of course, my Unwrap and Flatten Faces Extension for SketchUp is the ideal tool for doing that in SketchUp. In many cases, it can automatically unwrap a surface (i.e. multiple faces) and produce something usable. Of course there are some caveats for that to work and I will be covering some of those here.
SketchUp faces have a front and a back side (turn on “monochrome” face style in the View menu to see those orientations) where the front of a face is commonly colored white. The unfolding algorithm of my extension starts at one face and folds it along an edge to end up aligned with the next face at that edge (and then repeats from there).
This relies quite a bit on face orientation. As you can see in the image above, if all faces are aligned the same (as in the topmost model where all fronts point outwards), the three faces unwrap correctly. If neighboring faces have a different orientation, however, (as in the model in the lower half of the picture), then the unfolding process ends up placing faces on top of neighboring faces, which leads to an inaccurate result (SketchUp merges those overlapping faces in the process).
Best practice is therefore to always double-check that all faces that are to be unfolded together have their fronts oriented similarly. If a face needs to be reversed, then that can easily be done with the “reverse face” right-click tool. Do so before starting the unwrap tool.
In general terms, only flat shapes and developable surfaces (e.g. a cylinder) can be cleanly unwrapped and flattened. This means that if your object has double-curvature (as is the case for the semi-sphere and the shell in the image above), then my extension can fail because it is likely impossible to find a result for the entire object. In some cases, (as for the sphere), this tool may be able to “peel” a surface, which may or may not be a useful result.
It is always possible to select only part of an object (e.g. the curved top of the cone shown above) and unwrap that separately from the rest (the circular base in that case). For a double-curved surface (the rightmost object), the best way to unfold will likely consist of taking a horizontal or vertical set of faces and unwrapping in “strips”. This process is illustrated in the images below as well.
Double-Curved Surfaces and Material Stretch
When double-curved surfaces are unwrapped in strips (as shown for the sphere section above that was unwrapped in seven sections), then there are several ways to work with those to get a final result (see image below).
If the material has no stretch capacity (as with sheet metal, for example), then the best way to cut and reassemble is shown in the right example below. If, however, the material can stretch (as with fabric), then an educated guess must be made (because the unwrapping algorithm cannot at this point take stretching into account). All faces could be placed with a slight overlap (shown on left) and the outlining shape can be cut out. This assumes stretch to be possible in the vertical direction but assumes no horizontal stretch. This could be compensated for by scaling the shape horizontally a bit.
Thickness of the used material becomes an issue when two flattened and then cut pieces need to be joined. You can see this in the example above. If the material is of negligible thickness (much thinner than the overall material area) as is the case with paper, sheet metal, and the like, then flattened shapes can likely be reassembled just as they are cut (bottom example). When the material has some amount of thickness, however, as can be seen in the upper model in the image above, then mating edges must be treated (by tabbing, backcutting or chamfering) to allow for pieces to be reassembled as they were intended.
If you need to create overlaps at edges for glue or weld tabs, then the easy way is of course to simply overcut the flattened shape (while marking the original shape outline). For more control of tab size, one option is to use the tab tool in the Flattery extension that places tabs manually on single edges. An easier approach, however, is the Offset Contours tool in the Tools on Surface extension. As you can see in the image above, all edges can be offset in one go and a defined offset amount (1/2″ in my case) can be entered.