Surface Tools > Unroll Developable Surface
Surface > Unroll Developable Srf
Flattens ( develops) a surface or polysurface to a planar surface.
Steps:
Select surfaces or polysurfaces.
Select curves on the surface.
This is useful if you are trying to develop a trimmed polysurface. In situations in which the surface cannot be developed, remove the trim curves, develop the surface and curves, and then re-trim the developed surface with the trim curves.
Options
Explode
Yes
Resulting surfaces are not joined.
No
The resulting surfaces are joined along the same edges that were joined in the original polysurface.
Labels
Matching numbered dots are placed on the edges of the original polysurface and the flattened surfaces.
Notes
Use Gaussian curvature analysis to determine which areas of a surface have double curvature and are not developable.
Surfaces such as spheres, tori, and surfaces that are curved in two directions cannot be unrolled.
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Surface Tools > Unroll Developable Surface
Surface > Unroll Developable Srf |
Unrolls a surface without restriction to single-directional curvature.
Steps:
Select surfaces or polysurfaces.
Select curves on the surface.
Makes an approximate 2-D development of surfaces that have compound curvature.
This command can be used to deal with fabrics that have a certain amount of flexibility and stretch.
The Smash command is a modified version of the UnrollSrf command. With UnrollSrf, the surface has to be linear in one direction to unroll, and with the Smash command it does not.
Since it is not possible to flatten a double-curved object (like a half a coconut shell) to get a paper pattern, the answer is always inaccurate to some degree. This command is useful if the object you are flattening is not extremely curved and you want to make the pattern out of a stretchy material like rubber.
Options
LinearDirection
Natural
Allows the command to choose one direction to be the straight one.
U
Select the surface u-direction as the linear (straight) direction.
V
Select the surface v-direction as the linear (straight) direction.
Explode
Yes
Resulting surfaces are not joined.
No
The resulting surfaces are joined along the same edges that were joined in the original polysurface.
Labels
Matching numbered dots are placed on the edges of the original polysurface and the flattened surfaces.
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Surface Tools > Smash
Surface > Smash |
Creates 2-D curves from surface edges flattened onto the construction plane.
Steps:
Select a surface edge.
Select another edge.
Type a number for the sample point spacing.
Notes
The purpose of the FlattenSrf command is to draw a 2-D outline of a flattened view of a developable surface. This command cannot flatten all developable surfaces.
The result of the command is a group of four curves that define the outline of the flattened surface, that have one corner at the world coordinate origin 0,0 and that is drawn in the world x,y plane.
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Surface Tools > Flatten surface (Right click)
None |
Rhino has commands to create and unroll developable surfaces with holes and marking curves.
Developable surfaces are surfaces that can be formed by rolling a flat sheet of material such that the material does not stretch, tear, or wrinkle. Examples of this type of shape are cylinders, cones, and some steel ship hulls.
Developed surfaces used to make patterns for cutting steel
Picture trying to make your surface out of aluminum foil. If it crinkles or tears as you try to bend it into position, the surface is not developable. A sphere is an example of a surface that are not developable (try wrapping an apple with aluminum foil). So are most of the shapes on car bodies and most modern car windshields.
The Developable option of the Loft command creates surfaces that are linear in one direction only. Surfaces can be created in other ways, too, but they must still be linear in one direction for UnrollSrf to unroll them. You could lay a ruler along the surface and it would touch the surface from one edge to the other. These are called ruling lines.
Ruling lines
Because these surfaces are linear in one direction, the Gaussian curvature is zero at every point on the surface. If the Gaussian curvature is not zero, Rhino will not be able to unroll the surface. If the surface is not linear in one direction, Rhino will not be able to unroll the surface.
Since developable surfaces cannot be created from just any two curves, results from a developable style loft can be unpredictable. Curves of similar shape without kinks work best.
You can use Gaussian curvature analysis to determine which areas of a surface are not developable.
These developable surface tools work best for designing airfoil and hydrofoil type surfaces. They were not designed to be used for bending and unfolding sheet metal like duct work or for developing fabric patterns.
Using developable surfaces in Rhino and exporting planar Rhino geometry to DWG or DXF for NC cutting are the two areas that may have problems.
Developable surface pitfalls:
The Loft command with the Developable option is very sensitive to the makeup of the curves being lofted. You can get very different results depending on the complexity and similarity of the two edge curves. It is best if they are as simple as possible and have the same parameterization.
Picking different ends of the curves can give different results. You can sometimes combine the different results to get a better developable surface.
A developable surface is not necessarily a fair surface.
It is possible to make a developable surface that unrolls with a difference in area and ruling lengths from the 3 D surface. It is easy to miss the warning Rhino gives.
You can unroll surfaces that are not developable.
Rhino does not have any method of making a surface a bit 'more or less' developable. Some programs have a way of spreading out the concentrated fans of rulings that often occur, thereby smoothing the surface. Since metal has some elasticity (especially aluminum), you can deviate quite a bit from a mathematically correct developable surface and still plate it up. Some builders that use developable surfaces in their models, expand the plates, and then add up to 1" chord depth of radius to the flat sides in the sections to make the plate "taut." The expanded plates fit up to the changed sections. They have not figured out how to get this "blow" into their 3 D models, though.
One way to approach the problem is to use the CurvatureAnalysis command to analyze the Gaussian curvature as a guide to surface creation instead of the developable loft. But there is no way in Rhino to expand surfaces created this way (unless they happen to meet the requirements for the UnrollSrf command).
Exporting geometry for NC planar cutting from Rhino:
It is important to compare the geometry Rhino exports in DWG (or DXF) format to the original. The settings in the DWG export dialog are critical. It is easy to end up with arcs in Rhino translated into dense polylines, or curves in Rhino interpolated too loosely.
