Sunday, January 29, 2017

Ruled and Developable Surfaces and Forms for Fabrication


This post discusses ruled and developable surfaces and their use in digital fabrication. It defines the terms, discusses the generation and layout of parts, and methods of assembly to combine back into 3D form. It also shows methods of analyzing forms to determine if they are ruled and developable.

If you'd like to examine and experiment with the surfaces shown below you can download the Rhino file here.

Ruled Surfaces

A ruled surface is generated by sweeping a straight line through space. For every point on a ruled surface there is at least one straight line which lies on the surface. Examples are cylinders, lofts between two rotated circles, curves extruded along a straight line, or straight lines swept along a curve.

Doubly-Ruled Surfaces

A surface is doubly ruled if for every point on the surface there are two different lines that lie on the surface. An example of this is a hyperboloid, shown below. One of these can be generated by simply lofting two circles which have start points (curve seams) which are not aligned but rather are rotated relative to one another.

The loft is hyperbolic - saddle shaped - doubly curved in different directions:

The curvature circles (Rhino Curvature command) show the principal curvature at any point. Here you can see the circles curve in opposite directions. A later section of this post discusses the use of the Curvature command in more detail. 

Here you can see the two straight lines for every point:


Developable Surfaces

A special type of ruled surface is said to be developable. A developable surface can be unrolled onto a flat plane without tearing or stretching it. That is no distortion will be introduced when it is laid flat on the plane.

The Rhino command UnrollSrf is used to flatten a surface or polysurface with curvature in one direction onto a plane. In the image below you can see an unrolled cylinder, cone, truncated cone and extrusion. In these simple cases I think it's easy to visualize how those surfaces can be unrolled.

Some examples of ruled, developable surfaces are cylinders, cones, truncated cones, and curved shapes that are extruded along a straight line.


Curvature of Surfaces 

Principal Curvatures

The Curvature command shows the Principal Curvature for a surface. The principal curvatures measure the maximum and minimum bending of a surface at each point.

When the principal curvatures are positive that means they curvature is in the same direction (both positive or both negative). This means the curvature is bowl-shaped.

When the principal curvatures are in opposite directions (one positive and one negative) the result is a negative value as below. This means the surface is saddle-shaped.

When the principal curvatures are zero it means the surface is flat in one direction.

The principal curvatures are used to compute the Gaussian and Mean curvatures of the surface.

Gaussian Curvature

The Gaussian curvature of a surface at a point is the product of the principal curvatures at that point. The tangent plane of any point with positive Gaussian curvature touches the surface at a single point, whereas the tangent plane of any point with negative Gaussian curvature cuts the surface.

Positive Gaussian Curvature: A positive Gaussian curvature value means the surface is bowl-like. This surface is synclastic.


Negative Gaussian Curvature: A negative value means the surface is saddle-like. This type of surface is referred to as anticlastic.

Zero Gaussian Curvature: A zero value means the surface is flat in at least one direction. (Planes, cylinders, and cones have zero Gaussian curvature).

Curvature Terminology Summary

Below is a summary of the terminology used above:
  • Principal Curvature: The maximum and minimum curvature at a point on the surface. 
  • Gaussian Curvature: The product of the principal curvature values at a point. 
  • Synclastic Surface: A synclastic surface is one where the Gaussian curvature is positive everywhere. A point at which the Gaussian curvature is positive is called an elliptic point.
  • Anticlastic Surface: An anticlastic surface is one where the Gaussian curvature is negative everywhere. The surface is saddle shaped. 
  • Developable Surface: A developable surface is a surface with zero Gaussian curvature. That is, it is a "surface" that can be flattened onto a plane without distortion (i.e. "stretching" or "compressing").

Rhino Commands and Grasshopper Components for Creating Ruled Surfaces

There are many ways to create ruled surfaces in Rhino. The key is using straight lines as input to the commands. Here are a few examples:

Some of these commands are discussed below:
  • ExtrudeCrvToPoint: This command will generate conical forms which are ruled. It works because the curve is extruded up to a single point. Thus there's always a straight line from the curve to that point, and thus the surface is ruled. 
  • Sweep1: This command will generate a surface by moving a curve along a rail curve. If either of the curves is a straight line the surface will be ruled. 
  • Sweep2: A surface is generated by moving a curve between 2 rails. If the rails are straight lines, or the contour curves are, the surface will be ruled. 
  • Loft: When you loft between two curves you can generate a ruled surface. 
  • EdgeSrf: This command will generate a surface between 2, 3 or 4 open curves. If the input edges are straight lines a ruled surface results. 

UnRolling for Fabrication

It's a common practice to want to unroll forms, cut them from sheet material, then re-assemble them back into a 3D object. One method of doing this involves tabbing the edges of the surfaces.

The Rhino Polyhedra plug-in allows you to create an amazing variety of polyhedra. Those that can be generated as solids have planar faces. They can therefore be unrolled using the UnrollSrf command.

It is possible to take surfaces which aren't developable and make an approximation of them which are developable. Here's an example - a sphere - clearly not developable:

Un-Folding Mesh Forms

Meshes are composed of quads and triangles. These meshes can be unrolled. A free program to do this is Pepakura Designer. It takes 3D mesh models and will let you designate the seams where the model splits apart to unroll flat. It will also generate the tabs so you can secure them back together. This is a remarkable, easy to use program well worth checking out.

Here's an example of a Rhino Polyhedra, exported as a meshed OBJ, then loaded and unrolled in Pepakura.

Using the free version you can export the result to a PDF file (which will have the graphics in a vector format). You can then import the PDF into Rhino and modify the layers, linetypes, scale, etc. to get it ready for cutting out). 

If you purchase a commercial version of Pepakura you can export directly.

Programatically Unrolling

The UnrollSrf command can be used by a Python script or Grasshopper definition to unroll as part of a program.

A handy way to assemble unrolled forms is by tabbing the edges of the unrolled surfaces. These tabs can be made to interlock so the cut pieces can be reassembled. In this example a Grasshopper definition was used to generate a 3D form by sweeping a curve along a polygon rail. The rail is a triangle and thus the straight edges make a ruled, developable surface. The definition generates tabs along the edges.

The parts are cut on a CNC knife cutter. In this case several forms were cut from the same sheet of material - 0.03" thick PETG plastic.

The tabs lock the parts together and also ensure that the curvature remains true to the original form.

Here are the cut parts, with the plastic overlay removed, assembled by interlocking the tabs:
  

Hot Wire Cutting of Ruled Surfaces

Another application for ruled surface geometry is driving a robot with a hot wire cutter. The heated wire vaporizes the foam as it slowly moves through it. Because the wire is a straight line the resulting surfaces are inherently ruled.

Since we create the surfaces to drive the robot we do so by developing ruled surfaces in Rhino.

See this post - Hot Wire Foam Cutting Using the Robot - for an example of hot wire cutting a variety of ruled surfaces in expanded polystyrene foam with the robot.

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