Sunday, November 25, 2018

Rhombic Dodecahedron Stool

This latest chapter in a fascination with folding has been challenging. For several years now I've been interested in using CNC technology to create folded plywood objects.  Initially this involved applying piano hinges to pieces of plywood to make furniture.  You can see examples of this here and here

Over time, this method of cutting piano hinges and screwing together the joints with the hinge leaves felt complicated.  You had the many faces to cut and then you had the many hinges to cut. For something that was suppose to be automated, I felt like I was working on a methodology that was pretty involved.  The table design worked well for the most part because of the glass top that allowed one to peer down into the bristling interior.  I liked the clean star-like radiating lines but when I made the chair, the object felt too mechanical.  It was felt like a Frankenstein chair for a would-be Frankenstein.  It also wasn't all that comfortable.   

It was pretty clear what the simpler solution was...if possible.  I needed to "weaken"(but not break!) the plywood joints enough to steam bend them.  This would capture the wonderful simplicity associated with origami and create more supple and fluid shapes.  Undoubtedly this would come with a structural and mechanical price.  How far could one bend the steamed joint?  How strong would the joint remain after this bending? I didn't know if the thinness required for steaming would also render it too weak to support my weight. Would it break the first time someone sat on it? I'm happy to say it feels pretty strong after all the faces are connected.

The term "thick origami" is a phrase that has been emerging recently and in many ways the piano hinge furniture represents the clear geometric challenges associated with this way of constructing things. The plates, instead of all pivoting off one side of a cutout, pivot off the convex side of the fold.  This complicates the geometry of the cutout substantially. 

But this most recent furniture example is more challenging than objects made with this purely hinged approach. This stool - and hopefully future builds - does NOT exploit the use of fabric hinges or pivot hinging and their clear points of rotation. 

These "pliable sheet" builds hope to account for the anticipated deformation that occurs at steamed joints and compensates for this in the initial flat geometric layout. I've made several efforts to collect data in this regard. More data collection is in the works as I am working toward a strong 90 degree bend.

It is an exciting time to be working on this. Fabrication methods are evolving quickly in ways that strive to take advantage of both the structural and esthetic characteristics of the CNC approach. There are a lot of techniques to get comfortable with. Some examples of this:

What is the most efficient and accurate way to do a flip mill of a sheet of plywood? 

How does one tighten up the tolerances on the CNC machine when accuracy begins to degrade?

In parallel with these challenges there were, and remain, many questions about how to best modify the plywood to accomplish a good fold. Larger angles of folding remain a challenge and there are tradeoffs in seam width and depth with respect to flexibility and strength. This initial project was chosen, believe it or not, for its simplicity.  While the shape might appear somewhat complicated but it has this going for it:  All the dihedral angles (the angles between the faces) are 60 degrees.  

This made the initial folding ritual less complicated and did not impose too much stress on the joints.  Having said this, my first pass at this stool had several joints crack due to inadequate steaming.  I got discouraged and thought about giving up. I want to thank my son Jesse for telling me he could use one in his bedroom if I was able to finish it. This gave the thing purpose beyond the abstract and vainglorious desire to solve some large abstract design challenge.  There have been big improvements from where I started out, but the whole thing is definitely a work in progress.  The challenge of a good reliable steaming mechanism for the joints continues to be a large focus. Both the CNC and steaming techniques can use improvement.

Stay tuned for future pliable geometry.  

1 comment:

  1. Interesting! Your Grandpa Dimpfl designed and folded a number of geometric solids from paper for me when I developed an interest in crystals. We grew a number of crystals from solutions. Bill