Back to home page

Construction Story


The Big Roof: an adventure in computer aided design and creative geometry

I make a lot of three-dimensional things without using 3-D software. Most things can be constructed of parts that are essentially two-dimensional, and almost anything can be calculated with high-school geometry (if you were paying attention). I've found most 3-D software more difficult to learn and use than it was worth. It's usually a heck of a lot quicker to do some basic measurements and calculations and fake the rest. Or so I thought.

This project was a little different because the shape I decided on can be simply described as a 3-dimensional object (at least verbally), but the parts that make up the object contain compound curves which can't be easily described. The radius of the curves that make up the edges of the flattened sheets change continuously throughout every piece.

Describing the layout seemed a lot simpler in my head than when I tried to get it onto paper. The base of the roof is a 20 foot wide 9-sided equilateral polygon that is drawn up along 2 curves into a spiral that ends up being 14 feet tall. I pursued my usual method of drawing what I thought might work, printing it out, cutting it up and taping the paper together.

Despite about a dozen attempts, my models kept coming out lumpy and warped. No fun. I called the local big-time CAD vendor who eventually told me that they'd sell me somewhere between $6000 and $10,000 worth of software, but they weren't sure it would work. Time for plan B...

I spent the next week trying to draw this monster with Intellicad, my aging 3-D program that's roughly similar to AutoCad. After much head-banging I finally realized it was a lost cause. The main problem was that though it's possible to draw an undulating 3-D curve (known as a "spline"), once the spline has been created it's impossible to do anything useful with it. For example, I couldn't use the spline to define the edge of a surface. In addition, simply drawing the spline involved shifting the "viewport" of the program so that I was completely aligned with the axis of the curve. This involves a certain amount of cheating, because the splines that form the corners of the roof are basically asymmetrical helixes; they curve but also rotate. If this seems hard to understand, it's only because it is.

Since this wasn't getting me anywhere, I started looking at all the software I could find on the web. I installed demos of about 2 dozen different programs; most of which were lame, counter-intuitive, dysfunctional and next to useless. Some of the worst of the bunch had prices in the thousands. Two programs (both Japanese) that nearly got the job done were the cheapest.

HexaSuper2 (mac only) is very cool and only $54. Its interface and features are completely different than any drawing program I've ever seen. Ultimately, it may be more useful for rapidly drawing cartoon characters, but it does import and export 3-D dxf files, so using Hexa to create actual products isn't out of the question. It was also very useful for creating 3-D files that I could further manipulate in Tenkai...

Tenkai is amazing. (windows only) For $20 it imports 3-D data and generates an unfolded pattern that you can cut up, fold and tape together. I experimented with it quite a bit, but was a little leery that it would be capable of the accuracy that I needed. Since the output is faceted, it might be possible to take the output and trace over it to create smooth edges. It would probably work for smaller projects. Someday I'll try some more testing to see if it works.

Then I stumbled upon Rhino...

Robert McNeel's Rhinoceros (windows only) is utterly unbelievable. The company says it outperforms programs that cost 20 to 50 times more. I believe it. From the time I downloaded the demo to having a workable 3-D model of my project was 2 hours! This includes the time it took to figure out the program. Granted, I spent a few more days messing around with it and learning more about the program, but it was hard not to kick myself for missing this earlier. And not only could it draw my funny shape, but it also could take the surfaces of the shape and lay them out flat, so that I could import them into my CNC cutting program! All this for about $900. But since I'm so cheap, I was able to get by on the 25 free saves of the demo program. I almost feel guilty, but I keep telling myself that I'll buy the full program once I can charge it to a project.

Besides Rhino's capabilities, its interface was by far the most intelligently organized and intuitive. Controls are mostly where you'd think they should be, and it the commands are far less annoying than anything else I've found that's even vaguely similar. And to top it all off, Rhino is also far faster than any other 3-D program I've seen.

The flattened shapes did require some manipulation to smooth, but that was probably as much a function of my inexperience with the program as anything else.

Since the roof was constructed of 16 gauge CorTen Steel (only about 1/16" thick), I was very worried about the seams coming together closely enough to weld easily. But despite some difficulty pulling the pieces together, everything eventually fit beautifully.

The only real gripe I have about it is that there's no Mac version. It's really too bad, because I've become a big fan of OS X. Oh well.

I hope to find some more uses for this clever tool. And I apologize if this sounds like an ad, but it's hard to be restrained when something works so darn well.