Archive for May, 2011

Virtues of Rapid Prototyping and Strandbeests

STRANDBEESTS: If you don’t know Theo Jansen, he is an eclectic Dutch kinetic sculptor and physicist, who creates moving works of engineering and art that are hypnotically elaborate. These Strandbeests are designed to be easily moved by a light wind and can be typically found roaming the windy beaches of the Netherlands. These works are laboriously hand-constructed with electrical PVC tube, plywood, and other materials. When you see how freely and effortlessly his works move, you gain a supreme appreciation of how science can be applied into creating kinetic beauty. Here’s a video from TED a few years ago of the artist himself describing his work.

A short while ago, I had told a friend that rapid-prototyping is a toy and isn’t ready for prime-time, since the typical materials have it uses has poor mechanical properties. I’ve been quite a skeptic, but recently, I have had the luxury of having access to an industrial rapid-prototyping machine and have been trying to think of way to use it in a productive way. After using it and understanding the utility of them, I am beginning to believe…

With traditional machining, one of the most difficult things about mechanical design is figuring out how to make your parts machinable. In other words, can you make a square hole or square interior corner with a mill or lathe? Realistically, no. Or, how can I get the mill tool into the right area to remove material? Is the tool long or stiff enough? Can you make something hollow? You’d be surprised how many times you would come across problems like these in designing something complicated. Rapid prototyping can solve many of these problems, since the process doesn’t care how complicated a part is or how many features there are. But this isn’t to say that this is the end-all-be-all solution. It has it’s limitations as well: non-precision surfaces, less material strength, minimum thicknesses, and certain types of objects are unprintable, i.e. non-self-supported/floating or thin/wiry objects.

The most straightforward and obvious application of rapid prototyping is being able to physically play with and quickly test an idea, no matter how complicated the part. You take an idea, model it, print it, and it’s in your hands in less than a day. In stark contrast, machining a complicated part out of a solid block of metal can take days or more than week, mainly for planning and fixturing, depending on the number of features. Suppose you forgot to add a feature or sized the part wrong, just add it or scale it up, print it, and you’re done. With machining, you adjust your CNC code, if you’re so lucky, and re-machine a new part, which still can take a whole day or more.

But, probably the single-most greatest strength of rapid prototyping is that it doesn’t care how many parts you want to make or how different they are. You can design hundreds of small parts that all different, send it to the machine, and it will create them all, likely in one run. Just try asking a machine shop to do that in a day… (Sorry Mike!)

Theo Jansen applied these strengths to rapidly create small models of his Strandbeests in one step, which simplified his process incredibly. To create something like this in the traditional machine shop methods or even injection molding or casting, it would take as much time and effort as building a full size one out of his typical electrical tubing or wood. Here, the utility and the virtues of rapid prototyping can be fully realized.

While there are many types of rapid-prototyping methods, many of these machines are outside the reach of the home hobbyist since they can require expensive materials, as in powdered titanium alloys, and high powered equipment, as in lasers that not only can make you blind, but can also burn a hole through your eyeball. With these machines, you can easily create high resolution, accurate (around +/- 1mm), strong-ish parts (about half the strength and stiffness of traditional materials), made from high strength plastics like nylon, aluminum or glass-bead impregnated plastics, or even fused steel or titanium alloys. These machines are usually reserved for industry and professionals, but Shapeways provides this service for everyone at a reasonable cost, charging by the cubic centimeter. Just upload a solid model, pick the material (even stainless-steel), wait a week or so, and you got your part! Not exactly rapid, but better than nothing!

If you’d like your own at home, the popular open-source Reprap project is great place to start, but has limited capabilities. The project revolves around the idea of creating a ‘self-replicating’ machine, which currently lays melted, extruded plastic much like a hot-glue gun in a controlled CNC fashion. The objects it can create are like what you would expect, like they were made with a hot-glue gun, except a little harder and stronger. Great to screw around with, but the parts don’t have much mechanical utility since they are relatively soft and non-uniform, meaning that you wouldn’t want to use it for anything that would be under any type of force… at least not yet. Development and popularity of the project is high and I’m personally excited to see where it goes from here.