Way back in 2003 I managed to produce a complete set model for a production of Mozart's opera DON GIOVANNI on a 3D printer. (right) It was probably the first time this had been done, but I quickly realised that it was not the best use of this new technology for many parts, such as the base and any thin 'flat' elements, would have been far better made from wood or card in the traditional way. Also, some parts of the model are impossible to reach with a paint-brush, making it very difficult to paint. Now, many years later, I use a 3D printer to make just those parts that are difficult or time-consuming to make by conventional techniques, particularly detailed pieces of scale furniture, which can take a frustratingly long time to build by hand.
In 2003 3D printers were still a novelty, and I was lucky to have access to the Zcorp Z400 3D printer recently acquired by the IT department at the University of Alberta. It was a large, cumbersome and extremely expensive machine. Nowadays, if you do not have access to a 3D printer, you can get your stuff printed online. The process is very straight-forward: You log on to a web site, upload your file and select the size and material you require. You are immediately told if the file will produce a viable object and are given an estimate of the total cost including shipping. If you decide to proceed, you add your address and payment details in the usual way, and the model arrives by mail about a week later. I have used
http://www.shapeways.com and
http://www.i.materialise.com and both have given excellent service.(below left)
Desktop 3D printers are now available at more reasonable prices, and I have a Makerbot Replicator+ in my home studio (below) that can produce objects up to 25.5L x 19.5W x 16.0H cm. It uses PLA, - an organic, biodegradable plastic filament, fed from a spool at the back through a heated nozzle which moves rapidly over the build-plate extruding a very fine thread of molten plastic that hardens almost immediately and gradually builds up the model layer by layer. The objects it produces are remarkably strong and take paint pretty well. I usually apply a base coat of off-white acrylic, and finish with gouache.
The Makerbot Replicator+ has its limitations, but it is usually possible to work around them. The severest limitation is its inability to cope with large overhangs: the keyboard of a piano, for example. The software can construct temporary supports to be snapped off after removal from the printer, but I prefer to design complex models in separate parts to be assembled after printing. When designing a model such as a grand piano it is a simple matter to separate the legs etc. and add appropriate slots in the bottom of the case so the legs can easily be set into position and fixed permanently in place with a few dabs of superglue. Several objects can be printed at the same time, so all the parts of the grand piano can printed together, as in the photograph below. (An easily removable mesh or 'raft' is printed underneath the parts for better adhesion to the build-plate.)

Many software packages for 3D modelling are available, but my personal preference is for AutoCAD as this is the theatre industry's standard drafting software and the most widely used CAD software world-wide. It contains excellent 3D modelling and rendering capabilities, so items can be lifted directly from your plans and working drawings to form the basis of a 3D digital model which can then be exported to STL (stereolithography), - a standard format for 3D printing.
The STL files are imported into Makerbot's proprietary software, and you are shown a diagram of the printer's build-plate with your object(s) on it, so you can arrange them as you wish. (right) You can specify the thickness of the object's 'skin' and the amount of 'fill' to be used: 100% fill will produce an entirely solid object, but a fill of, say, 10% will create an open honeycomb inside the object that can save a considerable amount of filament when printing large objects. The software then prepares the objects for printing by digitally 'slicing' them into very thin layers and re-saves them in a format specific to the printer. It also estimates how long it will take to print and the amount of filament required. The resulting Makerbot file is sent to the printer via a wifi link or on a USB stick, then just hit 'PRINT' and off you go! The complete grand piano takes 2h. 40m. to print.
AutoCAD has excellent rendering capabilities that can produce a very realistic CGI version of your set, including painted detail, textures and lighting, from whatever viewpoint you choose. (right) A digital model can never completely replace a well-made physical model, but it does have some advantages: At the University of Alberta I have access to a 'Virtual Reality' room, where my digital set model can be presented so that, wearing stereoscopic visors, the cast and technicians can be given a convincing illusion of moving around in the full-sized set, long before the real set becomes available. (below)
The first 2D printer I owned many years ago printed only in black and at a very low-resolution. I longed for one that could turn out high quality images in full colour. This of course, has now become commonplace, and I believe the 3D printer now sitting on my model-making table is at about the same level of development as my first 2D printer, so it seems inevitable that much more sophisticated versions will soon be available, that will cost much less and be capable of producing very detailed models in full colour and a wide range of materials. 3D photo-booths are already appearing in shopping malls where visitors can be scanned and buy a small, full-colour 3D print of themselves. So now a set model can contain a miniature version of yourself or (more relevantly,) an actor in costume, to demonstrate the scale. (below)
THE THINGIVERSE is an interactive web site sponsored by Makerbot where anyone who has designed an object suitable for reproduction with a 3D printer may upload the STL files, which can then be freely downloaded and printed out by anyone interested. A vast number of objects is available, some of which (such as pieces of scale model furniture,) are of particular interest to theatre designers.
Click here to go to my own Thingiverse page, which includes the grand piano model described above, along with many other objects I have designed for 3D printing. Click on an object's picture to go the the Thingiverse download page containing further details and more pictures of the object concerned.