Stefanie Mueller, Patrick Baudisch
The invention of new input and output devices has been a longtime enabler of human-computer interaction research. In particular, a combination of analog-digital and digital-analog converters allowed us to replace analog solutions with digital solutions. For instance, the invention of 2D scanners/printers allowed ordinary people to create printed products, and similarly the invention of CD readers/writers allowed everyone to compose video and music in a simple digital editor without the use of expert analog tools.
One of the latest versions of this pattern is personal fabrication devices, also and especially in combination with scanners that digitize physical objects. Problems once solved using a specialized analog machine we can now digitize and solve using computer science techniques, and then translate the results back to the physical world. The transformation of the mechanical problem into a computer science problem allows computer scientists and especially human-computer interaction researchers to contribute, for example by developing interfaces that fill in domain knowledge.
The most popular personal fabrication tools right now are 3D printers. However, 3D printers are slow and thus limited to batch-style processing; the print job runs overnight and the user picks up the result the next morning. In contrast, laser cutters are faster by orders of magnitude: Within minutes, the user has a result and can continue iterating toward the best solution. While laser cutters are inherently limited to cutting 2D pieces, there are ways that allow them to solve 3D problems. Figure 2 shows one example, and we will explain different approaches in more detail below.
Laser cutters work by directing the output of a high-power laser through optics to the desired cutting location. For instance, in the laser cutter shown in Figure 1, the high-power laser is located at the back of the laser cutter and two diagonal mirrors reflect the laser beam toward the 2D sheet of material that needs to be cut. The position of the first mirror determines the y-location; the position of the second the x-location of the current cutting point.
A lens focuses the laser before it hits the workpiece. The focal length of the lens determines at which distance the laser is focused (and thus has its maximum impact and the narrowest cut). The workpiece needs to be located at that distance. To achieve this, most laser cutters have a table that can be moved up and down, either manually or automatically. Because the lens focuses the laser on a certain spot, the thickness of the sheets that can be cut is limited (with our laser cutter we can cut sheets up to 10mm thick). Also, when cutting a sheet, the cut will never be perfectly straight, but rather slightly slanted as the lens starts to defocus (this can be a problem when cutting joints that should fit).
Besides focusing on top of the sheet, the two most important parameters in laser cutting are the power at which the laser is operated and the speed at which the cutting head moves. The thicker the sheet, the more power is required. For more natural materials, such as wood, it can be difficult to find good settings, as they tend to have irregularities inside the material (e.g., knotholes).
While the laser cutter is cutting, material evaporates at the cut line, which needs to be removed from the laser cutter for two reasons: Particles in the way of the laser beam can catch fire and burn, leading to unclean cuts or smoke that can damage the lens. Plus, when inhaled, those particles can lead to health problems. The best way to remove them is to use a fume extractor in the form of a filter with suction. The filter often consists of several layers that have different granularities, from very coarse to very fine-grain. They need to be replaced from time to time.
The most common materials for laser cutting are acrylic, wood, and cardboard/paper. Since a sheet of acrylic can cost between $10 and $20 (depending on the thickness), it is often useful to prototype using cardboard. Small laser cutters typically used in human-computer interaction research, product design, and prototyping do not reach the temperatures required to cut metal, but there are high-power industrial devices that can also cut these materials. In the automotive industry, for example, many parts of a car body are laser cut; cutting steel of several millimeter thickness is common. Other materials are possible, but it is important to make sure they don't create toxic fumes when burned by the laser.
The most common operation of a laser cutter is to cut along the outline of a piece to release it from the overall sheet, something called the vector mode. However, laser cutters can also engrave images by using the gray values of the different pixels as values for how deep the laser engraves into the material. This results in a relief-like appearance, with a limited 3D effect, as the laser always comes from the top and cannot create undercuts. The laser engraves the image line by line in so-called raster mode, which takes more time than when cutting only an outline.
The input to a laser cutter is a simple 2D drawing that can be generated using any drawing software. The laser cutter is typically set up as a printer; to send the drawing to the laser cutter one only needs to print the file using the regular print dialog. Settings such as power and speed can be defined in the printing preferences, which typically open a custom laser cutter software dialog. Different colors and line widths in the drawing can be used to indicate different power and speed settings for different parts.
As mentioned earlier, laser cutters can create only 2D parts. A range of methods are available to create 3D objects from these 2D parts.
The most common one is to use joints, such as finger joints, that allow assembling pieces, for instance to make a box. Drawing the joints by hand can be cumbersome, thus sites such as BoxMaker help in generating matching joints; the user simply has to specify the dimensions of the box. Recent research supports users in building more complex objects: Enclosed generates entire enclosures with joints based on a .NET gadgeteer component layout , and SketchChair generates laser-cut files with matching joints to build entire chairs from a user's high-level drawing .
Laser cutters are faster than 3D printers by orders of magnitude.
A different option is to bend a 2D piece into a 3D shape. For thermoplastics such as acrylic, this can be done using a heat gun to make the piece compliant. While a heat gun is normally operated manually, LaserOrigami shows how to integrate the bending into the laser cutting itself by defocusing the laser . For other materials, such as wood, living hinges allow folding (see the designs by SNIJLAB).
Finally, 2D pieces can be stacked to achieve a 3D shape. Software such as Autodesk 123D Make allows the conversion of a 3D model into 2D parts, which can then be laser cut and glued. Recent research shows that stacking can also be automated by using the laser cutter not only to cut, but also to weld sheets together (LaserStacking ; Figure 3). Stacked objects can also contain moving parts, such as gears and linkages, which can be used to create mechanical automata  (Figure 2b).
The traditional way to use a laser cutter is to first create a 2D drawing in a digital editor and then, only at the very end when the design is finished, send it to the laser cutter for fabrication of the physical object.
Recently, researchers suggest taking a different approach: In interactive fabrication systems, physical output is created immediately after every user input . This allows us to verify the design immediately and thus build on top of previous steps. Laser cutters are especially suitable for interaction fabrication, as they can create physical output within a few seconds.
In constructable (Figure 4), for instance, users draw with a laser pointer directly on the piece of wood in the laser cutter; the laser then immediately follows and cuts . Different laser pointer tools support the user in creating precise constructions; for example, the polyline laser pointer only creates straight lines.
One reason why laser cutters are not yet as widely accepted as 3D printers is that they are not as simple and clean to operate (requiring fume extractors, should not be left unattended while cutting, etc.). However, more and more companies are investing in exploring the possibilities of laser cutting. For instance, former MakerBot CEOs Bre Pettis and Jenny Lawton, who made consumer 3D printing big, just recently invested in a low-cost laser cutter start-up, also joining the company as advisors. In the past few years, we have seen increased interest in laser cutters on crowdfunding platforms such as Kickstarter, with projects such as the MicroSlice Arduino-controlled laser cutter being successfully funded.
From the human-computer interaction side, one challenge that still needs to be overcome is the mapping from 3D to 2D: Though one wants to build a 3D object, one first has to decompose it into a layout of 2D parts (Figure 2a). While this challenge certainly can be solved in traditional digital CAD editing (e.g., Platener  and SketchChair ), it is less clear how to achieve this for interactive fabrication, in which the user works directly on the 2D sheet of material.
Finally, one of the major challenges is how to support people in creating more complex objects by filling in the necessary domain knowledge. While solutions for specific problems exist (e.g., designing mechanical characters by simply drawing the desired output motion ), it is unclear what the different classes of problems are and what a generic solution could look like.
4. Umapathi, U., Chen, H.-T., Mueller, S., Wall, L., Seufert, A., and Baudisch, P. LaserStacking: Creating non-planar objects using a laser cutter without need for manual assembly. Proc. of UIST'15 (to appear).
5. Coros, S., Thomaszewski, B., Noris, G., Sueda, S., Forberg, M., Sumner, R.W., Matusik, W., and Bickel, B. Computational design of mechanical characters. ACM Trans. on Graphics 32, 4, (2013), article 83.
Stefanie Mueller is a Ph.D. student working with Patrick Baudisch in the Human Computer Interaction Lab at Hasso Plattner Institute, Potsdam, Germany. In her research she develops novel ways to interactively edit physical matter using personal fabrication tools, such as laser cutters and 3D printers. email@example.com
Patrick Baudisch is a professor in computer science at Hasso Plattner Institute at Potsdam University, Potsdam, Germany, and chair of the Human Computer Interaction Lab. After years of research on natural UI and interactive devices, his current research focuses on interactive fabrication and haptics. firstname.lastname@example.org
Figure 2. (a) Platener allows the quick fabrication of 3D models by converting them into 2D parts that can be fabricated on a fast laser cutter . (b) Coros et al. show how to automatically generate mechanical characters with moving parts .
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