4. Digital fabrication principles
The techniques of digital fabrication generally fit into four main categories: cutting, subtraction, addition and formation. These headings are analogous to traditional processes used in architectural modelmaking and even full-sized prototyping, wherein materials are manually worked with tools to achieve the desired results.20 However, it is useful at this point to understand the differences between these procedures before we start to discuss the various equipment and machines that facilitate them.
Cutting
Perhaps the most accessible and commonly applied method of digital fabrication is ‘cutting’. There is a range of different cutting techniques, but essentially they all enable the production of flat components using a cutting head that follows instructions provided by digital design data to make shaped elements from sheet materials. The cutting head and sheet material move along two axes in relation to each other. This may be as a result of a moving cutting head, a moving bed upon which the material lies or, occasionally, a combination of both. Sometimes referred to as ‘two-dimensional fabrication’, CAD/CAM cutting techniques are usually limited by the thickness of material they can cut, which generates different cutting technologies for different materials. Laser-beam, plasma-arc and water-jet are all types of cutting technology that permit specific applications. The use of laser beams to cut materials is a widely known and available method, and will be dealt with in considerable detail later in this section. At this stage, it is only necessary to know that the process focuses a highintensity beam of infrared light mixed with a stream of highly pressurized gas, usually carbon dioxide, to cut materials. The plasma-arc cutting process passes an electric arc through a compressed gas jet in the machine’s cutting head, which heats the gas into very hightemperature plasma, a state change that subsequently reverses as the heat is transferred to cut the material. Water jets, as the name implies, force a high-pressured jet of water, mixed with an abrasive, through the cutting head, slicing the material in a precise manner.
Below and bottom
Digital fabrication has led to a revival of craft albeit with a new and interdisciplinary understanding pertaining to the algorithmic and precise qualities that digital technologies may couple with more conventional methods of creative practice. As such, the artist Kumiko Shimizu’s Angry House project posits the notion of the human and the digital in a fluctuating relationship that concerns a rich dialogue between both analogue and digital design and fabrication techniques. Envisaged as a full-size pavilion, it is shown here as a laser-cut acrylic model and CNC-milled timber prototype.
Fabrication methods using a subtractive process take material from an existing solid volume, leaving behind the desired features and components. The excess material is typically removed through a milling or routing process. These machines are available with a range of axially constrained cutting heads depending on the required task. Two-axis milling machines work by having the rotating drill bit move along X and Y axes, thereby subtracting two-dimensional patterns of material. The addition of another axis, as featured in three-axis machines, enables the drill bit to be moved up and down along the Z axis, allowing material to be subtracted volumetrically. This process is a logical extension of the two-axis method, and whilst it has greater application in CAD/CAM techniques it is still fairly limited in terms of the components it can make. It is therefore with four- and five-axis machines that complex forms and surface features may be produced, as these augment the milling or routing process through further manipulation of either the cutting head or the cutting bed by providing additional axes of rotation. Furthermore, the drill bits within the cutting heads come in different sizes, similar to conventional drills, allowing for a variety of finishes and accuracy. Likewise, the milling or routing may be applied at various speeds in relation to the material’s characteristics.
Subtractive processes using conventional machines have advantages in relation to: • Component size – as much larger elements can be made with these machines.
• Material selection – as a wider range of raw material can be used.
• Precision – as more accurate elements can be fabricated due to lower tolerances.
• Production – as these machines are typically more economical and faster for larger quantities of elements.
Addition
In direct contrast to the above methods, fabrication processes based on an additive technique slowly build up material in layers rather than steadily removing it. This category of digital fabrication is most commonly known as rapid prototyping, though this is actually an umbrella term that includes a number of different techniques. However, all additive processes work on the basis of translating digital design information into a series of two-dimensional layers. The data of each individual layer is then used to direct the head of the fabrication machine, and the physical object is made through an accumulative process of layering.
Additive processes have advantages in relation to:
• Bespoke components – the direct conversion from digital model (usually to an .stl file format) means that no additional devices such as jibs or moulds are required for the efficient fabrication of unique elements.
• Complex form – since material is deposited at all required points to produce the element, allowing sophisticated geometry and internal voids to be easily fabricated.
• Fabrication environment – rapid prototyping machines are typically enclosed and quiet in operation enabling them to be installed in design studios.
• Non-expert use – since techniques such as rapid prototyping do not require users to have obtained specialist programming languages or machine skills prior to application.
Formation
Rather than removing or building up material, formative fabrication processes utilize mechanical forces to reshape or deform materials into the required shape. Heat or steam is typically used in such processes, to render the material being formed more pliable and then to retain its new geometry once it has cooled.