Computer Numerically Controlled (CNC) milling and routing are two of the most firmly established digital fabrication techniques, with a 40-year history in relation to architecture. Although specifically used as a ‘prefix’ for these applications, the CNC process actually underlies most digital making technologies since it uses a computer system to generate coded instructions that in turn control the movements of a machine tool. This basic explanation shadows a complex procedure in which the CNC program coordinates a number of different tasks at any given time, including motion control, tool changes and spindle operation. Here we begin to encounter the vast array of options available to designers when organizing the movement and operation of the milling or routing head. This sequence of control functions is known as the ‘tool path’, and provides a set of instructions for the machine. The milling or routing of materials may be achieved in an almost endless number of ways, and even small variations in the instructions may produce significant differences in the end products. This is one of the main reasons that a skilled workshop technician or machine operator may prove valuable to the process, particularly where machinery has four or five axes in use and the operational parameters may become highly involved. CNC commands are effectively short computer scripts that tell the machine what to do. The majority of these functions are typically designated with a letter ‘G’, and as a consequence CAD/ CAM technicians often refer to CNC instructions as ‘G-code’. These instructions, also known as preparatory codes, are typically divided into several primary commands such as a rapid move, a controlled move in a straight line or arc, a series of controlled moves (resulting in a cut, hole or profile), or to set tool information. The command combinations directly affect the fabrication process and thus any manufactured components.
CNC milling or routing has two main roles. In its first, and perhaps most immediate, use it may be applied to remove material from a volume and fabricate components in a manner similar to carving, i.e. the material forms that remain on the machine bed are the desired design components. Architectural designers, keen to achieve complex geometries and fluid aesthetics with precision and efficiency, have readily exploited this process. Since the G-code interface may be used to maximize the arrangement and number of different components produced from a volume of material, this process may also reduce the amount of waste material and facilitates effective and relatively economical making of non-standard components. Such optimization is core to the growth of this digital fabrication technology within design disciplines, because it allows the material volume to be fully used within its limits. The second application for CNC milling or routing relates to the results of the process rather than being an end in itself. The high degree of accuracy and complexity of surfaces and forms that may be fabricated using this method means that it is also able to make geometrically sophisticated and very detailed moulds. These moulds may then be used to cast other materials, and are therefore often a key element in formative fabrication methods. Milling and routing processes are similar since they both use a rotating cutter to subtract material, but whereas milling is useful for metals amongst other materials, routing is typically applied only to wood and plastics. Because of the comparative density of these materials, the router cutting head is able to remove a much larger amount of them in a given time frame. As a direct consequence, specific machine types have been developed that increasingly differ from those used for working metals.
Routing is actually a parent term that includes numerous machine operations such as drilling, grooving and shaping of materials. The machine designs vary from relatively small tabletop versions appropriate for modelmaking to large machines able to handle and finish components typically up to 1.5m in each direction. Industrial manufacturing equipment may go well beyond these limits dependent on the production process and elements required. The most commonly used machines are three- and five-axis types. Although three-axis control should enable the cutting head to access any desired point within the work field, these operations are often limited due to obstruction by the actual component being fabricated. Five-axis machines therefore provide a further two rotational axes, perpendicular to one another, which facilitates the cutting head to reach internal areas or overhangs etc. Many applications of the routing process involve sheet materials, such as plywood or MDF, and therefore the work field is usually greater in the X and Y axes than the Z axis. Gantry configurations of machines are the most widespread, featuring either a fixed or moving worktable upon which the material is mounted. The advantages of moving worktables over their fixed counterparts are that they are often more economical and even precise but they are constrained by the size and weight of the element being fabricated. Fixed worktable set-ups overcome these limits through the use of a moving gantry but such operation needs to be able to move with precision and speed whilst withstanding the forces encountered during cutting.
CNC milling and routing machines come in a range of sizes and types, depending on the purpose and materials intended for processing.
The most common type of CNC milling and routing machines are the gantry type, shown here, which may be used to cut a variety of materials through a simple change of tool bits in the machine head.
TIP LAYOUT SHEETS
A simple error when transferring digital design data for use with sheet-material fabrication processes is the layout sheets not being ‘flat’ in CAD space. This can easily be rectified by aligning the Z axis to zero prior to machining.
This domestic garden shelter, designed by John Bridge, illustrates the design and fabrication process undertaken, with emphasis on CNC milling as an integral part of the manufacturing path.
1 The design was initially developed digitally in relation to the maximum size of plywood sheets able to fit on the CNC milling bed. Starting with a cuboid form, a sphere was ‘grown’ from the epicentre, and by applying modifiers the spherical form was pulled towards the internal, cuboid geometry.
2 Prior to full-size fabrication, the design was tested using a laser-cut MDF scale model that simulated the proposed method of manufacture and enabled potential issues to be identified. It transpired that structural stability was a major problem since the ribs were in one plane only. This led to the development of cross-sectional ribs, affording easy assembly and rigidity.
3 Once all the components were drawn in CAD software, sheets were prepared for ‘file-to-factory’ order. A specific package optimized the number of components from each sheet, reducing material waste.
4 Any drawing errors in the sheet files are typically identified in the CNC machine’s software, but care should be taken to ensure that the data is correctly formatted. Using the cutting patterns, or toolpaths, from the CAD/CAM files, the CNC milling machine cuts the full-size components from 18mm plywood sheets.
5 The components are slotted and jointed together. Assembly may be greatly assisted by using the CNC machine to provide reference numbers and pre-cut holes and slots.
6 To protect the wood from weathering, a UVi polythene skin was stapled and glued to the structural shell to allow outdoor usage.
The form and size of the cutting tool itself distinguishes the various machining operations possible with routers. As with the majority of CNC machines, tool changing is often an automatic procedure carried out under part-program control. First-generation three-axis machines provided general routing processes including drilling and edge profiling, but current five-axis configurations have extended this range to include sawing, contouring and carving.
The wide range of machining operations achievable with routers is informed by size and form of the cutting tool. Part-program control within the G-code instructions typically includes tool changing automatically as is the case with most CNC machines. While three-axis machines are effective for general routing procedures such as drilling and edge profiling, five-axis versions extend these operations further to facilitate carving, contouring and sawing. The primary process of routing uses a fluted cutter, typically less than 25mm in diameter, which rotates at high speed to remove material. An almost limitless combination and types of shapes and apertures may be achieved due to the spectrum of different cutter geometries available but it is worth understanding the main applications:
• Drilling or boring – this is perhaps the most familiar process. Small diameter holes are made using a corresponding sized router, which is sunk into the material as with a conventional drill. A large diameter hole may be produced by routing along its perimeter outline and then removing the waste disc left behind.
• Carving – this is implemented to fabricate contoured 3D surfaces that may feature complex curvilinear geometry. This process may involve several different types of cutter in any single application. Excess material is first removed using a ball-nose cutter and then once the rough shape has been carved, pointed detail tools are used to create intricate surface features and a high level of finish.
• Shaping – this is applied to fabricate profiles on edges, both internal and external, typically with a better level of finish and efficiency than routing. Cutter diameters may be up to 150mm and above, which coupled with interchangeable blades enable complex profiles to be produced.
• Sanding – this may be used to finish profiled edges etc. and is achieved through a sanding head which is similar to a shaping head, providing components ready for assembly.
• Sawing – where large quantities of linear cuts are required, routers may lose their efficiency in relation to other equipment such as circular saws but a number of tooling attachments are now available to augment routing operations if required.
• Squaring – this is possible by oscillating scraper blades across the work field and is used to remove significant excess material that would be inefficient or inaccessible to remove using other routing or cutting procedures.
CNC milling enables designers to fabricate components with highly intricate surface features, as shown in this work by Rupert Griffiths.
STEP BY STEP MAKING A PAVILION Prototype Unit, Manchester School of Architecture – Reflective Room, Manchester Museum, 2010.
This temporary structure provides a public ‘room’, formed by two interlocking wall elements, that includes a continuous bench, allowing users pause and a respite from the city. Designed and constructed by students from the Prototype Unit led by Ming Chung and Nick Tyson, the design and fabrication process utilized a range of media and techniques.
1 Initial design ideas used CAD software, producing laser-cut 1:10-scale models that enabled testing of three-dimensional structure and assembly methods.
2 Following this stage, full-size prototypes were fabricated using manual and digital tools to allow detail connections to be resolved and material properties to be assessed. Structural considerations were reviewed and tested at 1:1 alongside preliminary calculations by Atelier One Engineers. This facilitated the optimizing of both material effects and structural assembly.
3 Standard-grade external plywood was selected as an appropriate material for the structural carcass, and finer-grade birch ply for the skin. This design cycle of prototype making, testing and adjustment directly informed the development of two-dimensional CAD design drawings.
The two-dimensional design drawings, converted to a three-dimensional CAD model, were used to test the assembly of components and enable structural calculations to be finalized. A tectonic system was established that utilized components cut from a flat sheet material and a dry slot-jointed construction method that provided an overall structural matrix. Vertical ‘rib’ and horizontal ‘tab’ standard-grade plywood components formed an external carcass that supported a synclastic curved plywood interior skin, embedded with standard glazed ceramic tiles. The plywood components were precision machined using a CNC router, and assembled by hand. The router was selected as an appropriate machine tool owing to the precision required for the dry-slot assembly and its ability to meet production demands within the allocated timeframe. Individual components from the three-dimensional CAD model were converted to two-dimensional templates in ArtCAM to set tool paths and optimize layouts on plywood sheets.
4 A workshop-based prefabrication sequence for component parts was predetermined by the sequence of assembly on site, and established batching for manufacture and delivery.
5 Manual prefabrication methods of cross lamination for the larger ‘rib’ and ‘skin’ components were developed as a consequence of the standard sheet dimensions and the limitations of the CNC router. Site work commenced with the installation of ‘base plates’ that provided a precise setting-out template for manual assembly of the ‘rib’ and ‘tab’ components.
6 The ‘skin’ layers were manufactured oversized, allowing on-site adjustments to accommodate the geometries of the synclastic, curved surface. Manual construction skills allowed adjustment to take place in order to resolve problems found on site.