The router can cut large sheets of material such as plywood in much the same way a laser cutter cuts cardboard. CNC milling also can produce terrain models with large lengths and widths but relatively small elevation changes. Three-axis machines cut from one side only. Half-rounded objects are easy to machine, and fully round objects can be produced by rotating the pieces for milling on both sides.
The real Ballingdon Bridge, with its curving arches and elegant piers, is 40 meters (130 feet) long. It was designed by British architect Michael Stacey, of the Department of Architecture and Spatial Design, London Metropolitan University, formerly of Brookes Stacey Randall Architects. The bridge's primary structure is composed of two parallel, precast, arched concrete sections. The roadbed and sidewalks with railings were poured on top of the primary structure on site.
I worked with masters student Jonathan Friedman to create the scale model of the bridge. Stacey provided us with the original design model for the precast structure in 3D Studio format.
To that digital model we added blocks of solid material on each end representing stock material that would not be cut during the milling process. These would provide the means to secure the model to the work surface and to simplify turning the model over. We exported the 3D Studio model to stereolithography (STL) format. Then we cut the stock material and produced a precise 3D model of the physical stock.
Using Visual Mill software, we created the tool paths that would guide the CNC milling machine to cut the scale model from the stock. This process does not require knowing how to program tool paths, but it does require knowledge of the different tool-path strategies and of the appropriate tools to use with each strategy.
The Art of Milling
It is important to note that, unlike other rapid prototyping technologies, CNC milling is not an automatic process. How each step is executed determines what is possible, how long it will take to machine, and what quality of finish will be produced.
After experimenting with five tool paths, we chose a strategy with three passes. The first was a horizontal rough cut with a 1-inch (2.54-centimeter) flat mill. This tool path strategy leads to the removal of the bulk of the material using a flat-ended, drill-like cutter and cutting at constant horizontal levels to produce something resembling a stepped contour terrain model.
The second cut was reroughing with a 1/2-inch (1.27-centimeter) flat mill. Reroughing is like horizontal roughing, but this tool path continues on where horizontal roughing left off, by cutting smaller horizontal steps into the model.
Third and last cut was parallel finishing with a 3/8-inch (0.953-centimeter) ball mill. Parallel finishing is cut with a round-nosed drill-like cutter (flat end mill) with the cutter moving in parallel paths up and down over hills and valleys and taking the material down to the final surface.
Limiting the process to three tool paths halved the possible milling time. Selecting the tool path strategy is an art that can only be learned through experience. This is why CNC milling is not an automatic process but a creative and intellectually stimulating one. There is very little documentation on tool path strategies especially for wood and other nonmetallic materials.
We performed a test cut in rigid insulation of the top side of the bridge before cutting the final model in wood. We located the origin at one end of the stock so that the stock could be rotated about a plane of symmetry without shifting the origin. We used a straightened nail with a sharp point to locate the X and Y position of the origin. The vertical Z position of the tool was then set using the method of just being able to slip a piece of paper between the stock and the bottom of the tool.
In thinking through the clearances that would be needed by the tools and spindle assembly, we realized the underside of the bridge would prove challenging. From the surface of the blocks of stock at both ends, the model plunged straight down for 9.53 cm (3.75 inches) as did the piers of the bridge. If we had used a 3-inch (7.62-centimeter) cutter, we would have had to lower the spindle assembly, but that assembly would have struck the stock. The solution was to order tools that were long enough for the depth of the final cut and of constant diameter for that length.
Value of the Physical Model
The physical model provided a critical appreciation of the shape of the surfaces that could not have been obtained from viewing a rendered 3D virtual model. We did not really understand the nature of the undercut over the piers until we could see and hold the machined model.
We could have produced the model by other methods, such as 3D printing or stereolithography. But milling the model as a single piece saved time in comparison to making sections to fasten together. And the resulting scale model displays a more polished appearance for presentation purposes.
Machining scale models can also serve as a step in designing the fabrication of full-size objects. For example, there was a stage in the design of the actual bridge during which the formwork for the precast section was designed. CNC milling could have been used to model the individual precast sections and their joints.
While the curvilinear concrete formwork for Ballingdon Bridge was built manually from plywood by craftsmen, in the future, CNC milling might be used to create those forms. CNC machining can also be used for plastic injection molding to model and then build architectural components.
Moving from the mind's eye to physical scale models remains an important component of the design process. Three-axis CNC machining provides unique opportunities that we are just beginning to explore.