rapid prototyping techniques - cnc


This article is contributed by Gene Batten, an experienced CNC specialist.


CNC stands for “computer numeric control”. CNC machining is the earliest computer controlled rapid prototyping technique. First CNC systems were driven by simple controller, where a path was inputted as series of numeric combinations. Such controllers could be attached to a milling machine or a lathe, equipped with servo motors executing movements along x, y and z axis. During several decades CNC machines evolved into much more sophisticated systems, able to interpret a direct input from CAD based 3D files. Today computer controlled milling machines can precisely cut or carve flat two-dimensional as well as more complex three-dimensional shapes.

Unlike additive rapid prototyping methods such as FDM, SLA or SLS, CNC is a subtractive process. Milling machine is carving a desired shape out of the whole instead of building it. This concept gives the CNC method unique advantages, such as unlimited choice of materials and relatively low cost of the process. Practically any machinable material can be carved by CNC system. A list of materials includes, but not limited to wood, MDF, wooden and plastic composites, styrene, acrylic, PVC, PVC foam sheets and other machinable plastics, tooling boards, curable resins, Styrofoam, high density foam boards and blocks and many other materials.

Architecture of a typical CNC system is fairly simple. It includes a spindle or router motor turning a cutting tool at high speed, a cutting tool, and several computer driven servo motors, executing a movement of the cutting tool along the trajectory, which is a summary of series of movements along the x, y and z axis. A CNC control computer reads and interprets a “toolpath” file, specially prepared for producing each specific part and sends a series of commands, called G-code, to the servo motors. A CNC machine also includes a framework and a system of rails and guides serving to secure the material during processing.

Cutting tools and the cutting speed vary over a wide range. A CNC operator selects appropriate tool and speed depending on the material being cut and a part’s properties. Such wide variety of cutting tools (bits) and cutting speed range are also definite advantages of the CNC method.

CNC machines come in different sizes and configurations. They range from small desktop models to large commercial machines with beds up to 5' wide by 12' long or even more.

Fast, precise, dependable and inexpensive CNC technique became a friendly aid of professional model makers’ work decades ago and stays that way, offering new opportunities as the CNC systems continue to evolve.

Here are the most typical applications for the CNC method in professional model making.

Flat parts
Flat parts with low relief features are a classic case for CNC machining. Fast, precise carving turns flat sheet or block of material into complex part with recesses and holes of various depths, shapes and configurations. If needed, multiple parts can be produced using the same toolpath file, and all these parts will be accurate and identical.
Usually undercuts in such parts could pose a problem. However an experienced CNC programmer in most cases will be able to find the way around undercut by separating it into a new part and creating a nest for it in the main part. In a matter of hours CNC system does the job that otherwise would take days of manual fabrication.
Topographical models
CNC technique is irreplaceable for carving topographical models. Back in time creating topography by manually cutting and gluing together dozens or even hundreds of topographical line strips was an extremely time consuming process. Today CNC machine can carve a whole topography in a matter of hours with exceptional precision. Depending on model’s style topographical lines can be left to be or smoothed out during the carving. Small and medium size topographies can be carved out of lightweight but more expansive high density foam or cheap but much heavier MDF or other wooden composite. Large size topographies we carve out of Styrofoam sheets. Styrofoam topographical models are lightweight despite their size, and the material itself is unbelievably cheap. Selecting the appropriate cutting tool and speed allows representing even delicate features in soft and fragile Styrofoam.
CNC carved topography requires little finish. After carving a model’s surface can be reinforced by fiberglass, selectively textured and finished to specifications. Precise nests for applicable building and structures that can be pre-set in programming will make installation of these building and structures faster and more accurate.
Thermoform and fiberglass patterns
CNC carving is especially effective for creating shaped patterns for thermoforming (vacuum-forming) and fiberglass. Patterns can be carved from solid tooling or MDF boards or packs of such boards glued together. Practically unlimited choice of materials, one of the main advantages of the CNC method, allows producing such patterns directly out of materials that are able to withstand thermoforming or fiberglass layout.
Vacuum-form and fiberglass patterns’ surface can be sand, polished and coated after carving if needed in order to prolong the life of the pattern and make the production more effective.
Complex tree-dimensional shapes
Evolution of CNC controlling computer gave CNC machines the ability to read and interpret a direct input from CAD based 3D files. Modern CNC systems are able to produce complex configured 3D shapes and models, such as sculptures. A surface of CNC carved model or sculpture may be not as pristine and smooth as of SLA grown one, however wide selection of materials and low cost compensate for this disadvantage. As the cost of parts and sculptures produced by additive methods, such as SLA or FDM, dramatically rises with a size of the object, CNC carving in such case becomes the most cost effective technique. And a surface of a large model or sculpture can be finished to the desired standard after the carving.

Samples of the CNC carved Parts

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Unfinished CNC carved windows' grids for 1:5 scale model of MaxxPro Dash MRAP

CNC carved windows' grids for 1:5 scale  MaxxPro Dash MRAP, finished and installed

Carving of the topography model on large format CNC machine

CNC carved model of a military installation, finished and painted

Scale model of the military camp, CNC carved, finished and painted

Unfinished CNC carved model of the lidar discovery of Caralol - the largest Mayan City

The model of Caracol - finished, painted and mounted on mahogany display base

CNC carved letters for ornamental interior design map

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