CNC / Rapid Prototyping Guide

Building

Cutting / Milling





Building

Processes in this area are continuing to evolve but typically build objects in a series of layers between .01 and 1.0mm thick. This allows for the precise construction of complex geometry, which enables designers to explore forms that would previously have been too complex or expensive to model by hand.

Among these are Stereolithography (SLA), Selective Laser Sintering (SLS), Fused Deposition Modelling (FDM), Laminated Object Manufacturing (LOM), Inkjet Systems and Three Dimensional Printing (3DP). Although there is a maximum size for individual objects, it is possible to construct larger objects by connecting these together.

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Software for building

Industrial design and 3D modelling software is used and from an artistic viewpoint the most important consideration might be flexibility in modelling three-dimensional forms. An essential consideration with any software package should be its ability to export files in a format that is useful for RP – particularly .stl format .

For example, 3T RPD at Newbury accepts these formats;

.STL (Rapid Prototyping file [Binary with 0.01mm tolerance preferred])

.IGES (Initial Graphics Exchange Specification universal translator)

.EXP (Catia export file)

.PRT (Pro/Engineer)

.STEP (International Standard for Exchange of Product Model Data)

The Resources page has a list of software that is commonly used for industrial design and 3D Modelling

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Stereolithography (SLA)

This technology uses a UV laser to cure a liquid epoxy resin. The laser describes a shape for layers approx 0.1mm thick and these layers build to form an object as a build table drops within a tank. The resulting model / object is semi-transparent and generally has an off-white or yellow colour to which different finishes can be applied.

Developed by 3D Systems http://www.3dsystems.com

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Selective Laser Sintering (SLS)

This process is similar to SLA except that a powder is sintered (fused) by a laser. The powder is spread on a build table within a heated chamber and the laser provides the additional heat to sinter the shape it describes for each layer of the object. The powder comes in a variety of forms, such as Polystyrene, Nylon and Glass-filled Nylon and a variety of colours and finishes can be attained. At the end of the process the object is embedded within unsintered powder.

SLS was developed by DTM and EOS
(Lawnmower and springs image provided by 3T RPD)

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Direct Metal Laser Sintering (DMLS)

A fine layer of metallic powder is spread over a plate and is sintered by a laser beam which describes the data from a CAD file. Like other build technologies the object is constructed as consecutive fine layers or slices are built up while the object moves down the build chamber

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Thermojet Modelling (Wax)

This process builds a model / object from wax much like an ink-jet printer prints coloured ink but using heated wax droplets. The resulting objects can be fragile but are useful for investment casting, particularly suited for jewellery. It can also be a quick method of producing prototypes for design development and is easily carved by hand. The colour and the consistency of the wax give the objects an appearance that may not be suitable for finished objects.

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Other Techniques

There are a range of other techniques used for RP such the Envisiontec Perfactory Mini (Multi Lens) System, Pulsed Laser Deposition Pulsed laser deposition (PLD) and Fused Deposition Modelling (FDM). There is even a Rapid Freezing Prototyping (RFP) process currently being researched in the U.S.A.

For a comprehensive overview of RP techniques see: The Whole RP Family Tree at:
http://ltk.hut.fi/~koukka/RP/rptree.html

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Casting / Moulding

In some instances the basic RP object serves as the artwork but in may cases the object acts as a prototype for casting or moulding. Because of cost and the limited size of build chambers artist have generally been concerned with producing relatively small objects. However, lage objects can be produced by using adhesive to connect smaller parts or through welding cast parts together. Parts can be nested in the build chamber so as to save on cost.

Some secondary techniques for RP include;

Cast Metals
Investment Casting using a RP object to create a mould to cast aluminium, stainless steel, etc. Cast metals can also be CNC machined.

Injection Moulding
Generally a thermoplastic polymer is injected into metal mould, which can be opened to remove the object.

Vacuum Casting
Uses polyurethane resins to produce low cost models.

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Cutting / Milling

CNC cutting and milling has been in use for a longer period than RP and are relatively common in manufacturing. There are a range of applications for cutting, bending, hole-punching, milling, engraving, etc. and a number of different technologies, which are driven by CAD data are used. The advantages of CNC are its accuracy and speed.

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Software for cutting

All of the software packages listed above in the Building section can be used for cutting technologies and particularly for milling. In many instances however the commercial software file format is converted to that used by the software that interfaces with equipment and this generally requires some work (and a cost) for the CNC company. Where cutting involves materials in sheet form DXF file format can be used and cheaper vector-based packages such as Adobe Illustrator can export in this format.

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Routing

This technique can be used to cut a range of materials such as wood and aluminium, plastic and Styrofoam. The spindle on the machine follows a path described by a computer file and this allows for a high degree of accuracy and is particularly useful for the mass production of identical shapes. It is possible to build layered objects through stacking sections or profiles, although there is usually a limit to the thickness of material that can be cut. Internal corners are a problem as the diameter of the spindle means they will have a radius. CNC routing is widely used by industry and particularly wood workshops and sign writers, who often use it to cut plastic lettering. The objects in the adjacent image were cut using a router.

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Laser

This technique uses a laser to burn or melt a path described by computer files and is mostly used to cut metal, including stainless steel. There is a higher degree of accuracy and speed than other technologies and a CNC laser can also be for welding. The depth of cut can also be controlled with a laser so that forms can be made through the removal of material. The edge finish along the cut is better with a laser than with other techniques.

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Waterjet

This technique used a high-pressure waterjet forced through a narrow opening to cut a range of soft materials such as paper, carpet, cloths or foam. When an abrasive is added to the water (abrasivejet) then harder materials such as aluminium, stone or glass can be cut. There is a limit to the thickness of material and a finer cut than routing is possible. However, internal corners will still have a small radius.

The panels in the adjacent image were cut from aluminium sheet, anodised and fabricated so that the coloured sheets behind showed through holes cut in the front sheet.

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Wire

This technique is generally used to cut soft materials such as foam and there are basically two methods: One where a horizontal wire moves through a block of foam and describes a shape by moving up and down the vertical axis and another where a vertical wire describes a shape in a manner similar to a waterjet. The vertical system allows for thinner sheets to be cut.

The object in the adjacent image was built from layers of foam cut using a vertical wire system at Dunlop Foam in Brisbane.

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Fabrication

The accuracy of CNC cutting means that elements can fit together easily for the fabrication of artworks. Apart from cutting, CNC can be used for milling, drilling, tapping, bending, welding, grinding, etc and many industrial items are fabricated or assembled from components at the end of a CNC process. Although cutting processes typically work with sheet materials it is possible to use CNC machining centres to “carve” blocks of material such as stone or metal. It is also possible to build objects from layered sections of sheet material or to construct an armature for cladding.

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